ASTROBIOLOGYVolume 3 Number 4 2003copy Mary Ann Liebert Inc

Research Paper

Earthrsquos Earliest BiospheremdashA Proposal to Develop a Collection of Curated Archean Geologic

Reference Materials

JOHN F LINDSAY12 DAVID S MCKAY2 and CARLTON C ALLEN2

ABSTRACT

The discovery of evidence indicative of life in a Martian meteorite has led to an increase ininterest in astrobiology As a result of this discovery and the ensuing controversy it has be-come apparent that our knowledge of the early development of life on Earth is limitedArchean stratigraphic successions containing evidence of Earthrsquos early biosphere are well pre-served in the Pilbara Craton of Western Australia The craton includes part of a protoconti-nent consisting of granitoid complexes that were emplaced into and overlain by a 351ndash294Ga volcanigenic carapacemdashthe Pilbara Supergroup The craton is overlain by youngersupracrustal basins that form a time series recording Earth history from approximately 28 Gato approximately 19 Ga It is proposed that a well-documented suite of these ancient rocksbe collected as reference material for Archean and astrobiological research All samples wouldbe collected in a well-defined geological context in order to build a framework to test mod-els for the early evolution of life on Earth and to develop protocols for the search for life onother planets Key Words Geologic curated collectionmdashArcheanmdashWestern AustraliamdashEarlylife on Earth Astrobiology 3 739ndash758

739

INTRODUCTION

EVIDENCE SUGGESTING the presence of ancientlife in a Martian meteorite has drawn atten-

tion to the possibility of life on other planets(McKay et al 1996) However an important les-son learned from this study and from the subse-quent controversy is that determining biogenic-ity in the ancient rock record is extremelycomplex With the prospect of sample return fromMars in the relatively near future it is thus im-portant to develop an effective sampling strategy

and a set of systematic protocols well in advanceIt is also important that we have time to developthe special array of tools that will be needed forthe analysis of the returned samples especiallydistinctive and robust biomarkers One of themost cost-effective means of doing this is throughthe study of Earthrsquos early history and its earliestbiosphere

To that end we are proposing the developmentof a collection of reference samples from theArchean of Western Australia This would allowa systematic study of the earliest Earth and its

1Lunar and Planetary Institute Houston Texas2NASA Johnson Space Center Houston Texas

biosphere by making sets of well-documentedreference samples available to a large number ofqualified investigators at no cost

RATIONALE

It has long been assumed that life evolved op-portunistically on Earth in a simple interactive re-lationship with its environment sustained forthe most part by an exogenic energy source (theSun) There are however several threads of evi-dence that are converging to suggest that the bios-phere did not simply evolve on the Earthrsquos sur-face but was in a very general way drivenforward to greater complexity by the evolution ofthe Earth (Brasier and Lindsay 1998 Lindsay andBrasier 2002ab) That is biospheric evolutionwas driven by the Earthrsquos endogenic energy re-sources (as expressed in for example plate tec-tonics) and its long-term survival depends uponthose energy resources as well as those of the SunIf this is so it has fundamental implications notonly for life on Earth but for the more generalproblems surrounding the likelihood of life hav-ing evolved on other planetary bodies Smallplanets such as Mars had limited endogenic en-ergy resources to drive crustmantle interactionand are only likely to have supported life earlyin their history This hypothesis also implies thatlife should have evolved earlier on Mars than onEarth because it cooled more quickly and passedinto the optimal biospheric environmental win-dow earlier

These conclusions have major implications forany strategy to be established in the search for ex-traterrestrial life It implies that life on Mars ismore likely to have evolved and become extinctearly in the planetrsquos history This suggests that onMars we must search in the ancient rock recordand should be focusing on enduring biomarkersIt is also important that planetary dynamics beconsidered which suggest that samples must becollected in a well-constrained stratigraphic ortemporal context

EARTHrsquoS ANCIENT CRATONS

Exposures of rocks reflecting Earthrsquos earliesthistory are relatively rare at the Earthrsquos surfaceRocks older than 30 Ga form only 05 of thearea of the present continents (de Wit 1998)

However in the core the Pilbara and KaapvaalCratons of Western Australia and southernAfrica respectively (Fig 1) ancient rocks not onlysurvive but are only gently deformed and meta-morphosed The geology of the Pilbara Cratonhas been mapped in some detail (Fig 2) (egHickman and Lipple 1978 Van Kranendonk1998 2000) and provides an ideal opportunity tobuild a framework in which to test models for theearly evolution of life on Earth and to developprotocols for the search for life on other planetsespecially Mars

THE PILBARA CRATON

The present Australian continent consists oftwo major crustal components (Fig 1) (see Mur-ray et al 1989) Along the eastern margin of thecontinent the crust is Cambrian or younger theproduct of active margin processes during the Pa-leozoic The rest of the continent is much olderconsisting of a complex of terranes of Archean toMesoproterozoic age (Rutland 1976) The Pre-cambrian crust consists of several elements (Shawet al 1996) that were amalgamated in the Paleo-proterozoic and the early Mesoproterozoic West-ern Australia (Fig 1) which contains the oldestrocks consists of two well-defined Archeanblocks the Yilgarn and Pilbara Cratons whichwere sutured along the Capricorn Orogen at ap-proximately 20 Ga as part of an early episode ofsupercontinent assembly (Columbia) (Rogers andSantosh 2002) The Yilgarn and Pilbara Cratonsboth include Paleoproterozoic and Archeanrocks though the Pilbara incorporates a coherentautochthonous stratigraphy that is among theoldest on Earth extending from 35 to 28 Gathe critical period (Fig 3) during which theEarthrsquos crust was evolving rapidly and life wasbecoming more widespread Equally importantthe Pilbara also preserves a comprehensivesupracrustal succession covering the periodwhen the atmosphere and oceans were becom-ing oxygenated (28ndash17 Ga) (Fig 4) After 27Ga there is abundant evidence for photosyn-thesis in the form of fossils especially stroma-tolites (platform carbonates) (Walter 1976 Si-monson et al 1993ab) and stable isotopegeochemistry (Buick et al 1995a Lindsay andBrasier 2002ab) and from biomarkers includ-ing cyanobacterial hopanes and possibly eu-karyotic steranes (Brocks et al 1999)

LINDSAY ET AL740

EARTHrsquoS EARLIEST VOLCANO-SEDIMENTARY RECORD

The rocks of the Pilbara Craton form the ear-liest lithologic succession on the Australian con-tinent and along with the overlying supracrustalsuccession would form the focus of the proposedsampling program Most of the best exposures ofthis early sedimentary record occur in a rela-tively limited area covered by a single 1100000map sheet the North Shaw map sheet (Van Kra-nendonk 1998 2000) (Fig 2) which encompassesthe main study area considered in this paperFurther regional information from flanking areascan be obtained from the 1250000 Marble Barmap sheet (Hickman and Lipple 1978) which in-cludes the North Shaw sheet The area is some-what remote and only accessible by four-wheeldrive vehicle

THE NORTH SHAW STUDY AREA

Granitoid complexes

The study area is underlain by parts of theShaw Yule and Carlindi granitoid complexeswhich are broadly ovoid in plan 25ndash110 km indiameter and 40ndash60 km apart (Fig 2) The com-

plexes consist of several mappable plutonic orgneissic units (Bettenay et al 1981 Van Kra-nendonk 2000 Van Kranendonk et al 2002)Structural and geophysical analyses suggest thatthe granitoid complexes form structural domesas part of a continuous midcrustal layer ex-tending from 14 km beneath the interveningsupracrustal units (Hickman 1984 Van Kra-nendonk 2000) Components of the complexesare coeval with felsic volcanic horizons in thePilbara Supergroup (the carapace above thegranitoids) (Williams and Collins 1990) andhave intrusive or sheared intrusive contacts withthem indicating an intimate synchronous devel-opment

The plutons

Three granitoid intrusions are exposed withinthe supracrustal successionmdashthe North PoleMonzonite (3458 Ga) Strelley Granite (3238Ga) and Keep it Dark Monzonite (2936 Ga)(Fig 3)mdashand form discrete plutons unrelated tothe granitoid complexes (Fig 2) The geometry andgeochronology of these plutons suggest that theyare synvolcanic laccoliths formed along with thesupracrustal units and can be related directly to thelithostratigraphy and to the hydrothermal massivesulfide bodies discussed in a following section

ARCHEAN GEOLOGIC REFERENCE COLLECTION 741

FIG 1 The Pilbara and Yil-garn Cratons showing the de-formed suture zone of theCapricorn Orogen and the dis-tribution of younger sedi-mentary basins The proposedNorth Shaw study area isshown as a black rectangle

THE STRATIGRAPHIC RECORD

The term ldquogreenstonerdquo or ldquogreenstone beltsrdquohas generally been applied to the largely vol-canigenic stratigraphic intervals preserved as acarapace overlying the granitoid complexes TheArchean stratigraphic record preserved in thegreenstone carapace in the study area can be di-vided into five unconformity or intrusion-boundgroups or megasequences (Fig 3) They are tec-tonically defined units and not eustatically con-trolled (Van Kranendonk et al 2002) Themegasequences of the Pilbara Supergroup weredeposited over a time period between approxi-mately 351 and approximately 294 Ga Theywere deposited unconformably one above theother and consistently dip away from the domal

granitoid complexes The dips of the beddinggradually decrease with decreasing age suggest-ing that they were deposited as thickeningwedges adjacent to growing granitoid diapirs(Hickman 1975 1983 1984 Van Kranendonk2000) Granitoid diapirism appears to have con-tinued until after 27 Ga as the overlying rem-nants of the Hamersley Basin succession are pre-served as synclinorial remnants (Hickman 1984Van Kranendonk et al 2002)

Despite local complex faulting the supra-crustal megasequences are relatively well pre-served such that their ages and stratigraphic re-lationships are well established (Williams andCollins 1990 Van Kranendonk 2000) Metamor-phic grade decreases away from the granitoidcomplexes from a maximum of middle to lower

LINDSAY ET AL742

FIG 2 Simplified map of the pro-posed North Shaw study area show-ing the main lithologic units (groups)The sinuous Shaw River almost bisectsthe area Adapted from Van Kranen-donk (2000)

amphibolite facies at the contact to lower green-schist and prenhite-pumpellyite facies Lowergreenschist facies is by far the most widespreadmetamorphic grade in the study area

Megasequence 1mdashCoonterunah Group

The Coonterunah Group includes the earliestknown stratigraphic succession (3515 Ma) pre-served on the Pilbara Craton (Buick et al 1995bVan Kranendonk and Morant 1998) The lowercontact is intrusive with the 3468 Ma granitoidrocks of the Carlindi Granitoid Complex The suc-cession consists of three formations (Van Kra-nendonk 2000)

Tabletop Formation The Tabletop Formationconsists entirely of thick basaltic units They aremainly fine-grained tholeiitic lavas althoughsome gabbroic intrusions have been identifiedThe basalts are mostly featureless although pil-lowed units are present indicating a submarinesetting

Coucal Formation The Coucal Formation whichis 2 km thick at its maximum consists largely

ARCHEAN GEOLOGIC REFERENCE COLLECTION 743

of fine-grained doleritic andesite basalt and fel-sic volcanic units (Fig 5) Thin chert (Fig 6) andimpoverished cherty banded iron formation (BIF)units are present throughout the formation Theseare the earliest known sedimentary units in thePilbara Craton and offer the earliest opportunityfor the preservation of ancient life

Double Bar Formation The Double Bar Forma-tion which conformably overlies the Coucal For-mation consists of fine-grained tholeiitic basaltswith interbedded basaltic volcaniclastic rocksSome basalts are pillowed suggesting a subma-rine setting The volcanics retain their basaltictextures but have been altered significantly bylow-grade metamorphism (Van Kranendonk2000)

Megasequence 2mdashWarrawoona Group

The Warrawoona Group is separated from theunderlying Coonterunah Group by a widespreadangular unconformity the oldest evidence onEarth for emergence (Buick et al 1995b) (Fig 3)The Warrawoona Group is dominantly volcanic inorigin but includes some clastic sedimentary rocks

FIG 3 Simplified stratigraphy of theArchean rocks exposed in the proposedstudy area (adapted from Van Kranen-donk 2000) Arrows indicate dated periodof activity of the granitoid complexes as dis-tinct from the plutons which are shown incartoon form

McPhee Basalt The McPhee Basalt which isdated at 3477 Ma and reaches a maximum of 3km in thickness is relatively limited in outcrop(Van Kranendonk et al 2002) It consists largelyof basalts and schists and is only exposed in thesouthern part of the study area adjacent to theShaw Granitoid Complex where the contact is in-trusive A thin unit of impoverished cherty BIFforms a distinctive cap to the formation

Mount Ada Basalt The Mount Ada basalt con-sists of 3ndash4 km of massive tholeiitic basalt and do-

LINDSAY ET AL744

FIG 4 Simplified stratigraphy of the Hamersley and Ashburton Basins of Western Australia (after Lindsay andBrasier 2002)

lerite intrusions that are exposed in the south-eastern corner of the study area Its lower contactwith the Shaw Granitoid Complex and NorthPole Monzonite is intrusive For the most partvolcanic textures are well preserved in thebasalts

Dresser Formation The Dresser Formation isonly recognized in the North Pole Dome whereit reaches a maximum of 1000 m in thickness(Van Kranendonk et al 2002) The formation con-sists of massive and pillowed mafic volcanics in-

terbedded with units of chert sandstone con-glomerate barite and minor carbonate (Fig 7)The individual chert-barite beds vary from 5 to40 m in thickness Associated with these unusualsedimentary facies are structures that have beeninterpreted as the worldrsquos oldest stromatolites(Fig 8) and associated microfossils (Dunlop et al1978 Walter et al 1980 Buick et al 1981 Groveset al 1981 Walter 1983) It is perhaps significantthat this distinctive unit is fed by a deep-seatedsystem of hydrothermal dykes (Fig 9)

Duffer Formation The Duffer Formation is bestexposed in the southeastern portion of the studyarea where it consists largely of felsic schist andagglomerate The agglomerates which have been

locally interpreted as welded ignimbrites arebedded in meter-scale units The Duffer Forma-tion has a depositional age of approximately 347Ga (Thorpe et al 1992 McNaughton et al 1993Nelson 1997 1998 1999 2000 2001) and restsconformably on the Mount Ada Basalt

Apex Basalt The Apex Basalt is well exposed inthe northeastern corner of the study area and overlarge areas on adjacent map sheets The forma-tion is dominated by pillowed basalts which arelargely tholeiitic The pillows are well developedand preserve vesicular rims chilled margins andconcentric cooling cracks Hyaloclastic brecciasare also present within the assemblage Doleriticflows and intrusions are common and doleriticdykes occur locally

Interbedded with the volcanics are a numberof chert horizons which are 10 m in thickness

ARCHEAN GEOLOGIC REFERENCE COLLECTION 745

FIG 5 A measured section through the Coucal For-mation (locality A Fig 2) The formation is dominatedby basic volcanic rocks with some felsic volcanic rocksand thin (10 m) hydrothermal cherts between flows

FIG 6 Laminated cherts from the lower Coucal For-mation (locality A Fig 2) Cherts form the earliest sedi-mentary rocks in the Pilbara succession Some units areas much as 10 m thick Scale bar 5 10 cm

and usually well laminated mainly black andwhite chert Numerous chert dykes cut across theunderlying stratigraphy and terminate at thelevel of the chert horizons Close to Marble Barexposures of the Apex chert contain what was ini-tially believed to be Earthrsquos oldest morphologicalfossil assemblage (Fig 10) (Schopf and Packer1987 Schopf 1992 1993) a claim recently dis-puted by more detailed analysis (Brasier et al2002) (see following section on Paleontology)

Panorama Formation The Panorama Formation(3458ndash3434 Ma) which is best exposed in thenorthern part of the study area near the ShawRiver is dominated by felsic volcaniclastic rocks

with small volumes of felsic lavas and some chertinterbeds (Hickman 1983 Thorpe et al 1992 Nel-son 2000) The formation is strongly altered andintensely silicified having been exposed to tem-peratures as high as 350degC (Cullers et al 1993)A large volcanic vent within the Panorama For-mation is preserved in the northwestern part ofthe outcrop area (Van Kranendonk 2000)

Strelley Pool Chert The relationship of the Strel-ley Pool Chert (Lowe 1983) to the underlyingunits varies from conformable to unconformableIn the northeast the contact with the underlyingCoonterunah Group forms an angular unconfor-mity with pronounced paleorelief (Buick et al1995b) The formation consists of carbonate sili-cified carbonate and clastic rocks (Fig 11) Thecherts and carbonates both have well-developedsedimentary structure that have been describedas wavy laminated cherts or carbonates and stro-matolites (Fig 12) (Lowe 1983 Hofmann et al1999) although as discussed in a following sec-tion there is some concern as to whether they arebiogenic (Lowe 1994 Lindsay et al 2003ab)

Euro Basalt The Euro Basalt consists domi-nantly of basalts of tholeiitic and high-Mg com-position that rest conformably on the StrelleyPool Chert (Van Kranendonk 2000) The forma-tion reaches a thickness in excess of 9 km in thenortheastern part of the study area The basaltsare pillowed and bedded on a kilometer scaleThe formation also includes some komatiite andfelsic volcaniclastics as well as thin cherts andvolcaniclastic units

Megasequence 3mdashSulphur Springs Group

The Sulphur Springs Group consists of threeformations (Fig 3) that in contrast to the previ-ous two Groups contain a significant proportionof clastic sedimentary rocks some quartzose

Leilira Formation The Leilira Formation consistspredominantly of clastic sedimentary rocks withonly small volumes of felsic to intermediate ex-trusive rocks interbedded The clastic rocks arepredominantly sandstones but vary from silt-stone to coarser pebbly sandstone Composition-ally the sandstones are quartzose or volcaniclithic rocks Locally laminated cherts formed bythe silicification of felsic volcanic ash and silt-stone are abundant

LINDSAY ET AL746

FIG 7 Bedded sedimentary carbonate and silica fromthe Dresser Formation (locality B Fig 2) Scale bar 5 5cm The rock consists of well-bedded units that begin withorangeyellow iron-rich carbonates and grade upward topale green silica The unit reflects geochemical changes inresponse to pulsing or cycling of the hydrothermal sys-tem Elsewhere carbonate and chert form alternating beds

The Leilira Formation represents a significantchange in depositional patterns as it is the oldestunit in which clastic sediments are the dominantlithology It reflects the growing volcanic cara-pace and in particular the exposure of the grani-toid core to weathering and erosion the first signsthat a true crust was developing

Kunagunarrina Formation The KunagunarrinaFormation reaches a maximum of 24 km inthickness and consists predominantly of high-Mg basalts and komatiites Locally andesitic andfelsic units are present especially near the top ofthe formation A thick chert unit (10 m) in themiddle of the formation separates lower Mg

ARCHEAN GEOLOGIC REFERENCE COLLECTION 747

FIG 8 Stromatolitic structures in theDresser Formation (locality B Fig 2)The original barite has been largely re-placed by chert Scale bar 5 10 cm

FIG 9 Exposures of the Dresser Formation in the North Pole Dome area (locality B Fig 2) showing large chert-barite feeder dykes The dykes consist of black chert and coarse bladed barite Note the vehicle for scale

basalts at the base of the formation from ko-matiites towards the top

Kangaroo Caves Formation The Kangaroo CavesFormation which reaches a maximum thicknessof 15 km is best exposed towards the center ofthe study area The formation is complex and con-sists of a succession of andesitic volcanics anddacitic to rhyolitic sills and extrusives A chert in-terval up to 100 m thick forms a distinctivemarker at the top of the formation Close to thecenter of the study area at Sulphur Springs themassive chert is overlain by a polymict megabrec-cia that rests upon a sharp erosional contact (Hill1997 Vearncombe et al 1998)

Hosted within a 200-m dacite unit immediatelybeneath the marker chert are six sulfide ore bodiesspaced at regular intervals of 5ndash7 km around themargins of the Strelley Granite (Morant 1998) Theyconsist of pyrite sphalerite chalcopyrite and galenawith smaller amounts of other sulfides (Vearncombeet al 1995 1998 Vearncombe 1996 Morant 1998)The sulfides which were deposited in a blacksmoker hydrothermal setting occur in associationwith growth faults developed around the StrelleyGranite the intrusion of which was syndepositionalwith the Kangaroo Caves Formation (Vearncombe1996 Brauhart et al 1998 Brauhart 1999)

Megasequence 4mdashGorge Creek Group

The Gorge Creek Group consists of five for-mations (Fig 3) that show much greater litho-logic diversity than earlier megasequences Inparticular this interval includes for the firsttime a significant thickness of quartzose sand-stone as well as a variety of other clastic sedi-mentary rocks

Pincunah Hill Formation In the western part ofthe study area the basal Pincunah Hill Formationconsists of thinly bedded BIF with small amountsof laminated chert red to gray shale and silt-stone This basal unit is up to 1 km thick in thewestern part of the area but wedges out and dis-appears in a short distance eastward The upper

LINDSAY ET AL748

FIG 11 A measured section through the Strelley PoolChert showing the location of stromatolitic structuresand the carbonate interval (adapted from Van Kranen-donk 2000) (locality C Fig 2) The gamma log to theright shows continuity across the chert-carbonate

FIG 10 ldquoMicrofossilsrdquo from the Apex chert near Mar-ble Bar a computer-generated digital automontage ofputative beggiatoan Eoleptonema apex Holotype (Schopf1993) combining the most-sharply focused images fromsuccessive focal planes b and c Original manual pho-tomontage and interpretative sketch (Schopf 1993) whichomits lower structure at arrow in (b) d and e New sin-gle images showing continuity of original and newly im-aged and automontaged structures Reprinted with per-mission from Nature (Brasier et al 2002) Copyright 2002Macmillan Publishers Ltd

part of the formation consists largely of red orpurple shale (Van Kranendonk 2000)

Corboy Formation At its thickest in the easternpart of the study area the Corboy Formation in-cludes a number of distinct facies developedacross major growth faults (Wilhelmij and Dun-lop 1984 Wilhelmij 1986) Here the formationconsists of a basal quartzose sandstone abruptlyoverlain by conglomerate interbedded withgraded sandstone units red shale and coarsesandstone The formation is capped by massivesandstone and conglomerate The unit was de-posited as a series of small prograding shallowmarine fan deltas (Wilhelmij 1986 Hill 1997 VanKranendonk 2000)

Paddy Market Formation In the western part ofthe study area the Paddy Market Formation isdominantly cherty BIF with minor ferruginousshale Further east the formation becomes moreshale-rich although much of the shale has been re-placed by chert Sandstone is present in smallamounts in beds up to 10 m thick Towards thetop of the formation in the east the cherts containfelsic volcanic material that is overlain by up to 200m of massive dacite (Van Kranendonk 2000)

Honeyeater Basalt The Honeyeater Basaltwhich consists entirely of basalt and dolerite con-formably overlies the Paddy Market FormationThe lower part of the formation consists of high-Mg basalt interbedded with dolerite whereashigher in the formation the basalts are largelytholeiitic and interbedded with dolerite (Gliksonand Hickman 1981ab Glikson et al 1991)

Pyramid Hill Formation The Pyramid Hill For-mation is limited in areal extent It is found in thenorth central part of the study area east of theStrelley Granite The formation consists of redand black shales that when heavily silicifiedgrade into laminated black and white chert (VanKranendonk 2000) The Gorge Creek Group iscapped by a well-bedded BIF

Megasequence 5mdashDe Grey Group

The De Grey Group consists of a single for-mation the Lalla Rookh Sandstone (Fig 3)which is dominated by clastic sedimentary rocksthat contain a significant quartzose componentThe Lalla Rookh Sandstone which rests on awell-defined unconformity above the GorgeCreek Group reaches a maximum thickness of

ARCHEAN GEOLOGIC REFERENCE COLLECTION 749

FIG 12 Stromatolitic carbonates in the Strelley Pool Chert (locality C Fig 2) Laminae are isopachous and theapex of the cone is ruptured and veined by secondary carbonate (arrow) Scale bar 5 10 cm

3 km (Krapez 1984) In the north central part ofthe study area the formation consists entirely ofcoarse clastic sediments and minor shalewhereas to the south the predominate fill is lithicand feldspathic sandstone (Chan 1998) Theserocks provide the first clear evidence of streamactivity in the Pilbara succession (Krapez 1984Van Kranendonk and Collins 1998 Van Kra-nendonk 2000)

THE PILBARA SUPRACRUSTAL SUCCESSION

The late Archean and early Paleoproterozoicbasins that rest upon the ancient cratonic blocksof Western Australia (Fig 1) form a time seriesassociated with the formation and ultimate dis-assembly of one of Earthrsquos earliest major conti-nental masses This was followed by the suturingof the Pilbara and Yilgarn Cratons along theCapricorn Orogen as a new supercontinentevolved in the Paleoproterozoic (Rogers and San-tosh 2002) These basins contain an importantsedimentary record of the early Earth includingsome of Earthrsquos earliest stromatolites and plat-form carbonate deposits (Simonson et al1993ab) organic-rich black shales (Brocks et al1999) and BIF (Trendall 1983 1990) The strati-graphic and structural framework is well con-strained by radiometric ages (Arndt et al 1991Barley et al 1997 Trendall et al 1998) providinga record at a critical time in Earth history whenthe atmosphere was first becoming oxygen-rich(Lindsay and Brasier 2002ab)

The Late Archean-Early PaleoproterozoicHamersley Basin which lies close to the southernmargin of the Pilbara Block (Trendall 1983 1990Blake and Barley 1992) overlies a normal crustalthickness (30ndash40 km) (Drummond 1981) Thebasin began to subside at approximately 28 Gafollowing an episode of crustal extension The ar-chitecture of the basin fill is complex andpolyphase with major erosional surfaces separat-ing areally extensive megasequencessuperse-quences (Trendall 1990 Krapez 1996) DuringFortescue and Hamersley Group time (Fig 4)sedimentation began in a rift setting in a silici-clastic-starved environment The later stages ofHamersley Basin sedimentation (Turee CreekGroup times) record the early transition from apassive margin to a foreland basin setting andwith it the establishment of a terrigenous sedi-

ment supply (Tyler and Thorne 1990) The suc-cession is largely marine Evidence of eustasy ap-pears for the first time in the sedimentary suc-cession of the Hamersley Basin (Lindsay andBrasier 2002ab)

The geometry and sequence stacking pattern ofthe succession are both very similar to youngerintracratonic basins (see Lindsay et al 1993) TheHamersley Basin is thus the first basinal settingpreserved on the Australian craton in which ma-rine sediments and in particular platform car-bonate rocks have been preserved in a responseto broad regional crustal subsidence and wheredeposition is controlled by eustasy

The Ashburton Basin (22ndash18 Ga) discon-formably overlies the southern margin of theHamersley Basin along its the contact with theCapricorn Orogen (Fig 1) (Thorne and Seymour1991) The basin forms the northern margin of theCapricorn Orogen a deformed zone that devel-oped during ocean closure Loading of the oceanfloor by the approaching Yilgarn Craton led tothe development of a westndashnorthwest-orientedforeland basin [McGrath Trough (Horowitz1982)] with an uplifted orogenic margin to thesouthwest (Blake and Groves 1987 Blake andBarley 1992) with the result that sedimentationwas dominated by an abundant supply of ter-rigenous clastic material (Mount McGrath For-mation) Continued loading of the crust ulti-mately led to the supply of terrigenous materialsbeing disrupted and producing a prograding car-bonate shelf (Duck Creek Dolomite) Basin sedi-mentation was accompanied by active-marginmafic volcanism (Cheela Springs Basalt)

THE CAPRICORN OROGEN ANDASSOCIATED BASINS

The Yerrida (22ndash19 Ga) Bryah (20 Ga)Padbury (20 Ga) and Earaheedy (19ndash165Ga) Basins lie to the southeast of the Hamersleyand Ashburton Basins and overlie the CapricornOrogen (Fig 1) (Occhipinti et al 1997 1998 Pi-rajno et al 1998 Pirajno and Adamides 2000)These now fragmentary basins formed along thenorthern margin of the Yilgarn Craton in a back-arc setting and record the convergence and colli-sion of the Archean Pilbara and Yilgarn Cratons(Tyler and Thorne 1990 Thorne and Seymour1991 Occhipinti et al 1997 1998 Pirajno et al1998) Because of their active margin settings the

LINDSAY ET AL750

fill of these small basins is dominated by clasticsediments However platform carbonate unitsare preserved in the Yerrida Basin (Gee and Grey1993 Pirajno and Adamides 2000) and a signif-icant thickness of carbonate rocks also occurs inthe Earaheedy Basin

THE ARCHEAN BIOSPHERE

The end of the Archean is formally defined at25 Ga (Plumb 1991) In reality the Archean worldended at some point in time closer to 28 or 30 Gawhen the first large continental blocks began to as-semble The stratigraphic record preserved on theancient Australian Craton reflects this major tran-sition The rock record from the Early Archean isvery different from that of the Late Archean in thatthe early record is predominantly volcanic and hy-drothermal in nature (ldquogreenstonerdquo) while thelater record is largely clastic and more readily com-pared with the Phanerozoic record

Structures and textures of possible biogenicorigin that are preserved in the stratigraphic suc-cession in the Early Archean study area are wellknown and are believed to provide the earliestdirect evidence of Earthrsquos biosphere Stromatoliticstructures have been encountered at two levelswithin the section The Dresser Formation con-tains structures preserved in chert and barite in-tervals that have been interpreted as stromatolitesand oncolites (Dunlop et al 1978 Walter et al1980 Buick et al 1981 Groves et al 1981 Wal-ter 1983) The stromatolitic structures occur forthe most part in layered barite units that devel-oped over swarms of synvolcanic hydrothermalfeeder dykes along listric growth faults (Nijmanet al 1998 Van Kranendonk 2004) Although ithas been argued that these structures are biogenic(Walter et al 1980 Schopf and Walter 1983 Wal-ter 1983) their direct association with the feederdykes leaves room for doubt (Buick et al 1981)The setting is complex and there is considerablescope for more detailed contextual research

The second stromatolitic horizon occurs in theoverlying Strelley Pool Chert a complex unit con-sisting of at least five members (Lowe 1980 1983Hofmann et al 1999) The main stromatolitic unitoccurs as an 8-m-thick interval in the middle ofthe formation and appears to have a consistentand widespread pattern of deposition The struc-tures have been the focus of considerable discus-sion and their biogenicity has been questioned

(Buick et al 1981 Lowe 1995) The structures arecomplex and varied (Fig 12) leading to a morerecent re-evaluation again suggesting that theyare biogenic (Hofmann et al 1999) However thelaminae are largely isopachous an indication thatthey could be abiotic precipitates (cf Pope andGrotzinger 2000) Given the intense hydrother-mal activity that appears to have taken placethroughout the depositional time interval thestructure may well be the product of hydrother-mal deposition (Lindsay et al 2003ab) In all ourobservations suggest that considerably morework needs to be done to establish the deposi-tional context of these stromatolitic structures be-fore they can be confidently ascribed as biogenicin origin

Structures resembling remarkably preservedbacterial and cyanobacterial microfossils havebeen found at several localities in and close to thestudy area (Awramik et al 1983 Schopf andPacker 1987 Schopf 1992 1993 Rasmussen 2000)A complex biota of 11 species of filamentousprokaryotes which are compared with cyanobac-teria (Schopf and Packer 1987 Schopf 1992 1993)have been described from the Apex chert close toMarble Bar (21deg105579S 119deg418789E) The taxo-nomic interpretations are important in that theyprovide the earliest evidence of morphological lifeon Earth and support the hypothesis of an earlybeginning for oxygenic photosynthesis (Schopf1999) The structures are almost a billion yearsolder than the oldest known putative cyanobac-terial biomarkers (Summons et al 1999) and oc-cur well before the earliest evidence for the risein oxygen in the atmosphere (for a summary seeKnoll and Canfield 1998 Lindsay and Brasier2000 2002ab) However in a recent re-evaluationof the original Apex chert material it has been ar-gued that the filaments are not biogenic but aresecondary artifacts formed during diagenesisfrom abiogenic organic compounds generated inhydrothermal chert veins (Brasier et al 2002 VanKranendonk 2004)

Filaments believed to be biogenic in originhave also been reported from the 324 Ga Kan-garoo Caves Formation (Rasmussen 2000) wherethey form part of the Sulphur Springs deposit as-sociated with a black smoker hydrothermal ventsystem (Vearncombe et al 1995 1998) Ras-mussen (2000) argued that the filaments are rem-nants of a hydrothermal biota of thermophilicchemotrophic prokaryotes Sulfide textures areremarkably well preserved in these complex de-

ARCHEAN GEOLOGIC REFERENCE COLLECTION 751

posits and are worthy of much more detailedanalysis as they may well preserve evidence ofthe early biosphere

Finally there is evidence that hydrocarbons aretrapped in these ancient sedimentary rocks eitheras fluid inclusions in hydrothermal precipitates oras bituminous residues in larger cavities in theKangaroo Caves Formation and in the Lalla RookhSandstone (Dutkiewicz et al 1998 Rasmussen andBuick 2000) Buick et al (1998) also reported car-bonaceous globules in cherts from the lower War-rawoona Group In the Kangaroo Caves Forma-tion the hydrocarbons occur in hydrothermalbarite or within the sulfides of the volcanogenichydrothermal massive sulfide deposits that weresyndepositional with the formation (Rasmussenand Buick 2000) Textural information suggestthat these deposits were precipitated as blacksmoker chimneys in a submarine hydrothermalsetting driven by heat released from the intrudingStrelley Granite at approximately 324 Ga (Vearn-combe et al 1995) Fluorescing fluid inclusions oc-cur in quartz overgrowths in sandstones from theLalla Rookh Sandstone (Dutkiewicz et al 1998)Rasmussen and Buick (2000) and Dutkiewicz et al(1998) have argued for a biogenic origin for the hy-drocarbons however an abiotic origin involvingfor example Fischer-Tropsch-type synthesis ap-pears possible (Brasier et al 2002)

The Late Archean (and Early Paleoproterozoic)record preserved in the Hamersley and relatedbasins is stratigraphically much more coherentthan that preserved in the earlier successions TheHamersley Basin in particular provides a verycomprehensive record of the early oxygenation ofthe atmosphere in the form of large eustaticallycontrolled upward-shallowing sequences that in-clude black shales BIFs and ultimately at theirtops biogenic platform carbonates (Lindsay andBrasier 2002ab) These depositional cycles con-tain organic-rich shales at their base that recordevidence for some of Earthrsquos earliest biosphere(Brocks et al 1999) BIFs (Trendall 1983 1990)that record the rise of oxygen in the atmosphereand carbonates that document some of Earthrsquosearliest reef-like carbonate structures (Simonsonand Hassler 1996 1997 Simonson et al 1993ab)

SAMPLING STRATEGY

The Archean rocks of the Pilbara Craton arethus part of a protocontinent about which the

nascent Australian Craton developed It consistsof granitoid complexes overlain by a carapace ofsynchronously deposited volcano-sedimentarywedges Resting upon the early Archean proto-continent are a series of supracrustal basins Inall the sedimentary record of the Pilbara extendsfrom rocks as old as 354 Ga to some as young asapproximately 19 Ga As summarized in the fol-lowing paragraphs the sedimentary record of thePilbara records evidence of the major tectonic andgeological events that accompanied the evolutionof the Archean biosphere

It is here proposed that we develop a carefullydocumented and curated sample collection fromthis unique setting The collection would form thebasis for early Earth studies and a focus for astro-biology The sample set would be designed to re-flect the evolutionary succession described abovewith emphasis placed on cherts lower in the suc-cession and on clastic sediments higher in the suc-cession Any comprehensive reference collectionmust reflect diversity of the succession and includerocks from each of the sedimentary environmentsas well as igneous rocks reflecting the frameworkin which the protocontinent assembled

The five megasequences at the North Shawstudy area (Fig 3) are very different and reflectthe evolving Early Archean protocontinent Theearliest sedimentary record is limited and domi-nated by hydrothermally generated laminatedcherts and BIFs which appear as thin interbedswithin massive basalts (Coonterunah Group) (Fig5) Gradually clastic rocks appear dominated atfirst by volcanoclastics followed later by the firstevidence of sediments containing detrital quartzderived from true continental granitic crust Hy-drothermally generated laminated chert carbon-ate and barite units occur throughout the sectionThe first evidence of erosion appears at the top ofthe Coonterunah Group in the form of an angu-lar unconformity (Buick et al 1995b)

The basins that form the supracrustal succes-sion on the Pilbara Craton offer important in-sights into the closing phases of the Archean andthe immediate post-Archean world Because oftheir economic importance the basins are wellmapped and drill core is readily available (seeLindsay and Brasier 2002a) The succession pro-vides vital information about the early oxygena-tion of the atmosphere and the rapid growth ofthe biosphere that resulted from the assembly ofthe first continents and the initiation of the platetectonic cycle Platform carbonates whose geom-

LINDSAY ET AL752

etry was determined by eustasy are widespreadand contain considerable information on earlyoxygenic photosynthesis and the development ofthe first reef-like structure that are similar inmany ways to modern reefs (Simonson et al1993ab Lindsay and Brasier 2002ab)

The collection would consist of three groupingsof samples stratigraphic samples site-specificsamples and framework samples

Stratigraphic samples

The stratigraphic sample set would be collectedfrom well-defined settings such as carefully mea-sured and documented sections or from drill coreSamples would be collected from each of the fivestratigraphic groups or megasequences Thesupracrustal successions preserved in the Hamer-sley and associated basins would where possiblebe taken from existing drill core or from drill coresobtained for special projects Where drill core isnot available samples would be collected fromcarefully selected and measured outcrop sectionsThe samples would concentrate on rocks mostlikely to host biogenic material Samples from theEarly Archean volcaniclastic carapace would bedominated by cherts that are most likely to pre-serve fine biogenic structures but would also in-clude other sedimentary hydrothermal units suchas barite and carbonate that would be used in sta-ble isotope and geochemical studies

Site-specific sample

This sample set would focus on sites where po-tential biogenic structures have been documentedsuch as the stromatolitic intervals in the DresserFormation and the Strelley Pool Chert It wouldalso include detailed sampling of sites such asChinaman Creek and the Strelley Pool Chertwhere purported microfossils have been docu-mented (Schopf 1993) and the sulfide bodies inthe Kangaroo Caves Formation that contain pos-sible microfossils as well as evidence of hydro-carbons (Rasmussen 2000 Rasmussen and Buick2000) This would involve a broad samplingaround the sites in measured sections and drillcore to establish the geological context

Framework samples

While the focus of the collecting would be onmaterial that would help define the character of

the Earthrsquos earliest biosphere it is important thatthe broad context of the samples be defined TheArchean is a period when the Earthrsquos crust wasevolving rapidly and there is a growing body ofevidence to suggest that the evolution of theplanet drives the evolution of the atmosphere andbiosphere (Lindsay and Brasier 2002ab) Theframework sample set would include suites ofsample from the extrusive volcanic rocks col-lected along well-documented sections Sectionsthrough the volcanoclastic succession would em-phasize the dominant volcanic units (basalts ko-matiites andesites felsites and rhyolites) assources of information on the mantle and evolv-ing crust This sample set would also include adetailed sampling of the granitoid complexeswhich ultimately drove the entire process as wellas the later intrusions that were important in dri-ving the hydrothermal systems that may wellhave been the cradle of early life

SAMPLE CURATIONCHARACTERIZATION AND

DISTRIBUTION

All samples would be fully documented in thefield Samples would be photographed and theirlocations determined using a Global PositioningSystem receiver For specialized samples such asthose thought to contain easily contaminated bio-geochemical information the field collectionwould be modeled on techniques developed bythe NASANational Science FoundationSmith-sonian Institution Antarctic Search for Meteoritesprogram (Harvey and Schutt 1998)

Characterization

The petrologic and petrographic characteristicsof each sample would be determined using thin-section optical microscopy The quantitative bulkchemical composition of each sample would bemeasured by x-ray fluorescence spectrometry Fi-nally the qualitative bulk mineralogy of eachsample would be measured by powder x-ray dif-fraction

Data management

An Archean Geologic Reference Materials Data-base would be produced to fully document eachsample Field documentation would include sam-pling rationale supporting references and field

ARCHEAN GEOLOGIC REFERENCE COLLECTION 753

photographs Documentation would include theresults of all analyses along with images of rep-resentative sample surfaces and optical micro-graphs The Database would be produced in elec-tronic format and posted on the web Subsamplesof the Archean Geologic Reference Materialswould then be made available at no cost to re-searchers in the astrobiology community

DISCUSSION AND CONCLUSIONS

The Early Archean environment was uniqueRecent studies indicate that the atmosphere waslargely anoxic with oxygen levels 1025 of thepresent atmospheric level (Pavlov and Kasting2002) There were no large continents no activeor passive margins and by default no intracra-tonic basins Significantly perhaps there are nosigns of eustasy The crust was evolving rapidlyand heat flow was high leading to the intrusionof large granitoid bodies and the development ofa volcanic carapace above them In turn thismeant that the marine environment was domi-nated by hydrothermal activity

In the absence of large continental masses de-trital materials other than volcanoclastics are rel-atively rare in the Early Archean Significant vol-umes of quartz sand do not appear in the PilbaraCraton sedimentary record until after 32 Ga pre-sumably as the intruding igneous bodies begin tobe unroofed and eroded There are thus no ma-jor accumulations of sedimentary rocks that wenormally expect associated with passive conti-nental margins

With the assembly of the earliest large conti-nents beginning at approximately 30ndash28 Gathe Earthrsquos atmosphere hydrosphere and bios-phere changed rapidly Intracratonic basinssuch as the Hamersley Basin began to developon the new crustal blocks opening new shal-low-water environments that provided an idealsetting for photosynthesizing organisms andthe development of the first carbonate reefstructures (Lindsay and Brasier 2002ab) Hy-drothermal processes while still importantwere thus no longer the only sources of energyavailable to the evolving biosphere

The Archean was thus an unfamiliar environ-ment and unravelling the nature of the earliestbiosphere will require development of new ap-proaches and new techniques Without large con-tinental masses subareal exposures are likely to

be rare and short-lived There is no evidence forstream activity until the De Grey group at ap-proximately 29 Ga at which time braided streamgravels were preserved Thus models involvingearly biospheric evolution in ldquoa warm little pondrdquo(Darwin 1888) or in fresh water seem unlikelyInstead the focus in the search for the origins oflife on Earth should be directed to hydrothermalmarine settings involving high temperatures andwhat would today be hostile geochemical envi-ronments with significant concentrations of metal-lic ions and a largely reducing environment Therisks for the misidentification of abiotic structuresand geochemical signatures as signs of ancient lifeare significant as seen in recent discussions (egBrasier et al 2002 Fedo and Whitehouse 2002Schopf et al 2002)

These difficulties suggest that much could begained by the development of a well-documentedcollection of Archean rocks that could be madeavailable to all qualified investigators Until weunderstand the earliest stages of the evolution oflife on Earth we cannot hope to make significantprogress in the search for life on other planetssuch as Mars Until there is a sample return mis-sion to Mars we must make do with meteoritesthat come to us largely out of context An under-standing of the Archean Earth can help placethose limited samples in a planetary context

ACKNOWLEDGMENTS

Franco Pirajno Kath Grey and Martin VanKranendonk provided generous advice and as-sistance and the Geological Survey of WesternAustralia supported the field program We aregrateful to Peter Morant of Sipa Resources andAdrian Black of Newexco Perth for their adviceand for access to drill-core materials

ABBREVIATION

BIF banded iron formation

REFERENCES

Arndt NT Nelson DR Compston W Trendall AFand Thorne AM (1991) The age of the FortescueGroup Hamersley Basin Western Australia from ionmicroprobe zircon U-Pb results Aust J Earth Sci 38262ndash281

LINDSAY ET AL754

Awramik SM Schopf JW and Walter MR (1983) Fil-amentous fossil bacteria from the Archaean of WesternAustralia Precambrian Res 20 357ndash374

Barley ME Pickard AL and Sylvester PJ (1997) Em-placement of a large igneous province as a possiblecause of banded iron formation 245 billion years agoNature 385 55ndash58

Bettenay LF Bickle MJ Boulter CA Groves DIMorant P Blake TS and James BA (1981) Evolu-tion of the Shaw Batholithmdashan Archaean granitoid-gneiss dome in the eastern Pilbara Western AustraliaIn Special Publication 7 The Archaean edited by JEGlover and DI Groves Geological Society of WesternAustralia Perth pp 361ndash372

Blake TS and Barley ME (1992) Tectonic evolution ofthe Late Archaean to Early Proterozoic Mount BruceMegasequence Set Western Australia Tectonics 11 1415ndash1425

Blake TS and Groves DI (1987) Continental rifting andthe Archaean-Proterozoic transition Geology 15 229ndash232

Brasier MD and Lindsay JF (1998) A billion years ofenvironmental stability and the emergence of eukary-otes new data from Northern Australia Geology 26555ndash558

Brasier MD Green OR Jephcoat AP Kleppe AKVan Kranendonk MJ Lindsay JF Steele A andGrassineau NV (2002) Questioning the evidence forEarthrsquos oldest fossils Nature 416 76ndash81

Brauhart C (1999) Regional alteration systems associatedwith Archaean volcanogenic massive sulphide depositsat Panorama Pilbara Western Australia [PhD Thesis]University of Western Australia Perth

Brauhart C Groves DI and Morant P (1998) Regionalalteration systems associated with volcanogenic mas-sive sulfide mineralization at Panorama Pilbara West-ern Australia Econ Geol 93 292ndash302

Brocks J Logan GA Buick R and Summons RE(1999) Archean molecular fossils and the early rise ofeukaryotes Science 285 1033ndash1036

Buick R Dunlop JSR and Groves DI (1981) Stroma-tolite recognition in ancient rocks an appraisal of ir-regular laminated structures in an early Archaeanchert-barite unit from North Pole Western AustraliaAlcheringa 5 161ndash181

Buick R Des Marais DJ and Knoll AH (1995a) Sta-ble isotope compositions of carbonates from the Meso-proterozoic Bangemall Group northwestern AustraliaChem Geol 123 153ndash171

Buick R Thornett JR McNaughton NJ Smith JBBarley ME and Savage M (1995b) Record of emer-gent continental crust 35 billion years ago in the Pil-bara Craton of Australia Nature 375 574ndash577

Buick R Rasmussen B and Krapez B (1998) Archaeanoil evidence for extensive hydrocarbon generation andmigration 25ndash35 Ga Am Assoc Petroleum Geol Bull82 50ndash69

Chan M (1998) Sedimentology and structure of threesmall congolmeratic basins in the Lalla Rookh-WesternShaw structural corridor Pilbara Craton Western Aus-

tralia [BSc Honours Thesis] University of WesternAustralia Perth

Cullers RL DiMarco MJ Lowe DR and Stone J(1993) Geochemistry of a silicified felsic volcaniclasticsuite from the Early Archaean Panorama FormationPilbara Block Western Australiamdashan evaluation of de-positional and post-depositional processes with specialemphasis on the rare-Earth elements Precambrian Res60 99ndash116

Darwin F ed (1888) The Life and Letters of Charles Dar-win Vol 3 John Murray London

de Wit MJ (1998) On Archaean granites greenstonescratons and tectonics does the evidence demand a ver-dict Precambrian Geol 91 181ndash226

Drummond BJ (1981) Crustal structure of the Precam-brian terrains of northwest Australia from seismic re-fraction data BMR J Geol Geophys 6 123ndash135

Dunlop JSR Muir M Milne VA and Groves DI(1978) A new microfossil assemblage from the Ar-chaean of Western Australia Nature 274 676ndash678

Dutkiewicz A Rasmussen B and Buick R (1998) Oilpreserved in fluid inclusions in Archaean sandstonesNature 395 885ndash888

Fedo CM and Whitehouse MJ (2002) Metasomatic ori-gin of quartz-pyroxene rock Akilia Greenland and im-plications for Earthrsquos earliest life Science 296 1448ndash1452

Gee RD and Grey K (1993) Report 41 Proterozoic Rockson the Glengarry 1250 000 SheetmdashStratigraphy Structureand Stromatolite Biostratigraphy Geological Survey ofWestern Australia Perth

Glikson AY and Hickman AH (1981a) Chemicalstratigraphy and petrogenesis of Archaean basic-ultra-basic volcanic units eastern Pilbara Block WesternAustralia In Special Publication 7 Archaean Geologyedited by JE Groves and DI Groves Geological Soci-ety of Western Australia Perth pp 287ndash300

Glikson AY and Hickman AH (1981b) Record 198136Geochemistry of Archaean Volcanic Successions EasternPilbara Block Western Australia Australian Bureau ofMineral Resources Canberra

Glikson AY Davy R and Hickman AH (1991) Record199146 Trace Metal Distribution Patterns in Basalts Pil-bara Craton Western Australia with Stratigraphic-Geo-chemical Implications Australian Bureau of Mineral Re-sources Canberra

Groves DI Dunlop JSR and Buick R (1981) An earlyhabitat of life Sci Am 245 64ndash73

Harvey RP and Schutt JW (1998) Meteorite recoveryand reconnaissance in the Allan Hills-David Glacierand Darwin Glacier regions 1996ndash1997 Antarctic J US 32 25ndash27

Hickman AH (1975) Annual Report 1974 PrecambrianStructural Geology of Part of the Pilbara Region Geologi-cal Survey of Western Australia Perth pp 68ndash73

Hickman AH (1983) Bulletin 127 Geology of the PilbaraBlock and Its Environs Geological Survey of WesternAustralia Perth

Hickman AH (1984) Archaean diapirism in the PilbaraBlock Western Australia In Precambrian Tectonics Il-lustrated edited by A Kroner and R Greiling E

ARCHEAN GEOLOGIC REFERENCE COLLECTION 755

biosphere by making sets of well-documentedreference samples available to a large number ofqualified investigators at no cost

RATIONALE

It has long been assumed that life evolved op-portunistically on Earth in a simple interactive re-lationship with its environment sustained forthe most part by an exogenic energy source (theSun) There are however several threads of evi-dence that are converging to suggest that the bios-phere did not simply evolve on the Earthrsquos sur-face but was in a very general way drivenforward to greater complexity by the evolution ofthe Earth (Brasier and Lindsay 1998 Lindsay andBrasier 2002ab) That is biospheric evolutionwas driven by the Earthrsquos endogenic energy re-sources (as expressed in for example plate tec-tonics) and its long-term survival depends uponthose energy resources as well as those of the SunIf this is so it has fundamental implications notonly for life on Earth but for the more generalproblems surrounding the likelihood of life hav-ing evolved on other planetary bodies Smallplanets such as Mars had limited endogenic en-ergy resources to drive crustmantle interactionand are only likely to have supported life earlyin their history This hypothesis also implies thatlife should have evolved earlier on Mars than onEarth because it cooled more quickly and passedinto the optimal biospheric environmental win-dow earlier

These conclusions have major implications forany strategy to be established in the search for ex-traterrestrial life It implies that life on Mars ismore likely to have evolved and become extinctearly in the planetrsquos history This suggests that onMars we must search in the ancient rock recordand should be focusing on enduring biomarkersIt is also important that planetary dynamics beconsidered which suggest that samples must becollected in a well-constrained stratigraphic ortemporal context

EARTHrsquoS ANCIENT CRATONS

Exposures of rocks reflecting Earthrsquos earliesthistory are relatively rare at the Earthrsquos surfaceRocks older than 30 Ga form only 05 of thearea of the present continents (de Wit 1998)

However in the core the Pilbara and KaapvaalCratons of Western Australia and southernAfrica respectively (Fig 1) ancient rocks not onlysurvive but are only gently deformed and meta-morphosed The geology of the Pilbara Cratonhas been mapped in some detail (Fig 2) (egHickman and Lipple 1978 Van Kranendonk1998 2000) and provides an ideal opportunity tobuild a framework in which to test models for theearly evolution of life on Earth and to developprotocols for the search for life on other planetsespecially Mars

THE PILBARA CRATON

The present Australian continent consists oftwo major crustal components (Fig 1) (see Mur-ray et al 1989) Along the eastern margin of thecontinent the crust is Cambrian or younger theproduct of active margin processes during the Pa-leozoic The rest of the continent is much olderconsisting of a complex of terranes of Archean toMesoproterozoic age (Rutland 1976) The Pre-cambrian crust consists of several elements (Shawet al 1996) that were amalgamated in the Paleo-proterozoic and the early Mesoproterozoic West-ern Australia (Fig 1) which contains the oldestrocks consists of two well-defined Archeanblocks the Yilgarn and Pilbara Cratons whichwere sutured along the Capricorn Orogen at ap-proximately 20 Ga as part of an early episode ofsupercontinent assembly (Columbia) (Rogers andSantosh 2002) The Yilgarn and Pilbara Cratonsboth include Paleoproterozoic and Archeanrocks though the Pilbara incorporates a coherentautochthonous stratigraphy that is among theoldest on Earth extending from 35 to 28 Gathe critical period (Fig 3) during which theEarthrsquos crust was evolving rapidly and life wasbecoming more widespread Equally importantthe Pilbara also preserves a comprehensivesupracrustal succession covering the periodwhen the atmosphere and oceans were becom-ing oxygenated (28ndash17 Ga) (Fig 4) After 27Ga there is abundant evidence for photosyn-thesis in the form of fossils especially stroma-tolites (platform carbonates) (Walter 1976 Si-monson et al 1993ab) and stable isotopegeochemistry (Buick et al 1995a Lindsay andBrasier 2002ab) and from biomarkers includ-ing cyanobacterial hopanes and possibly eu-karyotic steranes (Brocks et al 1999)

LINDSAY ET AL740

EARTHrsquoS EARLIEST VOLCANO-SEDIMENTARY RECORD

The rocks of the Pilbara Craton form the ear-liest lithologic succession on the Australian con-tinent and along with the overlying supracrustalsuccession would form the focus of the proposedsampling program Most of the best exposures ofthis early sedimentary record occur in a rela-tively limited area covered by a single 1100000map sheet the North Shaw map sheet (Van Kra-nendonk 1998 2000) (Fig 2) which encompassesthe main study area considered in this paperFurther regional information from flanking areascan be obtained from the 1250000 Marble Barmap sheet (Hickman and Lipple 1978) which in-cludes the North Shaw sheet The area is some-what remote and only accessible by four-wheeldrive vehicle

THE NORTH SHAW STUDY AREA

Granitoid complexes

The study area is underlain by parts of theShaw Yule and Carlindi granitoid complexeswhich are broadly ovoid in plan 25ndash110 km indiameter and 40ndash60 km apart (Fig 2) The com-

plexes consist of several mappable plutonic orgneissic units (Bettenay et al 1981 Van Kra-nendonk 2000 Van Kranendonk et al 2002)Structural and geophysical analyses suggest thatthe granitoid complexes form structural domesas part of a continuous midcrustal layer ex-tending from 14 km beneath the interveningsupracrustal units (Hickman 1984 Van Kra-nendonk 2000) Components of the complexesare coeval with felsic volcanic horizons in thePilbara Supergroup (the carapace above thegranitoids) (Williams and Collins 1990) andhave intrusive or sheared intrusive contacts withthem indicating an intimate synchronous devel-opment

The plutons

Three granitoid intrusions are exposed withinthe supracrustal successionmdashthe North PoleMonzonite (3458 Ga) Strelley Granite (3238Ga) and Keep it Dark Monzonite (2936 Ga)(Fig 3)mdashand form discrete plutons unrelated tothe granitoid complexes (Fig 2) The geometry andgeochronology of these plutons suggest that theyare synvolcanic laccoliths formed along with thesupracrustal units and can be related directly to thelithostratigraphy and to the hydrothermal massivesulfide bodies discussed in a following section

ARCHEAN GEOLOGIC REFERENCE COLLECTION 741

FIG 1 The Pilbara and Yil-garn Cratons showing the de-formed suture zone of theCapricorn Orogen and the dis-tribution of younger sedi-mentary basins The proposedNorth Shaw study area isshown as a black rectangle

THE STRATIGRAPHIC RECORD

The term ldquogreenstonerdquo or ldquogreenstone beltsrdquohas generally been applied to the largely vol-canigenic stratigraphic intervals preserved as acarapace overlying the granitoid complexes TheArchean stratigraphic record preserved in thegreenstone carapace in the study area can be di-vided into five unconformity or intrusion-boundgroups or megasequences (Fig 3) They are tec-tonically defined units and not eustatically con-trolled (Van Kranendonk et al 2002) Themegasequences of the Pilbara Supergroup weredeposited over a time period between approxi-mately 351 and approximately 294 Ga Theywere deposited unconformably one above theother and consistently dip away from the domal

granitoid complexes The dips of the beddinggradually decrease with decreasing age suggest-ing that they were deposited as thickeningwedges adjacent to growing granitoid diapirs(Hickman 1975 1983 1984 Van Kranendonk2000) Granitoid diapirism appears to have con-tinued until after 27 Ga as the overlying rem-nants of the Hamersley Basin succession are pre-served as synclinorial remnants (Hickman 1984Van Kranendonk et al 2002)

Despite local complex faulting the supra-crustal megasequences are relatively well pre-served such that their ages and stratigraphic re-lationships are well established (Williams andCollins 1990 Van Kranendonk 2000) Metamor-phic grade decreases away from the granitoidcomplexes from a maximum of middle to lower

LINDSAY ET AL742

FIG 2 Simplified map of the pro-posed North Shaw study area show-ing the main lithologic units (groups)The sinuous Shaw River almost bisectsthe area Adapted from Van Kranen-donk (2000)

amphibolite facies at the contact to lower green-schist and prenhite-pumpellyite facies Lowergreenschist facies is by far the most widespreadmetamorphic grade in the study area

Megasequence 1mdashCoonterunah Group

The Coonterunah Group includes the earliestknown stratigraphic succession (3515 Ma) pre-served on the Pilbara Craton (Buick et al 1995bVan Kranendonk and Morant 1998) The lowercontact is intrusive with the 3468 Ma granitoidrocks of the Carlindi Granitoid Complex The suc-cession consists of three formations (Van Kra-nendonk 2000)

Tabletop Formation The Tabletop Formationconsists entirely of thick basaltic units They aremainly fine-grained tholeiitic lavas althoughsome gabbroic intrusions have been identifiedThe basalts are mostly featureless although pil-lowed units are present indicating a submarinesetting

Coucal Formation The Coucal Formation whichis 2 km thick at its maximum consists largely

ARCHEAN GEOLOGIC REFERENCE COLLECTION 743

of fine-grained doleritic andesite basalt and fel-sic volcanic units (Fig 5) Thin chert (Fig 6) andimpoverished cherty banded iron formation (BIF)units are present throughout the formation Theseare the earliest known sedimentary units in thePilbara Craton and offer the earliest opportunityfor the preservation of ancient life

Double Bar Formation The Double Bar Forma-tion which conformably overlies the Coucal For-mation consists of fine-grained tholeiitic basaltswith interbedded basaltic volcaniclastic rocksSome basalts are pillowed suggesting a subma-rine setting The volcanics retain their basaltictextures but have been altered significantly bylow-grade metamorphism (Van Kranendonk2000)

Megasequence 2mdashWarrawoona Group

The Warrawoona Group is separated from theunderlying Coonterunah Group by a widespreadangular unconformity the oldest evidence onEarth for emergence (Buick et al 1995b) (Fig 3)The Warrawoona Group is dominantly volcanic inorigin but includes some clastic sedimentary rocks

FIG 3 Simplified stratigraphy of theArchean rocks exposed in the proposedstudy area (adapted from Van Kranen-donk 2000) Arrows indicate dated periodof activity of the granitoid complexes as dis-tinct from the plutons which are shown incartoon form

McPhee Basalt The McPhee Basalt which isdated at 3477 Ma and reaches a maximum of 3km in thickness is relatively limited in outcrop(Van Kranendonk et al 2002) It consists largelyof basalts and schists and is only exposed in thesouthern part of the study area adjacent to theShaw Granitoid Complex where the contact is in-trusive A thin unit of impoverished cherty BIFforms a distinctive cap to the formation

Mount Ada Basalt The Mount Ada basalt con-sists of 3ndash4 km of massive tholeiitic basalt and do-

LINDSAY ET AL744

FIG 4 Simplified stratigraphy of the Hamersley and Ashburton Basins of Western Australia (after Lindsay andBrasier 2002)

lerite intrusions that are exposed in the south-eastern corner of the study area Its lower contactwith the Shaw Granitoid Complex and NorthPole Monzonite is intrusive For the most partvolcanic textures are well preserved in thebasalts

Dresser Formation The Dresser Formation isonly recognized in the North Pole Dome whereit reaches a maximum of 1000 m in thickness(Van Kranendonk et al 2002) The formation con-sists of massive and pillowed mafic volcanics in-

terbedded with units of chert sandstone con-glomerate barite and minor carbonate (Fig 7)The individual chert-barite beds vary from 5 to40 m in thickness Associated with these unusualsedimentary facies are structures that have beeninterpreted as the worldrsquos oldest stromatolites(Fig 8) and associated microfossils (Dunlop et al1978 Walter et al 1980 Buick et al 1981 Groveset al 1981 Walter 1983) It is perhaps significantthat this distinctive unit is fed by a deep-seatedsystem of hydrothermal dykes (Fig 9)

Duffer Formation The Duffer Formation is bestexposed in the southeastern portion of the studyarea where it consists largely of felsic schist andagglomerate The agglomerates which have been

locally interpreted as welded ignimbrites arebedded in meter-scale units The Duffer Forma-tion has a depositional age of approximately 347Ga (Thorpe et al 1992 McNaughton et al 1993Nelson 1997 1998 1999 2000 2001) and restsconformably on the Mount Ada Basalt

Apex Basalt The Apex Basalt is well exposed inthe northeastern corner of the study area and overlarge areas on adjacent map sheets The forma-tion is dominated by pillowed basalts which arelargely tholeiitic The pillows are well developedand preserve vesicular rims chilled margins andconcentric cooling cracks Hyaloclastic brecciasare also present within the assemblage Doleriticflows and intrusions are common and doleriticdykes occur locally

Interbedded with the volcanics are a numberof chert horizons which are 10 m in thickness

ARCHEAN GEOLOGIC REFERENCE COLLECTION 745

FIG 5 A measured section through the Coucal For-mation (locality A Fig 2) The formation is dominatedby basic volcanic rocks with some felsic volcanic rocksand thin (10 m) hydrothermal cherts between flows

FIG 6 Laminated cherts from the lower Coucal For-mation (locality A Fig 2) Cherts form the earliest sedi-mentary rocks in the Pilbara succession Some units areas much as 10 m thick Scale bar 5 10 cm

and usually well laminated mainly black andwhite chert Numerous chert dykes cut across theunderlying stratigraphy and terminate at thelevel of the chert horizons Close to Marble Barexposures of the Apex chert contain what was ini-tially believed to be Earthrsquos oldest morphologicalfossil assemblage (Fig 10) (Schopf and Packer1987 Schopf 1992 1993) a claim recently dis-puted by more detailed analysis (Brasier et al2002) (see following section on Paleontology)

Panorama Formation The Panorama Formation(3458ndash3434 Ma) which is best exposed in thenorthern part of the study area near the ShawRiver is dominated by felsic volcaniclastic rocks

with small volumes of felsic lavas and some chertinterbeds (Hickman 1983 Thorpe et al 1992 Nel-son 2000) The formation is strongly altered andintensely silicified having been exposed to tem-peratures as high as 350degC (Cullers et al 1993)A large volcanic vent within the Panorama For-mation is preserved in the northwestern part ofthe outcrop area (Van Kranendonk 2000)

Strelley Pool Chert The relationship of the Strel-ley Pool Chert (Lowe 1983) to the underlyingunits varies from conformable to unconformableIn the northeast the contact with the underlyingCoonterunah Group forms an angular unconfor-mity with pronounced paleorelief (Buick et al1995b) The formation consists of carbonate sili-cified carbonate and clastic rocks (Fig 11) Thecherts and carbonates both have well-developedsedimentary structure that have been describedas wavy laminated cherts or carbonates and stro-matolites (Fig 12) (Lowe 1983 Hofmann et al1999) although as discussed in a following sec-tion there is some concern as to whether they arebiogenic (Lowe 1994 Lindsay et al 2003ab)

Euro Basalt The Euro Basalt consists domi-nantly of basalts of tholeiitic and high-Mg com-position that rest conformably on the StrelleyPool Chert (Van Kranendonk 2000) The forma-tion reaches a thickness in excess of 9 km in thenortheastern part of the study area The basaltsare pillowed and bedded on a kilometer scaleThe formation also includes some komatiite andfelsic volcaniclastics as well as thin cherts andvolcaniclastic units

Megasequence 3mdashSulphur Springs Group

The Sulphur Springs Group consists of threeformations (Fig 3) that in contrast to the previ-ous two Groups contain a significant proportionof clastic sedimentary rocks some quartzose

Leilira Formation The Leilira Formation consistspredominantly of clastic sedimentary rocks withonly small volumes of felsic to intermediate ex-trusive rocks interbedded The clastic rocks arepredominantly sandstones but vary from silt-stone to coarser pebbly sandstone Composition-ally the sandstones are quartzose or volcaniclithic rocks Locally laminated cherts formed bythe silicification of felsic volcanic ash and silt-stone are abundant

LINDSAY ET AL746

FIG 7 Bedded sedimentary carbonate and silica fromthe Dresser Formation (locality B Fig 2) Scale bar 5 5cm The rock consists of well-bedded units that begin withorangeyellow iron-rich carbonates and grade upward topale green silica The unit reflects geochemical changes inresponse to pulsing or cycling of the hydrothermal sys-tem Elsewhere carbonate and chert form alternating beds

The Leilira Formation represents a significantchange in depositional patterns as it is the oldestunit in which clastic sediments are the dominantlithology It reflects the growing volcanic cara-pace and in particular the exposure of the grani-toid core to weathering and erosion the first signsthat a true crust was developing

Kunagunarrina Formation The KunagunarrinaFormation reaches a maximum of 24 km inthickness and consists predominantly of high-Mg basalts and komatiites Locally andesitic andfelsic units are present especially near the top ofthe formation A thick chert unit (10 m) in themiddle of the formation separates lower Mg

ARCHEAN GEOLOGIC REFERENCE COLLECTION 747

FIG 8 Stromatolitic structures in theDresser Formation (locality B Fig 2)The original barite has been largely re-placed by chert Scale bar 5 10 cm

FIG 9 Exposures of the Dresser Formation in the North Pole Dome area (locality B Fig 2) showing large chert-barite feeder dykes The dykes consist of black chert and coarse bladed barite Note the vehicle for scale

basalts at the base of the formation from ko-matiites towards the top

Kangaroo Caves Formation The Kangaroo CavesFormation which reaches a maximum thicknessof 15 km is best exposed towards the center ofthe study area The formation is complex and con-sists of a succession of andesitic volcanics anddacitic to rhyolitic sills and extrusives A chert in-terval up to 100 m thick forms a distinctivemarker at the top of the formation Close to thecenter of the study area at Sulphur Springs themassive chert is overlain by a polymict megabrec-cia that rests upon a sharp erosional contact (Hill1997 Vearncombe et al 1998)

Hosted within a 200-m dacite unit immediatelybeneath the marker chert are six sulfide ore bodiesspaced at regular intervals of 5ndash7 km around themargins of the Strelley Granite (Morant 1998) Theyconsist of pyrite sphalerite chalcopyrite and galenawith smaller amounts of other sulfides (Vearncombeet al 1995 1998 Vearncombe 1996 Morant 1998)The sulfides which were deposited in a blacksmoker hydrothermal setting occur in associationwith growth faults developed around the StrelleyGranite the intrusion of which was syndepositionalwith the Kangaroo Caves Formation (Vearncombe1996 Brauhart et al 1998 Brauhart 1999)

Megasequence 4mdashGorge Creek Group

The Gorge Creek Group consists of five for-mations (Fig 3) that show much greater litho-logic diversity than earlier megasequences Inparticular this interval includes for the firsttime a significant thickness of quartzose sand-stone as well as a variety of other clastic sedi-mentary rocks

Pincunah Hill Formation In the western part ofthe study area the basal Pincunah Hill Formationconsists of thinly bedded BIF with small amountsof laminated chert red to gray shale and silt-stone This basal unit is up to 1 km thick in thewestern part of the area but wedges out and dis-appears in a short distance eastward The upper

LINDSAY ET AL748

FIG 11 A measured section through the Strelley PoolChert showing the location of stromatolitic structuresand the carbonate interval (adapted from Van Kranen-donk 2000) (locality C Fig 2) The gamma log to theright shows continuity across the chert-carbonate

FIG 10 ldquoMicrofossilsrdquo from the Apex chert near Mar-ble Bar a computer-generated digital automontage ofputative beggiatoan Eoleptonema apex Holotype (Schopf1993) combining the most-sharply focused images fromsuccessive focal planes b and c Original manual pho-tomontage and interpretative sketch (Schopf 1993) whichomits lower structure at arrow in (b) d and e New sin-gle images showing continuity of original and newly im-aged and automontaged structures Reprinted with per-mission from Nature (Brasier et al 2002) Copyright 2002Macmillan Publishers Ltd

part of the formation consists largely of red orpurple shale (Van Kranendonk 2000)

Corboy Formation At its thickest in the easternpart of the study area the Corboy Formation in-cludes a number of distinct facies developedacross major growth faults (Wilhelmij and Dun-lop 1984 Wilhelmij 1986) Here the formationconsists of a basal quartzose sandstone abruptlyoverlain by conglomerate interbedded withgraded sandstone units red shale and coarsesandstone The formation is capped by massivesandstone and conglomerate The unit was de-posited as a series of small prograding shallowmarine fan deltas (Wilhelmij 1986 Hill 1997 VanKranendonk 2000)

Paddy Market Formation In the western part ofthe study area the Paddy Market Formation isdominantly cherty BIF with minor ferruginousshale Further east the formation becomes moreshale-rich although much of the shale has been re-placed by chert Sandstone is present in smallamounts in beds up to 10 m thick Towards thetop of the formation in the east the cherts containfelsic volcanic material that is overlain by up to 200m of massive dacite (Van Kranendonk 2000)

Honeyeater Basalt The Honeyeater Basaltwhich consists entirely of basalt and dolerite con-formably overlies the Paddy Market FormationThe lower part of the formation consists of high-Mg basalt interbedded with dolerite whereashigher in the formation the basalts are largelytholeiitic and interbedded with dolerite (Gliksonand Hickman 1981ab Glikson et al 1991)

Pyramid Hill Formation The Pyramid Hill For-mation is limited in areal extent It is found in thenorth central part of the study area east of theStrelley Granite The formation consists of redand black shales that when heavily silicifiedgrade into laminated black and white chert (VanKranendonk 2000) The Gorge Creek Group iscapped by a well-bedded BIF

Megasequence 5mdashDe Grey Group

The De Grey Group consists of a single for-mation the Lalla Rookh Sandstone (Fig 3)which is dominated by clastic sedimentary rocksthat contain a significant quartzose componentThe Lalla Rookh Sandstone which rests on awell-defined unconformity above the GorgeCreek Group reaches a maximum thickness of

ARCHEAN GEOLOGIC REFERENCE COLLECTION 749

FIG 12 Stromatolitic carbonates in the Strelley Pool Chert (locality C Fig 2) Laminae are isopachous and theapex of the cone is ruptured and veined by secondary carbonate (arrow) Scale bar 5 10 cm

3 km (Krapez 1984) In the north central part ofthe study area the formation consists entirely ofcoarse clastic sediments and minor shalewhereas to the south the predominate fill is lithicand feldspathic sandstone (Chan 1998) Theserocks provide the first clear evidence of streamactivity in the Pilbara succession (Krapez 1984Van Kranendonk and Collins 1998 Van Kra-nendonk 2000)

THE PILBARA SUPRACRUSTAL SUCCESSION

The late Archean and early Paleoproterozoicbasins that rest upon the ancient cratonic blocksof Western Australia (Fig 1) form a time seriesassociated with the formation and ultimate dis-assembly of one of Earthrsquos earliest major conti-nental masses This was followed by the suturingof the Pilbara and Yilgarn Cratons along theCapricorn Orogen as a new supercontinentevolved in the Paleoproterozoic (Rogers and San-tosh 2002) These basins contain an importantsedimentary record of the early Earth includingsome of Earthrsquos earliest stromatolites and plat-form carbonate deposits (Simonson et al1993ab) organic-rich black shales (Brocks et al1999) and BIF (Trendall 1983 1990) The strati-graphic and structural framework is well con-strained by radiometric ages (Arndt et al 1991Barley et al 1997 Trendall et al 1998) providinga record at a critical time in Earth history whenthe atmosphere was first becoming oxygen-rich(Lindsay and Brasier 2002ab)

The Late Archean-Early PaleoproterozoicHamersley Basin which lies close to the southernmargin of the Pilbara Block (Trendall 1983 1990Blake and Barley 1992) overlies a normal crustalthickness (30ndash40 km) (Drummond 1981) Thebasin began to subside at approximately 28 Gafollowing an episode of crustal extension The ar-chitecture of the basin fill is complex andpolyphase with major erosional surfaces separat-ing areally extensive megasequencessuperse-quences (Trendall 1990 Krapez 1996) DuringFortescue and Hamersley Group time (Fig 4)sedimentation began in a rift setting in a silici-clastic-starved environment The later stages ofHamersley Basin sedimentation (Turee CreekGroup times) record the early transition from apassive margin to a foreland basin setting andwith it the establishment of a terrigenous sedi-

ment supply (Tyler and Thorne 1990) The suc-cession is largely marine Evidence of eustasy ap-pears for the first time in the sedimentary suc-cession of the Hamersley Basin (Lindsay andBrasier 2002ab)

The geometry and sequence stacking pattern ofthe succession are both very similar to youngerintracratonic basins (see Lindsay et al 1993) TheHamersley Basin is thus the first basinal settingpreserved on the Australian craton in which ma-rine sediments and in particular platform car-bonate rocks have been preserved in a responseto broad regional crustal subsidence and wheredeposition is controlled by eustasy

The Ashburton Basin (22ndash18 Ga) discon-formably overlies the southern margin of theHamersley Basin along its the contact with theCapricorn Orogen (Fig 1) (Thorne and Seymour1991) The basin forms the northern margin of theCapricorn Orogen a deformed zone that devel-oped during ocean closure Loading of the oceanfloor by the approaching Yilgarn Craton led tothe development of a westndashnorthwest-orientedforeland basin [McGrath Trough (Horowitz1982)] with an uplifted orogenic margin to thesouthwest (Blake and Groves 1987 Blake andBarley 1992) with the result that sedimentationwas dominated by an abundant supply of ter-rigenous clastic material (Mount McGrath For-mation) Continued loading of the crust ulti-mately led to the supply of terrigenous materialsbeing disrupted and producing a prograding car-bonate shelf (Duck Creek Dolomite) Basin sedi-mentation was accompanied by active-marginmafic volcanism (Cheela Springs Basalt)

THE CAPRICORN OROGEN ANDASSOCIATED BASINS

The Yerrida (22ndash19 Ga) Bryah (20 Ga)Padbury (20 Ga) and Earaheedy (19ndash165Ga) Basins lie to the southeast of the Hamersleyand Ashburton Basins and overlie the CapricornOrogen (Fig 1) (Occhipinti et al 1997 1998 Pi-rajno et al 1998 Pirajno and Adamides 2000)These now fragmentary basins formed along thenorthern margin of the Yilgarn Craton in a back-arc setting and record the convergence and colli-sion of the Archean Pilbara and Yilgarn Cratons(Tyler and Thorne 1990 Thorne and Seymour1991 Occhipinti et al 1997 1998 Pirajno et al1998) Because of their active margin settings the

LINDSAY ET AL750

fill of these small basins is dominated by clasticsediments However platform carbonate unitsare preserved in the Yerrida Basin (Gee and Grey1993 Pirajno and Adamides 2000) and a signif-icant thickness of carbonate rocks also occurs inthe Earaheedy Basin

THE ARCHEAN BIOSPHERE

The end of the Archean is formally defined at25 Ga (Plumb 1991) In reality the Archean worldended at some point in time closer to 28 or 30 Gawhen the first large continental blocks began to as-semble The stratigraphic record preserved on theancient Australian Craton reflects this major tran-sition The rock record from the Early Archean isvery different from that of the Late Archean in thatthe early record is predominantly volcanic and hy-drothermal in nature (ldquogreenstonerdquo) while thelater record is largely clastic and more readily com-pared with the Phanerozoic record

Structures and textures of possible biogenicorigin that are preserved in the stratigraphic suc-cession in the Early Archean study area are wellknown and are believed to provide the earliestdirect evidence of Earthrsquos biosphere Stromatoliticstructures have been encountered at two levelswithin the section The Dresser Formation con-tains structures preserved in chert and barite in-tervals that have been interpreted as stromatolitesand oncolites (Dunlop et al 1978 Walter et al1980 Buick et al 1981 Groves et al 1981 Wal-ter 1983) The stromatolitic structures occur forthe most part in layered barite units that devel-oped over swarms of synvolcanic hydrothermalfeeder dykes along listric growth faults (Nijmanet al 1998 Van Kranendonk 2004) Although ithas been argued that these structures are biogenic(Walter et al 1980 Schopf and Walter 1983 Wal-ter 1983) their direct association with the feederdykes leaves room for doubt (Buick et al 1981)The setting is complex and there is considerablescope for more detailed contextual research

The second stromatolitic horizon occurs in theoverlying Strelley Pool Chert a complex unit con-sisting of at least five members (Lowe 1980 1983Hofmann et al 1999) The main stromatolitic unitoccurs as an 8-m-thick interval in the middle ofthe formation and appears to have a consistentand widespread pattern of deposition The struc-tures have been the focus of considerable discus-sion and their biogenicity has been questioned

(Buick et al 1981 Lowe 1995) The structures arecomplex and varied (Fig 12) leading to a morerecent re-evaluation again suggesting that theyare biogenic (Hofmann et al 1999) However thelaminae are largely isopachous an indication thatthey could be abiotic precipitates (cf Pope andGrotzinger 2000) Given the intense hydrother-mal activity that appears to have taken placethroughout the depositional time interval thestructure may well be the product of hydrother-mal deposition (Lindsay et al 2003ab) In all ourobservations suggest that considerably morework needs to be done to establish the deposi-tional context of these stromatolitic structures be-fore they can be confidently ascribed as biogenicin origin

Structures resembling remarkably preservedbacterial and cyanobacterial microfossils havebeen found at several localities in and close to thestudy area (Awramik et al 1983 Schopf andPacker 1987 Schopf 1992 1993 Rasmussen 2000)A complex biota of 11 species of filamentousprokaryotes which are compared with cyanobac-teria (Schopf and Packer 1987 Schopf 1992 1993)have been described from the Apex chert close toMarble Bar (21deg105579S 119deg418789E) The taxo-nomic interpretations are important in that theyprovide the earliest evidence of morphological lifeon Earth and support the hypothesis of an earlybeginning for oxygenic photosynthesis (Schopf1999) The structures are almost a billion yearsolder than the oldest known putative cyanobac-terial biomarkers (Summons et al 1999) and oc-cur well before the earliest evidence for the risein oxygen in the atmosphere (for a summary seeKnoll and Canfield 1998 Lindsay and Brasier2000 2002ab) However in a recent re-evaluationof the original Apex chert material it has been ar-gued that the filaments are not biogenic but aresecondary artifacts formed during diagenesisfrom abiogenic organic compounds generated inhydrothermal chert veins (Brasier et al 2002 VanKranendonk 2004)

Filaments believed to be biogenic in originhave also been reported from the 324 Ga Kan-garoo Caves Formation (Rasmussen 2000) wherethey form part of the Sulphur Springs deposit as-sociated with a black smoker hydrothermal ventsystem (Vearncombe et al 1995 1998) Ras-mussen (2000) argued that the filaments are rem-nants of a hydrothermal biota of thermophilicchemotrophic prokaryotes Sulfide textures areremarkably well preserved in these complex de-

ARCHEAN GEOLOGIC REFERENCE COLLECTION 751

posits and are worthy of much more detailedanalysis as they may well preserve evidence ofthe early biosphere

Finally there is evidence that hydrocarbons aretrapped in these ancient sedimentary rocks eitheras fluid inclusions in hydrothermal precipitates oras bituminous residues in larger cavities in theKangaroo Caves Formation and in the Lalla RookhSandstone (Dutkiewicz et al 1998 Rasmussen andBuick 2000) Buick et al (1998) also reported car-bonaceous globules in cherts from the lower War-rawoona Group In the Kangaroo Caves Forma-tion the hydrocarbons occur in hydrothermalbarite or within the sulfides of the volcanogenichydrothermal massive sulfide deposits that weresyndepositional with the formation (Rasmussenand Buick 2000) Textural information suggestthat these deposits were precipitated as blacksmoker chimneys in a submarine hydrothermalsetting driven by heat released from the intrudingStrelley Granite at approximately 324 Ga (Vearn-combe et al 1995) Fluorescing fluid inclusions oc-cur in quartz overgrowths in sandstones from theLalla Rookh Sandstone (Dutkiewicz et al 1998)Rasmussen and Buick (2000) and Dutkiewicz et al(1998) have argued for a biogenic origin for the hy-drocarbons however an abiotic origin involvingfor example Fischer-Tropsch-type synthesis ap-pears possible (Brasier et al 2002)

The Late Archean (and Early Paleoproterozoic)record preserved in the Hamersley and relatedbasins is stratigraphically much more coherentthan that preserved in the earlier successions TheHamersley Basin in particular provides a verycomprehensive record of the early oxygenation ofthe atmosphere in the form of large eustaticallycontrolled upward-shallowing sequences that in-clude black shales BIFs and ultimately at theirtops biogenic platform carbonates (Lindsay andBrasier 2002ab) These depositional cycles con-tain organic-rich shales at their base that recordevidence for some of Earthrsquos earliest biosphere(Brocks et al 1999) BIFs (Trendall 1983 1990)that record the rise of oxygen in the atmosphereand carbonates that document some of Earthrsquosearliest reef-like carbonate structures (Simonsonand Hassler 1996 1997 Simonson et al 1993ab)

SAMPLING STRATEGY

The Archean rocks of the Pilbara Craton arethus part of a protocontinent about which the

nascent Australian Craton developed It consistsof granitoid complexes overlain by a carapace ofsynchronously deposited volcano-sedimentarywedges Resting upon the early Archean proto-continent are a series of supracrustal basins Inall the sedimentary record of the Pilbara extendsfrom rocks as old as 354 Ga to some as young asapproximately 19 Ga As summarized in the fol-lowing paragraphs the sedimentary record of thePilbara records evidence of the major tectonic andgeological events that accompanied the evolutionof the Archean biosphere

It is here proposed that we develop a carefullydocumented and curated sample collection fromthis unique setting The collection would form thebasis for early Earth studies and a focus for astro-biology The sample set would be designed to re-flect the evolutionary succession described abovewith emphasis placed on cherts lower in the suc-cession and on clastic sediments higher in the suc-cession Any comprehensive reference collectionmust reflect diversity of the succession and includerocks from each of the sedimentary environmentsas well as igneous rocks reflecting the frameworkin which the protocontinent assembled

The five megasequences at the North Shawstudy area (Fig 3) are very different and reflectthe evolving Early Archean protocontinent Theearliest sedimentary record is limited and domi-nated by hydrothermally generated laminatedcherts and BIFs which appear as thin interbedswithin massive basalts (Coonterunah Group) (Fig5) Gradually clastic rocks appear dominated atfirst by volcanoclastics followed later by the firstevidence of sediments containing detrital quartzderived from true continental granitic crust Hy-drothermally generated laminated chert carbon-ate and barite units occur throughout the sectionThe first evidence of erosion appears at the top ofthe Coonterunah Group in the form of an angu-lar unconformity (Buick et al 1995b)

The basins that form the supracrustal succes-sion on the Pilbara Craton offer important in-sights into the closing phases of the Archean andthe immediate post-Archean world Because oftheir economic importance the basins are wellmapped and drill core is readily available (seeLindsay and Brasier 2002a) The succession pro-vides vital information about the early oxygena-tion of the atmosphere and the rapid growth ofthe biosphere that resulted from the assembly ofthe first continents and the initiation of the platetectonic cycle Platform carbonates whose geom-

LINDSAY ET AL752

etry was determined by eustasy are widespreadand contain considerable information on earlyoxygenic photosynthesis and the development ofthe first reef-like structure that are similar inmany ways to modern reefs (Simonson et al1993ab Lindsay and Brasier 2002ab)

The collection would consist of three groupingsof samples stratigraphic samples site-specificsamples and framework samples

Stratigraphic samples

The stratigraphic sample set would be collectedfrom well-defined settings such as carefully mea-sured and documented sections or from drill coreSamples would be collected from each of the fivestratigraphic groups or megasequences Thesupracrustal successions preserved in the Hamer-sley and associated basins would where possiblebe taken from existing drill core or from drill coresobtained for special projects Where drill core isnot available samples would be collected fromcarefully selected and measured outcrop sectionsThe samples would concentrate on rocks mostlikely to host biogenic material Samples from theEarly Archean volcaniclastic carapace would bedominated by cherts that are most likely to pre-serve fine biogenic structures but would also in-clude other sedimentary hydrothermal units suchas barite and carbonate that would be used in sta-ble isotope and geochemical studies

Site-specific sample

This sample set would focus on sites where po-tential biogenic structures have been documentedsuch as the stromatolitic intervals in the DresserFormation and the Strelley Pool Chert It wouldalso include detailed sampling of sites such asChinaman Creek and the Strelley Pool Chertwhere purported microfossils have been docu-mented (Schopf 1993) and the sulfide bodies inthe Kangaroo Caves Formation that contain pos-sible microfossils as well as evidence of hydro-carbons (Rasmussen 2000 Rasmussen and Buick2000) This would involve a broad samplingaround the sites in measured sections and drillcore to establish the geological context

Framework samples

While the focus of the collecting would be onmaterial that would help define the character of

the Earthrsquos earliest biosphere it is important thatthe broad context of the samples be defined TheArchean is a period when the Earthrsquos crust wasevolving rapidly and there is a growing body ofevidence to suggest that the evolution of theplanet drives the evolution of the atmosphere andbiosphere (Lindsay and Brasier 2002ab) Theframework sample set would include suites ofsample from the extrusive volcanic rocks col-lected along well-documented sections Sectionsthrough the volcanoclastic succession would em-phasize the dominant volcanic units (basalts ko-matiites andesites felsites and rhyolites) assources of information on the mantle and evolv-ing crust This sample set would also include adetailed sampling of the granitoid complexeswhich ultimately drove the entire process as wellas the later intrusions that were important in dri-ving the hydrothermal systems that may wellhave been the cradle of early life

SAMPLE CURATIONCHARACTERIZATION AND

DISTRIBUTION

All samples would be fully documented in thefield Samples would be photographed and theirlocations determined using a Global PositioningSystem receiver For specialized samples such asthose thought to contain easily contaminated bio-geochemical information the field collectionwould be modeled on techniques developed bythe NASANational Science FoundationSmith-sonian Institution Antarctic Search for Meteoritesprogram (Harvey and Schutt 1998)

Characterization

The petrologic and petrographic characteristicsof each sample would be determined using thin-section optical microscopy The quantitative bulkchemical composition of each sample would bemeasured by x-ray fluorescence spectrometry Fi-nally the qualitative bulk mineralogy of eachsample would be measured by powder x-ray dif-fraction

Data management

An Archean Geologic Reference Materials Data-base would be produced to fully document eachsample Field documentation would include sam-pling rationale supporting references and field

ARCHEAN GEOLOGIC REFERENCE COLLECTION 753

photographs Documentation would include theresults of all analyses along with images of rep-resentative sample surfaces and optical micro-graphs The Database would be produced in elec-tronic format and posted on the web Subsamplesof the Archean Geologic Reference Materialswould then be made available at no cost to re-searchers in the astrobiology community

DISCUSSION AND CONCLUSIONS

The Early Archean environment was uniqueRecent studies indicate that the atmosphere waslargely anoxic with oxygen levels 1025 of thepresent atmospheric level (Pavlov and Kasting2002) There were no large continents no activeor passive margins and by default no intracra-tonic basins Significantly perhaps there are nosigns of eustasy The crust was evolving rapidlyand heat flow was high leading to the intrusionof large granitoid bodies and the development ofa volcanic carapace above them In turn thismeant that the marine environment was domi-nated by hydrothermal activity

In the absence of large continental masses de-trital materials other than volcanoclastics are rel-atively rare in the Early Archean Significant vol-umes of quartz sand do not appear in the PilbaraCraton sedimentary record until after 32 Ga pre-sumably as the intruding igneous bodies begin tobe unroofed and eroded There are thus no ma-jor accumulations of sedimentary rocks that wenormally expect associated with passive conti-nental margins

With the assembly of the earliest large conti-nents beginning at approximately 30ndash28 Gathe Earthrsquos atmosphere hydrosphere and bios-phere changed rapidly Intracratonic basinssuch as the Hamersley Basin began to developon the new crustal blocks opening new shal-low-water environments that provided an idealsetting for photosynthesizing organisms andthe development of the first carbonate reefstructures (Lindsay and Brasier 2002ab) Hy-drothermal processes while still importantwere thus no longer the only sources of energyavailable to the evolving biosphere

The Archean was thus an unfamiliar environ-ment and unravelling the nature of the earliestbiosphere will require development of new ap-proaches and new techniques Without large con-tinental masses subareal exposures are likely to

be rare and short-lived There is no evidence forstream activity until the De Grey group at ap-proximately 29 Ga at which time braided streamgravels were preserved Thus models involvingearly biospheric evolution in ldquoa warm little pondrdquo(Darwin 1888) or in fresh water seem unlikelyInstead the focus in the search for the origins oflife on Earth should be directed to hydrothermalmarine settings involving high temperatures andwhat would today be hostile geochemical envi-ronments with significant concentrations of metal-lic ions and a largely reducing environment Therisks for the misidentification of abiotic structuresand geochemical signatures as signs of ancient lifeare significant as seen in recent discussions (egBrasier et al 2002 Fedo and Whitehouse 2002Schopf et al 2002)

These difficulties suggest that much could begained by the development of a well-documentedcollection of Archean rocks that could be madeavailable to all qualified investigators Until weunderstand the earliest stages of the evolution oflife on Earth we cannot hope to make significantprogress in the search for life on other planetssuch as Mars Until there is a sample return mis-sion to Mars we must make do with meteoritesthat come to us largely out of context An under-standing of the Archean Earth can help placethose limited samples in a planetary context

ACKNOWLEDGMENTS

Franco Pirajno Kath Grey and Martin VanKranendonk provided generous advice and as-sistance and the Geological Survey of WesternAustralia supported the field program We aregrateful to Peter Morant of Sipa Resources andAdrian Black of Newexco Perth for their adviceand for access to drill-core materials

ABBREVIATION

BIF banded iron formation

REFERENCES

Arndt NT Nelson DR Compston W Trendall AFand Thorne AM (1991) The age of the FortescueGroup Hamersley Basin Western Australia from ionmicroprobe zircon U-Pb results Aust J Earth Sci 38262ndash281

LINDSAY ET AL754

Awramik SM Schopf JW and Walter MR (1983) Fil-amentous fossil bacteria from the Archaean of WesternAustralia Precambrian Res 20 357ndash374

Barley ME Pickard AL and Sylvester PJ (1997) Em-placement of a large igneous province as a possiblecause of banded iron formation 245 billion years agoNature 385 55ndash58

Bettenay LF Bickle MJ Boulter CA Groves DIMorant P Blake TS and James BA (1981) Evolu-tion of the Shaw Batholithmdashan Archaean granitoid-gneiss dome in the eastern Pilbara Western AustraliaIn Special Publication 7 The Archaean edited by JEGlover and DI Groves Geological Society of WesternAustralia Perth pp 361ndash372

Blake TS and Barley ME (1992) Tectonic evolution ofthe Late Archaean to Early Proterozoic Mount BruceMegasequence Set Western Australia Tectonics 11 1415ndash1425

Blake TS and Groves DI (1987) Continental rifting andthe Archaean-Proterozoic transition Geology 15 229ndash232

Brasier MD and Lindsay JF (1998) A billion years ofenvironmental stability and the emergence of eukary-otes new data from Northern Australia Geology 26555ndash558

Brasier MD Green OR Jephcoat AP Kleppe AKVan Kranendonk MJ Lindsay JF Steele A andGrassineau NV (2002) Questioning the evidence forEarthrsquos oldest fossils Nature 416 76ndash81

Brauhart C (1999) Regional alteration systems associatedwith Archaean volcanogenic massive sulphide depositsat Panorama Pilbara Western Australia [PhD Thesis]University of Western Australia Perth

Brauhart C Groves DI and Morant P (1998) Regionalalteration systems associated with volcanogenic mas-sive sulfide mineralization at Panorama Pilbara West-ern Australia Econ Geol 93 292ndash302

Brocks J Logan GA Buick R and Summons RE(1999) Archean molecular fossils and the early rise ofeukaryotes Science 285 1033ndash1036

Buick R Dunlop JSR and Groves DI (1981) Stroma-tolite recognition in ancient rocks an appraisal of ir-regular laminated structures in an early Archaeanchert-barite unit from North Pole Western AustraliaAlcheringa 5 161ndash181

Buick R Des Marais DJ and Knoll AH (1995a) Sta-ble isotope compositions of carbonates from the Meso-proterozoic Bangemall Group northwestern AustraliaChem Geol 123 153ndash171

Buick R Thornett JR McNaughton NJ Smith JBBarley ME and Savage M (1995b) Record of emer-gent continental crust 35 billion years ago in the Pil-bara Craton of Australia Nature 375 574ndash577

Buick R Rasmussen B and Krapez B (1998) Archaeanoil evidence for extensive hydrocarbon generation andmigration 25ndash35 Ga Am Assoc Petroleum Geol Bull82 50ndash69

Chan M (1998) Sedimentology and structure of threesmall congolmeratic basins in the Lalla Rookh-WesternShaw structural corridor Pilbara Craton Western Aus-

tralia [BSc Honours Thesis] University of WesternAustralia Perth

Cullers RL DiMarco MJ Lowe DR and Stone J(1993) Geochemistry of a silicified felsic volcaniclasticsuite from the Early Archaean Panorama FormationPilbara Block Western Australiamdashan evaluation of de-positional and post-depositional processes with specialemphasis on the rare-Earth elements Precambrian Res60 99ndash116

Darwin F ed (1888) The Life and Letters of Charles Dar-win Vol 3 John Murray London

de Wit MJ (1998) On Archaean granites greenstonescratons and tectonics does the evidence demand a ver-dict Precambrian Geol 91 181ndash226

Drummond BJ (1981) Crustal structure of the Precam-brian terrains of northwest Australia from seismic re-fraction data BMR J Geol Geophys 6 123ndash135

Dunlop JSR Muir M Milne VA and Groves DI(1978) A new microfossil assemblage from the Ar-chaean of Western Australia Nature 274 676ndash678

Dutkiewicz A Rasmussen B and Buick R (1998) Oilpreserved in fluid inclusions in Archaean sandstonesNature 395 885ndash888

Fedo CM and Whitehouse MJ (2002) Metasomatic ori-gin of quartz-pyroxene rock Akilia Greenland and im-plications for Earthrsquos earliest life Science 296 1448ndash1452

Gee RD and Grey K (1993) Report 41 Proterozoic Rockson the Glengarry 1250 000 SheetmdashStratigraphy Structureand Stromatolite Biostratigraphy Geological Survey ofWestern Australia Perth

Glikson AY and Hickman AH (1981a) Chemicalstratigraphy and petrogenesis of Archaean basic-ultra-basic volcanic units eastern Pilbara Block WesternAustralia In Special Publication 7 Archaean Geologyedited by JE Groves and DI Groves Geological Soci-ety of Western Australia Perth pp 287ndash300

Glikson AY and Hickman AH (1981b) Record 198136Geochemistry of Archaean Volcanic Successions EasternPilbara Block Western Australia Australian Bureau ofMineral Resources Canberra

Glikson AY Davy R and Hickman AH (1991) Record199146 Trace Metal Distribution Patterns in Basalts Pil-bara Craton Western Australia with Stratigraphic-Geo-chemical Implications Australian Bureau of Mineral Re-sources Canberra

Groves DI Dunlop JSR and Buick R (1981) An earlyhabitat of life Sci Am 245 64ndash73

Harvey RP and Schutt JW (1998) Meteorite recoveryand reconnaissance in the Allan Hills-David Glacierand Darwin Glacier regions 1996ndash1997 Antarctic J US 32 25ndash27

Hickman AH (1975) Annual Report 1974 PrecambrianStructural Geology of Part of the Pilbara Region Geologi-cal Survey of Western Australia Perth pp 68ndash73

Hickman AH (1983) Bulletin 127 Geology of the PilbaraBlock and Its Environs Geological Survey of WesternAustralia Perth

Hickman AH (1984) Archaean diapirism in the PilbaraBlock Western Australia In Precambrian Tectonics Il-lustrated edited by A Kroner and R Greiling E

ARCHEAN GEOLOGIC REFERENCE COLLECTION 755

EARTHrsquoS EARLIEST VOLCANO-SEDIMENTARY RECORD

The rocks of the Pilbara Craton form the ear-liest lithologic succession on the Australian con-tinent and along with the overlying supracrustalsuccession would form the focus of the proposedsampling program Most of the best exposures ofthis early sedimentary record occur in a rela-tively limited area covered by a single 1100000map sheet the North Shaw map sheet (Van Kra-nendonk 1998 2000) (Fig 2) which encompassesthe main study area considered in this paperFurther regional information from flanking areascan be obtained from the 1250000 Marble Barmap sheet (Hickman and Lipple 1978) which in-cludes the North Shaw sheet The area is some-what remote and only accessible by four-wheeldrive vehicle

THE NORTH SHAW STUDY AREA

Granitoid complexes

The study area is underlain by parts of theShaw Yule and Carlindi granitoid complexeswhich are broadly ovoid in plan 25ndash110 km indiameter and 40ndash60 km apart (Fig 2) The com-

plexes consist of several mappable plutonic orgneissic units (Bettenay et al 1981 Van Kra-nendonk 2000 Van Kranendonk et al 2002)Structural and geophysical analyses suggest thatthe granitoid complexes form structural domesas part of a continuous midcrustal layer ex-tending from 14 km beneath the interveningsupracrustal units (Hickman 1984 Van Kra-nendonk 2000) Components of the complexesare coeval with felsic volcanic horizons in thePilbara Supergroup (the carapace above thegranitoids) (Williams and Collins 1990) andhave intrusive or sheared intrusive contacts withthem indicating an intimate synchronous devel-opment

The plutons

Three granitoid intrusions are exposed withinthe supracrustal successionmdashthe North PoleMonzonite (3458 Ga) Strelley Granite (3238Ga) and Keep it Dark Monzonite (2936 Ga)(Fig 3)mdashand form discrete plutons unrelated tothe granitoid complexes (Fig 2) The geometry andgeochronology of these plutons suggest that theyare synvolcanic laccoliths formed along with thesupracrustal units and can be related directly to thelithostratigraphy and to the hydrothermal massivesulfide bodies discussed in a following section

ARCHEAN GEOLOGIC REFERENCE COLLECTION 741

FIG 1 The Pilbara and Yil-garn Cratons showing the de-formed suture zone of theCapricorn Orogen and the dis-tribution of younger sedi-mentary basins The proposedNorth Shaw study area isshown as a black rectangle

THE STRATIGRAPHIC RECORD

The term ldquogreenstonerdquo or ldquogreenstone beltsrdquohas generally been applied to the largely vol-canigenic stratigraphic intervals preserved as acarapace overlying the granitoid complexes TheArchean stratigraphic record preserved in thegreenstone carapace in the study area can be di-vided into five unconformity or intrusion-boundgroups or megasequences (Fig 3) They are tec-tonically defined units and not eustatically con-trolled (Van Kranendonk et al 2002) Themegasequences of the Pilbara Supergroup weredeposited over a time period between approxi-mately 351 and approximately 294 Ga Theywere deposited unconformably one above theother and consistently dip away from the domal

granitoid complexes The dips of the beddinggradually decrease with decreasing age suggest-ing that they were deposited as thickeningwedges adjacent to growing granitoid diapirs(Hickman 1975 1983 1984 Van Kranendonk2000) Granitoid diapirism appears to have con-tinued until after 27 Ga as the overlying rem-nants of the Hamersley Basin succession are pre-served as synclinorial remnants (Hickman 1984Van Kranendonk et al 2002)

Despite local complex faulting the supra-crustal megasequences are relatively well pre-served such that their ages and stratigraphic re-lationships are well established (Williams andCollins 1990 Van Kranendonk 2000) Metamor-phic grade decreases away from the granitoidcomplexes from a maximum of middle to lower

LINDSAY ET AL742

FIG 2 Simplified map of the pro-posed North Shaw study area show-ing the main lithologic units (groups)The sinuous Shaw River almost bisectsthe area Adapted from Van Kranen-donk (2000)

amphibolite facies at the contact to lower green-schist and prenhite-pumpellyite facies Lowergreenschist facies is by far the most widespreadmetamorphic grade in the study area

Megasequence 1mdashCoonterunah Group

The Coonterunah Group includes the earliestknown stratigraphic succession (3515 Ma) pre-served on the Pilbara Craton (Buick et al 1995bVan Kranendonk and Morant 1998) The lowercontact is intrusive with the 3468 Ma granitoidrocks of the Carlindi Granitoid Complex The suc-cession consists of three formations (Van Kra-nendonk 2000)

Tabletop Formation The Tabletop Formationconsists entirely of thick basaltic units They aremainly fine-grained tholeiitic lavas althoughsome gabbroic intrusions have been identifiedThe basalts are mostly featureless although pil-lowed units are present indicating a submarinesetting

Coucal Formation The Coucal Formation whichis 2 km thick at its maximum consists largely

ARCHEAN GEOLOGIC REFERENCE COLLECTION 743

of fine-grained doleritic andesite basalt and fel-sic volcanic units (Fig 5) Thin chert (Fig 6) andimpoverished cherty banded iron formation (BIF)units are present throughout the formation Theseare the earliest known sedimentary units in thePilbara Craton and offer the earliest opportunityfor the preservation of ancient life

Double Bar Formation The Double Bar Forma-tion which conformably overlies the Coucal For-mation consists of fine-grained tholeiitic basaltswith interbedded basaltic volcaniclastic rocksSome basalts are pillowed suggesting a subma-rine setting The volcanics retain their basaltictextures but have been altered significantly bylow-grade metamorphism (Van Kranendonk2000)

Megasequence 2mdashWarrawoona Group

The Warrawoona Group is separated from theunderlying Coonterunah Group by a widespreadangular unconformity the oldest evidence onEarth for emergence (Buick et al 1995b) (Fig 3)The Warrawoona Group is dominantly volcanic inorigin but includes some clastic sedimentary rocks

FIG 3 Simplified stratigraphy of theArchean rocks exposed in the proposedstudy area (adapted from Van Kranen-donk 2000) Arrows indicate dated periodof activity of the granitoid complexes as dis-tinct from the plutons which are shown incartoon form

McPhee Basalt The McPhee Basalt which isdated at 3477 Ma and reaches a maximum of 3km in thickness is relatively limited in outcrop(Van Kranendonk et al 2002) It consists largelyof basalts and schists and is only exposed in thesouthern part of the study area adjacent to theShaw Granitoid Complex where the contact is in-trusive A thin unit of impoverished cherty BIFforms a distinctive cap to the formation

Mount Ada Basalt The Mount Ada basalt con-sists of 3ndash4 km of massive tholeiitic basalt and do-

LINDSAY ET AL744

FIG 4 Simplified stratigraphy of the Hamersley and Ashburton Basins of Western Australia (after Lindsay andBrasier 2002)

lerite intrusions that are exposed in the south-eastern corner of the study area Its lower contactwith the Shaw Granitoid Complex and NorthPole Monzonite is intrusive For the most partvolcanic textures are well preserved in thebasalts

Dresser Formation The Dresser Formation isonly recognized in the North Pole Dome whereit reaches a maximum of 1000 m in thickness(Van Kranendonk et al 2002) The formation con-sists of massive and pillowed mafic volcanics in-

terbedded with units of chert sandstone con-glomerate barite and minor carbonate (Fig 7)The individual chert-barite beds vary from 5 to40 m in thickness Associated with these unusualsedimentary facies are structures that have beeninterpreted as the worldrsquos oldest stromatolites(Fig 8) and associated microfossils (Dunlop et al1978 Walter et al 1980 Buick et al 1981 Groveset al 1981 Walter 1983) It is perhaps significantthat this distinctive unit is fed by a deep-seatedsystem of hydrothermal dykes (Fig 9)

Duffer Formation The Duffer Formation is bestexposed in the southeastern portion of the studyarea where it consists largely of felsic schist andagglomerate The agglomerates which have been

locally interpreted as welded ignimbrites arebedded in meter-scale units The Duffer Forma-tion has a depositional age of approximately 347Ga (Thorpe et al 1992 McNaughton et al 1993Nelson 1997 1998 1999 2000 2001) and restsconformably on the Mount Ada Basalt

Apex Basalt The Apex Basalt is well exposed inthe northeastern corner of the study area and overlarge areas on adjacent map sheets The forma-tion is dominated by pillowed basalts which arelargely tholeiitic The pillows are well developedand preserve vesicular rims chilled margins andconcentric cooling cracks Hyaloclastic brecciasare also present within the assemblage Doleriticflows and intrusions are common and doleriticdykes occur locally

Interbedded with the volcanics are a numberof chert horizons which are 10 m in thickness

ARCHEAN GEOLOGIC REFERENCE COLLECTION 745

FIG 5 A measured section through the Coucal For-mation (locality A Fig 2) The formation is dominatedby basic volcanic rocks with some felsic volcanic rocksand thin (10 m) hydrothermal cherts between flows

FIG 6 Laminated cherts from the lower Coucal For-mation (locality A Fig 2) Cherts form the earliest sedi-mentary rocks in the Pilbara succession Some units areas much as 10 m thick Scale bar 5 10 cm

and usually well laminated mainly black andwhite chert Numerous chert dykes cut across theunderlying stratigraphy and terminate at thelevel of the chert horizons Close to Marble Barexposures of the Apex chert contain what was ini-tially believed to be Earthrsquos oldest morphologicalfossil assemblage (Fig 10) (Schopf and Packer1987 Schopf 1992 1993) a claim recently dis-puted by more detailed analysis (Brasier et al2002) (see following section on Paleontology)

Panorama Formation The Panorama Formation(3458ndash3434 Ma) which is best exposed in thenorthern part of the study area near the ShawRiver is dominated by felsic volcaniclastic rocks

with small volumes of felsic lavas and some chertinterbeds (Hickman 1983 Thorpe et al 1992 Nel-son 2000) The formation is strongly altered andintensely silicified having been exposed to tem-peratures as high as 350degC (Cullers et al 1993)A large volcanic vent within the Panorama For-mation is preserved in the northwestern part ofthe outcrop area (Van Kranendonk 2000)

Strelley Pool Chert The relationship of the Strel-ley Pool Chert (Lowe 1983) to the underlyingunits varies from conformable to unconformableIn the northeast the contact with the underlyingCoonterunah Group forms an angular unconfor-mity with pronounced paleorelief (Buick et al1995b) The formation consists of carbonate sili-cified carbonate and clastic rocks (Fig 11) Thecherts and carbonates both have well-developedsedimentary structure that have been describedas wavy laminated cherts or carbonates and stro-matolites (Fig 12) (Lowe 1983 Hofmann et al1999) although as discussed in a following sec-tion there is some concern as to whether they arebiogenic (Lowe 1994 Lindsay et al 2003ab)

Euro Basalt The Euro Basalt consists domi-nantly of basalts of tholeiitic and high-Mg com-position that rest conformably on the StrelleyPool Chert (Van Kranendonk 2000) The forma-tion reaches a thickness in excess of 9 km in thenortheastern part of the study area The basaltsare pillowed and bedded on a kilometer scaleThe formation also includes some komatiite andfelsic volcaniclastics as well as thin cherts andvolcaniclastic units

Megasequence 3mdashSulphur Springs Group

The Sulphur Springs Group consists of threeformations (Fig 3) that in contrast to the previ-ous two Groups contain a significant proportionof clastic sedimentary rocks some quartzose

Leilira Formation The Leilira Formation consistspredominantly of clastic sedimentary rocks withonly small volumes of felsic to intermediate ex-trusive rocks interbedded The clastic rocks arepredominantly sandstones but vary from silt-stone to coarser pebbly sandstone Composition-ally the sandstones are quartzose or volcaniclithic rocks Locally laminated cherts formed bythe silicification of felsic volcanic ash and silt-stone are abundant

LINDSAY ET AL746

FIG 7 Bedded sedimentary carbonate and silica fromthe Dresser Formation (locality B Fig 2) Scale bar 5 5cm The rock consists of well-bedded units that begin withorangeyellow iron-rich carbonates and grade upward topale green silica The unit reflects geochemical changes inresponse to pulsing or cycling of the hydrothermal sys-tem Elsewhere carbonate and chert form alternating beds

The Leilira Formation represents a significantchange in depositional patterns as it is the oldestunit in which clastic sediments are the dominantlithology It reflects the growing volcanic cara-pace and in particular the exposure of the grani-toid core to weathering and erosion the first signsthat a true crust was developing

Kunagunarrina Formation The KunagunarrinaFormation reaches a maximum of 24 km inthickness and consists predominantly of high-Mg basalts and komatiites Locally andesitic andfelsic units are present especially near the top ofthe formation A thick chert unit (10 m) in themiddle of the formation separates lower Mg

ARCHEAN GEOLOGIC REFERENCE COLLECTION 747

FIG 8 Stromatolitic structures in theDresser Formation (locality B Fig 2)The original barite has been largely re-placed by chert Scale bar 5 10 cm

FIG 9 Exposures of the Dresser Formation in the North Pole Dome area (locality B Fig 2) showing large chert-barite feeder dykes The dykes consist of black chert and coarse bladed barite Note the vehicle for scale

basalts at the base of the formation from ko-matiites towards the top

Kangaroo Caves Formation The Kangaroo CavesFormation which reaches a maximum thicknessof 15 km is best exposed towards the center ofthe study area The formation is complex and con-sists of a succession of andesitic volcanics anddacitic to rhyolitic sills and extrusives A chert in-terval up to 100 m thick forms a distinctivemarker at the top of the formation Close to thecenter of the study area at Sulphur Springs themassive chert is overlain by a polymict megabrec-cia that rests upon a sharp erosional contact (Hill1997 Vearncombe et al 1998)

Hosted within a 200-m dacite unit immediatelybeneath the marker chert are six sulfide ore bodiesspaced at regular intervals of 5ndash7 km around themargins of the Strelley Granite (Morant 1998) Theyconsist of pyrite sphalerite chalcopyrite and galenawith smaller amounts of other sulfides (Vearncombeet al 1995 1998 Vearncombe 1996 Morant 1998)The sulfides which were deposited in a blacksmoker hydrothermal setting occur in associationwith growth faults developed around the StrelleyGranite the intrusion of which was syndepositionalwith the Kangaroo Caves Formation (Vearncombe1996 Brauhart et al 1998 Brauhart 1999)

Megasequence 4mdashGorge Creek Group

The Gorge Creek Group consists of five for-mations (Fig 3) that show much greater litho-logic diversity than earlier megasequences Inparticular this interval includes for the firsttime a significant thickness of quartzose sand-stone as well as a variety of other clastic sedi-mentary rocks

Pincunah Hill Formation In the western part ofthe study area the basal Pincunah Hill Formationconsists of thinly bedded BIF with small amountsof laminated chert red to gray shale and silt-stone This basal unit is up to 1 km thick in thewestern part of the area but wedges out and dis-appears in a short distance eastward The upper

LINDSAY ET AL748

FIG 11 A measured section through the Strelley PoolChert showing the location of stromatolitic structuresand the carbonate interval (adapted from Van Kranen-donk 2000) (locality C Fig 2) The gamma log to theright shows continuity across the chert-carbonate

FIG 10 ldquoMicrofossilsrdquo from the Apex chert near Mar-ble Bar a computer-generated digital automontage ofputative beggiatoan Eoleptonema apex Holotype (Schopf1993) combining the most-sharply focused images fromsuccessive focal planes b and c Original manual pho-tomontage and interpretative sketch (Schopf 1993) whichomits lower structure at arrow in (b) d and e New sin-gle images showing continuity of original and newly im-aged and automontaged structures Reprinted with per-mission from Nature (Brasier et al 2002) Copyright 2002Macmillan Publishers Ltd

part of the formation consists largely of red orpurple shale (Van Kranendonk 2000)

Corboy Formation At its thickest in the easternpart of the study area the Corboy Formation in-cludes a number of distinct facies developedacross major growth faults (Wilhelmij and Dun-lop 1984 Wilhelmij 1986) Here the formationconsists of a basal quartzose sandstone abruptlyoverlain by conglomerate interbedded withgraded sandstone units red shale and coarsesandstone The formation is capped by massivesandstone and conglomerate The unit was de-posited as a series of small prograding shallowmarine fan deltas (Wilhelmij 1986 Hill 1997 VanKranendonk 2000)

Paddy Market Formation In the western part ofthe study area the Paddy Market Formation isdominantly cherty BIF with minor ferruginousshale Further east the formation becomes moreshale-rich although much of the shale has been re-placed by chert Sandstone is present in smallamounts in beds up to 10 m thick Towards thetop of the formation in the east the cherts containfelsic volcanic material that is overlain by up to 200m of massive dacite (Van Kranendonk 2000)

Honeyeater Basalt The Honeyeater Basaltwhich consists entirely of basalt and dolerite con-formably overlies the Paddy Market FormationThe lower part of the formation consists of high-Mg basalt interbedded with dolerite whereashigher in the formation the basalts are largelytholeiitic and interbedded with dolerite (Gliksonand Hickman 1981ab Glikson et al 1991)

Pyramid Hill Formation The Pyramid Hill For-mation is limited in areal extent It is found in thenorth central part of the study area east of theStrelley Granite The formation consists of redand black shales that when heavily silicifiedgrade into laminated black and white chert (VanKranendonk 2000) The Gorge Creek Group iscapped by a well-bedded BIF

Megasequence 5mdashDe Grey Group

The De Grey Group consists of a single for-mation the Lalla Rookh Sandstone (Fig 3)which is dominated by clastic sedimentary rocksthat contain a significant quartzose componentThe Lalla Rookh Sandstone which rests on awell-defined unconformity above the GorgeCreek Group reaches a maximum thickness of

ARCHEAN GEOLOGIC REFERENCE COLLECTION 749

FIG 12 Stromatolitic carbonates in the Strelley Pool Chert (locality C Fig 2) Laminae are isopachous and theapex of the cone is ruptured and veined by secondary carbonate (arrow) Scale bar 5 10 cm

3 km (Krapez 1984) In the north central part ofthe study area the formation consists entirely ofcoarse clastic sediments and minor shalewhereas to the south the predominate fill is lithicand feldspathic sandstone (Chan 1998) Theserocks provide the first clear evidence of streamactivity in the Pilbara succession (Krapez 1984Van Kranendonk and Collins 1998 Van Kra-nendonk 2000)

THE PILBARA SUPRACRUSTAL SUCCESSION

The late Archean and early Paleoproterozoicbasins that rest upon the ancient cratonic blocksof Western Australia (Fig 1) form a time seriesassociated with the formation and ultimate dis-assembly of one of Earthrsquos earliest major conti-nental masses This was followed by the suturingof the Pilbara and Yilgarn Cratons along theCapricorn Orogen as a new supercontinentevolved in the Paleoproterozoic (Rogers and San-tosh 2002) These basins contain an importantsedimentary record of the early Earth includingsome of Earthrsquos earliest stromatolites and plat-form carbonate deposits (Simonson et al1993ab) organic-rich black shales (Brocks et al1999) and BIF (Trendall 1983 1990) The strati-graphic and structural framework is well con-strained by radiometric ages (Arndt et al 1991Barley et al 1997 Trendall et al 1998) providinga record at a critical time in Earth history whenthe atmosphere was first becoming oxygen-rich(Lindsay and Brasier 2002ab)

The Late Archean-Early PaleoproterozoicHamersley Basin which lies close to the southernmargin of the Pilbara Block (Trendall 1983 1990Blake and Barley 1992) overlies a normal crustalthickness (30ndash40 km) (Drummond 1981) Thebasin began to subside at approximately 28 Gafollowing an episode of crustal extension The ar-chitecture of the basin fill is complex andpolyphase with major erosional surfaces separat-ing areally extensive megasequencessuperse-quences (Trendall 1990 Krapez 1996) DuringFortescue and Hamersley Group time (Fig 4)sedimentation began in a rift setting in a silici-clastic-starved environment The later stages ofHamersley Basin sedimentation (Turee CreekGroup times) record the early transition from apassive margin to a foreland basin setting andwith it the establishment of a terrigenous sedi-

ment supply (Tyler and Thorne 1990) The suc-cession is largely marine Evidence of eustasy ap-pears for the first time in the sedimentary suc-cession of the Hamersley Basin (Lindsay andBrasier 2002ab)

The geometry and sequence stacking pattern ofthe succession are both very similar to youngerintracratonic basins (see Lindsay et al 1993) TheHamersley Basin is thus the first basinal settingpreserved on the Australian craton in which ma-rine sediments and in particular platform car-bonate rocks have been preserved in a responseto broad regional crustal subsidence and wheredeposition is controlled by eustasy

The Ashburton Basin (22ndash18 Ga) discon-formably overlies the southern margin of theHamersley Basin along its the contact with theCapricorn Orogen (Fig 1) (Thorne and Seymour1991) The basin forms the northern margin of theCapricorn Orogen a deformed zone that devel-oped during ocean closure Loading of the oceanfloor by the approaching Yilgarn Craton led tothe development of a westndashnorthwest-orientedforeland basin [McGrath Trough (Horowitz1982)] with an uplifted orogenic margin to thesouthwest (Blake and Groves 1987 Blake andBarley 1992) with the result that sedimentationwas dominated by an abundant supply of ter-rigenous clastic material (Mount McGrath For-mation) Continued loading of the crust ulti-mately led to the supply of terrigenous materialsbeing disrupted and producing a prograding car-bonate shelf (Duck Creek Dolomite) Basin sedi-mentation was accompanied by active-marginmafic volcanism (Cheela Springs Basalt)

THE CAPRICORN OROGEN ANDASSOCIATED BASINS

The Yerrida (22ndash19 Ga) Bryah (20 Ga)Padbury (20 Ga) and Earaheedy (19ndash165Ga) Basins lie to the southeast of the Hamersleyand Ashburton Basins and overlie the CapricornOrogen (Fig 1) (Occhipinti et al 1997 1998 Pi-rajno et al 1998 Pirajno and Adamides 2000)These now fragmentary basins formed along thenorthern margin of the Yilgarn Craton in a back-arc setting and record the convergence and colli-sion of the Archean Pilbara and Yilgarn Cratons(Tyler and Thorne 1990 Thorne and Seymour1991 Occhipinti et al 1997 1998 Pirajno et al1998) Because of their active margin settings the

LINDSAY ET AL750

fill of these small basins is dominated by clasticsediments However platform carbonate unitsare preserved in the Yerrida Basin (Gee and Grey1993 Pirajno and Adamides 2000) and a signif-icant thickness of carbonate rocks also occurs inthe Earaheedy Basin

THE ARCHEAN BIOSPHERE

The end of the Archean is formally defined at25 Ga (Plumb 1991) In reality the Archean worldended at some point in time closer to 28 or 30 Gawhen the first large continental blocks began to as-semble The stratigraphic record preserved on theancient Australian Craton reflects this major tran-sition The rock record from the Early Archean isvery different from that of the Late Archean in thatthe early record is predominantly volcanic and hy-drothermal in nature (ldquogreenstonerdquo) while thelater record is largely clastic and more readily com-pared with the Phanerozoic record

Structures and textures of possible biogenicorigin that are preserved in the stratigraphic suc-cession in the Early Archean study area are wellknown and are believed to provide the earliestdirect evidence of Earthrsquos biosphere Stromatoliticstructures have been encountered at two levelswithin the section The Dresser Formation con-tains structures preserved in chert and barite in-tervals that have been interpreted as stromatolitesand oncolites (Dunlop et al 1978 Walter et al1980 Buick et al 1981 Groves et al 1981 Wal-ter 1983) The stromatolitic structures occur forthe most part in layered barite units that devel-oped over swarms of synvolcanic hydrothermalfeeder dykes along listric growth faults (Nijmanet al 1998 Van Kranendonk 2004) Although ithas been argued that these structures are biogenic(Walter et al 1980 Schopf and Walter 1983 Wal-ter 1983) their direct association with the feederdykes leaves room for doubt (Buick et al 1981)The setting is complex and there is considerablescope for more detailed contextual research

The second stromatolitic horizon occurs in theoverlying Strelley Pool Chert a complex unit con-sisting of at least five members (Lowe 1980 1983Hofmann et al 1999) The main stromatolitic unitoccurs as an 8-m-thick interval in the middle ofthe formation and appears to have a consistentand widespread pattern of deposition The struc-tures have been the focus of considerable discus-sion and their biogenicity has been questioned

(Buick et al 1981 Lowe 1995) The structures arecomplex and varied (Fig 12) leading to a morerecent re-evaluation again suggesting that theyare biogenic (Hofmann et al 1999) However thelaminae are largely isopachous an indication thatthey could be abiotic precipitates (cf Pope andGrotzinger 2000) Given the intense hydrother-mal activity that appears to have taken placethroughout the depositional time interval thestructure may well be the product of hydrother-mal deposition (Lindsay et al 2003ab) In all ourobservations suggest that considerably morework needs to be done to establish the deposi-tional context of these stromatolitic structures be-fore they can be confidently ascribed as biogenicin origin

Structures resembling remarkably preservedbacterial and cyanobacterial microfossils havebeen found at several localities in and close to thestudy area (Awramik et al 1983 Schopf andPacker 1987 Schopf 1992 1993 Rasmussen 2000)A complex biota of 11 species of filamentousprokaryotes which are compared with cyanobac-teria (Schopf and Packer 1987 Schopf 1992 1993)have been described from the Apex chert close toMarble Bar (21deg105579S 119deg418789E) The taxo-nomic interpretations are important in that theyprovide the earliest evidence of morphological lifeon Earth and support the hypothesis of an earlybeginning for oxygenic photosynthesis (Schopf1999) The structures are almost a billion yearsolder than the oldest known putative cyanobac-terial biomarkers (Summons et al 1999) and oc-cur well before the earliest evidence for the risein oxygen in the atmosphere (for a summary seeKnoll and Canfield 1998 Lindsay and Brasier2000 2002ab) However in a recent re-evaluationof the original Apex chert material it has been ar-gued that the filaments are not biogenic but aresecondary artifacts formed during diagenesisfrom abiogenic organic compounds generated inhydrothermal chert veins (Brasier et al 2002 VanKranendonk 2004)

Filaments believed to be biogenic in originhave also been reported from the 324 Ga Kan-garoo Caves Formation (Rasmussen 2000) wherethey form part of the Sulphur Springs deposit as-sociated with a black smoker hydrothermal ventsystem (Vearncombe et al 1995 1998) Ras-mussen (2000) argued that the filaments are rem-nants of a hydrothermal biota of thermophilicchemotrophic prokaryotes Sulfide textures areremarkably well preserved in these complex de-

ARCHEAN GEOLOGIC REFERENCE COLLECTION 751

posits and are worthy of much more detailedanalysis as they may well preserve evidence ofthe early biosphere

Finally there is evidence that hydrocarbons aretrapped in these ancient sedimentary rocks eitheras fluid inclusions in hydrothermal precipitates oras bituminous residues in larger cavities in theKangaroo Caves Formation and in the Lalla RookhSandstone (Dutkiewicz et al 1998 Rasmussen andBuick 2000) Buick et al (1998) also reported car-bonaceous globules in cherts from the lower War-rawoona Group In the Kangaroo Caves Forma-tion the hydrocarbons occur in hydrothermalbarite or within the sulfides of the volcanogenichydrothermal massive sulfide deposits that weresyndepositional with the formation (Rasmussenand Buick 2000) Textural information suggestthat these deposits were precipitated as blacksmoker chimneys in a submarine hydrothermalsetting driven by heat released from the intrudingStrelley Granite at approximately 324 Ga (Vearn-combe et al 1995) Fluorescing fluid inclusions oc-cur in quartz overgrowths in sandstones from theLalla Rookh Sandstone (Dutkiewicz et al 1998)Rasmussen and Buick (2000) and Dutkiewicz et al(1998) have argued for a biogenic origin for the hy-drocarbons however an abiotic origin involvingfor example Fischer-Tropsch-type synthesis ap-pears possible (Brasier et al 2002)

The Late Archean (and Early Paleoproterozoic)record preserved in the Hamersley and relatedbasins is stratigraphically much more coherentthan that preserved in the earlier successions TheHamersley Basin in particular provides a verycomprehensive record of the early oxygenation ofthe atmosphere in the form of large eustaticallycontrolled upward-shallowing sequences that in-clude black shales BIFs and ultimately at theirtops biogenic platform carbonates (Lindsay andBrasier 2002ab) These depositional cycles con-tain organic-rich shales at their base that recordevidence for some of Earthrsquos earliest biosphere(Brocks et al 1999) BIFs (Trendall 1983 1990)that record the rise of oxygen in the atmosphereand carbonates that document some of Earthrsquosearliest reef-like carbonate structures (Simonsonand Hassler 1996 1997 Simonson et al 1993ab)

SAMPLING STRATEGY

The Archean rocks of the Pilbara Craton arethus part of a protocontinent about which the

nascent Australian Craton developed It consistsof granitoid complexes overlain by a carapace ofsynchronously deposited volcano-sedimentarywedges Resting upon the early Archean proto-continent are a series of supracrustal basins Inall the sedimentary record of the Pilbara extendsfrom rocks as old as 354 Ga to some as young asapproximately 19 Ga As summarized in the fol-lowing paragraphs the sedimentary record of thePilbara records evidence of the major tectonic andgeological events that accompanied the evolutionof the Archean biosphere

It is here proposed that we develop a carefullydocumented and curated sample collection fromthis unique setting The collection would form thebasis for early Earth studies and a focus for astro-biology The sample set would be designed to re-flect the evolutionary succession described abovewith emphasis placed on cherts lower in the suc-cession and on clastic sediments higher in the suc-cession Any comprehensive reference collectionmust reflect diversity of the succession and includerocks from each of the sedimentary environmentsas well as igneous rocks reflecting the frameworkin which the protocontinent assembled

The five megasequences at the North Shawstudy area (Fig 3) are very different and reflectthe evolving Early Archean protocontinent Theearliest sedimentary record is limited and domi-nated by hydrothermally generated laminatedcherts and BIFs which appear as thin interbedswithin massive basalts (Coonterunah Group) (Fig5) Gradually clastic rocks appear dominated atfirst by volcanoclastics followed later by the firstevidence of sediments containing detrital quartzderived from true continental granitic crust Hy-drothermally generated laminated chert carbon-ate and barite units occur throughout the sectionThe first evidence of erosion appears at the top ofthe Coonterunah Group in the form of an angu-lar unconformity (Buick et al 1995b)

The basins that form the supracrustal succes-sion on the Pilbara Craton offer important in-sights into the closing phases of the Archean andthe immediate post-Archean world Because oftheir economic importance the basins are wellmapped and drill core is readily available (seeLindsay and Brasier 2002a) The succession pro-vides vital information about the early oxygena-tion of the atmosphere and the rapid growth ofthe biosphere that resulted from the assembly ofthe first continents and the initiation of the platetectonic cycle Platform carbonates whose geom-

LINDSAY ET AL752

etry was determined by eustasy are widespreadand contain considerable information on earlyoxygenic photosynthesis and the development ofthe first reef-like structure that are similar inmany ways to modern reefs (Simonson et al1993ab Lindsay and Brasier 2002ab)

The collection would consist of three groupingsof samples stratigraphic samples site-specificsamples and framework samples

Stratigraphic samples

The stratigraphic sample set would be collectedfrom well-defined settings such as carefully mea-sured and documented sections or from drill coreSamples would be collected from each of the fivestratigraphic groups or megasequences Thesupracrustal successions preserved in the Hamer-sley and associated basins would where possiblebe taken from existing drill core or from drill coresobtained for special projects Where drill core isnot available samples would be collected fromcarefully selected and measured outcrop sectionsThe samples would concentrate on rocks mostlikely to host biogenic material Samples from theEarly Archean volcaniclastic carapace would bedominated by cherts that are most likely to pre-serve fine biogenic structures but would also in-clude other sedimentary hydrothermal units suchas barite and carbonate that would be used in sta-ble isotope and geochemical studies

Site-specific sample

This sample set would focus on sites where po-tential biogenic structures have been documentedsuch as the stromatolitic intervals in the DresserFormation and the Strelley Pool Chert It wouldalso include detailed sampling of sites such asChinaman Creek and the Strelley Pool Chertwhere purported microfossils have been docu-mented (Schopf 1993) and the sulfide bodies inthe Kangaroo Caves Formation that contain pos-sible microfossils as well as evidence of hydro-carbons (Rasmussen 2000 Rasmussen and Buick2000) This would involve a broad samplingaround the sites in measured sections and drillcore to establish the geological context

Framework samples

While the focus of the collecting would be onmaterial that would help define the character of

the Earthrsquos earliest biosphere it is important thatthe broad context of the samples be defined TheArchean is a period when the Earthrsquos crust wasevolving rapidly and there is a growing body ofevidence to suggest that the evolution of theplanet drives the evolution of the atmosphere andbiosphere (Lindsay and Brasier 2002ab) Theframework sample set would include suites ofsample from the extrusive volcanic rocks col-lected along well-documented sections Sectionsthrough the volcanoclastic succession would em-phasize the dominant volcanic units (basalts ko-matiites andesites felsites and rhyolites) assources of information on the mantle and evolv-ing crust This sample set would also include adetailed sampling of the granitoid complexeswhich ultimately drove the entire process as wellas the later intrusions that were important in dri-ving the hydrothermal systems that may wellhave been the cradle of early life

SAMPLE CURATIONCHARACTERIZATION AND

DISTRIBUTION

All samples would be fully documented in thefield Samples would be photographed and theirlocations determined using a Global PositioningSystem receiver For specialized samples such asthose thought to contain easily contaminated bio-geochemical information the field collectionwould be modeled on techniques developed bythe NASANational Science FoundationSmith-sonian Institution Antarctic Search for Meteoritesprogram (Harvey and Schutt 1998)

Characterization

The petrologic and petrographic characteristicsof each sample would be determined using thin-section optical microscopy The quantitative bulkchemical composition of each sample would bemeasured by x-ray fluorescence spectrometry Fi-nally the qualitative bulk mineralogy of eachsample would be measured by powder x-ray dif-fraction

Data management

An Archean Geologic Reference Materials Data-base would be produced to fully document eachsample Field documentation would include sam-pling rationale supporting references and field

ARCHEAN GEOLOGIC REFERENCE COLLECTION 753

photographs Documentation would include theresults of all analyses along with images of rep-resentative sample surfaces and optical micro-graphs The Database would be produced in elec-tronic format and posted on the web Subsamplesof the Archean Geologic Reference Materialswould then be made available at no cost to re-searchers in the astrobiology community

DISCUSSION AND CONCLUSIONS

The Early Archean environment was uniqueRecent studies indicate that the atmosphere waslargely anoxic with oxygen levels 1025 of thepresent atmospheric level (Pavlov and Kasting2002) There were no large continents no activeor passive margins and by default no intracra-tonic basins Significantly perhaps there are nosigns of eustasy The crust was evolving rapidlyand heat flow was high leading to the intrusionof large granitoid bodies and the development ofa volcanic carapace above them In turn thismeant that the marine environment was domi-nated by hydrothermal activity

In the absence of large continental masses de-trital materials other than volcanoclastics are rel-atively rare in the Early Archean Significant vol-umes of quartz sand do not appear in the PilbaraCraton sedimentary record until after 32 Ga pre-sumably as the intruding igneous bodies begin tobe unroofed and eroded There are thus no ma-jor accumulations of sedimentary rocks that wenormally expect associated with passive conti-nental margins

With the assembly of the earliest large conti-nents beginning at approximately 30ndash28 Gathe Earthrsquos atmosphere hydrosphere and bios-phere changed rapidly Intracratonic basinssuch as the Hamersley Basin began to developon the new crustal blocks opening new shal-low-water environments that provided an idealsetting for photosynthesizing organisms andthe development of the first carbonate reefstructures (Lindsay and Brasier 2002ab) Hy-drothermal processes while still importantwere thus no longer the only sources of energyavailable to the evolving biosphere

The Archean was thus an unfamiliar environ-ment and unravelling the nature of the earliestbiosphere will require development of new ap-proaches and new techniques Without large con-tinental masses subareal exposures are likely to

be rare and short-lived There is no evidence forstream activity until the De Grey group at ap-proximately 29 Ga at which time braided streamgravels were preserved Thus models involvingearly biospheric evolution in ldquoa warm little pondrdquo(Darwin 1888) or in fresh water seem unlikelyInstead the focus in the search for the origins oflife on Earth should be directed to hydrothermalmarine settings involving high temperatures andwhat would today be hostile geochemical envi-ronments with significant concentrations of metal-lic ions and a largely reducing environment Therisks for the misidentification of abiotic structuresand geochemical signatures as signs of ancient lifeare significant as seen in recent discussions (egBrasier et al 2002 Fedo and Whitehouse 2002Schopf et al 2002)

These difficulties suggest that much could begained by the development of a well-documentedcollection of Archean rocks that could be madeavailable to all qualified investigators Until weunderstand the earliest stages of the evolution oflife on Earth we cannot hope to make significantprogress in the search for life on other planetssuch as Mars Until there is a sample return mis-sion to Mars we must make do with meteoritesthat come to us largely out of context An under-standing of the Archean Earth can help placethose limited samples in a planetary context

ACKNOWLEDGMENTS

Franco Pirajno Kath Grey and Martin VanKranendonk provided generous advice and as-sistance and the Geological Survey of WesternAustralia supported the field program We aregrateful to Peter Morant of Sipa Resources andAdrian Black of Newexco Perth for their adviceand for access to drill-core materials

ABBREVIATION

BIF banded iron formation

REFERENCES

Arndt NT Nelson DR Compston W Trendall AFand Thorne AM (1991) The age of the FortescueGroup Hamersley Basin Western Australia from ionmicroprobe zircon U-Pb results Aust J Earth Sci 38262ndash281

LINDSAY ET AL754

Awramik SM Schopf JW and Walter MR (1983) Fil-amentous fossil bacteria from the Archaean of WesternAustralia Precambrian Res 20 357ndash374

Barley ME Pickard AL and Sylvester PJ (1997) Em-placement of a large igneous province as a possiblecause of banded iron formation 245 billion years agoNature 385 55ndash58

Bettenay LF Bickle MJ Boulter CA Groves DIMorant P Blake TS and James BA (1981) Evolu-tion of the Shaw Batholithmdashan Archaean granitoid-gneiss dome in the eastern Pilbara Western AustraliaIn Special Publication 7 The Archaean edited by JEGlover and DI Groves Geological Society of WesternAustralia Perth pp 361ndash372

Blake TS and Barley ME (1992) Tectonic evolution ofthe Late Archaean to Early Proterozoic Mount BruceMegasequence Set Western Australia Tectonics 11 1415ndash1425

Blake TS and Groves DI (1987) Continental rifting andthe Archaean-Proterozoic transition Geology 15 229ndash232

Brasier MD and Lindsay JF (1998) A billion years ofenvironmental stability and the emergence of eukary-otes new data from Northern Australia Geology 26555ndash558

Brasier MD Green OR Jephcoat AP Kleppe AKVan Kranendonk MJ Lindsay JF Steele A andGrassineau NV (2002) Questioning the evidence forEarthrsquos oldest fossils Nature 416 76ndash81

Brauhart C (1999) Regional alteration systems associatedwith Archaean volcanogenic massive sulphide depositsat Panorama Pilbara Western Australia [PhD Thesis]University of Western Australia Perth

Brauhart C Groves DI and Morant P (1998) Regionalalteration systems associated with volcanogenic mas-sive sulfide mineralization at Panorama Pilbara West-ern Australia Econ Geol 93 292ndash302

Brocks J Logan GA Buick R and Summons RE(1999) Archean molecular fossils and the early rise ofeukaryotes Science 285 1033ndash1036

Buick R Dunlop JSR and Groves DI (1981) Stroma-tolite recognition in ancient rocks an appraisal of ir-regular laminated structures in an early Archaeanchert-barite unit from North Pole Western AustraliaAlcheringa 5 161ndash181

Buick R Des Marais DJ and Knoll AH (1995a) Sta-ble isotope compositions of carbonates from the Meso-proterozoic Bangemall Group northwestern AustraliaChem Geol 123 153ndash171

Buick R Thornett JR McNaughton NJ Smith JBBarley ME and Savage M (1995b) Record of emer-gent continental crust 35 billion years ago in the Pil-bara Craton of Australia Nature 375 574ndash577

Buick R Rasmussen B and Krapez B (1998) Archaeanoil evidence for extensive hydrocarbon generation andmigration 25ndash35 Ga Am Assoc Petroleum Geol Bull82 50ndash69

Chan M (1998) Sedimentology and structure of threesmall congolmeratic basins in the Lalla Rookh-WesternShaw structural corridor Pilbara Craton Western Aus-

tralia [BSc Honours Thesis] University of WesternAustralia Perth

Cullers RL DiMarco MJ Lowe DR and Stone J(1993) Geochemistry of a silicified felsic volcaniclasticsuite from the Early Archaean Panorama FormationPilbara Block Western Australiamdashan evaluation of de-positional and post-depositional processes with specialemphasis on the rare-Earth elements Precambrian Res60 99ndash116

Darwin F ed (1888) The Life and Letters of Charles Dar-win Vol 3 John Murray London

de Wit MJ (1998) On Archaean granites greenstonescratons and tectonics does the evidence demand a ver-dict Precambrian Geol 91 181ndash226

Drummond BJ (1981) Crustal structure of the Precam-brian terrains of northwest Australia from seismic re-fraction data BMR J Geol Geophys 6 123ndash135

Dunlop JSR Muir M Milne VA and Groves DI(1978) A new microfossil assemblage from the Ar-chaean of Western Australia Nature 274 676ndash678

Dutkiewicz A Rasmussen B and Buick R (1998) Oilpreserved in fluid inclusions in Archaean sandstonesNature 395 885ndash888

Fedo CM and Whitehouse MJ (2002) Metasomatic ori-gin of quartz-pyroxene rock Akilia Greenland and im-plications for Earthrsquos earliest life Science 296 1448ndash1452

Gee RD and Grey K (1993) Report 41 Proterozoic Rockson the Glengarry 1250 000 SheetmdashStratigraphy Structureand Stromatolite Biostratigraphy Geological Survey ofWestern Australia Perth

Glikson AY and Hickman AH (1981a) Chemicalstratigraphy and petrogenesis of Archaean basic-ultra-basic volcanic units eastern Pilbara Block WesternAustralia In Special Publication 7 Archaean Geologyedited by JE Groves and DI Groves Geological Soci-ety of Western Australia Perth pp 287ndash300

Glikson AY and Hickman AH (1981b) Record 198136Geochemistry of Archaean Volcanic Successions EasternPilbara Block Western Australia Australian Bureau ofMineral Resources Canberra

Glikson AY Davy R and Hickman AH (1991) Record199146 Trace Metal Distribution Patterns in Basalts Pil-bara Craton Western Australia with Stratigraphic-Geo-chemical Implications Australian Bureau of Mineral Re-sources Canberra

Groves DI Dunlop JSR and Buick R (1981) An earlyhabitat of life Sci Am 245 64ndash73

Harvey RP and Schutt JW (1998) Meteorite recoveryand reconnaissance in the Allan Hills-David Glacierand Darwin Glacier regions 1996ndash1997 Antarctic J US 32 25ndash27

Hickman AH (1975) Annual Report 1974 PrecambrianStructural Geology of Part of the Pilbara Region Geologi-cal Survey of Western Australia Perth pp 68ndash73

Hickman AH (1983) Bulletin 127 Geology of the PilbaraBlock and Its Environs Geological Survey of WesternAustralia Perth

Hickman AH (1984) Archaean diapirism in the PilbaraBlock Western Australia In Precambrian Tectonics Il-lustrated edited by A Kroner and R Greiling E

ARCHEAN GEOLOGIC REFERENCE COLLECTION 755

THE STRATIGRAPHIC RECORD

The term ldquogreenstonerdquo or ldquogreenstone beltsrdquohas generally been applied to the largely vol-canigenic stratigraphic intervals preserved as acarapace overlying the granitoid complexes TheArchean stratigraphic record preserved in thegreenstone carapace in the study area can be di-vided into five unconformity or intrusion-boundgroups or megasequences (Fig 3) They are tec-tonically defined units and not eustatically con-trolled (Van Kranendonk et al 2002) Themegasequences of the Pilbara Supergroup weredeposited over a time period between approxi-mately 351 and approximately 294 Ga Theywere deposited unconformably one above theother and consistently dip away from the domal

granitoid complexes The dips of the beddinggradually decrease with decreasing age suggest-ing that they were deposited as thickeningwedges adjacent to growing granitoid diapirs(Hickman 1975 1983 1984 Van Kranendonk2000) Granitoid diapirism appears to have con-tinued until after 27 Ga as the overlying rem-nants of the Hamersley Basin succession are pre-served as synclinorial remnants (Hickman 1984Van Kranendonk et al 2002)

Despite local complex faulting the supra-crustal megasequences are relatively well pre-served such that their ages and stratigraphic re-lationships are well established (Williams andCollins 1990 Van Kranendonk 2000) Metamor-phic grade decreases away from the granitoidcomplexes from a maximum of middle to lower

LINDSAY ET AL742

FIG 2 Simplified map of the pro-posed North Shaw study area show-ing the main lithologic units (groups)The sinuous Shaw River almost bisectsthe area Adapted from Van Kranen-donk (2000)

amphibolite facies at the contact to lower green-schist and prenhite-pumpellyite facies Lowergreenschist facies is by far the most widespreadmetamorphic grade in the study area

Megasequence 1mdashCoonterunah Group

The Coonterunah Group includes the earliestknown stratigraphic succession (3515 Ma) pre-served on the Pilbara Craton (Buick et al 1995bVan Kranendonk and Morant 1998) The lowercontact is intrusive with the 3468 Ma granitoidrocks of the Carlindi Granitoid Complex The suc-cession consists of three formations (Van Kra-nendonk 2000)

Tabletop Formation The Tabletop Formationconsists entirely of thick basaltic units They aremainly fine-grained tholeiitic lavas althoughsome gabbroic intrusions have been identifiedThe basalts are mostly featureless although pil-lowed units are present indicating a submarinesetting

Coucal Formation The Coucal Formation whichis 2 km thick at its maximum consists largely

ARCHEAN GEOLOGIC REFERENCE COLLECTION 743

of fine-grained doleritic andesite basalt and fel-sic volcanic units (Fig 5) Thin chert (Fig 6) andimpoverished cherty banded iron formation (BIF)units are present throughout the formation Theseare the earliest known sedimentary units in thePilbara Craton and offer the earliest opportunityfor the preservation of ancient life

Double Bar Formation The Double Bar Forma-tion which conformably overlies the Coucal For-mation consists of fine-grained tholeiitic basaltswith interbedded basaltic volcaniclastic rocksSome basalts are pillowed suggesting a subma-rine setting The volcanics retain their basaltictextures but have been altered significantly bylow-grade metamorphism (Van Kranendonk2000)

Megasequence 2mdashWarrawoona Group

The Warrawoona Group is separated from theunderlying Coonterunah Group by a widespreadangular unconformity the oldest evidence onEarth for emergence (Buick et al 1995b) (Fig 3)The Warrawoona Group is dominantly volcanic inorigin but includes some clastic sedimentary rocks

FIG 3 Simplified stratigraphy of theArchean rocks exposed in the proposedstudy area (adapted from Van Kranen-donk 2000) Arrows indicate dated periodof activity of the granitoid complexes as dis-tinct from the plutons which are shown incartoon form

McPhee Basalt The McPhee Basalt which isdated at 3477 Ma and reaches a maximum of 3km in thickness is relatively limited in outcrop(Van Kranendonk et al 2002) It consists largelyof basalts and schists and is only exposed in thesouthern part of the study area adjacent to theShaw Granitoid Complex where the contact is in-trusive A thin unit of impoverished cherty BIFforms a distinctive cap to the formation

Mount Ada Basalt The Mount Ada basalt con-sists of 3ndash4 km of massive tholeiitic basalt and do-

LINDSAY ET AL744

FIG 4 Simplified stratigraphy of the Hamersley and Ashburton Basins of Western Australia (after Lindsay andBrasier 2002)

lerite intrusions that are exposed in the south-eastern corner of the study area Its lower contactwith the Shaw Granitoid Complex and NorthPole Monzonite is intrusive For the most partvolcanic textures are well preserved in thebasalts

Dresser Formation The Dresser Formation isonly recognized in the North Pole Dome whereit reaches a maximum of 1000 m in thickness(Van Kranendonk et al 2002) The formation con-sists of massive and pillowed mafic volcanics in-

terbedded with units of chert sandstone con-glomerate barite and minor carbonate (Fig 7)The individual chert-barite beds vary from 5 to40 m in thickness Associated with these unusualsedimentary facies are structures that have beeninterpreted as the worldrsquos oldest stromatolites(Fig 8) and associated microfossils (Dunlop et al1978 Walter et al 1980 Buick et al 1981 Groveset al 1981 Walter 1983) It is perhaps significantthat this distinctive unit is fed by a deep-seatedsystem of hydrothermal dykes (Fig 9)

Duffer Formation The Duffer Formation is bestexposed in the southeastern portion of the studyarea where it consists largely of felsic schist andagglomerate The agglomerates which have been

locally interpreted as welded ignimbrites arebedded in meter-scale units The Duffer Forma-tion has a depositional age of approximately 347Ga (Thorpe et al 1992 McNaughton et al 1993Nelson 1997 1998 1999 2000 2001) and restsconformably on the Mount Ada Basalt

Apex Basalt The Apex Basalt is well exposed inthe northeastern corner of the study area and overlarge areas on adjacent map sheets The forma-tion is dominated by pillowed basalts which arelargely tholeiitic The pillows are well developedand preserve vesicular rims chilled margins andconcentric cooling cracks Hyaloclastic brecciasare also present within the assemblage Doleriticflows and intrusions are common and doleriticdykes occur locally

Interbedded with the volcanics are a numberof chert horizons which are 10 m in thickness

ARCHEAN GEOLOGIC REFERENCE COLLECTION 745

FIG 5 A measured section through the Coucal For-mation (locality A Fig 2) The formation is dominatedby basic volcanic rocks with some felsic volcanic rocksand thin (10 m) hydrothermal cherts between flows

FIG 6 Laminated cherts from the lower Coucal For-mation (locality A Fig 2) Cherts form the earliest sedi-mentary rocks in the Pilbara succession Some units areas much as 10 m thick Scale bar 5 10 cm

and usually well laminated mainly black andwhite chert Numerous chert dykes cut across theunderlying stratigraphy and terminate at thelevel of the chert horizons Close to Marble Barexposures of the Apex chert contain what was ini-tially believed to be Earthrsquos oldest morphologicalfossil assemblage (Fig 10) (Schopf and Packer1987 Schopf 1992 1993) a claim recently dis-puted by more detailed analysis (Brasier et al2002) (see following section on Paleontology)

Panorama Formation The Panorama Formation(3458ndash3434 Ma) which is best exposed in thenorthern part of the study area near the ShawRiver is dominated by felsic volcaniclastic rocks

with small volumes of felsic lavas and some chertinterbeds (Hickman 1983 Thorpe et al 1992 Nel-son 2000) The formation is strongly altered andintensely silicified having been exposed to tem-peratures as high as 350degC (Cullers et al 1993)A large volcanic vent within the Panorama For-mation is preserved in the northwestern part ofthe outcrop area (Van Kranendonk 2000)

Strelley Pool Chert The relationship of the Strel-ley Pool Chert (Lowe 1983) to the underlyingunits varies from conformable to unconformableIn the northeast the contact with the underlyingCoonterunah Group forms an angular unconfor-mity with pronounced paleorelief (Buick et al1995b) The formation consists of carbonate sili-cified carbonate and clastic rocks (Fig 11) Thecherts and carbonates both have well-developedsedimentary structure that have been describedas wavy laminated cherts or carbonates and stro-matolites (Fig 12) (Lowe 1983 Hofmann et al1999) although as discussed in a following sec-tion there is some concern as to whether they arebiogenic (Lowe 1994 Lindsay et al 2003ab)

Euro Basalt The Euro Basalt consists domi-nantly of basalts of tholeiitic and high-Mg com-position that rest conformably on the StrelleyPool Chert (Van Kranendonk 2000) The forma-tion reaches a thickness in excess of 9 km in thenortheastern part of the study area The basaltsare pillowed and bedded on a kilometer scaleThe formation also includes some komatiite andfelsic volcaniclastics as well as thin cherts andvolcaniclastic units

Megasequence 3mdashSulphur Springs Group

The Sulphur Springs Group consists of threeformations (Fig 3) that in contrast to the previ-ous two Groups contain a significant proportionof clastic sedimentary rocks some quartzose

Leilira Formation The Leilira Formation consistspredominantly of clastic sedimentary rocks withonly small volumes of felsic to intermediate ex-trusive rocks interbedded The clastic rocks arepredominantly sandstones but vary from silt-stone to coarser pebbly sandstone Composition-ally the sandstones are quartzose or volcaniclithic rocks Locally laminated cherts formed bythe silicification of felsic volcanic ash and silt-stone are abundant

LINDSAY ET AL746

FIG 7 Bedded sedimentary carbonate and silica fromthe Dresser Formation (locality B Fig 2) Scale bar 5 5cm The rock consists of well-bedded units that begin withorangeyellow iron-rich carbonates and grade upward topale green silica The unit reflects geochemical changes inresponse to pulsing or cycling of the hydrothermal sys-tem Elsewhere carbonate and chert form alternating beds

The Leilira Formation represents a significantchange in depositional patterns as it is the oldestunit in which clastic sediments are the dominantlithology It reflects the growing volcanic cara-pace and in particular the exposure of the grani-toid core to weathering and erosion the first signsthat a true crust was developing

Kunagunarrina Formation The KunagunarrinaFormation reaches a maximum of 24 km inthickness and consists predominantly of high-Mg basalts and komatiites Locally andesitic andfelsic units are present especially near the top ofthe formation A thick chert unit (10 m) in themiddle of the formation separates lower Mg

ARCHEAN GEOLOGIC REFERENCE COLLECTION 747

FIG 8 Stromatolitic structures in theDresser Formation (locality B Fig 2)The original barite has been largely re-placed by chert Scale bar 5 10 cm

FIG 9 Exposures of the Dresser Formation in the North Pole Dome area (locality B Fig 2) showing large chert-barite feeder dykes The dykes consist of black chert and coarse bladed barite Note the vehicle for scale

basalts at the base of the formation from ko-matiites towards the top

Kangaroo Caves Formation The Kangaroo CavesFormation which reaches a maximum thicknessof 15 km is best exposed towards the center ofthe study area The formation is complex and con-sists of a succession of andesitic volcanics anddacitic to rhyolitic sills and extrusives A chert in-terval up to 100 m thick forms a distinctivemarker at the top of the formation Close to thecenter of the study area at Sulphur Springs themassive chert is overlain by a polymict megabrec-cia that rests upon a sharp erosional contact (Hill1997 Vearncombe et al 1998)

Hosted within a 200-m dacite unit immediatelybeneath the marker chert are six sulfide ore bodiesspaced at regular intervals of 5ndash7 km around themargins of the Strelley Granite (Morant 1998) Theyconsist of pyrite sphalerite chalcopyrite and galenawith smaller amounts of other sulfides (Vearncombeet al 1995 1998 Vearncombe 1996 Morant 1998)The sulfides which were deposited in a blacksmoker hydrothermal setting occur in associationwith growth faults developed around the StrelleyGranite the intrusion of which was syndepositionalwith the Kangaroo Caves Formation (Vearncombe1996 Brauhart et al 1998 Brauhart 1999)

Megasequence 4mdashGorge Creek Group

The Gorge Creek Group consists of five for-mations (Fig 3) that show much greater litho-logic diversity than earlier megasequences Inparticular this interval includes for the firsttime a significant thickness of quartzose sand-stone as well as a variety of other clastic sedi-mentary rocks

Pincunah Hill Formation In the western part ofthe study area the basal Pincunah Hill Formationconsists of thinly bedded BIF with small amountsof laminated chert red to gray shale and silt-stone This basal unit is up to 1 km thick in thewestern part of the area but wedges out and dis-appears in a short distance eastward The upper

LINDSAY ET AL748

FIG 11 A measured section through the Strelley PoolChert showing the location of stromatolitic structuresand the carbonate interval (adapted from Van Kranen-donk 2000) (locality C Fig 2) The gamma log to theright shows continuity across the chert-carbonate

FIG 10 ldquoMicrofossilsrdquo from the Apex chert near Mar-ble Bar a computer-generated digital automontage ofputative beggiatoan Eoleptonema apex Holotype (Schopf1993) combining the most-sharply focused images fromsuccessive focal planes b and c Original manual pho-tomontage and interpretative sketch (Schopf 1993) whichomits lower structure at arrow in (b) d and e New sin-gle images showing continuity of original and newly im-aged and automontaged structures Reprinted with per-mission from Nature (Brasier et al 2002) Copyright 2002Macmillan Publishers Ltd

part of the formation consists largely of red orpurple shale (Van Kranendonk 2000)

Corboy Formation At its thickest in the easternpart of the study area the Corboy Formation in-cludes a number of distinct facies developedacross major growth faults (Wilhelmij and Dun-lop 1984 Wilhelmij 1986) Here the formationconsists of a basal quartzose sandstone abruptlyoverlain by conglomerate interbedded withgraded sandstone units red shale and coarsesandstone The formation is capped by massivesandstone and conglomerate The unit was de-posited as a series of small prograding shallowmarine fan deltas (Wilhelmij 1986 Hill 1997 VanKranendonk 2000)

Paddy Market Formation In the western part ofthe study area the Paddy Market Formation isdominantly cherty BIF with minor ferruginousshale Further east the formation becomes moreshale-rich although much of the shale has been re-placed by chert Sandstone is present in smallamounts in beds up to 10 m thick Towards thetop of the formation in the east the cherts containfelsic volcanic material that is overlain by up to 200m of massive dacite (Van Kranendonk 2000)

Honeyeater Basalt The Honeyeater Basaltwhich consists entirely of basalt and dolerite con-formably overlies the Paddy Market FormationThe lower part of the formation consists of high-Mg basalt interbedded with dolerite whereashigher in the formation the basalts are largelytholeiitic and interbedded with dolerite (Gliksonand Hickman 1981ab Glikson et al 1991)

Pyramid Hill Formation The Pyramid Hill For-mation is limited in areal extent It is found in thenorth central part of the study area east of theStrelley Granite The formation consists of redand black shales that when heavily silicifiedgrade into laminated black and white chert (VanKranendonk 2000) The Gorge Creek Group iscapped by a well-bedded BIF

Megasequence 5mdashDe Grey Group

The De Grey Group consists of a single for-mation the Lalla Rookh Sandstone (Fig 3)which is dominated by clastic sedimentary rocksthat contain a significant quartzose componentThe Lalla Rookh Sandstone which rests on awell-defined unconformity above the GorgeCreek Group reaches a maximum thickness of

ARCHEAN GEOLOGIC REFERENCE COLLECTION 749

FIG 12 Stromatolitic carbonates in the Strelley Pool Chert (locality C Fig 2) Laminae are isopachous and theapex of the cone is ruptured and veined by secondary carbonate (arrow) Scale bar 5 10 cm

3 km (Krapez 1984) In the north central part ofthe study area the formation consists entirely ofcoarse clastic sediments and minor shalewhereas to the south the predominate fill is lithicand feldspathic sandstone (Chan 1998) Theserocks provide the first clear evidence of streamactivity in the Pilbara succession (Krapez 1984Van Kranendonk and Collins 1998 Van Kra-nendonk 2000)

THE PILBARA SUPRACRUSTAL SUCCESSION

The late Archean and early Paleoproterozoicbasins that rest upon the ancient cratonic blocksof Western Australia (Fig 1) form a time seriesassociated with the formation and ultimate dis-assembly of one of Earthrsquos earliest major conti-nental masses This was followed by the suturingof the Pilbara and Yilgarn Cratons along theCapricorn Orogen as a new supercontinentevolved in the Paleoproterozoic (Rogers and San-tosh 2002) These basins contain an importantsedimentary record of the early Earth includingsome of Earthrsquos earliest stromatolites and plat-form carbonate deposits (Simonson et al1993ab) organic-rich black shales (Brocks et al1999) and BIF (Trendall 1983 1990) The strati-graphic and structural framework is well con-strained by radiometric ages (Arndt et al 1991Barley et al 1997 Trendall et al 1998) providinga record at a critical time in Earth history whenthe atmosphere was first becoming oxygen-rich(Lindsay and Brasier 2002ab)

The Late Archean-Early PaleoproterozoicHamersley Basin which lies close to the southernmargin of the Pilbara Block (Trendall 1983 1990Blake and Barley 1992) overlies a normal crustalthickness (30ndash40 km) (Drummond 1981) Thebasin began to subside at approximately 28 Gafollowing an episode of crustal extension The ar-chitecture of the basin fill is complex andpolyphase with major erosional surfaces separat-ing areally extensive megasequencessuperse-quences (Trendall 1990 Krapez 1996) DuringFortescue and Hamersley Group time (Fig 4)sedimentation began in a rift setting in a silici-clastic-starved environment The later stages ofHamersley Basin sedimentation (Turee CreekGroup times) record the early transition from apassive margin to a foreland basin setting andwith it the establishment of a terrigenous sedi-

ment supply (Tyler and Thorne 1990) The suc-cession is largely marine Evidence of eustasy ap-pears for the first time in the sedimentary suc-cession of the Hamersley Basin (Lindsay andBrasier 2002ab)

The geometry and sequence stacking pattern ofthe succession are both very similar to youngerintracratonic basins (see Lindsay et al 1993) TheHamersley Basin is thus the first basinal settingpreserved on the Australian craton in which ma-rine sediments and in particular platform car-bonate rocks have been preserved in a responseto broad regional crustal subsidence and wheredeposition is controlled by eustasy

The Ashburton Basin (22ndash18 Ga) discon-formably overlies the southern margin of theHamersley Basin along its the contact with theCapricorn Orogen (Fig 1) (Thorne and Seymour1991) The basin forms the northern margin of theCapricorn Orogen a deformed zone that devel-oped during ocean closure Loading of the oceanfloor by the approaching Yilgarn Craton led tothe development of a westndashnorthwest-orientedforeland basin [McGrath Trough (Horowitz1982)] with an uplifted orogenic margin to thesouthwest (Blake and Groves 1987 Blake andBarley 1992) with the result that sedimentationwas dominated by an abundant supply of ter-rigenous clastic material (Mount McGrath For-mation) Continued loading of the crust ulti-mately led to the supply of terrigenous materialsbeing disrupted and producing a prograding car-bonate shelf (Duck Creek Dolomite) Basin sedi-mentation was accompanied by active-marginmafic volcanism (Cheela Springs Basalt)

THE CAPRICORN OROGEN ANDASSOCIATED BASINS

The Yerrida (22ndash19 Ga) Bryah (20 Ga)Padbury (20 Ga) and Earaheedy (19ndash165Ga) Basins lie to the southeast of the Hamersleyand Ashburton Basins and overlie the CapricornOrogen (Fig 1) (Occhipinti et al 1997 1998 Pi-rajno et al 1998 Pirajno and Adamides 2000)These now fragmentary basins formed along thenorthern margin of the Yilgarn Craton in a back-arc setting and record the convergence and colli-sion of the Archean Pilbara and Yilgarn Cratons(Tyler and Thorne 1990 Thorne and Seymour1991 Occhipinti et al 1997 1998 Pirajno et al1998) Because of their active margin settings the

LINDSAY ET AL750

fill of these small basins is dominated by clasticsediments However platform carbonate unitsare preserved in the Yerrida Basin (Gee and Grey1993 Pirajno and Adamides 2000) and a signif-icant thickness of carbonate rocks also occurs inthe Earaheedy Basin

THE ARCHEAN BIOSPHERE

The end of the Archean is formally defined at25 Ga (Plumb 1991) In reality the Archean worldended at some point in time closer to 28 or 30 Gawhen the first large continental blocks began to as-semble The stratigraphic record preserved on theancient Australian Craton reflects this major tran-sition The rock record from the Early Archean isvery different from that of the Late Archean in thatthe early record is predominantly volcanic and hy-drothermal in nature (ldquogreenstonerdquo) while thelater record is largely clastic and more readily com-pared with the Phanerozoic record

Structures and textures of possible biogenicorigin that are preserved in the stratigraphic suc-cession in the Early Archean study area are wellknown and are believed to provide the earliestdirect evidence of Earthrsquos biosphere Stromatoliticstructures have been encountered at two levelswithin the section The Dresser Formation con-tains structures preserved in chert and barite in-tervals that have been interpreted as stromatolitesand oncolites (Dunlop et al 1978 Walter et al1980 Buick et al 1981 Groves et al 1981 Wal-ter 1983) The stromatolitic structures occur forthe most part in layered barite units that devel-oped over swarms of synvolcanic hydrothermalfeeder dykes along listric growth faults (Nijmanet al 1998 Van Kranendonk 2004) Although ithas been argued that these structures are biogenic(Walter et al 1980 Schopf and Walter 1983 Wal-ter 1983) their direct association with the feederdykes leaves room for doubt (Buick et al 1981)The setting is complex and there is considerablescope for more detailed contextual research

The second stromatolitic horizon occurs in theoverlying Strelley Pool Chert a complex unit con-sisting of at least five members (Lowe 1980 1983Hofmann et al 1999) The main stromatolitic unitoccurs as an 8-m-thick interval in the middle ofthe formation and appears to have a consistentand widespread pattern of deposition The struc-tures have been the focus of considerable discus-sion and their biogenicity has been questioned

(Buick et al 1981 Lowe 1995) The structures arecomplex and varied (Fig 12) leading to a morerecent re-evaluation again suggesting that theyare biogenic (Hofmann et al 1999) However thelaminae are largely isopachous an indication thatthey could be abiotic precipitates (cf Pope andGrotzinger 2000) Given the intense hydrother-mal activity that appears to have taken placethroughout the depositional time interval thestructure may well be the product of hydrother-mal deposition (Lindsay et al 2003ab) In all ourobservations suggest that considerably morework needs to be done to establish the deposi-tional context of these stromatolitic structures be-fore they can be confidently ascribed as biogenicin origin

Structures resembling remarkably preservedbacterial and cyanobacterial microfossils havebeen found at several localities in and close to thestudy area (Awramik et al 1983 Schopf andPacker 1987 Schopf 1992 1993 Rasmussen 2000)A complex biota of 11 species of filamentousprokaryotes which are compared with cyanobac-teria (Schopf and Packer 1987 Schopf 1992 1993)have been described from the Apex chert close toMarble Bar (21deg105579S 119deg418789E) The taxo-nomic interpretations are important in that theyprovide the earliest evidence of morphological lifeon Earth and support the hypothesis of an earlybeginning for oxygenic photosynthesis (Schopf1999) The structures are almost a billion yearsolder than the oldest known putative cyanobac-terial biomarkers (Summons et al 1999) and oc-cur well before the earliest evidence for the risein oxygen in the atmosphere (for a summary seeKnoll and Canfield 1998 Lindsay and Brasier2000 2002ab) However in a recent re-evaluationof the original Apex chert material it has been ar-gued that the filaments are not biogenic but aresecondary artifacts formed during diagenesisfrom abiogenic organic compounds generated inhydrothermal chert veins (Brasier et al 2002 VanKranendonk 2004)

Filaments believed to be biogenic in originhave also been reported from the 324 Ga Kan-garoo Caves Formation (Rasmussen 2000) wherethey form part of the Sulphur Springs deposit as-sociated with a black smoker hydrothermal ventsystem (Vearncombe et al 1995 1998) Ras-mussen (2000) argued that the filaments are rem-nants of a hydrothermal biota of thermophilicchemotrophic prokaryotes Sulfide textures areremarkably well preserved in these complex de-

ARCHEAN GEOLOGIC REFERENCE COLLECTION 751

posits and are worthy of much more detailedanalysis as they may well preserve evidence ofthe early biosphere

Finally there is evidence that hydrocarbons aretrapped in these ancient sedimentary rocks eitheras fluid inclusions in hydrothermal precipitates oras bituminous residues in larger cavities in theKangaroo Caves Formation and in the Lalla RookhSandstone (Dutkiewicz et al 1998 Rasmussen andBuick 2000) Buick et al (1998) also reported car-bonaceous globules in cherts from the lower War-rawoona Group In the Kangaroo Caves Forma-tion the hydrocarbons occur in hydrothermalbarite or within the sulfides of the volcanogenichydrothermal massive sulfide deposits that weresyndepositional with the formation (Rasmussenand Buick 2000) Textural information suggestthat these deposits were precipitated as blacksmoker chimneys in a submarine hydrothermalsetting driven by heat released from the intrudingStrelley Granite at approximately 324 Ga (Vearn-combe et al 1995) Fluorescing fluid inclusions oc-cur in quartz overgrowths in sandstones from theLalla Rookh Sandstone (Dutkiewicz et al 1998)Rasmussen and Buick (2000) and Dutkiewicz et al(1998) have argued for a biogenic origin for the hy-drocarbons however an abiotic origin involvingfor example Fischer-Tropsch-type synthesis ap-pears possible (Brasier et al 2002)

The Late Archean (and Early Paleoproterozoic)record preserved in the Hamersley and relatedbasins is stratigraphically much more coherentthan that preserved in the earlier successions TheHamersley Basin in particular provides a verycomprehensive record of the early oxygenation ofthe atmosphere in the form of large eustaticallycontrolled upward-shallowing sequences that in-clude black shales BIFs and ultimately at theirtops biogenic platform carbonates (Lindsay andBrasier 2002ab) These depositional cycles con-tain organic-rich shales at their base that recordevidence for some of Earthrsquos earliest biosphere(Brocks et al 1999) BIFs (Trendall 1983 1990)that record the rise of oxygen in the atmosphereand carbonates that document some of Earthrsquosearliest reef-like carbonate structures (Simonsonand Hassler 1996 1997 Simonson et al 1993ab)

SAMPLING STRATEGY

The Archean rocks of the Pilbara Craton arethus part of a protocontinent about which the

nascent Australian Craton developed It consistsof granitoid complexes overlain by a carapace ofsynchronously deposited volcano-sedimentarywedges Resting upon the early Archean proto-continent are a series of supracrustal basins Inall the sedimentary record of the Pilbara extendsfrom rocks as old as 354 Ga to some as young asapproximately 19 Ga As summarized in the fol-lowing paragraphs the sedimentary record of thePilbara records evidence of the major tectonic andgeological events that accompanied the evolutionof the Archean biosphere

It is here proposed that we develop a carefullydocumented and curated sample collection fromthis unique setting The collection would form thebasis for early Earth studies and a focus for astro-biology The sample set would be designed to re-flect the evolutionary succession described abovewith emphasis placed on cherts lower in the suc-cession and on clastic sediments higher in the suc-cession Any comprehensive reference collectionmust reflect diversity of the succession and includerocks from each of the sedimentary environmentsas well as igneous rocks reflecting the frameworkin which the protocontinent assembled

The five megasequences at the North Shawstudy area (Fig 3) are very different and reflectthe evolving Early Archean protocontinent Theearliest sedimentary record is limited and domi-nated by hydrothermally generated laminatedcherts and BIFs which appear as thin interbedswithin massive basalts (Coonterunah Group) (Fig5) Gradually clastic rocks appear dominated atfirst by volcanoclastics followed later by the firstevidence of sediments containing detrital quartzderived from true continental granitic crust Hy-drothermally generated laminated chert carbon-ate and barite units occur throughout the sectionThe first evidence of erosion appears at the top ofthe Coonterunah Group in the form of an angu-lar unconformity (Buick et al 1995b)

The basins that form the supracrustal succes-sion on the Pilbara Craton offer important in-sights into the closing phases of the Archean andthe immediate post-Archean world Because oftheir economic importance the basins are wellmapped and drill core is readily available (seeLindsay and Brasier 2002a) The succession pro-vides vital information about the early oxygena-tion of the atmosphere and the rapid growth ofthe biosphere that resulted from the assembly ofthe first continents and the initiation of the platetectonic cycle Platform carbonates whose geom-

LINDSAY ET AL752

etry was determined by eustasy are widespreadand contain considerable information on earlyoxygenic photosynthesis and the development ofthe first reef-like structure that are similar inmany ways to modern reefs (Simonson et al1993ab Lindsay and Brasier 2002ab)

The collection would consist of three groupingsof samples stratigraphic samples site-specificsamples and framework samples

Stratigraphic samples

The stratigraphic sample set would be collectedfrom well-defined settings such as carefully mea-sured and documented sections or from drill coreSamples would be collected from each of the fivestratigraphic groups or megasequences Thesupracrustal successions preserved in the Hamer-sley and associated basins would where possiblebe taken from existing drill core or from drill coresobtained for special projects Where drill core isnot available samples would be collected fromcarefully selected and measured outcrop sectionsThe samples would concentrate on rocks mostlikely to host biogenic material Samples from theEarly Archean volcaniclastic carapace would bedominated by cherts that are most likely to pre-serve fine biogenic structures but would also in-clude other sedimentary hydrothermal units suchas barite and carbonate that would be used in sta-ble isotope and geochemical studies

Site-specific sample

This sample set would focus on sites where po-tential biogenic structures have been documentedsuch as the stromatolitic intervals in the DresserFormation and the Strelley Pool Chert It wouldalso include detailed sampling of sites such asChinaman Creek and the Strelley Pool Chertwhere purported microfossils have been docu-mented (Schopf 1993) and the sulfide bodies inthe Kangaroo Caves Formation that contain pos-sible microfossils as well as evidence of hydro-carbons (Rasmussen 2000 Rasmussen and Buick2000) This would involve a broad samplingaround the sites in measured sections and drillcore to establish the geological context

Framework samples

While the focus of the collecting would be onmaterial that would help define the character of

the Earthrsquos earliest biosphere it is important thatthe broad context of the samples be defined TheArchean is a period when the Earthrsquos crust wasevolving rapidly and there is a growing body ofevidence to suggest that the evolution of theplanet drives the evolution of the atmosphere andbiosphere (Lindsay and Brasier 2002ab) Theframework sample set would include suites ofsample from the extrusive volcanic rocks col-lected along well-documented sections Sectionsthrough the volcanoclastic succession would em-phasize the dominant volcanic units (basalts ko-matiites andesites felsites and rhyolites) assources of information on the mantle and evolv-ing crust This sample set would also include adetailed sampling of the granitoid complexeswhich ultimately drove the entire process as wellas the later intrusions that were important in dri-ving the hydrothermal systems that may wellhave been the cradle of early life

SAMPLE CURATIONCHARACTERIZATION AND

DISTRIBUTION

All samples would be fully documented in thefield Samples would be photographed and theirlocations determined using a Global PositioningSystem receiver For specialized samples such asthose thought to contain easily contaminated bio-geochemical information the field collectionwould be modeled on techniques developed bythe NASANational Science FoundationSmith-sonian Institution Antarctic Search for Meteoritesprogram (Harvey and Schutt 1998)

Characterization

The petrologic and petrographic characteristicsof each sample would be determined using thin-section optical microscopy The quantitative bulkchemical composition of each sample would bemeasured by x-ray fluorescence spectrometry Fi-nally the qualitative bulk mineralogy of eachsample would be measured by powder x-ray dif-fraction

Data management

An Archean Geologic Reference Materials Data-base would be produced to fully document eachsample Field documentation would include sam-pling rationale supporting references and field

ARCHEAN GEOLOGIC REFERENCE COLLECTION 753

photographs Documentation would include theresults of all analyses along with images of rep-resentative sample surfaces and optical micro-graphs The Database would be produced in elec-tronic format and posted on the web Subsamplesof the Archean Geologic Reference Materialswould then be made available at no cost to re-searchers in the astrobiology community

DISCUSSION AND CONCLUSIONS

The Early Archean environment was uniqueRecent studies indicate that the atmosphere waslargely anoxic with oxygen levels 1025 of thepresent atmospheric level (Pavlov and Kasting2002) There were no large continents no activeor passive margins and by default no intracra-tonic basins Significantly perhaps there are nosigns of eustasy The crust was evolving rapidlyand heat flow was high leading to the intrusionof large granitoid bodies and the development ofa volcanic carapace above them In turn thismeant that the marine environment was domi-nated by hydrothermal activity

In the absence of large continental masses de-trital materials other than volcanoclastics are rel-atively rare in the Early Archean Significant vol-umes of quartz sand do not appear in the PilbaraCraton sedimentary record until after 32 Ga pre-sumably as the intruding igneous bodies begin tobe unroofed and eroded There are thus no ma-jor accumulations of sedimentary rocks that wenormally expect associated with passive conti-nental margins

With the assembly of the earliest large conti-nents beginning at approximately 30ndash28 Gathe Earthrsquos atmosphere hydrosphere and bios-phere changed rapidly Intracratonic basinssuch as the Hamersley Basin began to developon the new crustal blocks opening new shal-low-water environments that provided an idealsetting for photosynthesizing organisms andthe development of the first carbonate reefstructures (Lindsay and Brasier 2002ab) Hy-drothermal processes while still importantwere thus no longer the only sources of energyavailable to the evolving biosphere

The Archean was thus an unfamiliar environ-ment and unravelling the nature of the earliestbiosphere will require development of new ap-proaches and new techniques Without large con-tinental masses subareal exposures are likely to

be rare and short-lived There is no evidence forstream activity until the De Grey group at ap-proximately 29 Ga at which time braided streamgravels were preserved Thus models involvingearly biospheric evolution in ldquoa warm little pondrdquo(Darwin 1888) or in fresh water seem unlikelyInstead the focus in the search for the origins oflife on Earth should be directed to hydrothermalmarine settings involving high temperatures andwhat would today be hostile geochemical envi-ronments with significant concentrations of metal-lic ions and a largely reducing environment Therisks for the misidentification of abiotic structuresand geochemical signatures as signs of ancient lifeare significant as seen in recent discussions (egBrasier et al 2002 Fedo and Whitehouse 2002Schopf et al 2002)

These difficulties suggest that much could begained by the development of a well-documentedcollection of Archean rocks that could be madeavailable to all qualified investigators Until weunderstand the earliest stages of the evolution oflife on Earth we cannot hope to make significantprogress in the search for life on other planetssuch as Mars Until there is a sample return mis-sion to Mars we must make do with meteoritesthat come to us largely out of context An under-standing of the Archean Earth can help placethose limited samples in a planetary context

ACKNOWLEDGMENTS

Franco Pirajno Kath Grey and Martin VanKranendonk provided generous advice and as-sistance and the Geological Survey of WesternAustralia supported the field program We aregrateful to Peter Morant of Sipa Resources andAdrian Black of Newexco Perth for their adviceand for access to drill-core materials

ABBREVIATION

BIF banded iron formation

REFERENCES

Arndt NT Nelson DR Compston W Trendall AFand Thorne AM (1991) The age of the FortescueGroup Hamersley Basin Western Australia from ionmicroprobe zircon U-Pb results Aust J Earth Sci 38262ndash281

LINDSAY ET AL754

Awramik SM Schopf JW and Walter MR (1983) Fil-amentous fossil bacteria from the Archaean of WesternAustralia Precambrian Res 20 357ndash374

Barley ME Pickard AL and Sylvester PJ (1997) Em-placement of a large igneous province as a possiblecause of banded iron formation 245 billion years agoNature 385 55ndash58

Bettenay LF Bickle MJ Boulter CA Groves DIMorant P Blake TS and James BA (1981) Evolu-tion of the Shaw Batholithmdashan Archaean granitoid-gneiss dome in the eastern Pilbara Western AustraliaIn Special Publication 7 The Archaean edited by JEGlover and DI Groves Geological Society of WesternAustralia Perth pp 361ndash372

Blake TS and Barley ME (1992) Tectonic evolution ofthe Late Archaean to Early Proterozoic Mount BruceMegasequence Set Western Australia Tectonics 11 1415ndash1425

Blake TS and Groves DI (1987) Continental rifting andthe Archaean-Proterozoic transition Geology 15 229ndash232

Brasier MD and Lindsay JF (1998) A billion years ofenvironmental stability and the emergence of eukary-otes new data from Northern Australia Geology 26555ndash558

Brasier MD Green OR Jephcoat AP Kleppe AKVan Kranendonk MJ Lindsay JF Steele A andGrassineau NV (2002) Questioning the evidence forEarthrsquos oldest fossils Nature 416 76ndash81

Brauhart C (1999) Regional alteration systems associatedwith Archaean volcanogenic massive sulphide depositsat Panorama Pilbara Western Australia [PhD Thesis]University of Western Australia Perth

Brauhart C Groves DI and Morant P (1998) Regionalalteration systems associated with volcanogenic mas-sive sulfide mineralization at Panorama Pilbara West-ern Australia Econ Geol 93 292ndash302

Brocks J Logan GA Buick R and Summons RE(1999) Archean molecular fossils and the early rise ofeukaryotes Science 285 1033ndash1036

Buick R Dunlop JSR and Groves DI (1981) Stroma-tolite recognition in ancient rocks an appraisal of ir-regular laminated structures in an early Archaeanchert-barite unit from North Pole Western AustraliaAlcheringa 5 161ndash181

Buick R Des Marais DJ and Knoll AH (1995a) Sta-ble isotope compositions of carbonates from the Meso-proterozoic Bangemall Group northwestern AustraliaChem Geol 123 153ndash171

Buick R Thornett JR McNaughton NJ Smith JBBarley ME and Savage M (1995b) Record of emer-gent continental crust 35 billion years ago in the Pil-bara Craton of Australia Nature 375 574ndash577

Buick R Rasmussen B and Krapez B (1998) Archaeanoil evidence for extensive hydrocarbon generation andmigration 25ndash35 Ga Am Assoc Petroleum Geol Bull82 50ndash69

Chan M (1998) Sedimentology and structure of threesmall congolmeratic basins in the Lalla Rookh-WesternShaw structural corridor Pilbara Craton Western Aus-

tralia [BSc Honours Thesis] University of WesternAustralia Perth

Cullers RL DiMarco MJ Lowe DR and Stone J(1993) Geochemistry of a silicified felsic volcaniclasticsuite from the Early Archaean Panorama FormationPilbara Block Western Australiamdashan evaluation of de-positional and post-depositional processes with specialemphasis on the rare-Earth elements Precambrian Res60 99ndash116

Darwin F ed (1888) The Life and Letters of Charles Dar-win Vol 3 John Murray London

de Wit MJ (1998) On Archaean granites greenstonescratons and tectonics does the evidence demand a ver-dict Precambrian Geol 91 181ndash226

Drummond BJ (1981) Crustal structure of the Precam-brian terrains of northwest Australia from seismic re-fraction data BMR J Geol Geophys 6 123ndash135

Dunlop JSR Muir M Milne VA and Groves DI(1978) A new microfossil assemblage from the Ar-chaean of Western Australia Nature 274 676ndash678

Dutkiewicz A Rasmussen B and Buick R (1998) Oilpreserved in fluid inclusions in Archaean sandstonesNature 395 885ndash888

Fedo CM and Whitehouse MJ (2002) Metasomatic ori-gin of quartz-pyroxene rock Akilia Greenland and im-plications for Earthrsquos earliest life Science 296 1448ndash1452

Gee RD and Grey K (1993) Report 41 Proterozoic Rockson the Glengarry 1250 000 SheetmdashStratigraphy Structureand Stromatolite Biostratigraphy Geological Survey ofWestern Australia Perth

Glikson AY and Hickman AH (1981a) Chemicalstratigraphy and petrogenesis of Archaean basic-ultra-basic volcanic units eastern Pilbara Block WesternAustralia In Special Publication 7 Archaean Geologyedited by JE Groves and DI Groves Geological Soci-ety of Western Australia Perth pp 287ndash300

Glikson AY and Hickman AH (1981b) Record 198136Geochemistry of Archaean Volcanic Successions EasternPilbara Block Western Australia Australian Bureau ofMineral Resources Canberra

Glikson AY Davy R and Hickman AH (1991) Record199146 Trace Metal Distribution Patterns in Basalts Pil-bara Craton Western Australia with Stratigraphic-Geo-chemical Implications Australian Bureau of Mineral Re-sources Canberra

Groves DI Dunlop JSR and Buick R (1981) An earlyhabitat of life Sci Am 245 64ndash73

Harvey RP and Schutt JW (1998) Meteorite recoveryand reconnaissance in the Allan Hills-David Glacierand Darwin Glacier regions 1996ndash1997 Antarctic J US 32 25ndash27

Hickman AH (1975) Annual Report 1974 PrecambrianStructural Geology of Part of the Pilbara Region Geologi-cal Survey of Western Australia Perth pp 68ndash73

Hickman AH (1983) Bulletin 127 Geology of the PilbaraBlock and Its Environs Geological Survey of WesternAustralia Perth

Hickman AH (1984) Archaean diapirism in the PilbaraBlock Western Australia In Precambrian Tectonics Il-lustrated edited by A Kroner and R Greiling E

ARCHEAN GEOLOGIC REFERENCE COLLECTION 755

amphibolite facies at the contact to lower green-schist and prenhite-pumpellyite facies Lowergreenschist facies is by far the most widespreadmetamorphic grade in the study area

Megasequence 1mdashCoonterunah Group

The Coonterunah Group includes the earliestknown stratigraphic succession (3515 Ma) pre-served on the Pilbara Craton (Buick et al 1995bVan Kranendonk and Morant 1998) The lowercontact is intrusive with the 3468 Ma granitoidrocks of the Carlindi Granitoid Complex The suc-cession consists of three formations (Van Kra-nendonk 2000)

Tabletop Formation The Tabletop Formationconsists entirely of thick basaltic units They aremainly fine-grained tholeiitic lavas althoughsome gabbroic intrusions have been identifiedThe basalts are mostly featureless although pil-lowed units are present indicating a submarinesetting

Coucal Formation The Coucal Formation whichis 2 km thick at its maximum consists largely

ARCHEAN GEOLOGIC REFERENCE COLLECTION 743

of fine-grained doleritic andesite basalt and fel-sic volcanic units (Fig 5) Thin chert (Fig 6) andimpoverished cherty banded iron formation (BIF)units are present throughout the formation Theseare the earliest known sedimentary units in thePilbara Craton and offer the earliest opportunityfor the preservation of ancient life

Double Bar Formation The Double Bar Forma-tion which conformably overlies the Coucal For-mation consists of fine-grained tholeiitic basaltswith interbedded basaltic volcaniclastic rocksSome basalts are pillowed suggesting a subma-rine setting The volcanics retain their basaltictextures but have been altered significantly bylow-grade metamorphism (Van Kranendonk2000)

Megasequence 2mdashWarrawoona Group

The Warrawoona Group is separated from theunderlying Coonterunah Group by a widespreadangular unconformity the oldest evidence onEarth for emergence (Buick et al 1995b) (Fig 3)The Warrawoona Group is dominantly volcanic inorigin but includes some clastic sedimentary rocks

FIG 3 Simplified stratigraphy of theArchean rocks exposed in the proposedstudy area (adapted from Van Kranen-donk 2000) Arrows indicate dated periodof activity of the granitoid complexes as dis-tinct from the plutons which are shown incartoon form

McPhee Basalt The McPhee Basalt which isdated at 3477 Ma and reaches a maximum of 3km in thickness is relatively limited in outcrop(Van Kranendonk et al 2002) It consists largelyof basalts and schists and is only exposed in thesouthern part of the study area adjacent to theShaw Granitoid Complex where the contact is in-trusive A thin unit of impoverished cherty BIFforms a distinctive cap to the formation

Mount Ada Basalt The Mount Ada basalt con-sists of 3ndash4 km of massive tholeiitic basalt and do-

LINDSAY ET AL744

FIG 4 Simplified stratigraphy of the Hamersley and Ashburton Basins of Western Australia (after Lindsay andBrasier 2002)

lerite intrusions that are exposed in the south-eastern corner of the study area Its lower contactwith the Shaw Granitoid Complex and NorthPole Monzonite is intrusive For the most partvolcanic textures are well preserved in thebasalts

Dresser Formation The Dresser Formation isonly recognized in the North Pole Dome whereit reaches a maximum of 1000 m in thickness(Van Kranendonk et al 2002) The formation con-sists of massive and pillowed mafic volcanics in-

terbedded with units of chert sandstone con-glomerate barite and minor carbonate (Fig 7)The individual chert-barite beds vary from 5 to40 m in thickness Associated with these unusualsedimentary facies are structures that have beeninterpreted as the worldrsquos oldest stromatolites(Fig 8) and associated microfossils (Dunlop et al1978 Walter et al 1980 Buick et al 1981 Groveset al 1981 Walter 1983) It is perhaps significantthat this distinctive unit is fed by a deep-seatedsystem of hydrothermal dykes (Fig 9)

Duffer Formation The Duffer Formation is bestexposed in the southeastern portion of the studyarea where it consists largely of felsic schist andagglomerate The agglomerates which have been

locally interpreted as welded ignimbrites arebedded in meter-scale units The Duffer Forma-tion has a depositional age of approximately 347Ga (Thorpe et al 1992 McNaughton et al 1993Nelson 1997 1998 1999 2000 2001) and restsconformably on the Mount Ada Basalt

Apex Basalt The Apex Basalt is well exposed inthe northeastern corner of the study area and overlarge areas on adjacent map sheets The forma-tion is dominated by pillowed basalts which arelargely tholeiitic The pillows are well developedand preserve vesicular rims chilled margins andconcentric cooling cracks Hyaloclastic brecciasare also present within the assemblage Doleriticflows and intrusions are common and doleriticdykes occur locally

Interbedded with the volcanics are a numberof chert horizons which are 10 m in thickness

ARCHEAN GEOLOGIC REFERENCE COLLECTION 745

FIG 5 A measured section through the Coucal For-mation (locality A Fig 2) The formation is dominatedby basic volcanic rocks with some felsic volcanic rocksand thin (10 m) hydrothermal cherts between flows

FIG 6 Laminated cherts from the lower Coucal For-mation (locality A Fig 2) Cherts form the earliest sedi-mentary rocks in the Pilbara succession Some units areas much as 10 m thick Scale bar 5 10 cm

and usually well laminated mainly black andwhite chert Numerous chert dykes cut across theunderlying stratigraphy and terminate at thelevel of the chert horizons Close to Marble Barexposures of the Apex chert contain what was ini-tially believed to be Earthrsquos oldest morphologicalfossil assemblage (Fig 10) (Schopf and Packer1987 Schopf 1992 1993) a claim recently dis-puted by more detailed analysis (Brasier et al2002) (see following section on Paleontology)

Panorama Formation The Panorama Formation(3458ndash3434 Ma) which is best exposed in thenorthern part of the study area near the ShawRiver is dominated by felsic volcaniclastic rocks

with small volumes of felsic lavas and some chertinterbeds (Hickman 1983 Thorpe et al 1992 Nel-son 2000) The formation is strongly altered andintensely silicified having been exposed to tem-peratures as high as 350degC (Cullers et al 1993)A large volcanic vent within the Panorama For-mation is preserved in the northwestern part ofthe outcrop area (Van Kranendonk 2000)

Strelley Pool Chert The relationship of the Strel-ley Pool Chert (Lowe 1983) to the underlyingunits varies from conformable to unconformableIn the northeast the contact with the underlyingCoonterunah Group forms an angular unconfor-mity with pronounced paleorelief (Buick et al1995b) The formation consists of carbonate sili-cified carbonate and clastic rocks (Fig 11) Thecherts and carbonates both have well-developedsedimentary structure that have been describedas wavy laminated cherts or carbonates and stro-matolites (Fig 12) (Lowe 1983 Hofmann et al1999) although as discussed in a following sec-tion there is some concern as to whether they arebiogenic (Lowe 1994 Lindsay et al 2003ab)

Euro Basalt The Euro Basalt consists domi-nantly of basalts of tholeiitic and high-Mg com-position that rest conformably on the StrelleyPool Chert (Van Kranendonk 2000) The forma-tion reaches a thickness in excess of 9 km in thenortheastern part of the study area The basaltsare pillowed and bedded on a kilometer scaleThe formation also includes some komatiite andfelsic volcaniclastics as well as thin cherts andvolcaniclastic units

Megasequence 3mdashSulphur Springs Group

The Sulphur Springs Group consists of threeformations (Fig 3) that in contrast to the previ-ous two Groups contain a significant proportionof clastic sedimentary rocks some quartzose

Leilira Formation The Leilira Formation consistspredominantly of clastic sedimentary rocks withonly small volumes of felsic to intermediate ex-trusive rocks interbedded The clastic rocks arepredominantly sandstones but vary from silt-stone to coarser pebbly sandstone Composition-ally the sandstones are quartzose or volcaniclithic rocks Locally laminated cherts formed bythe silicification of felsic volcanic ash and silt-stone are abundant

LINDSAY ET AL746

FIG 7 Bedded sedimentary carbonate and silica fromthe Dresser Formation (locality B Fig 2) Scale bar 5 5cm The rock consists of well-bedded units that begin withorangeyellow iron-rich carbonates and grade upward topale green silica The unit reflects geochemical changes inresponse to pulsing or cycling of the hydrothermal sys-tem Elsewhere carbonate and chert form alternating beds

The Leilira Formation represents a significantchange in depositional patterns as it is the oldestunit in which clastic sediments are the dominantlithology It reflects the growing volcanic cara-pace and in particular the exposure of the grani-toid core to weathering and erosion the first signsthat a true crust was developing

Kunagunarrina Formation The KunagunarrinaFormation reaches a maximum of 24 km inthickness and consists predominantly of high-Mg basalts and komatiites Locally andesitic andfelsic units are present especially near the top ofthe formation A thick chert unit (10 m) in themiddle of the formation separates lower Mg

ARCHEAN GEOLOGIC REFERENCE COLLECTION 747

FIG 8 Stromatolitic structures in theDresser Formation (locality B Fig 2)The original barite has been largely re-placed by chert Scale bar 5 10 cm

FIG 9 Exposures of the Dresser Formation in the North Pole Dome area (locality B Fig 2) showing large chert-barite feeder dykes The dykes consist of black chert and coarse bladed barite Note the vehicle for scale

basalts at the base of the formation from ko-matiites towards the top

Kangaroo Caves Formation The Kangaroo CavesFormation which reaches a maximum thicknessof 15 km is best exposed towards the center ofthe study area The formation is complex and con-sists of a succession of andesitic volcanics anddacitic to rhyolitic sills and extrusives A chert in-terval up to 100 m thick forms a distinctivemarker at the top of the formation Close to thecenter of the study area at Sulphur Springs themassive chert is overlain by a polymict megabrec-cia that rests upon a sharp erosional contact (Hill1997 Vearncombe et al 1998)

Hosted within a 200-m dacite unit immediatelybeneath the marker chert are six sulfide ore bodiesspaced at regular intervals of 5ndash7 km around themargins of the Strelley Granite (Morant 1998) Theyconsist of pyrite sphalerite chalcopyrite and galenawith smaller amounts of other sulfides (Vearncombeet al 1995 1998 Vearncombe 1996 Morant 1998)The sulfides which were deposited in a blacksmoker hydrothermal setting occur in associationwith growth faults developed around the StrelleyGranite the intrusion of which was syndepositionalwith the Kangaroo Caves Formation (Vearncombe1996 Brauhart et al 1998 Brauhart 1999)

Megasequence 4mdashGorge Creek Group

The Gorge Creek Group consists of five for-mations (Fig 3) that show much greater litho-logic diversity than earlier megasequences Inparticular this interval includes for the firsttime a significant thickness of quartzose sand-stone as well as a variety of other clastic sedi-mentary rocks

Pincunah Hill Formation In the western part ofthe study area the basal Pincunah Hill Formationconsists of thinly bedded BIF with small amountsof laminated chert red to gray shale and silt-stone This basal unit is up to 1 km thick in thewestern part of the area but wedges out and dis-appears in a short distance eastward The upper

LINDSAY ET AL748

FIG 11 A measured section through the Strelley PoolChert showing the location of stromatolitic structuresand the carbonate interval (adapted from Van Kranen-donk 2000) (locality C Fig 2) The gamma log to theright shows continuity across the chert-carbonate

FIG 10 ldquoMicrofossilsrdquo from the Apex chert near Mar-ble Bar a computer-generated digital automontage ofputative beggiatoan Eoleptonema apex Holotype (Schopf1993) combining the most-sharply focused images fromsuccessive focal planes b and c Original manual pho-tomontage and interpretative sketch (Schopf 1993) whichomits lower structure at arrow in (b) d and e New sin-gle images showing continuity of original and newly im-aged and automontaged structures Reprinted with per-mission from Nature (Brasier et al 2002) Copyright 2002Macmillan Publishers Ltd

part of the formation consists largely of red orpurple shale (Van Kranendonk 2000)

Corboy Formation At its thickest in the easternpart of the study area the Corboy Formation in-cludes a number of distinct facies developedacross major growth faults (Wilhelmij and Dun-lop 1984 Wilhelmij 1986) Here the formationconsists of a basal quartzose sandstone abruptlyoverlain by conglomerate interbedded withgraded sandstone units red shale and coarsesandstone The formation is capped by massivesandstone and conglomerate The unit was de-posited as a series of small prograding shallowmarine fan deltas (Wilhelmij 1986 Hill 1997 VanKranendonk 2000)

Paddy Market Formation In the western part ofthe study area the Paddy Market Formation isdominantly cherty BIF with minor ferruginousshale Further east the formation becomes moreshale-rich although much of the shale has been re-placed by chert Sandstone is present in smallamounts in beds up to 10 m thick Towards thetop of the formation in the east the cherts containfelsic volcanic material that is overlain by up to 200m of massive dacite (Van Kranendonk 2000)

Honeyeater Basalt The Honeyeater Basaltwhich consists entirely of basalt and dolerite con-formably overlies the Paddy Market FormationThe lower part of the formation consists of high-Mg basalt interbedded with dolerite whereashigher in the formation the basalts are largelytholeiitic and interbedded with dolerite (Gliksonand Hickman 1981ab Glikson et al 1991)

Pyramid Hill Formation The Pyramid Hill For-mation is limited in areal extent It is found in thenorth central part of the study area east of theStrelley Granite The formation consists of redand black shales that when heavily silicifiedgrade into laminated black and white chert (VanKranendonk 2000) The Gorge Creek Group iscapped by a well-bedded BIF

Megasequence 5mdashDe Grey Group

The De Grey Group consists of a single for-mation the Lalla Rookh Sandstone (Fig 3)which is dominated by clastic sedimentary rocksthat contain a significant quartzose componentThe Lalla Rookh Sandstone which rests on awell-defined unconformity above the GorgeCreek Group reaches a maximum thickness of

ARCHEAN GEOLOGIC REFERENCE COLLECTION 749

FIG 12 Stromatolitic carbonates in the Strelley Pool Chert (locality C Fig 2) Laminae are isopachous and theapex of the cone is ruptured and veined by secondary carbonate (arrow) Scale bar 5 10 cm

3 km (Krapez 1984) In the north central part ofthe study area the formation consists entirely ofcoarse clastic sediments and minor shalewhereas to the south the predominate fill is lithicand feldspathic sandstone (Chan 1998) Theserocks provide the first clear evidence of streamactivity in the Pilbara succession (Krapez 1984Van Kranendonk and Collins 1998 Van Kra-nendonk 2000)

THE PILBARA SUPRACRUSTAL SUCCESSION

The late Archean and early Paleoproterozoicbasins that rest upon the ancient cratonic blocksof Western Australia (Fig 1) form a time seriesassociated with the formation and ultimate dis-assembly of one of Earthrsquos earliest major conti-nental masses This was followed by the suturingof the Pilbara and Yilgarn Cratons along theCapricorn Orogen as a new supercontinentevolved in the Paleoproterozoic (Rogers and San-tosh 2002) These basins contain an importantsedimentary record of the early Earth includingsome of Earthrsquos earliest stromatolites and plat-form carbonate deposits (Simonson et al1993ab) organic-rich black shales (Brocks et al1999) and BIF (Trendall 1983 1990) The strati-graphic and structural framework is well con-strained by radiometric ages (Arndt et al 1991Barley et al 1997 Trendall et al 1998) providinga record at a critical time in Earth history whenthe atmosphere was first becoming oxygen-rich(Lindsay and Brasier 2002ab)

The Late Archean-Early PaleoproterozoicHamersley Basin which lies close to the southernmargin of the Pilbara Block (Trendall 1983 1990Blake and Barley 1992) overlies a normal crustalthickness (30ndash40 km) (Drummond 1981) Thebasin began to subside at approximately 28 Gafollowing an episode of crustal extension The ar-chitecture of the basin fill is complex andpolyphase with major erosional surfaces separat-ing areally extensive megasequencessuperse-quences (Trendall 1990 Krapez 1996) DuringFortescue and Hamersley Group time (Fig 4)sedimentation began in a rift setting in a silici-clastic-starved environment The later stages ofHamersley Basin sedimentation (Turee CreekGroup times) record the early transition from apassive margin to a foreland basin setting andwith it the establishment of a terrigenous sedi-

ment supply (Tyler and Thorne 1990) The suc-cession is largely marine Evidence of eustasy ap-pears for the first time in the sedimentary suc-cession of the Hamersley Basin (Lindsay andBrasier 2002ab)

The geometry and sequence stacking pattern ofthe succession are both very similar to youngerintracratonic basins (see Lindsay et al 1993) TheHamersley Basin is thus the first basinal settingpreserved on the Australian craton in which ma-rine sediments and in particular platform car-bonate rocks have been preserved in a responseto broad regional crustal subsidence and wheredeposition is controlled by eustasy

The Ashburton Basin (22ndash18 Ga) discon-formably overlies the southern margin of theHamersley Basin along its the contact with theCapricorn Orogen (Fig 1) (Thorne and Seymour1991) The basin forms the northern margin of theCapricorn Orogen a deformed zone that devel-oped during ocean closure Loading of the oceanfloor by the approaching Yilgarn Craton led tothe development of a westndashnorthwest-orientedforeland basin [McGrath Trough (Horowitz1982)] with an uplifted orogenic margin to thesouthwest (Blake and Groves 1987 Blake andBarley 1992) with the result that sedimentationwas dominated by an abundant supply of ter-rigenous clastic material (Mount McGrath For-mation) Continued loading of the crust ulti-mately led to the supply of terrigenous materialsbeing disrupted and producing a prograding car-bonate shelf (Duck Creek Dolomite) Basin sedi-mentation was accompanied by active-marginmafic volcanism (Cheela Springs Basalt)

THE CAPRICORN OROGEN ANDASSOCIATED BASINS

The Yerrida (22ndash19 Ga) Bryah (20 Ga)Padbury (20 Ga) and Earaheedy (19ndash165Ga) Basins lie to the southeast of the Hamersleyand Ashburton Basins and overlie the CapricornOrogen (Fig 1) (Occhipinti et al 1997 1998 Pi-rajno et al 1998 Pirajno and Adamides 2000)These now fragmentary basins formed along thenorthern margin of the Yilgarn Craton in a back-arc setting and record the convergence and colli-sion of the Archean Pilbara and Yilgarn Cratons(Tyler and Thorne 1990 Thorne and Seymour1991 Occhipinti et al 1997 1998 Pirajno et al1998) Because of their active margin settings the

LINDSAY ET AL750

fill of these small basins is dominated by clasticsediments However platform carbonate unitsare preserved in the Yerrida Basin (Gee and Grey1993 Pirajno and Adamides 2000) and a signif-icant thickness of carbonate rocks also occurs inthe Earaheedy Basin

THE ARCHEAN BIOSPHERE

The end of the Archean is formally defined at25 Ga (Plumb 1991) In reality the Archean worldended at some point in time closer to 28 or 30 Gawhen the first large continental blocks began to as-semble The stratigraphic record preserved on theancient Australian Craton reflects this major tran-sition The rock record from the Early Archean isvery different from that of the Late Archean in thatthe early record is predominantly volcanic and hy-drothermal in nature (ldquogreenstonerdquo) while thelater record is largely clastic and more readily com-pared with the Phanerozoic record

Structures and textures of possible biogenicorigin that are preserved in the stratigraphic suc-cession in the Early Archean study area are wellknown and are believed to provide the earliestdirect evidence of Earthrsquos biosphere Stromatoliticstructures have been encountered at two levelswithin the section The Dresser Formation con-tains structures preserved in chert and barite in-tervals that have been interpreted as stromatolitesand oncolites (Dunlop et al 1978 Walter et al1980 Buick et al 1981 Groves et al 1981 Wal-ter 1983) The stromatolitic structures occur forthe most part in layered barite units that devel-oped over swarms of synvolcanic hydrothermalfeeder dykes along listric growth faults (Nijmanet al 1998 Van Kranendonk 2004) Although ithas been argued that these structures are biogenic(Walter et al 1980 Schopf and Walter 1983 Wal-ter 1983) their direct association with the feederdykes leaves room for doubt (Buick et al 1981)The setting is complex and there is considerablescope for more detailed contextual research

The second stromatolitic horizon occurs in theoverlying Strelley Pool Chert a complex unit con-sisting of at least five members (Lowe 1980 1983Hofmann et al 1999) The main stromatolitic unitoccurs as an 8-m-thick interval in the middle ofthe formation and appears to have a consistentand widespread pattern of deposition The struc-tures have been the focus of considerable discus-sion and their biogenicity has been questioned

(Buick et al 1981 Lowe 1995) The structures arecomplex and varied (Fig 12) leading to a morerecent re-evaluation again suggesting that theyare biogenic (Hofmann et al 1999) However thelaminae are largely isopachous an indication thatthey could be abiotic precipitates (cf Pope andGrotzinger 2000) Given the intense hydrother-mal activity that appears to have taken placethroughout the depositional time interval thestructure may well be the product of hydrother-mal deposition (Lindsay et al 2003ab) In all ourobservations suggest that considerably morework needs to be done to establish the deposi-tional context of these stromatolitic structures be-fore they can be confidently ascribed as biogenicin origin

Structures resembling remarkably preservedbacterial and cyanobacterial microfossils havebeen found at several localities in and close to thestudy area (Awramik et al 1983 Schopf andPacker 1987 Schopf 1992 1993 Rasmussen 2000)A complex biota of 11 species of filamentousprokaryotes which are compared with cyanobac-teria (Schopf and Packer 1987 Schopf 1992 1993)have been described from the Apex chert close toMarble Bar (21deg105579S 119deg418789E) The taxo-nomic interpretations are important in that theyprovide the earliest evidence of morphological lifeon Earth and support the hypothesis of an earlybeginning for oxygenic photosynthesis (Schopf1999) The structures are almost a billion yearsolder than the oldest known putative cyanobac-terial biomarkers (Summons et al 1999) and oc-cur well before the earliest evidence for the risein oxygen in the atmosphere (for a summary seeKnoll and Canfield 1998 Lindsay and Brasier2000 2002ab) However in a recent re-evaluationof the original Apex chert material it has been ar-gued that the filaments are not biogenic but aresecondary artifacts formed during diagenesisfrom abiogenic organic compounds generated inhydrothermal chert veins (Brasier et al 2002 VanKranendonk 2004)

Filaments believed to be biogenic in originhave also been reported from the 324 Ga Kan-garoo Caves Formation (Rasmussen 2000) wherethey form part of the Sulphur Springs deposit as-sociated with a black smoker hydrothermal ventsystem (Vearncombe et al 1995 1998) Ras-mussen (2000) argued that the filaments are rem-nants of a hydrothermal biota of thermophilicchemotrophic prokaryotes Sulfide textures areremarkably well preserved in these complex de-

ARCHEAN GEOLOGIC REFERENCE COLLECTION 751

posits and are worthy of much more detailedanalysis as they may well preserve evidence ofthe early biosphere

Finally there is evidence that hydrocarbons aretrapped in these ancient sedimentary rocks eitheras fluid inclusions in hydrothermal precipitates oras bituminous residues in larger cavities in theKangaroo Caves Formation and in the Lalla RookhSandstone (Dutkiewicz et al 1998 Rasmussen andBuick 2000) Buick et al (1998) also reported car-bonaceous globules in cherts from the lower War-rawoona Group In the Kangaroo Caves Forma-tion the hydrocarbons occur in hydrothermalbarite or within the sulfides of the volcanogenichydrothermal massive sulfide deposits that weresyndepositional with the formation (Rasmussenand Buick 2000) Textural information suggestthat these deposits were precipitated as blacksmoker chimneys in a submarine hydrothermalsetting driven by heat released from the intrudingStrelley Granite at approximately 324 Ga (Vearn-combe et al 1995) Fluorescing fluid inclusions oc-cur in quartz overgrowths in sandstones from theLalla Rookh Sandstone (Dutkiewicz et al 1998)Rasmussen and Buick (2000) and Dutkiewicz et al(1998) have argued for a biogenic origin for the hy-drocarbons however an abiotic origin involvingfor example Fischer-Tropsch-type synthesis ap-pears possible (Brasier et al 2002)

The Late Archean (and Early Paleoproterozoic)record preserved in the Hamersley and relatedbasins is stratigraphically much more coherentthan that preserved in the earlier successions TheHamersley Basin in particular provides a verycomprehensive record of the early oxygenation ofthe atmosphere in the form of large eustaticallycontrolled upward-shallowing sequences that in-clude black shales BIFs and ultimately at theirtops biogenic platform carbonates (Lindsay andBrasier 2002ab) These depositional cycles con-tain organic-rich shales at their base that recordevidence for some of Earthrsquos earliest biosphere(Brocks et al 1999) BIFs (Trendall 1983 1990)that record the rise of oxygen in the atmosphereand carbonates that document some of Earthrsquosearliest reef-like carbonate structures (Simonsonand Hassler 1996 1997 Simonson et al 1993ab)

SAMPLING STRATEGY

The Archean rocks of the Pilbara Craton arethus part of a protocontinent about which the

nascent Australian Craton developed It consistsof granitoid complexes overlain by a carapace ofsynchronously deposited volcano-sedimentarywedges Resting upon the early Archean proto-continent are a series of supracrustal basins Inall the sedimentary record of the Pilbara extendsfrom rocks as old as 354 Ga to some as young asapproximately 19 Ga As summarized in the fol-lowing paragraphs the sedimentary record of thePilbara records evidence of the major tectonic andgeological events that accompanied the evolutionof the Archean biosphere

It is here proposed that we develop a carefullydocumented and curated sample collection fromthis unique setting The collection would form thebasis for early Earth studies and a focus for astro-biology The sample set would be designed to re-flect the evolutionary succession described abovewith emphasis placed on cherts lower in the suc-cession and on clastic sediments higher in the suc-cession Any comprehensive reference collectionmust reflect diversity of the succession and includerocks from each of the sedimentary environmentsas well as igneous rocks reflecting the frameworkin which the protocontinent assembled

The five megasequences at the North Shawstudy area (Fig 3) are very different and reflectthe evolving Early Archean protocontinent Theearliest sedimentary record is limited and domi-nated by hydrothermally generated laminatedcherts and BIFs which appear as thin interbedswithin massive basalts (Coonterunah Group) (Fig5) Gradually clastic rocks appear dominated atfirst by volcanoclastics followed later by the firstevidence of sediments containing detrital quartzderived from true continental granitic crust Hy-drothermally generated laminated chert carbon-ate and barite units occur throughout the sectionThe first evidence of erosion appears at the top ofthe Coonterunah Group in the form of an angu-lar unconformity (Buick et al 1995b)

The basins that form the supracrustal succes-sion on the Pilbara Craton offer important in-sights into the closing phases of the Archean andthe immediate post-Archean world Because oftheir economic importance the basins are wellmapped and drill core is readily available (seeLindsay and Brasier 2002a) The succession pro-vides vital information about the early oxygena-tion of the atmosphere and the rapid growth ofthe biosphere that resulted from the assembly ofthe first continents and the initiation of the platetectonic cycle Platform carbonates whose geom-

LINDSAY ET AL752

etry was determined by eustasy are widespreadand contain considerable information on earlyoxygenic photosynthesis and the development ofthe first reef-like structure that are similar inmany ways to modern reefs (Simonson et al1993ab Lindsay and Brasier 2002ab)

The collection would consist of three groupingsof samples stratigraphic samples site-specificsamples and framework samples

Stratigraphic samples

The stratigraphic sample set would be collectedfrom well-defined settings such as carefully mea-sured and documented sections or from drill coreSamples would be collected from each of the fivestratigraphic groups or megasequences Thesupracrustal successions preserved in the Hamer-sley and associated basins would where possiblebe taken from existing drill core or from drill coresobtained for special projects Where drill core isnot available samples would be collected fromcarefully selected and measured outcrop sectionsThe samples would concentrate on rocks mostlikely to host biogenic material Samples from theEarly Archean volcaniclastic carapace would bedominated by cherts that are most likely to pre-serve fine biogenic structures but would also in-clude other sedimentary hydrothermal units suchas barite and carbonate that would be used in sta-ble isotope and geochemical studies

Site-specific sample

This sample set would focus on sites where po-tential biogenic structures have been documentedsuch as the stromatolitic intervals in the DresserFormation and the Strelley Pool Chert It wouldalso include detailed sampling of sites such asChinaman Creek and the Strelley Pool Chertwhere purported microfossils have been docu-mented (Schopf 1993) and the sulfide bodies inthe Kangaroo Caves Formation that contain pos-sible microfossils as well as evidence of hydro-carbons (Rasmussen 2000 Rasmussen and Buick2000) This would involve a broad samplingaround the sites in measured sections and drillcore to establish the geological context

Framework samples

While the focus of the collecting would be onmaterial that would help define the character of

the Earthrsquos earliest biosphere it is important thatthe broad context of the samples be defined TheArchean is a period when the Earthrsquos crust wasevolving rapidly and there is a growing body ofevidence to suggest that the evolution of theplanet drives the evolution of the atmosphere andbiosphere (Lindsay and Brasier 2002ab) Theframework sample set would include suites ofsample from the extrusive volcanic rocks col-lected along well-documented sections Sectionsthrough the volcanoclastic succession would em-phasize the dominant volcanic units (basalts ko-matiites andesites felsites and rhyolites) assources of information on the mantle and evolv-ing crust This sample set would also include adetailed sampling of the granitoid complexeswhich ultimately drove the entire process as wellas the later intrusions that were important in dri-ving the hydrothermal systems that may wellhave been the cradle of early life

SAMPLE CURATIONCHARACTERIZATION AND

DISTRIBUTION

All samples would be fully documented in thefield Samples would be photographed and theirlocations determined using a Global PositioningSystem receiver For specialized samples such asthose thought to contain easily contaminated bio-geochemical information the field collectionwould be modeled on techniques developed bythe NASANational Science FoundationSmith-sonian Institution Antarctic Search for Meteoritesprogram (Harvey and Schutt 1998)

Characterization

The petrologic and petrographic characteristicsof each sample would be determined using thin-section optical microscopy The quantitative bulkchemical composition of each sample would bemeasured by x-ray fluorescence spectrometry Fi-nally the qualitative bulk mineralogy of eachsample would be measured by powder x-ray dif-fraction

Data management

An Archean Geologic Reference Materials Data-base would be produced to fully document eachsample Field documentation would include sam-pling rationale supporting references and field

ARCHEAN GEOLOGIC REFERENCE COLLECTION 753

photographs Documentation would include theresults of all analyses along with images of rep-resentative sample surfaces and optical micro-graphs The Database would be produced in elec-tronic format and posted on the web Subsamplesof the Archean Geologic Reference Materialswould then be made available at no cost to re-searchers in the astrobiology community

DISCUSSION AND CONCLUSIONS

The Early Archean environment was uniqueRecent studies indicate that the atmosphere waslargely anoxic with oxygen levels 1025 of thepresent atmospheric level (Pavlov and Kasting2002) There were no large continents no activeor passive margins and by default no intracra-tonic basins Significantly perhaps there are nosigns of eustasy The crust was evolving rapidlyand heat flow was high leading to the intrusionof large granitoid bodies and the development ofa volcanic carapace above them In turn thismeant that the marine environment was domi-nated by hydrothermal activity

In the absence of large continental masses de-trital materials other than volcanoclastics are rel-atively rare in the Early Archean Significant vol-umes of quartz sand do not appear in the PilbaraCraton sedimentary record until after 32 Ga pre-sumably as the intruding igneous bodies begin tobe unroofed and eroded There are thus no ma-jor accumulations of sedimentary rocks that wenormally expect associated with passive conti-nental margins

With the assembly of the earliest large conti-nents beginning at approximately 30ndash28 Gathe Earthrsquos atmosphere hydrosphere and bios-phere changed rapidly Intracratonic basinssuch as the Hamersley Basin began to developon the new crustal blocks opening new shal-low-water environments that provided an idealsetting for photosynthesizing organisms andthe development of the first carbonate reefstructures (Lindsay and Brasier 2002ab) Hy-drothermal processes while still importantwere thus no longer the only sources of energyavailable to the evolving biosphere

The Archean was thus an unfamiliar environ-ment and unravelling the nature of the earliestbiosphere will require development of new ap-proaches and new techniques Without large con-tinental masses subareal exposures are likely to

be rare and short-lived There is no evidence forstream activity until the De Grey group at ap-proximately 29 Ga at which time braided streamgravels were preserved Thus models involvingearly biospheric evolution in ldquoa warm little pondrdquo(Darwin 1888) or in fresh water seem unlikelyInstead the focus in the search for the origins oflife on Earth should be directed to hydrothermalmarine settings involving high temperatures andwhat would today be hostile geochemical envi-ronments with significant concentrations of metal-lic ions and a largely reducing environment Therisks for the misidentification of abiotic structuresand geochemical signatures as signs of ancient lifeare significant as seen in recent discussions (egBrasier et al 2002 Fedo and Whitehouse 2002Schopf et al 2002)

These difficulties suggest that much could begained by the development of a well-documentedcollection of Archean rocks that could be madeavailable to all qualified investigators Until weunderstand the earliest stages of the evolution oflife on Earth we cannot hope to make significantprogress in the search for life on other planetssuch as Mars Until there is a sample return mis-sion to Mars we must make do with meteoritesthat come to us largely out of context An under-standing of the Archean Earth can help placethose limited samples in a planetary context

ACKNOWLEDGMENTS

Franco Pirajno Kath Grey and Martin VanKranendonk provided generous advice and as-sistance and the Geological Survey of WesternAustralia supported the field program We aregrateful to Peter Morant of Sipa Resources andAdrian Black of Newexco Perth for their adviceand for access to drill-core materials

ABBREVIATION

BIF banded iron formation

REFERENCES

Arndt NT Nelson DR Compston W Trendall AFand Thorne AM (1991) The age of the FortescueGroup Hamersley Basin Western Australia from ionmicroprobe zircon U-Pb results Aust J Earth Sci 38262ndash281

LINDSAY ET AL754

Awramik SM Schopf JW and Walter MR (1983) Fil-amentous fossil bacteria from the Archaean of WesternAustralia Precambrian Res 20 357ndash374

Barley ME Pickard AL and Sylvester PJ (1997) Em-placement of a large igneous province as a possiblecause of banded iron formation 245 billion years agoNature 385 55ndash58

Bettenay LF Bickle MJ Boulter CA Groves DIMorant P Blake TS and James BA (1981) Evolu-tion of the Shaw Batholithmdashan Archaean granitoid-gneiss dome in the eastern Pilbara Western AustraliaIn Special Publication 7 The Archaean edited by JEGlover and DI Groves Geological Society of WesternAustralia Perth pp 361ndash372

Blake TS and Barley ME (1992) Tectonic evolution ofthe Late Archaean to Early Proterozoic Mount BruceMegasequence Set Western Australia Tectonics 11 1415ndash1425

Blake TS and Groves DI (1987) Continental rifting andthe Archaean-Proterozoic transition Geology 15 229ndash232

Brasier MD and Lindsay JF (1998) A billion years ofenvironmental stability and the emergence of eukary-otes new data from Northern Australia Geology 26555ndash558

Brasier MD Green OR Jephcoat AP Kleppe AKVan Kranendonk MJ Lindsay JF Steele A andGrassineau NV (2002) Questioning the evidence forEarthrsquos oldest fossils Nature 416 76ndash81

Brauhart C (1999) Regional alteration systems associatedwith Archaean volcanogenic massive sulphide depositsat Panorama Pilbara Western Australia [PhD Thesis]University of Western Australia Perth

Brauhart C Groves DI and Morant P (1998) Regionalalteration systems associated with volcanogenic mas-sive sulfide mineralization at Panorama Pilbara West-ern Australia Econ Geol 93 292ndash302

Brocks J Logan GA Buick R and Summons RE(1999) Archean molecular fossils and the early rise ofeukaryotes Science 285 1033ndash1036

Buick R Dunlop JSR and Groves DI (1981) Stroma-tolite recognition in ancient rocks an appraisal of ir-regular laminated structures in an early Archaeanchert-barite unit from North Pole Western AustraliaAlcheringa 5 161ndash181

Buick R Des Marais DJ and Knoll AH (1995a) Sta-ble isotope compositions of carbonates from the Meso-proterozoic Bangemall Group northwestern AustraliaChem Geol 123 153ndash171

Buick R Thornett JR McNaughton NJ Smith JBBarley ME and Savage M (1995b) Record of emer-gent continental crust 35 billion years ago in the Pil-bara Craton of Australia Nature 375 574ndash577

Buick R Rasmussen B and Krapez B (1998) Archaeanoil evidence for extensive hydrocarbon generation andmigration 25ndash35 Ga Am Assoc Petroleum Geol Bull82 50ndash69

Chan M (1998) Sedimentology and structure of threesmall congolmeratic basins in the Lalla Rookh-WesternShaw structural corridor Pilbara Craton Western Aus-

tralia [BSc Honours Thesis] University of WesternAustralia Perth

Cullers RL DiMarco MJ Lowe DR and Stone J(1993) Geochemistry of a silicified felsic volcaniclasticsuite from the Early Archaean Panorama FormationPilbara Block Western Australiamdashan evaluation of de-positional and post-depositional processes with specialemphasis on the rare-Earth elements Precambrian Res60 99ndash116

Darwin F ed (1888) The Life and Letters of Charles Dar-win Vol 3 John Murray London

de Wit MJ (1998) On Archaean granites greenstonescratons and tectonics does the evidence demand a ver-dict Precambrian Geol 91 181ndash226

Drummond BJ (1981) Crustal structure of the Precam-brian terrains of northwest Australia from seismic re-fraction data BMR J Geol Geophys 6 123ndash135

Dunlop JSR Muir M Milne VA and Groves DI(1978) A new microfossil assemblage from the Ar-chaean of Western Australia Nature 274 676ndash678

Dutkiewicz A Rasmussen B and Buick R (1998) Oilpreserved in fluid inclusions in Archaean sandstonesNature 395 885ndash888

Fedo CM and Whitehouse MJ (2002) Metasomatic ori-gin of quartz-pyroxene rock Akilia Greenland and im-plications for Earthrsquos earliest life Science 296 1448ndash1452

Gee RD and Grey K (1993) Report 41 Proterozoic Rockson the Glengarry 1250 000 SheetmdashStratigraphy Structureand Stromatolite Biostratigraphy Geological Survey ofWestern Australia Perth

Glikson AY and Hickman AH (1981a) Chemicalstratigraphy and petrogenesis of Archaean basic-ultra-basic volcanic units eastern Pilbara Block WesternAustralia In Special Publication 7 Archaean Geologyedited by JE Groves and DI Groves Geological Soci-ety of Western Australia Perth pp 287ndash300

Glikson AY and Hickman AH (1981b) Record 198136Geochemistry of Archaean Volcanic Successions EasternPilbara Block Western Australia Australian Bureau ofMineral Resources Canberra

Glikson AY Davy R and Hickman AH (1991) Record199146 Trace Metal Distribution Patterns in Basalts Pil-bara Craton Western Australia with Stratigraphic-Geo-chemical Implications Australian Bureau of Mineral Re-sources Canberra

Groves DI Dunlop JSR and Buick R (1981) An earlyhabitat of life Sci Am 245 64ndash73

Harvey RP and Schutt JW (1998) Meteorite recoveryand reconnaissance in the Allan Hills-David Glacierand Darwin Glacier regions 1996ndash1997 Antarctic J US 32 25ndash27

Hickman AH (1975) Annual Report 1974 PrecambrianStructural Geology of Part of the Pilbara Region Geologi-cal Survey of Western Australia Perth pp 68ndash73

Hickman AH (1983) Bulletin 127 Geology of the PilbaraBlock and Its Environs Geological Survey of WesternAustralia Perth

Hickman AH (1984) Archaean diapirism in the PilbaraBlock Western Australia In Precambrian Tectonics Il-lustrated edited by A Kroner and R Greiling E

ARCHEAN GEOLOGIC REFERENCE COLLECTION 755

McPhee Basalt The McPhee Basalt which isdated at 3477 Ma and reaches a maximum of 3km in thickness is relatively limited in outcrop(Van Kranendonk et al 2002) It consists largelyof basalts and schists and is only exposed in thesouthern part of the study area adjacent to theShaw Granitoid Complex where the contact is in-trusive A thin unit of impoverished cherty BIFforms a distinctive cap to the formation

Mount Ada Basalt The Mount Ada basalt con-sists of 3ndash4 km of massive tholeiitic basalt and do-

LINDSAY ET AL744

FIG 4 Simplified stratigraphy of the Hamersley and Ashburton Basins of Western Australia (after Lindsay andBrasier 2002)

lerite intrusions that are exposed in the south-eastern corner of the study area Its lower contactwith the Shaw Granitoid Complex and NorthPole Monzonite is intrusive For the most partvolcanic textures are well preserved in thebasalts

Dresser Formation The Dresser Formation isonly recognized in the North Pole Dome whereit reaches a maximum of 1000 m in thickness(Van Kranendonk et al 2002) The formation con-sists of massive and pillowed mafic volcanics in-

terbedded with units of chert sandstone con-glomerate barite and minor carbonate (Fig 7)The individual chert-barite beds vary from 5 to40 m in thickness Associated with these unusualsedimentary facies are structures that have beeninterpreted as the worldrsquos oldest stromatolites(Fig 8) and associated microfossils (Dunlop et al1978 Walter et al 1980 Buick et al 1981 Groveset al 1981 Walter 1983) It is perhaps significantthat this distinctive unit is fed by a deep-seatedsystem of hydrothermal dykes (Fig 9)

Duffer Formation The Duffer Formation is bestexposed in the southeastern portion of the studyarea where it consists largely of felsic schist andagglomerate The agglomerates which have been

locally interpreted as welded ignimbrites arebedded in meter-scale units The Duffer Forma-tion has a depositional age of approximately 347Ga (Thorpe et al 1992 McNaughton et al 1993Nelson 1997 1998 1999 2000 2001) and restsconformably on the Mount Ada Basalt

Apex Basalt The Apex Basalt is well exposed inthe northeastern corner of the study area and overlarge areas on adjacent map sheets The forma-tion is dominated by pillowed basalts which arelargely tholeiitic The pillows are well developedand preserve vesicular rims chilled margins andconcentric cooling cracks Hyaloclastic brecciasare also present within the assemblage Doleriticflows and intrusions are common and doleriticdykes occur locally

Interbedded with the volcanics are a numberof chert horizons which are 10 m in thickness

ARCHEAN GEOLOGIC REFERENCE COLLECTION 745

FIG 5 A measured section through the Coucal For-mation (locality A Fig 2) The formation is dominatedby basic volcanic rocks with some felsic volcanic rocksand thin (10 m) hydrothermal cherts between flows

FIG 6 Laminated cherts from the lower Coucal For-mation (locality A Fig 2) Cherts form the earliest sedi-mentary rocks in the Pilbara succession Some units areas much as 10 m thick Scale bar 5 10 cm

and usually well laminated mainly black andwhite chert Numerous chert dykes cut across theunderlying stratigraphy and terminate at thelevel of the chert horizons Close to Marble Barexposures of the Apex chert contain what was ini-tially believed to be Earthrsquos oldest morphologicalfossil assemblage (Fig 10) (Schopf and Packer1987 Schopf 1992 1993) a claim recently dis-puted by more detailed analysis (Brasier et al2002) (see following section on Paleontology)

Panorama Formation The Panorama Formation(3458ndash3434 Ma) which is best exposed in thenorthern part of the study area near the ShawRiver is dominated by felsic volcaniclastic rocks

with small volumes of felsic lavas and some chertinterbeds (Hickman 1983 Thorpe et al 1992 Nel-son 2000) The formation is strongly altered andintensely silicified having been exposed to tem-peratures as high as 350degC (Cullers et al 1993)A large volcanic vent within the Panorama For-mation is preserved in the northwestern part ofthe outcrop area (Van Kranendonk 2000)

Strelley Pool Chert The relationship of the Strel-ley Pool Chert (Lowe 1983) to the underlyingunits varies from conformable to unconformableIn the northeast the contact with the underlyingCoonterunah Group forms an angular unconfor-mity with pronounced paleorelief (Buick et al1995b) The formation consists of carbonate sili-cified carbonate and clastic rocks (Fig 11) Thecherts and carbonates both have well-developedsedimentary structure that have been describedas wavy laminated cherts or carbonates and stro-matolites (Fig 12) (Lowe 1983 Hofmann et al1999) although as discussed in a following sec-tion there is some concern as to whether they arebiogenic (Lowe 1994 Lindsay et al 2003ab)

Euro Basalt The Euro Basalt consists domi-nantly of basalts of tholeiitic and high-Mg com-position that rest conformably on the StrelleyPool Chert (Van Kranendonk 2000) The forma-tion reaches a thickness in excess of 9 km in thenortheastern part of the study area The basaltsare pillowed and bedded on a kilometer scaleThe formation also includes some komatiite andfelsic volcaniclastics as well as thin cherts andvolcaniclastic units

Megasequence 3mdashSulphur Springs Group

The Sulphur Springs Group consists of threeformations (Fig 3) that in contrast to the previ-ous two Groups contain a significant proportionof clastic sedimentary rocks some quartzose

Leilira Formation The Leilira Formation consistspredominantly of clastic sedimentary rocks withonly small volumes of felsic to intermediate ex-trusive rocks interbedded The clastic rocks arepredominantly sandstones but vary from silt-stone to coarser pebbly sandstone Composition-ally the sandstones are quartzose or volcaniclithic rocks Locally laminated cherts formed bythe silicification of felsic volcanic ash and silt-stone are abundant

LINDSAY ET AL746

FIG 7 Bedded sedimentary carbonate and silica fromthe Dresser Formation (locality B Fig 2) Scale bar 5 5cm The rock consists of well-bedded units that begin withorangeyellow iron-rich carbonates and grade upward topale green silica The unit reflects geochemical changes inresponse to pulsing or cycling of the hydrothermal sys-tem Elsewhere carbonate and chert form alternating beds

The Leilira Formation represents a significantchange in depositional patterns as it is the oldestunit in which clastic sediments are the dominantlithology It reflects the growing volcanic cara-pace and in particular the exposure of the grani-toid core to weathering and erosion the first signsthat a true crust was developing

Kunagunarrina Formation The KunagunarrinaFormation reaches a maximum of 24 km inthickness and consists predominantly of high-Mg basalts and komatiites Locally andesitic andfelsic units are present especially near the top ofthe formation A thick chert unit (10 m) in themiddle of the formation separates lower Mg

ARCHEAN GEOLOGIC REFERENCE COLLECTION 747

FIG 8 Stromatolitic structures in theDresser Formation (locality B Fig 2)The original barite has been largely re-placed by chert Scale bar 5 10 cm

FIG 9 Exposures of the Dresser Formation in the North Pole Dome area (locality B Fig 2) showing large chert-barite feeder dykes The dykes consist of black chert and coarse bladed barite Note the vehicle for scale

basalts at the base of the formation from ko-matiites towards the top

Kangaroo Caves Formation The Kangaroo CavesFormation which reaches a maximum thicknessof 15 km is best exposed towards the center ofthe study area The formation is complex and con-sists of a succession of andesitic volcanics anddacitic to rhyolitic sills and extrusives A chert in-terval up to 100 m thick forms a distinctivemarker at the top of the formation Close to thecenter of the study area at Sulphur Springs themassive chert is overlain by a polymict megabrec-cia that rests upon a sharp erosional contact (Hill1997 Vearncombe et al 1998)

Hosted within a 200-m dacite unit immediatelybeneath the marker chert are six sulfide ore bodiesspaced at regular intervals of 5ndash7 km around themargins of the Strelley Granite (Morant 1998) Theyconsist of pyrite sphalerite chalcopyrite and galenawith smaller amounts of other sulfides (Vearncombeet al 1995 1998 Vearncombe 1996 Morant 1998)The sulfides which were deposited in a blacksmoker hydrothermal setting occur in associationwith growth faults developed around the StrelleyGranite the intrusion of which was syndepositionalwith the Kangaroo Caves Formation (Vearncombe1996 Brauhart et al 1998 Brauhart 1999)

Megasequence 4mdashGorge Creek Group

The Gorge Creek Group consists of five for-mations (Fig 3) that show much greater litho-logic diversity than earlier megasequences Inparticular this interval includes for the firsttime a significant thickness of quartzose sand-stone as well as a variety of other clastic sedi-mentary rocks

Pincunah Hill Formation In the western part ofthe study area the basal Pincunah Hill Formationconsists of thinly bedded BIF with small amountsof laminated chert red to gray shale and silt-stone This basal unit is up to 1 km thick in thewestern part of the area but wedges out and dis-appears in a short distance eastward The upper

LINDSAY ET AL748

FIG 11 A measured section through the Strelley PoolChert showing the location of stromatolitic structuresand the carbonate interval (adapted from Van Kranen-donk 2000) (locality C Fig 2) The gamma log to theright shows continuity across the chert-carbonate

FIG 10 ldquoMicrofossilsrdquo from the Apex chert near Mar-ble Bar a computer-generated digital automontage ofputative beggiatoan Eoleptonema apex Holotype (Schopf1993) combining the most-sharply focused images fromsuccessive focal planes b and c Original manual pho-tomontage and interpretative sketch (Schopf 1993) whichomits lower structure at arrow in (b) d and e New sin-gle images showing continuity of original and newly im-aged and automontaged structures Reprinted with per-mission from Nature (Brasier et al 2002) Copyright 2002Macmillan Publishers Ltd

part of the formation consists largely of red orpurple shale (Van Kranendonk 2000)

Corboy Formation At its thickest in the easternpart of the study area the Corboy Formation in-cludes a number of distinct facies developedacross major growth faults (Wilhelmij and Dun-lop 1984 Wilhelmij 1986) Here the formationconsists of a basal quartzose sandstone abruptlyoverlain by conglomerate interbedded withgraded sandstone units red shale and coarsesandstone The formation is capped by massivesandstone and conglomerate The unit was de-posited as a series of small prograding shallowmarine fan deltas (Wilhelmij 1986 Hill 1997 VanKranendonk 2000)

Paddy Market Formation In the western part ofthe study area the Paddy Market Formation isdominantly cherty BIF with minor ferruginousshale Further east the formation becomes moreshale-rich although much of the shale has been re-placed by chert Sandstone is present in smallamounts in beds up to 10 m thick Towards thetop of the formation in the east the cherts containfelsic volcanic material that is overlain by up to 200m of massive dacite (Van Kranendonk 2000)

Honeyeater Basalt The Honeyeater Basaltwhich consists entirely of basalt and dolerite con-formably overlies the Paddy Market FormationThe lower part of the formation consists of high-Mg basalt interbedded with dolerite whereashigher in the formation the basalts are largelytholeiitic and interbedded with dolerite (Gliksonand Hickman 1981ab Glikson et al 1991)

Pyramid Hill Formation The Pyramid Hill For-mation is limited in areal extent It is found in thenorth central part of the study area east of theStrelley Granite The formation consists of redand black shales that when heavily silicifiedgrade into laminated black and white chert (VanKranendonk 2000) The Gorge Creek Group iscapped by a well-bedded BIF

Megasequence 5mdashDe Grey Group

The De Grey Group consists of a single for-mation the Lalla Rookh Sandstone (Fig 3)which is dominated by clastic sedimentary rocksthat contain a significant quartzose componentThe Lalla Rookh Sandstone which rests on awell-defined unconformity above the GorgeCreek Group reaches a maximum thickness of

ARCHEAN GEOLOGIC REFERENCE COLLECTION 749

FIG 12 Stromatolitic carbonates in the Strelley Pool Chert (locality C Fig 2) Laminae are isopachous and theapex of the cone is ruptured and veined by secondary carbonate (arrow) Scale bar 5 10 cm

3 km (Krapez 1984) In the north central part ofthe study area the formation consists entirely ofcoarse clastic sediments and minor shalewhereas to the south the predominate fill is lithicand feldspathic sandstone (Chan 1998) Theserocks provide the first clear evidence of streamactivity in the Pilbara succession (Krapez 1984Van Kranendonk and Collins 1998 Van Kra-nendonk 2000)

THE PILBARA SUPRACRUSTAL SUCCESSION

The late Archean and early Paleoproterozoicbasins that rest upon the ancient cratonic blocksof Western Australia (Fig 1) form a time seriesassociated with the formation and ultimate dis-assembly of one of Earthrsquos earliest major conti-nental masses This was followed by the suturingof the Pilbara and Yilgarn Cratons along theCapricorn Orogen as a new supercontinentevolved in the Paleoproterozoic (Rogers and San-tosh 2002) These basins contain an importantsedimentary record of the early Earth includingsome of Earthrsquos earliest stromatolites and plat-form carbonate deposits (Simonson et al1993ab) organic-rich black shales (Brocks et al1999) and BIF (Trendall 1983 1990) The strati-graphic and structural framework is well con-strained by radiometric ages (Arndt et al 1991Barley et al 1997 Trendall et al 1998) providinga record at a critical time in Earth history whenthe atmosphere was first becoming oxygen-rich(Lindsay and Brasier 2002ab)

The Late Archean-Early PaleoproterozoicHamersley Basin which lies close to the southernmargin of the Pilbara Block (Trendall 1983 1990Blake and Barley 1992) overlies a normal crustalthickness (30ndash40 km) (Drummond 1981) Thebasin began to subside at approximately 28 Gafollowing an episode of crustal extension The ar-chitecture of the basin fill is complex andpolyphase with major erosional surfaces separat-ing areally extensive megasequencessuperse-quences (Trendall 1990 Krapez 1996) DuringFortescue and Hamersley Group time (Fig 4)sedimentation began in a rift setting in a silici-clastic-starved environment The later stages ofHamersley Basin sedimentation (Turee CreekGroup times) record the early transition from apassive margin to a foreland basin setting andwith it the establishment of a terrigenous sedi-

ment supply (Tyler and Thorne 1990) The suc-cession is largely marine Evidence of eustasy ap-pears for the first time in the sedimentary suc-cession of the Hamersley Basin (Lindsay andBrasier 2002ab)

The geometry and sequence stacking pattern ofthe succession are both very similar to youngerintracratonic basins (see Lindsay et al 1993) TheHamersley Basin is thus the first basinal settingpreserved on the Australian craton in which ma-rine sediments and in particular platform car-bonate rocks have been preserved in a responseto broad regional crustal subsidence and wheredeposition is controlled by eustasy

The Ashburton Basin (22ndash18 Ga) discon-formably overlies the southern margin of theHamersley Basin along its the contact with theCapricorn Orogen (Fig 1) (Thorne and Seymour1991) The basin forms the northern margin of theCapricorn Orogen a deformed zone that devel-oped during ocean closure Loading of the oceanfloor by the approaching Yilgarn Craton led tothe development of a westndashnorthwest-orientedforeland basin [McGrath Trough (Horowitz1982)] with an uplifted orogenic margin to thesouthwest (Blake and Groves 1987 Blake andBarley 1992) with the result that sedimentationwas dominated by an abundant supply of ter-rigenous clastic material (Mount McGrath For-mation) Continued loading of the crust ulti-mately led to the supply of terrigenous materialsbeing disrupted and producing a prograding car-bonate shelf (Duck Creek Dolomite) Basin sedi-mentation was accompanied by active-marginmafic volcanism (Cheela Springs Basalt)

THE CAPRICORN OROGEN ANDASSOCIATED BASINS

The Yerrida (22ndash19 Ga) Bryah (20 Ga)Padbury (20 Ga) and Earaheedy (19ndash165Ga) Basins lie to the southeast of the Hamersleyand Ashburton Basins and overlie the CapricornOrogen (Fig 1) (Occhipinti et al 1997 1998 Pi-rajno et al 1998 Pirajno and Adamides 2000)These now fragmentary basins formed along thenorthern margin of the Yilgarn Craton in a back-arc setting and record the convergence and colli-sion of the Archean Pilbara and Yilgarn Cratons(Tyler and Thorne 1990 Thorne and Seymour1991 Occhipinti et al 1997 1998 Pirajno et al1998) Because of their active margin settings the

LINDSAY ET AL750

fill of these small basins is dominated by clasticsediments However platform carbonate unitsare preserved in the Yerrida Basin (Gee and Grey1993 Pirajno and Adamides 2000) and a signif-icant thickness of carbonate rocks also occurs inthe Earaheedy Basin

THE ARCHEAN BIOSPHERE

The end of the Archean is formally defined at25 Ga (Plumb 1991) In reality the Archean worldended at some point in time closer to 28 or 30 Gawhen the first large continental blocks began to as-semble The stratigraphic record preserved on theancient Australian Craton reflects this major tran-sition The rock record from the Early Archean isvery different from that of the Late Archean in thatthe early record is predominantly volcanic and hy-drothermal in nature (ldquogreenstonerdquo) while thelater record is largely clastic and more readily com-pared with the Phanerozoic record

Structures and textures of possible biogenicorigin that are preserved in the stratigraphic suc-cession in the Early Archean study area are wellknown and are believed to provide the earliestdirect evidence of Earthrsquos biosphere Stromatoliticstructures have been encountered at two levelswithin the section The Dresser Formation con-tains structures preserved in chert and barite in-tervals that have been interpreted as stromatolitesand oncolites (Dunlop et al 1978 Walter et al1980 Buick et al 1981 Groves et al 1981 Wal-ter 1983) The stromatolitic structures occur forthe most part in layered barite units that devel-oped over swarms of synvolcanic hydrothermalfeeder dykes along listric growth faults (Nijmanet al 1998 Van Kranendonk 2004) Although ithas been argued that these structures are biogenic(Walter et al 1980 Schopf and Walter 1983 Wal-ter 1983) their direct association with the feederdykes leaves room for doubt (Buick et al 1981)The setting is complex and there is considerablescope for more detailed contextual research

The second stromatolitic horizon occurs in theoverlying Strelley Pool Chert a complex unit con-sisting of at least five members (Lowe 1980 1983Hofmann et al 1999) The main stromatolitic unitoccurs as an 8-m-thick interval in the middle ofthe formation and appears to have a consistentand widespread pattern of deposition The struc-tures have been the focus of considerable discus-sion and their biogenicity has been questioned

(Buick et al 1981 Lowe 1995) The structures arecomplex and varied (Fig 12) leading to a morerecent re-evaluation again suggesting that theyare biogenic (Hofmann et al 1999) However thelaminae are largely isopachous an indication thatthey could be abiotic precipitates (cf Pope andGrotzinger 2000) Given the intense hydrother-mal activity that appears to have taken placethroughout the depositional time interval thestructure may well be the product of hydrother-mal deposition (Lindsay et al 2003ab) In all ourobservations suggest that considerably morework needs to be done to establish the deposi-tional context of these stromatolitic structures be-fore they can be confidently ascribed as biogenicin origin

Structures resembling remarkably preservedbacterial and cyanobacterial microfossils havebeen found at several localities in and close to thestudy area (Awramik et al 1983 Schopf andPacker 1987 Schopf 1992 1993 Rasmussen 2000)A complex biota of 11 species of filamentousprokaryotes which are compared with cyanobac-teria (Schopf and Packer 1987 Schopf 1992 1993)have been described from the Apex chert close toMarble Bar (21deg105579S 119deg418789E) The taxo-nomic interpretations are important in that theyprovide the earliest evidence of morphological lifeon Earth and support the hypothesis of an earlybeginning for oxygenic photosynthesis (Schopf1999) The structures are almost a billion yearsolder than the oldest known putative cyanobac-terial biomarkers (Summons et al 1999) and oc-cur well before the earliest evidence for the risein oxygen in the atmosphere (for a summary seeKnoll and Canfield 1998 Lindsay and Brasier2000 2002ab) However in a recent re-evaluationof the original Apex chert material it has been ar-gued that the filaments are not biogenic but aresecondary artifacts formed during diagenesisfrom abiogenic organic compounds generated inhydrothermal chert veins (Brasier et al 2002 VanKranendonk 2004)

Filaments believed to be biogenic in originhave also been reported from the 324 Ga Kan-garoo Caves Formation (Rasmussen 2000) wherethey form part of the Sulphur Springs deposit as-sociated with a black smoker hydrothermal ventsystem (Vearncombe et al 1995 1998) Ras-mussen (2000) argued that the filaments are rem-nants of a hydrothermal biota of thermophilicchemotrophic prokaryotes Sulfide textures areremarkably well preserved in these complex de-

ARCHEAN GEOLOGIC REFERENCE COLLECTION 751

posits and are worthy of much more detailedanalysis as they may well preserve evidence ofthe early biosphere

Finally there is evidence that hydrocarbons aretrapped in these ancient sedimentary rocks eitheras fluid inclusions in hydrothermal precipitates oras bituminous residues in larger cavities in theKangaroo Caves Formation and in the Lalla RookhSandstone (Dutkiewicz et al 1998 Rasmussen andBuick 2000) Buick et al (1998) also reported car-bonaceous globules in cherts from the lower War-rawoona Group In the Kangaroo Caves Forma-tion the hydrocarbons occur in hydrothermalbarite or within the sulfides of the volcanogenichydrothermal massive sulfide deposits that weresyndepositional with the formation (Rasmussenand Buick 2000) Textural information suggestthat these deposits were precipitated as blacksmoker chimneys in a submarine hydrothermalsetting driven by heat released from the intrudingStrelley Granite at approximately 324 Ga (Vearn-combe et al 1995) Fluorescing fluid inclusions oc-cur in quartz overgrowths in sandstones from theLalla Rookh Sandstone (Dutkiewicz et al 1998)Rasmussen and Buick (2000) and Dutkiewicz et al(1998) have argued for a biogenic origin for the hy-drocarbons however an abiotic origin involvingfor example Fischer-Tropsch-type synthesis ap-pears possible (Brasier et al 2002)

The Late Archean (and Early Paleoproterozoic)record preserved in the Hamersley and relatedbasins is stratigraphically much more coherentthan that preserved in the earlier successions TheHamersley Basin in particular provides a verycomprehensive record of the early oxygenation ofthe atmosphere in the form of large eustaticallycontrolled upward-shallowing sequences that in-clude black shales BIFs and ultimately at theirtops biogenic platform carbonates (Lindsay andBrasier 2002ab) These depositional cycles con-tain organic-rich shales at their base that recordevidence for some of Earthrsquos earliest biosphere(Brocks et al 1999) BIFs (Trendall 1983 1990)that record the rise of oxygen in the atmosphereand carbonates that document some of Earthrsquosearliest reef-like carbonate structures (Simonsonand Hassler 1996 1997 Simonson et al 1993ab)

SAMPLING STRATEGY

The Archean rocks of the Pilbara Craton arethus part of a protocontinent about which the

nascent Australian Craton developed It consistsof granitoid complexes overlain by a carapace ofsynchronously deposited volcano-sedimentarywedges Resting upon the early Archean proto-continent are a series of supracrustal basins Inall the sedimentary record of the Pilbara extendsfrom rocks as old as 354 Ga to some as young asapproximately 19 Ga As summarized in the fol-lowing paragraphs the sedimentary record of thePilbara records evidence of the major tectonic andgeological events that accompanied the evolutionof the Archean biosphere

It is here proposed that we develop a carefullydocumented and curated sample collection fromthis unique setting The collection would form thebasis for early Earth studies and a focus for astro-biology The sample set would be designed to re-flect the evolutionary succession described abovewith emphasis placed on cherts lower in the suc-cession and on clastic sediments higher in the suc-cession Any comprehensive reference collectionmust reflect diversity of the succession and includerocks from each of the sedimentary environmentsas well as igneous rocks reflecting the frameworkin which the protocontinent assembled

The five megasequences at the North Shawstudy area (Fig 3) are very different and reflectthe evolving Early Archean protocontinent Theearliest sedimentary record is limited and domi-nated by hydrothermally generated laminatedcherts and BIFs which appear as thin interbedswithin massive basalts (Coonterunah Group) (Fig5) Gradually clastic rocks appear dominated atfirst by volcanoclastics followed later by the firstevidence of sediments containing detrital quartzderived from true continental granitic crust Hy-drothermally generated laminated chert carbon-ate and barite units occur throughout the sectionThe first evidence of erosion appears at the top ofthe Coonterunah Group in the form of an angu-lar unconformity (Buick et al 1995b)

The basins that form the supracrustal succes-sion on the Pilbara Craton offer important in-sights into the closing phases of the Archean andthe immediate post-Archean world Because oftheir economic importance the basins are wellmapped and drill core is readily available (seeLindsay and Brasier 2002a) The succession pro-vides vital information about the early oxygena-tion of the atmosphere and the rapid growth ofthe biosphere that resulted from the assembly ofthe first continents and the initiation of the platetectonic cycle Platform carbonates whose geom-

LINDSAY ET AL752

etry was determined by eustasy are widespreadand contain considerable information on earlyoxygenic photosynthesis and the development ofthe first reef-like structure that are similar inmany ways to modern reefs (Simonson et al1993ab Lindsay and Brasier 2002ab)

The collection would consist of three groupingsof samples stratigraphic samples site-specificsamples and framework samples

Stratigraphic samples

The stratigraphic sample set would be collectedfrom well-defined settings such as carefully mea-sured and documented sections or from drill coreSamples would be collected from each of the fivestratigraphic groups or megasequences Thesupracrustal successions preserved in the Hamer-sley and associated basins would where possiblebe taken from existing drill core or from drill coresobtained for special projects Where drill core isnot available samples would be collected fromcarefully selected and measured outcrop sectionsThe samples would concentrate on rocks mostlikely to host biogenic material Samples from theEarly Archean volcaniclastic carapace would bedominated by cherts that are most likely to pre-serve fine biogenic structures but would also in-clude other sedimentary hydrothermal units suchas barite and carbonate that would be used in sta-ble isotope and geochemical studies

Site-specific sample

This sample set would focus on sites where po-tential biogenic structures have been documentedsuch as the stromatolitic intervals in the DresserFormation and the Strelley Pool Chert It wouldalso include detailed sampling of sites such asChinaman Creek and the Strelley Pool Chertwhere purported microfossils have been docu-mented (Schopf 1993) and the sulfide bodies inthe Kangaroo Caves Formation that contain pos-sible microfossils as well as evidence of hydro-carbons (Rasmussen 2000 Rasmussen and Buick2000) This would involve a broad samplingaround the sites in measured sections and drillcore to establish the geological context

Framework samples

While the focus of the collecting would be onmaterial that would help define the character of

the Earthrsquos earliest biosphere it is important thatthe broad context of the samples be defined TheArchean is a period when the Earthrsquos crust wasevolving rapidly and there is a growing body ofevidence to suggest that the evolution of theplanet drives the evolution of the atmosphere andbiosphere (Lindsay and Brasier 2002ab) Theframework sample set would include suites ofsample from the extrusive volcanic rocks col-lected along well-documented sections Sectionsthrough the volcanoclastic succession would em-phasize the dominant volcanic units (basalts ko-matiites andesites felsites and rhyolites) assources of information on the mantle and evolv-ing crust This sample set would also include adetailed sampling of the granitoid complexeswhich ultimately drove the entire process as wellas the later intrusions that were important in dri-ving the hydrothermal systems that may wellhave been the cradle of early life

SAMPLE CURATIONCHARACTERIZATION AND

DISTRIBUTION

All samples would be fully documented in thefield Samples would be photographed and theirlocations determined using a Global PositioningSystem receiver For specialized samples such asthose thought to contain easily contaminated bio-geochemical information the field collectionwould be modeled on techniques developed bythe NASANational Science FoundationSmith-sonian Institution Antarctic Search for Meteoritesprogram (Harvey and Schutt 1998)

Characterization

The petrologic and petrographic characteristicsof each sample would be determined using thin-section optical microscopy The quantitative bulkchemical composition of each sample would bemeasured by x-ray fluorescence spectrometry Fi-nally the qualitative bulk mineralogy of eachsample would be measured by powder x-ray dif-fraction

Data management

An Archean Geologic Reference Materials Data-base would be produced to fully document eachsample Field documentation would include sam-pling rationale supporting references and field

ARCHEAN GEOLOGIC REFERENCE COLLECTION 753

photographs Documentation would include theresults of all analyses along with images of rep-resentative sample surfaces and optical micro-graphs The Database would be produced in elec-tronic format and posted on the web Subsamplesof the Archean Geologic Reference Materialswould then be made available at no cost to re-searchers in the astrobiology community

DISCUSSION AND CONCLUSIONS

The Early Archean environment was uniqueRecent studies indicate that the atmosphere waslargely anoxic with oxygen levels 1025 of thepresent atmospheric level (Pavlov and Kasting2002) There were no large continents no activeor passive margins and by default no intracra-tonic basins Significantly perhaps there are nosigns of eustasy The crust was evolving rapidlyand heat flow was high leading to the intrusionof large granitoid bodies and the development ofa volcanic carapace above them In turn thismeant that the marine environment was domi-nated by hydrothermal activity

In the absence of large continental masses de-trital materials other than volcanoclastics are rel-atively rare in the Early Archean Significant vol-umes of quartz sand do not appear in the PilbaraCraton sedimentary record until after 32 Ga pre-sumably as the intruding igneous bodies begin tobe unroofed and eroded There are thus no ma-jor accumulations of sedimentary rocks that wenormally expect associated with passive conti-nental margins

With the assembly of the earliest large conti-nents beginning at approximately 30ndash28 Gathe Earthrsquos atmosphere hydrosphere and bios-phere changed rapidly Intracratonic basinssuch as the Hamersley Basin began to developon the new crustal blocks opening new shal-low-water environments that provided an idealsetting for photosynthesizing organisms andthe development of the first carbonate reefstructures (Lindsay and Brasier 2002ab) Hy-drothermal processes while still importantwere thus no longer the only sources of energyavailable to the evolving biosphere

The Archean was thus an unfamiliar environ-ment and unravelling the nature of the earliestbiosphere will require development of new ap-proaches and new techniques Without large con-tinental masses subareal exposures are likely to

be rare and short-lived There is no evidence forstream activity until the De Grey group at ap-proximately 29 Ga at which time braided streamgravels were preserved Thus models involvingearly biospheric evolution in ldquoa warm little pondrdquo(Darwin 1888) or in fresh water seem unlikelyInstead the focus in the search for the origins oflife on Earth should be directed to hydrothermalmarine settings involving high temperatures andwhat would today be hostile geochemical envi-ronments with significant concentrations of metal-lic ions and a largely reducing environment Therisks for the misidentification of abiotic structuresand geochemical signatures as signs of ancient lifeare significant as seen in recent discussions (egBrasier et al 2002 Fedo and Whitehouse 2002Schopf et al 2002)

These difficulties suggest that much could begained by the development of a well-documentedcollection of Archean rocks that could be madeavailable to all qualified investigators Until weunderstand the earliest stages of the evolution oflife on Earth we cannot hope to make significantprogress in the search for life on other planetssuch as Mars Until there is a sample return mis-sion to Mars we must make do with meteoritesthat come to us largely out of context An under-standing of the Archean Earth can help placethose limited samples in a planetary context

ACKNOWLEDGMENTS

Franco Pirajno Kath Grey and Martin VanKranendonk provided generous advice and as-sistance and the Geological Survey of WesternAustralia supported the field program We aregrateful to Peter Morant of Sipa Resources andAdrian Black of Newexco Perth for their adviceand for access to drill-core materials

ABBREVIATION

BIF banded iron formation

REFERENCES

Arndt NT Nelson DR Compston W Trendall AFand Thorne AM (1991) The age of the FortescueGroup Hamersley Basin Western Australia from ionmicroprobe zircon U-Pb results Aust J Earth Sci 38262ndash281

LINDSAY ET AL754

Awramik SM Schopf JW and Walter MR (1983) Fil-amentous fossil bacteria from the Archaean of WesternAustralia Precambrian Res 20 357ndash374

Barley ME Pickard AL and Sylvester PJ (1997) Em-placement of a large igneous province as a possiblecause of banded iron formation 245 billion years agoNature 385 55ndash58

Bettenay LF Bickle MJ Boulter CA Groves DIMorant P Blake TS and James BA (1981) Evolu-tion of the Shaw Batholithmdashan Archaean granitoid-gneiss dome in the eastern Pilbara Western AustraliaIn Special Publication 7 The Archaean edited by JEGlover and DI Groves Geological Society of WesternAustralia Perth pp 361ndash372

Blake TS and Barley ME (1992) Tectonic evolution ofthe Late Archaean to Early Proterozoic Mount BruceMegasequence Set Western Australia Tectonics 11 1415ndash1425

Blake TS and Groves DI (1987) Continental rifting andthe Archaean-Proterozoic transition Geology 15 229ndash232

Brasier MD and Lindsay JF (1998) A billion years ofenvironmental stability and the emergence of eukary-otes new data from Northern Australia Geology 26555ndash558

Brasier MD Green OR Jephcoat AP Kleppe AKVan Kranendonk MJ Lindsay JF Steele A andGrassineau NV (2002) Questioning the evidence forEarthrsquos oldest fossils Nature 416 76ndash81

Brauhart C (1999) Regional alteration systems associatedwith Archaean volcanogenic massive sulphide depositsat Panorama Pilbara Western Australia [PhD Thesis]University of Western Australia Perth

Brauhart C Groves DI and Morant P (1998) Regionalalteration systems associated with volcanogenic mas-sive sulfide mineralization at Panorama Pilbara West-ern Australia Econ Geol 93 292ndash302

Brocks J Logan GA Buick R and Summons RE(1999) Archean molecular fossils and the early rise ofeukaryotes Science 285 1033ndash1036

Buick R Dunlop JSR and Groves DI (1981) Stroma-tolite recognition in ancient rocks an appraisal of ir-regular laminated structures in an early Archaeanchert-barite unit from North Pole Western AustraliaAlcheringa 5 161ndash181

Buick R Des Marais DJ and Knoll AH (1995a) Sta-ble isotope compositions of carbonates from the Meso-proterozoic Bangemall Group northwestern AustraliaChem Geol 123 153ndash171

Buick R Thornett JR McNaughton NJ Smith JBBarley ME and Savage M (1995b) Record of emer-gent continental crust 35 billion years ago in the Pil-bara Craton of Australia Nature 375 574ndash577

Buick R Rasmussen B and Krapez B (1998) Archaeanoil evidence for extensive hydrocarbon generation andmigration 25ndash35 Ga Am Assoc Petroleum Geol Bull82 50ndash69

Chan M (1998) Sedimentology and structure of threesmall congolmeratic basins in the Lalla Rookh-WesternShaw structural corridor Pilbara Craton Western Aus-

tralia [BSc Honours Thesis] University of WesternAustralia Perth

Cullers RL DiMarco MJ Lowe DR and Stone J(1993) Geochemistry of a silicified felsic volcaniclasticsuite from the Early Archaean Panorama FormationPilbara Block Western Australiamdashan evaluation of de-positional and post-depositional processes with specialemphasis on the rare-Earth elements Precambrian Res60 99ndash116

Darwin F ed (1888) The Life and Letters of Charles Dar-win Vol 3 John Murray London

de Wit MJ (1998) On Archaean granites greenstonescratons and tectonics does the evidence demand a ver-dict Precambrian Geol 91 181ndash226

Drummond BJ (1981) Crustal structure of the Precam-brian terrains of northwest Australia from seismic re-fraction data BMR J Geol Geophys 6 123ndash135

Dunlop JSR Muir M Milne VA and Groves DI(1978) A new microfossil assemblage from the Ar-chaean of Western Australia Nature 274 676ndash678

Dutkiewicz A Rasmussen B and Buick R (1998) Oilpreserved in fluid inclusions in Archaean sandstonesNature 395 885ndash888

Fedo CM and Whitehouse MJ (2002) Metasomatic ori-gin of quartz-pyroxene rock Akilia Greenland and im-plications for Earthrsquos earliest life Science 296 1448ndash1452

Gee RD and Grey K (1993) Report 41 Proterozoic Rockson the Glengarry 1250 000 SheetmdashStratigraphy Structureand Stromatolite Biostratigraphy Geological Survey ofWestern Australia Perth

Glikson AY and Hickman AH (1981a) Chemicalstratigraphy and petrogenesis of Archaean basic-ultra-basic volcanic units eastern Pilbara Block WesternAustralia In Special Publication 7 Archaean Geologyedited by JE Groves and DI Groves Geological Soci-ety of Western Australia Perth pp 287ndash300

Glikson AY and Hickman AH (1981b) Record 198136Geochemistry of Archaean Volcanic Successions EasternPilbara Block Western Australia Australian Bureau ofMineral Resources Canberra

Glikson AY Davy R and Hickman AH (1991) Record199146 Trace Metal Distribution Patterns in Basalts Pil-bara Craton Western Australia with Stratigraphic-Geo-chemical Implications Australian Bureau of Mineral Re-sources Canberra

Groves DI Dunlop JSR and Buick R (1981) An earlyhabitat of life Sci Am 245 64ndash73

Harvey RP and Schutt JW (1998) Meteorite recoveryand reconnaissance in the Allan Hills-David Glacierand Darwin Glacier regions 1996ndash1997 Antarctic J US 32 25ndash27

Hickman AH (1975) Annual Report 1974 PrecambrianStructural Geology of Part of the Pilbara Region Geologi-cal Survey of Western Australia Perth pp 68ndash73

Hickman AH (1983) Bulletin 127 Geology of the PilbaraBlock and Its Environs Geological Survey of WesternAustralia Perth

Hickman AH (1984) Archaean diapirism in the PilbaraBlock Western Australia In Precambrian Tectonics Il-lustrated edited by A Kroner and R Greiling E

ARCHEAN GEOLOGIC REFERENCE COLLECTION 755

terbedded with units of chert sandstone con-glomerate barite and minor carbonate (Fig 7)The individual chert-barite beds vary from 5 to40 m in thickness Associated with these unusualsedimentary facies are structures that have beeninterpreted as the worldrsquos oldest stromatolites(Fig 8) and associated microfossils (Dunlop et al1978 Walter et al 1980 Buick et al 1981 Groveset al 1981 Walter 1983) It is perhaps significantthat this distinctive unit is fed by a deep-seatedsystem of hydrothermal dykes (Fig 9)

Duffer Formation The Duffer Formation is bestexposed in the southeastern portion of the studyarea where it consists largely of felsic schist andagglomerate The agglomerates which have been

locally interpreted as welded ignimbrites arebedded in meter-scale units The Duffer Forma-tion has a depositional age of approximately 347Ga (Thorpe et al 1992 McNaughton et al 1993Nelson 1997 1998 1999 2000 2001) and restsconformably on the Mount Ada Basalt

Apex Basalt The Apex Basalt is well exposed inthe northeastern corner of the study area and overlarge areas on adjacent map sheets The forma-tion is dominated by pillowed basalts which arelargely tholeiitic The pillows are well developedand preserve vesicular rims chilled margins andconcentric cooling cracks Hyaloclastic brecciasare also present within the assemblage Doleriticflows and intrusions are common and doleriticdykes occur locally

Interbedded with the volcanics are a numberof chert horizons which are 10 m in thickness

ARCHEAN GEOLOGIC REFERENCE COLLECTION 745

FIG 5 A measured section through the Coucal For-mation (locality A Fig 2) The formation is dominatedby basic volcanic rocks with some felsic volcanic rocksand thin (10 m) hydrothermal cherts between flows

FIG 6 Laminated cherts from the lower Coucal For-mation (locality A Fig 2) Cherts form the earliest sedi-mentary rocks in the Pilbara succession Some units areas much as 10 m thick Scale bar 5 10 cm

and usually well laminated mainly black andwhite chert Numerous chert dykes cut across theunderlying stratigraphy and terminate at thelevel of the chert horizons Close to Marble Barexposures of the Apex chert contain what was ini-tially believed to be Earthrsquos oldest morphologicalfossil assemblage (Fig 10) (Schopf and Packer1987 Schopf 1992 1993) a claim recently dis-puted by more detailed analysis (Brasier et al2002) (see following section on Paleontology)

Panorama Formation The Panorama Formation(3458ndash3434 Ma) which is best exposed in thenorthern part of the study area near the ShawRiver is dominated by felsic volcaniclastic rocks

with small volumes of felsic lavas and some chertinterbeds (Hickman 1983 Thorpe et al 1992 Nel-son 2000) The formation is strongly altered andintensely silicified having been exposed to tem-peratures as high as 350degC (Cullers et al 1993)A large volcanic vent within the Panorama For-mation is preserved in the northwestern part ofthe outcrop area (Van Kranendonk 2000)

Strelley Pool Chert The relationship of the Strel-ley Pool Chert (Lowe 1983) to the underlyingunits varies from conformable to unconformableIn the northeast the contact with the underlyingCoonterunah Group forms an angular unconfor-mity with pronounced paleorelief (Buick et al1995b) The formation consists of carbonate sili-cified carbonate and clastic rocks (Fig 11) Thecherts and carbonates both have well-developedsedimentary structure that have been describedas wavy laminated cherts or carbonates and stro-matolites (Fig 12) (Lowe 1983 Hofmann et al1999) although as discussed in a following sec-tion there is some concern as to whether they arebiogenic (Lowe 1994 Lindsay et al 2003ab)

Euro Basalt The Euro Basalt consists domi-nantly of basalts of tholeiitic and high-Mg com-position that rest conformably on the StrelleyPool Chert (Van Kranendonk 2000) The forma-tion reaches a thickness in excess of 9 km in thenortheastern part of the study area The basaltsare pillowed and bedded on a kilometer scaleThe formation also includes some komatiite andfelsic volcaniclastics as well as thin cherts andvolcaniclastic units

Megasequence 3mdashSulphur Springs Group

The Sulphur Springs Group consists of threeformations (Fig 3) that in contrast to the previ-ous two Groups contain a significant proportionof clastic sedimentary rocks some quartzose

Leilira Formation The Leilira Formation consistspredominantly of clastic sedimentary rocks withonly small volumes of felsic to intermediate ex-trusive rocks interbedded The clastic rocks arepredominantly sandstones but vary from silt-stone to coarser pebbly sandstone Composition-ally the sandstones are quartzose or volcaniclithic rocks Locally laminated cherts formed bythe silicification of felsic volcanic ash and silt-stone are abundant

LINDSAY ET AL746

FIG 7 Bedded sedimentary carbonate and silica fromthe Dresser Formation (locality B Fig 2) Scale bar 5 5cm The rock consists of well-bedded units that begin withorangeyellow iron-rich carbonates and grade upward topale green silica The unit reflects geochemical changes inresponse to pulsing or cycling of the hydrothermal sys-tem Elsewhere carbonate and chert form alternating beds

The Leilira Formation represents a significantchange in depositional patterns as it is the oldestunit in which clastic sediments are the dominantlithology It reflects the growing volcanic cara-pace and in particular the exposure of the grani-toid core to weathering and erosion the first signsthat a true crust was developing

Kunagunarrina Formation The KunagunarrinaFormation reaches a maximum of 24 km inthickness and consists predominantly of high-Mg basalts and komatiites Locally andesitic andfelsic units are present especially near the top ofthe formation A thick chert unit (10 m) in themiddle of the formation separates lower Mg

ARCHEAN GEOLOGIC REFERENCE COLLECTION 747

FIG 8 Stromatolitic structures in theDresser Formation (locality B Fig 2)The original barite has been largely re-placed by chert Scale bar 5 10 cm

FIG 9 Exposures of the Dresser Formation in the North Pole Dome area (locality B Fig 2) showing large chert-barite feeder dykes The dykes consist of black chert and coarse bladed barite Note the vehicle for scale

basalts at the base of the formation from ko-matiites towards the top

Kangaroo Caves Formation The Kangaroo CavesFormation which reaches a maximum thicknessof 15 km is best exposed towards the center ofthe study area The formation is complex and con-sists of a succession of andesitic volcanics anddacitic to rhyolitic sills and extrusives A chert in-terval up to 100 m thick forms a distinctivemarker at the top of the formation Close to thecenter of the study area at Sulphur Springs themassive chert is overlain by a polymict megabrec-cia that rests upon a sharp erosional contact (Hill1997 Vearncombe et al 1998)

Hosted within a 200-m dacite unit immediatelybeneath the marker chert are six sulfide ore bodiesspaced at regular intervals of 5ndash7 km around themargins of the Strelley Granite (Morant 1998) Theyconsist of pyrite sphalerite chalcopyrite and galenawith smaller amounts of other sulfides (Vearncombeet al 1995 1998 Vearncombe 1996 Morant 1998)The sulfides which were deposited in a blacksmoker hydrothermal setting occur in associationwith growth faults developed around the StrelleyGranite the intrusion of which was syndepositionalwith the Kangaroo Caves Formation (Vearncombe1996 Brauhart et al 1998 Brauhart 1999)

Megasequence 4mdashGorge Creek Group

The Gorge Creek Group consists of five for-mations (Fig 3) that show much greater litho-logic diversity than earlier megasequences Inparticular this interval includes for the firsttime a significant thickness of quartzose sand-stone as well as a variety of other clastic sedi-mentary rocks

Pincunah Hill Formation In the western part ofthe study area the basal Pincunah Hill Formationconsists of thinly bedded BIF with small amountsof laminated chert red to gray shale and silt-stone This basal unit is up to 1 km thick in thewestern part of the area but wedges out and dis-appears in a short distance eastward The upper

LINDSAY ET AL748

FIG 11 A measured section through the Strelley PoolChert showing the location of stromatolitic structuresand the carbonate interval (adapted from Van Kranen-donk 2000) (locality C Fig 2) The gamma log to theright shows continuity across the chert-carbonate

FIG 10 ldquoMicrofossilsrdquo from the Apex chert near Mar-ble Bar a computer-generated digital automontage ofputative beggiatoan Eoleptonema apex Holotype (Schopf1993) combining the most-sharply focused images fromsuccessive focal planes b and c Original manual pho-tomontage and interpretative sketch (Schopf 1993) whichomits lower structure at arrow in (b) d and e New sin-gle images showing continuity of original and newly im-aged and automontaged structures Reprinted with per-mission from Nature (Brasier et al 2002) Copyright 2002Macmillan Publishers Ltd

part of the formation consists largely of red orpurple shale (Van Kranendonk 2000)

Corboy Formation At its thickest in the easternpart of the study area the Corboy Formation in-cludes a number of distinct facies developedacross major growth faults (Wilhelmij and Dun-lop 1984 Wilhelmij 1986) Here the formationconsists of a basal quartzose sandstone abruptlyoverlain by conglomerate interbedded withgraded sandstone units red shale and coarsesandstone The formation is capped by massivesandstone and conglomerate The unit was de-posited as a series of small prograding shallowmarine fan deltas (Wilhelmij 1986 Hill 1997 VanKranendonk 2000)

Paddy Market Formation In the western part ofthe study area the Paddy Market Formation isdominantly cherty BIF with minor ferruginousshale Further east the formation becomes moreshale-rich although much of the shale has been re-placed by chert Sandstone is present in smallamounts in beds up to 10 m thick Towards thetop of the formation in the east the cherts containfelsic volcanic material that is overlain by up to 200m of massive dacite (Van Kranendonk 2000)

Honeyeater Basalt The Honeyeater Basaltwhich consists entirely of basalt and dolerite con-formably overlies the Paddy Market FormationThe lower part of the formation consists of high-Mg basalt interbedded with dolerite whereashigher in the formation the basalts are largelytholeiitic and interbedded with dolerite (Gliksonand Hickman 1981ab Glikson et al 1991)

Pyramid Hill Formation The Pyramid Hill For-mation is limited in areal extent It is found in thenorth central part of the study area east of theStrelley Granite The formation consists of redand black shales that when heavily silicifiedgrade into laminated black and white chert (VanKranendonk 2000) The Gorge Creek Group iscapped by a well-bedded BIF

Megasequence 5mdashDe Grey Group

The De Grey Group consists of a single for-mation the Lalla Rookh Sandstone (Fig 3)which is dominated by clastic sedimentary rocksthat contain a significant quartzose componentThe Lalla Rookh Sandstone which rests on awell-defined unconformity above the GorgeCreek Group reaches a maximum thickness of

ARCHEAN GEOLOGIC REFERENCE COLLECTION 749

FIG 12 Stromatolitic carbonates in the Strelley Pool Chert (locality C Fig 2) Laminae are isopachous and theapex of the cone is ruptured and veined by secondary carbonate (arrow) Scale bar 5 10 cm

3 km (Krapez 1984) In the north central part ofthe study area the formation consists entirely ofcoarse clastic sediments and minor shalewhereas to the south the predominate fill is lithicand feldspathic sandstone (Chan 1998) Theserocks provide the first clear evidence of streamactivity in the Pilbara succession (Krapez 1984Van Kranendonk and Collins 1998 Van Kra-nendonk 2000)

THE PILBARA SUPRACRUSTAL SUCCESSION

The late Archean and early Paleoproterozoicbasins that rest upon the ancient cratonic blocksof Western Australia (Fig 1) form a time seriesassociated with the formation and ultimate dis-assembly of one of Earthrsquos earliest major conti-nental masses This was followed by the suturingof the Pilbara and Yilgarn Cratons along theCapricorn Orogen as a new supercontinentevolved in the Paleoproterozoic (Rogers and San-tosh 2002) These basins contain an importantsedimentary record of the early Earth includingsome of Earthrsquos earliest stromatolites and plat-form carbonate deposits (Simonson et al1993ab) organic-rich black shales (Brocks et al1999) and BIF (Trendall 1983 1990) The strati-graphic and structural framework is well con-strained by radiometric ages (Arndt et al 1991Barley et al 1997 Trendall et al 1998) providinga record at a critical time in Earth history whenthe atmosphere was first becoming oxygen-rich(Lindsay and Brasier 2002ab)

The Late Archean-Early PaleoproterozoicHamersley Basin which lies close to the southernmargin of the Pilbara Block (Trendall 1983 1990Blake and Barley 1992) overlies a normal crustalthickness (30ndash40 km) (Drummond 1981) Thebasin began to subside at approximately 28 Gafollowing an episode of crustal extension The ar-chitecture of the basin fill is complex andpolyphase with major erosional surfaces separat-ing areally extensive megasequencessuperse-quences (Trendall 1990 Krapez 1996) DuringFortescue and Hamersley Group time (Fig 4)sedimentation began in a rift setting in a silici-clastic-starved environment The later stages ofHamersley Basin sedimentation (Turee CreekGroup times) record the early transition from apassive margin to a foreland basin setting andwith it the establishment of a terrigenous sedi-

ment supply (Tyler and Thorne 1990) The suc-cession is largely marine Evidence of eustasy ap-pears for the first time in the sedimentary suc-cession of the Hamersley Basin (Lindsay andBrasier 2002ab)

The geometry and sequence stacking pattern ofthe succession are both very similar to youngerintracratonic basins (see Lindsay et al 1993) TheHamersley Basin is thus the first basinal settingpreserved on the Australian craton in which ma-rine sediments and in particular platform car-bonate rocks have been preserved in a responseto broad regional crustal subsidence and wheredeposition is controlled by eustasy

The Ashburton Basin (22ndash18 Ga) discon-formably overlies the southern margin of theHamersley Basin along its the contact with theCapricorn Orogen (Fig 1) (Thorne and Seymour1991) The basin forms the northern margin of theCapricorn Orogen a deformed zone that devel-oped during ocean closure Loading of the oceanfloor by the approaching Yilgarn Craton led tothe development of a westndashnorthwest-orientedforeland basin [McGrath Trough (Horowitz1982)] with an uplifted orogenic margin to thesouthwest (Blake and Groves 1987 Blake andBarley 1992) with the result that sedimentationwas dominated by an abundant supply of ter-rigenous clastic material (Mount McGrath For-mation) Continued loading of the crust ulti-mately led to the supply of terrigenous materialsbeing disrupted and producing a prograding car-bonate shelf (Duck Creek Dolomite) Basin sedi-mentation was accompanied by active-marginmafic volcanism (Cheela Springs Basalt)

THE CAPRICORN OROGEN ANDASSOCIATED BASINS

The Yerrida (22ndash19 Ga) Bryah (20 Ga)Padbury (20 Ga) and Earaheedy (19ndash165Ga) Basins lie to the southeast of the Hamersleyand Ashburton Basins and overlie the CapricornOrogen (Fig 1) (Occhipinti et al 1997 1998 Pi-rajno et al 1998 Pirajno and Adamides 2000)These now fragmentary basins formed along thenorthern margin of the Yilgarn Craton in a back-arc setting and record the convergence and colli-sion of the Archean Pilbara and Yilgarn Cratons(Tyler and Thorne 1990 Thorne and Seymour1991 Occhipinti et al 1997 1998 Pirajno et al1998) Because of their active margin settings the

LINDSAY ET AL750

fill of these small basins is dominated by clasticsediments However platform carbonate unitsare preserved in the Yerrida Basin (Gee and Grey1993 Pirajno and Adamides 2000) and a signif-icant thickness of carbonate rocks also occurs inthe Earaheedy Basin

THE ARCHEAN BIOSPHERE

The end of the Archean is formally defined at25 Ga (Plumb 1991) In reality the Archean worldended at some point in time closer to 28 or 30 Gawhen the first large continental blocks began to as-semble The stratigraphic record preserved on theancient Australian Craton reflects this major tran-sition The rock record from the Early Archean isvery different from that of the Late Archean in thatthe early record is predominantly volcanic and hy-drothermal in nature (ldquogreenstonerdquo) while thelater record is largely clastic and more readily com-pared with the Phanerozoic record

Structures and textures of possible biogenicorigin that are preserved in the stratigraphic suc-cession in the Early Archean study area are wellknown and are believed to provide the earliestdirect evidence of Earthrsquos biosphere Stromatoliticstructures have been encountered at two levelswithin the section The Dresser Formation con-tains structures preserved in chert and barite in-tervals that have been interpreted as stromatolitesand oncolites (Dunlop et al 1978 Walter et al1980 Buick et al 1981 Groves et al 1981 Wal-ter 1983) The stromatolitic structures occur forthe most part in layered barite units that devel-oped over swarms of synvolcanic hydrothermalfeeder dykes along listric growth faults (Nijmanet al 1998 Van Kranendonk 2004) Although ithas been argued that these structures are biogenic(Walter et al 1980 Schopf and Walter 1983 Wal-ter 1983) their direct association with the feederdykes leaves room for doubt (Buick et al 1981)The setting is complex and there is considerablescope for more detailed contextual research

The second stromatolitic horizon occurs in theoverlying Strelley Pool Chert a complex unit con-sisting of at least five members (Lowe 1980 1983Hofmann et al 1999) The main stromatolitic unitoccurs as an 8-m-thick interval in the middle ofthe formation and appears to have a consistentand widespread pattern of deposition The struc-tures have been the focus of considerable discus-sion and their biogenicity has been questioned

(Buick et al 1981 Lowe 1995) The structures arecomplex and varied (Fig 12) leading to a morerecent re-evaluation again suggesting that theyare biogenic (Hofmann et al 1999) However thelaminae are largely isopachous an indication thatthey could be abiotic precipitates (cf Pope andGrotzinger 2000) Given the intense hydrother-mal activity that appears to have taken placethroughout the depositional time interval thestructure may well be the product of hydrother-mal deposition (Lindsay et al 2003ab) In all ourobservations suggest that considerably morework needs to be done to establish the deposi-tional context of these stromatolitic structures be-fore they can be confidently ascribed as biogenicin origin

Structures resembling remarkably preservedbacterial and cyanobacterial microfossils havebeen found at several localities in and close to thestudy area (Awramik et al 1983 Schopf andPacker 1987 Schopf 1992 1993 Rasmussen 2000)A complex biota of 11 species of filamentousprokaryotes which are compared with cyanobac-teria (Schopf and Packer 1987 Schopf 1992 1993)have been described from the Apex chert close toMarble Bar (21deg105579S 119deg418789E) The taxo-nomic interpretations are important in that theyprovide the earliest evidence of morphological lifeon Earth and support the hypothesis of an earlybeginning for oxygenic photosynthesis (Schopf1999) The structures are almost a billion yearsolder than the oldest known putative cyanobac-terial biomarkers (Summons et al 1999) and oc-cur well before the earliest evidence for the risein oxygen in the atmosphere (for a summary seeKnoll and Canfield 1998 Lindsay and Brasier2000 2002ab) However in a recent re-evaluationof the original Apex chert material it has been ar-gued that the filaments are not biogenic but aresecondary artifacts formed during diagenesisfrom abiogenic organic compounds generated inhydrothermal chert veins (Brasier et al 2002 VanKranendonk 2004)

Filaments believed to be biogenic in originhave also been reported from the 324 Ga Kan-garoo Caves Formation (Rasmussen 2000) wherethey form part of the Sulphur Springs deposit as-sociated with a black smoker hydrothermal ventsystem (Vearncombe et al 1995 1998) Ras-mussen (2000) argued that the filaments are rem-nants of a hydrothermal biota of thermophilicchemotrophic prokaryotes Sulfide textures areremarkably well preserved in these complex de-

ARCHEAN GEOLOGIC REFERENCE COLLECTION 751

posits and are worthy of much more detailedanalysis as they may well preserve evidence ofthe early biosphere

Finally there is evidence that hydrocarbons aretrapped in these ancient sedimentary rocks eitheras fluid inclusions in hydrothermal precipitates oras bituminous residues in larger cavities in theKangaroo Caves Formation and in the Lalla RookhSandstone (Dutkiewicz et al 1998 Rasmussen andBuick 2000) Buick et al (1998) also reported car-bonaceous globules in cherts from the lower War-rawoona Group In the Kangaroo Caves Forma-tion the hydrocarbons occur in hydrothermalbarite or within the sulfides of the volcanogenichydrothermal massive sulfide deposits that weresyndepositional with the formation (Rasmussenand Buick 2000) Textural information suggestthat these deposits were precipitated as blacksmoker chimneys in a submarine hydrothermalsetting driven by heat released from the intrudingStrelley Granite at approximately 324 Ga (Vearn-combe et al 1995) Fluorescing fluid inclusions oc-cur in quartz overgrowths in sandstones from theLalla Rookh Sandstone (Dutkiewicz et al 1998)Rasmussen and Buick (2000) and Dutkiewicz et al(1998) have argued for a biogenic origin for the hy-drocarbons however an abiotic origin involvingfor example Fischer-Tropsch-type synthesis ap-pears possible (Brasier et al 2002)

The Late Archean (and Early Paleoproterozoic)record preserved in the Hamersley and relatedbasins is stratigraphically much more coherentthan that preserved in the earlier successions TheHamersley Basin in particular provides a verycomprehensive record of the early oxygenation ofthe atmosphere in the form of large eustaticallycontrolled upward-shallowing sequences that in-clude black shales BIFs and ultimately at theirtops biogenic platform carbonates (Lindsay andBrasier 2002ab) These depositional cycles con-tain organic-rich shales at their base that recordevidence for some of Earthrsquos earliest biosphere(Brocks et al 1999) BIFs (Trendall 1983 1990)that record the rise of oxygen in the atmosphereand carbonates that document some of Earthrsquosearliest reef-like carbonate structures (Simonsonand Hassler 1996 1997 Simonson et al 1993ab)

SAMPLING STRATEGY

The Archean rocks of the Pilbara Craton arethus part of a protocontinent about which the

nascent Australian Craton developed It consistsof granitoid complexes overlain by a carapace ofsynchronously deposited volcano-sedimentarywedges Resting upon the early Archean proto-continent are a series of supracrustal basins Inall the sedimentary record of the Pilbara extendsfrom rocks as old as 354 Ga to some as young asapproximately 19 Ga As summarized in the fol-lowing paragraphs the sedimentary record of thePilbara records evidence of the major tectonic andgeological events that accompanied the evolutionof the Archean biosphere

It is here proposed that we develop a carefullydocumented and curated sample collection fromthis unique setting The collection would form thebasis for early Earth studies and a focus for astro-biology The sample set would be designed to re-flect the evolutionary succession described abovewith emphasis placed on cherts lower in the suc-cession and on clastic sediments higher in the suc-cession Any comprehensive reference collectionmust reflect diversity of the succession and includerocks from each of the sedimentary environmentsas well as igneous rocks reflecting the frameworkin which the protocontinent assembled

The five megasequences at the North Shawstudy area (Fig 3) are very different and reflectthe evolving Early Archean protocontinent Theearliest sedimentary record is limited and domi-nated by hydrothermally generated laminatedcherts and BIFs which appear as thin interbedswithin massive basalts (Coonterunah Group) (Fig5) Gradually clastic rocks appear dominated atfirst by volcanoclastics followed later by the firstevidence of sediments containing detrital quartzderived from true continental granitic crust Hy-drothermally generated laminated chert carbon-ate and barite units occur throughout the sectionThe first evidence of erosion appears at the top ofthe Coonterunah Group in the form of an angu-lar unconformity (Buick et al 1995b)

The basins that form the supracrustal succes-sion on the Pilbara Craton offer important in-sights into the closing phases of the Archean andthe immediate post-Archean world Because oftheir economic importance the basins are wellmapped and drill core is readily available (seeLindsay and Brasier 2002a) The succession pro-vides vital information about the early oxygena-tion of the atmosphere and the rapid growth ofthe biosphere that resulted from the assembly ofthe first continents and the initiation of the platetectonic cycle Platform carbonates whose geom-

LINDSAY ET AL752

etry was determined by eustasy are widespreadand contain considerable information on earlyoxygenic photosynthesis and the development ofthe first reef-like structure that are similar inmany ways to modern reefs (Simonson et al1993ab Lindsay and Brasier 2002ab)

The collection would consist of three groupingsof samples stratigraphic samples site-specificsamples and framework samples

Stratigraphic samples

The stratigraphic sample set would be collectedfrom well-defined settings such as carefully mea-sured and documented sections or from drill coreSamples would be collected from each of the fivestratigraphic groups or megasequences Thesupracrustal successions preserved in the Hamer-sley and associated basins would where possiblebe taken from existing drill core or from drill coresobtained for special projects Where drill core isnot available samples would be collected fromcarefully selected and measured outcrop sectionsThe samples would concentrate on rocks mostlikely to host biogenic material Samples from theEarly Archean volcaniclastic carapace would bedominated by cherts that are most likely to pre-serve fine biogenic structures but would also in-clude other sedimentary hydrothermal units suchas barite and carbonate that would be used in sta-ble isotope and geochemical studies

Site-specific sample

This sample set would focus on sites where po-tential biogenic structures have been documentedsuch as the stromatolitic intervals in the DresserFormation and the Strelley Pool Chert It wouldalso include detailed sampling of sites such asChinaman Creek and the Strelley Pool Chertwhere purported microfossils have been docu-mented (Schopf 1993) and the sulfide bodies inthe Kangaroo Caves Formation that contain pos-sible microfossils as well as evidence of hydro-carbons (Rasmussen 2000 Rasmussen and Buick2000) This would involve a broad samplingaround the sites in measured sections and drillcore to establish the geological context

Framework samples

While the focus of the collecting would be onmaterial that would help define the character of

the Earthrsquos earliest biosphere it is important thatthe broad context of the samples be defined TheArchean is a period when the Earthrsquos crust wasevolving rapidly and there is a growing body ofevidence to suggest that the evolution of theplanet drives the evolution of the atmosphere andbiosphere (Lindsay and Brasier 2002ab) Theframework sample set would include suites ofsample from the extrusive volcanic rocks col-lected along well-documented sections Sectionsthrough the volcanoclastic succession would em-phasize the dominant volcanic units (basalts ko-matiites andesites felsites and rhyolites) assources of information on the mantle and evolv-ing crust This sample set would also include adetailed sampling of the granitoid complexeswhich ultimately drove the entire process as wellas the later intrusions that were important in dri-ving the hydrothermal systems that may wellhave been the cradle of early life

SAMPLE CURATIONCHARACTERIZATION AND

DISTRIBUTION

All samples would be fully documented in thefield Samples would be photographed and theirlocations determined using a Global PositioningSystem receiver For specialized samples such asthose thought to contain easily contaminated bio-geochemical information the field collectionwould be modeled on techniques developed bythe NASANational Science FoundationSmith-sonian Institution Antarctic Search for Meteoritesprogram (Harvey and Schutt 1998)

Characterization

The petrologic and petrographic characteristicsof each sample would be determined using thin-section optical microscopy The quantitative bulkchemical composition of each sample would bemeasured by x-ray fluorescence spectrometry Fi-nally the qualitative bulk mineralogy of eachsample would be measured by powder x-ray dif-fraction

Data management

An Archean Geologic Reference Materials Data-base would be produced to fully document eachsample Field documentation would include sam-pling rationale supporting references and field

ARCHEAN GEOLOGIC REFERENCE COLLECTION 753

photographs Documentation would include theresults of all analyses along with images of rep-resentative sample surfaces and optical micro-graphs The Database would be produced in elec-tronic format and posted on the web Subsamplesof the Archean Geologic Reference Materialswould then be made available at no cost to re-searchers in the astrobiology community

DISCUSSION AND CONCLUSIONS

The Early Archean environment was uniqueRecent studies indicate that the atmosphere waslargely anoxic with oxygen levels 1025 of thepresent atmospheric level (Pavlov and Kasting2002) There were no large continents no activeor passive margins and by default no intracra-tonic basins Significantly perhaps there are nosigns of eustasy The crust was evolving rapidlyand heat flow was high leading to the intrusionof large granitoid bodies and the development ofa volcanic carapace above them In turn thismeant that the marine environment was domi-nated by hydrothermal activity

In the absence of large continental masses de-trital materials other than volcanoclastics are rel-atively rare in the Early Archean Significant vol-umes of quartz sand do not appear in the PilbaraCraton sedimentary record until after 32 Ga pre-sumably as the intruding igneous bodies begin tobe unroofed and eroded There are thus no ma-jor accumulations of sedimentary rocks that wenormally expect associated with passive conti-nental margins

With the assembly of the earliest large conti-nents beginning at approximately 30ndash28 Gathe Earthrsquos atmosphere hydrosphere and bios-phere changed rapidly Intracratonic basinssuch as the Hamersley Basin began to developon the new crustal blocks opening new shal-low-water environments that provided an idealsetting for photosynthesizing organisms andthe development of the first carbonate reefstructures (Lindsay and Brasier 2002ab) Hy-drothermal processes while still importantwere thus no longer the only sources of energyavailable to the evolving biosphere

The Archean was thus an unfamiliar environ-ment and unravelling the nature of the earliestbiosphere will require development of new ap-proaches and new techniques Without large con-tinental masses subareal exposures are likely to

be rare and short-lived There is no evidence forstream activity until the De Grey group at ap-proximately 29 Ga at which time braided streamgravels were preserved Thus models involvingearly biospheric evolution in ldquoa warm little pondrdquo(Darwin 1888) or in fresh water seem unlikelyInstead the focus in the search for the origins oflife on Earth should be directed to hydrothermalmarine settings involving high temperatures andwhat would today be hostile geochemical envi-ronments with significant concentrations of metal-lic ions and a largely reducing environment Therisks for the misidentification of abiotic structuresand geochemical signatures as signs of ancient lifeare significant as seen in recent discussions (egBrasier et al 2002 Fedo and Whitehouse 2002Schopf et al 2002)

These difficulties suggest that much could begained by the development of a well-documentedcollection of Archean rocks that could be madeavailable to all qualified investigators Until weunderstand the earliest stages of the evolution oflife on Earth we cannot hope to make significantprogress in the search for life on other planetssuch as Mars Until there is a sample return mis-sion to Mars we must make do with meteoritesthat come to us largely out of context An under-standing of the Archean Earth can help placethose limited samples in a planetary context

ACKNOWLEDGMENTS

Franco Pirajno Kath Grey and Martin VanKranendonk provided generous advice and as-sistance and the Geological Survey of WesternAustralia supported the field program We aregrateful to Peter Morant of Sipa Resources andAdrian Black of Newexco Perth for their adviceand for access to drill-core materials

ABBREVIATION

BIF banded iron formation

REFERENCES

Arndt NT Nelson DR Compston W Trendall AFand Thorne AM (1991) The age of the FortescueGroup Hamersley Basin Western Australia from ionmicroprobe zircon U-Pb results Aust J Earth Sci 38262ndash281

LINDSAY ET AL754

Awramik SM Schopf JW and Walter MR (1983) Fil-amentous fossil bacteria from the Archaean of WesternAustralia Precambrian Res 20 357ndash374

Barley ME Pickard AL and Sylvester PJ (1997) Em-placement of a large igneous province as a possiblecause of banded iron formation 245 billion years agoNature 385 55ndash58

Bettenay LF Bickle MJ Boulter CA Groves DIMorant P Blake TS and James BA (1981) Evolu-tion of the Shaw Batholithmdashan Archaean granitoid-gneiss dome in the eastern Pilbara Western AustraliaIn Special Publication 7 The Archaean edited by JEGlover and DI Groves Geological Society of WesternAustralia Perth pp 361ndash372

Blake TS and Barley ME (1992) Tectonic evolution ofthe Late Archaean to Early Proterozoic Mount BruceMegasequence Set Western Australia Tectonics 11 1415ndash1425

Blake TS and Groves DI (1987) Continental rifting andthe Archaean-Proterozoic transition Geology 15 229ndash232

Brasier MD and Lindsay JF (1998) A billion years ofenvironmental stability and the emergence of eukary-otes new data from Northern Australia Geology 26555ndash558

Brasier MD Green OR Jephcoat AP Kleppe AKVan Kranendonk MJ Lindsay JF Steele A andGrassineau NV (2002) Questioning the evidence forEarthrsquos oldest fossils Nature 416 76ndash81

Brauhart C (1999) Regional alteration systems associatedwith Archaean volcanogenic massive sulphide depositsat Panorama Pilbara Western Australia [PhD Thesis]University of Western Australia Perth

Brauhart C Groves DI and Morant P (1998) Regionalalteration systems associated with volcanogenic mas-sive sulfide mineralization at Panorama Pilbara West-ern Australia Econ Geol 93 292ndash302

Brocks J Logan GA Buick R and Summons RE(1999) Archean molecular fossils and the early rise ofeukaryotes Science 285 1033ndash1036

Buick R Dunlop JSR and Groves DI (1981) Stroma-tolite recognition in ancient rocks an appraisal of ir-regular laminated structures in an early Archaeanchert-barite unit from North Pole Western AustraliaAlcheringa 5 161ndash181

Buick R Des Marais DJ and Knoll AH (1995a) Sta-ble isotope compositions of carbonates from the Meso-proterozoic Bangemall Group northwestern AustraliaChem Geol 123 153ndash171

Buick R Thornett JR McNaughton NJ Smith JBBarley ME and Savage M (1995b) Record of emer-gent continental crust 35 billion years ago in the Pil-bara Craton of Australia Nature 375 574ndash577

Buick R Rasmussen B and Krapez B (1998) Archaeanoil evidence for extensive hydrocarbon generation andmigration 25ndash35 Ga Am Assoc Petroleum Geol Bull82 50ndash69

Chan M (1998) Sedimentology and structure of threesmall congolmeratic basins in the Lalla Rookh-WesternShaw structural corridor Pilbara Craton Western Aus-

tralia [BSc Honours Thesis] University of WesternAustralia Perth

Cullers RL DiMarco MJ Lowe DR and Stone J(1993) Geochemistry of a silicified felsic volcaniclasticsuite from the Early Archaean Panorama FormationPilbara Block Western Australiamdashan evaluation of de-positional and post-depositional processes with specialemphasis on the rare-Earth elements Precambrian Res60 99ndash116

Darwin F ed (1888) The Life and Letters of Charles Dar-win Vol 3 John Murray London

de Wit MJ (1998) On Archaean granites greenstonescratons and tectonics does the evidence demand a ver-dict Precambrian Geol 91 181ndash226

Drummond BJ (1981) Crustal structure of the Precam-brian terrains of northwest Australia from seismic re-fraction data BMR J Geol Geophys 6 123ndash135

Dunlop JSR Muir M Milne VA and Groves DI(1978) A new microfossil assemblage from the Ar-chaean of Western Australia Nature 274 676ndash678

Dutkiewicz A Rasmussen B and Buick R (1998) Oilpreserved in fluid inclusions in Archaean sandstonesNature 395 885ndash888

Fedo CM and Whitehouse MJ (2002) Metasomatic ori-gin of quartz-pyroxene rock Akilia Greenland and im-plications for Earthrsquos earliest life Science 296 1448ndash1452

Gee RD and Grey K (1993) Report 41 Proterozoic Rockson the Glengarry 1250 000 SheetmdashStratigraphy Structureand Stromatolite Biostratigraphy Geological Survey ofWestern Australia Perth

Glikson AY and Hickman AH (1981a) Chemicalstratigraphy and petrogenesis of Archaean basic-ultra-basic volcanic units eastern Pilbara Block WesternAustralia In Special Publication 7 Archaean Geologyedited by JE Groves and DI Groves Geological Soci-ety of Western Australia Perth pp 287ndash300

Glikson AY and Hickman AH (1981b) Record 198136Geochemistry of Archaean Volcanic Successions EasternPilbara Block Western Australia Australian Bureau ofMineral Resources Canberra

Glikson AY Davy R and Hickman AH (1991) Record199146 Trace Metal Distribution Patterns in Basalts Pil-bara Craton Western Australia with Stratigraphic-Geo-chemical Implications Australian Bureau of Mineral Re-sources Canberra

Groves DI Dunlop JSR and Buick R (1981) An earlyhabitat of life Sci Am 245 64ndash73

Harvey RP and Schutt JW (1998) Meteorite recoveryand reconnaissance in the Allan Hills-David Glacierand Darwin Glacier regions 1996ndash1997 Antarctic J US 32 25ndash27

Hickman AH (1975) Annual Report 1974 PrecambrianStructural Geology of Part of the Pilbara Region Geologi-cal Survey of Western Australia Perth pp 68ndash73

Hickman AH (1983) Bulletin 127 Geology of the PilbaraBlock and Its Environs Geological Survey of WesternAustralia Perth

Hickman AH (1984) Archaean diapirism in the PilbaraBlock Western Australia In Precambrian Tectonics Il-lustrated edited by A Kroner and R Greiling E

ARCHEAN GEOLOGIC REFERENCE COLLECTION 755

and usually well laminated mainly black andwhite chert Numerous chert dykes cut across theunderlying stratigraphy and terminate at thelevel of the chert horizons Close to Marble Barexposures of the Apex chert contain what was ini-tially believed to be Earthrsquos oldest morphologicalfossil assemblage (Fig 10) (Schopf and Packer1987 Schopf 1992 1993) a claim recently dis-puted by more detailed analysis (Brasier et al2002) (see following section on Paleontology)

Panorama Formation The Panorama Formation(3458ndash3434 Ma) which is best exposed in thenorthern part of the study area near the ShawRiver is dominated by felsic volcaniclastic rocks

with small volumes of felsic lavas and some chertinterbeds (Hickman 1983 Thorpe et al 1992 Nel-son 2000) The formation is strongly altered andintensely silicified having been exposed to tem-peratures as high as 350degC (Cullers et al 1993)A large volcanic vent within the Panorama For-mation is preserved in the northwestern part ofthe outcrop area (Van Kranendonk 2000)

Strelley Pool Chert The relationship of the Strel-ley Pool Chert (Lowe 1983) to the underlyingunits varies from conformable to unconformableIn the northeast the contact with the underlyingCoonterunah Group forms an angular unconfor-mity with pronounced paleorelief (Buick et al1995b) The formation consists of carbonate sili-cified carbonate and clastic rocks (Fig 11) Thecherts and carbonates both have well-developedsedimentary structure that have been describedas wavy laminated cherts or carbonates and stro-matolites (Fig 12) (Lowe 1983 Hofmann et al1999) although as discussed in a following sec-tion there is some concern as to whether they arebiogenic (Lowe 1994 Lindsay et al 2003ab)

Euro Basalt The Euro Basalt consists domi-nantly of basalts of tholeiitic and high-Mg com-position that rest conformably on the StrelleyPool Chert (Van Kranendonk 2000) The forma-tion reaches a thickness in excess of 9 km in thenortheastern part of the study area The basaltsare pillowed and bedded on a kilometer scaleThe formation also includes some komatiite andfelsic volcaniclastics as well as thin cherts andvolcaniclastic units

Megasequence 3mdashSulphur Springs Group

The Sulphur Springs Group consists of threeformations (Fig 3) that in contrast to the previ-ous two Groups contain a significant proportionof clastic sedimentary rocks some quartzose

Leilira Formation The Leilira Formation consistspredominantly of clastic sedimentary rocks withonly small volumes of felsic to intermediate ex-trusive rocks interbedded The clastic rocks arepredominantly sandstones but vary from silt-stone to coarser pebbly sandstone Composition-ally the sandstones are quartzose or volcaniclithic rocks Locally laminated cherts formed bythe silicification of felsic volcanic ash and silt-stone are abundant

LINDSAY ET AL746

FIG 7 Bedded sedimentary carbonate and silica fromthe Dresser Formation (locality B Fig 2) Scale bar 5 5cm The rock consists of well-bedded units that begin withorangeyellow iron-rich carbonates and grade upward topale green silica The unit reflects geochemical changes inresponse to pulsing or cycling of the hydrothermal sys-tem Elsewhere carbonate and chert form alternating beds

The Leilira Formation represents a significantchange in depositional patterns as it is the oldestunit in which clastic sediments are the dominantlithology It reflects the growing volcanic cara-pace and in particular the exposure of the grani-toid core to weathering and erosion the first signsthat a true crust was developing

Kunagunarrina Formation The KunagunarrinaFormation reaches a maximum of 24 km inthickness and consists predominantly of high-Mg basalts and komatiites Locally andesitic andfelsic units are present especially near the top ofthe formation A thick chert unit (10 m) in themiddle of the formation separates lower Mg

ARCHEAN GEOLOGIC REFERENCE COLLECTION 747

FIG 8 Stromatolitic structures in theDresser Formation (locality B Fig 2)The original barite has been largely re-placed by chert Scale bar 5 10 cm

FIG 9 Exposures of the Dresser Formation in the North Pole Dome area (locality B Fig 2) showing large chert-barite feeder dykes The dykes consist of black chert and coarse bladed barite Note the vehicle for scale

basalts at the base of the formation from ko-matiites towards the top

Kangaroo Caves Formation The Kangaroo CavesFormation which reaches a maximum thicknessof 15 km is best exposed towards the center ofthe study area The formation is complex and con-sists of a succession of andesitic volcanics anddacitic to rhyolitic sills and extrusives A chert in-terval up to 100 m thick forms a distinctivemarker at the top of the formation Close to thecenter of the study area at Sulphur Springs themassive chert is overlain by a polymict megabrec-cia that rests upon a sharp erosional contact (Hill1997 Vearncombe et al 1998)

Hosted within a 200-m dacite unit immediatelybeneath the marker chert are six sulfide ore bodiesspaced at regular intervals of 5ndash7 km around themargins of the Strelley Granite (Morant 1998) Theyconsist of pyrite sphalerite chalcopyrite and galenawith smaller amounts of other sulfides (Vearncombeet al 1995 1998 Vearncombe 1996 Morant 1998)The sulfides which were deposited in a blacksmoker hydrothermal setting occur in associationwith growth faults developed around the StrelleyGranite the intrusion of which was syndepositionalwith the Kangaroo Caves Formation (Vearncombe1996 Brauhart et al 1998 Brauhart 1999)

Megasequence 4mdashGorge Creek Group

The Gorge Creek Group consists of five for-mations (Fig 3) that show much greater litho-logic diversity than earlier megasequences Inparticular this interval includes for the firsttime a significant thickness of quartzose sand-stone as well as a variety of other clastic sedi-mentary rocks

Pincunah Hill Formation In the western part ofthe study area the basal Pincunah Hill Formationconsists of thinly bedded BIF with small amountsof laminated chert red to gray shale and silt-stone This basal unit is up to 1 km thick in thewestern part of the area but wedges out and dis-appears in a short distance eastward The upper

LINDSAY ET AL748

FIG 11 A measured section through the Strelley PoolChert showing the location of stromatolitic structuresand the carbonate interval (adapted from Van Kranen-donk 2000) (locality C Fig 2) The gamma log to theright shows continuity across the chert-carbonate

FIG 10 ldquoMicrofossilsrdquo from the Apex chert near Mar-ble Bar a computer-generated digital automontage ofputative beggiatoan Eoleptonema apex Holotype (Schopf1993) combining the most-sharply focused images fromsuccessive focal planes b and c Original manual pho-tomontage and interpretative sketch (Schopf 1993) whichomits lower structure at arrow in (b) d and e New sin-gle images showing continuity of original and newly im-aged and automontaged structures Reprinted with per-mission from Nature (Brasier et al 2002) Copyright 2002Macmillan Publishers Ltd

part of the formation consists largely of red orpurple shale (Van Kranendonk 2000)

Corboy Formation At its thickest in the easternpart of the study area the Corboy Formation in-cludes a number of distinct facies developedacross major growth faults (Wilhelmij and Dun-lop 1984 Wilhelmij 1986) Here the formationconsists of a basal quartzose sandstone abruptlyoverlain by conglomerate interbedded withgraded sandstone units red shale and coarsesandstone The formation is capped by massivesandstone and conglomerate The unit was de-posited as a series of small prograding shallowmarine fan deltas (Wilhelmij 1986 Hill 1997 VanKranendonk 2000)

Paddy Market Formation In the western part ofthe study area the Paddy Market Formation isdominantly cherty BIF with minor ferruginousshale Further east the formation becomes moreshale-rich although much of the shale has been re-placed by chert Sandstone is present in smallamounts in beds up to 10 m thick Towards thetop of the formation in the east the cherts containfelsic volcanic material that is overlain by up to 200m of massive dacite (Van Kranendonk 2000)

Honeyeater Basalt The Honeyeater Basaltwhich consists entirely of basalt and dolerite con-formably overlies the Paddy Market FormationThe lower part of the formation consists of high-Mg basalt interbedded with dolerite whereashigher in the formation the basalts are largelytholeiitic and interbedded with dolerite (Gliksonand Hickman 1981ab Glikson et al 1991)

Pyramid Hill Formation The Pyramid Hill For-mation is limited in areal extent It is found in thenorth central part of the study area east of theStrelley Granite The formation consists of redand black shales that when heavily silicifiedgrade into laminated black and white chert (VanKranendonk 2000) The Gorge Creek Group iscapped by a well-bedded BIF

Megasequence 5mdashDe Grey Group

The De Grey Group consists of a single for-mation the Lalla Rookh Sandstone (Fig 3)which is dominated by clastic sedimentary rocksthat contain a significant quartzose componentThe Lalla Rookh Sandstone which rests on awell-defined unconformity above the GorgeCreek Group reaches a maximum thickness of

ARCHEAN GEOLOGIC REFERENCE COLLECTION 749

FIG 12 Stromatolitic carbonates in the Strelley Pool Chert (locality C Fig 2) Laminae are isopachous and theapex of the cone is ruptured and veined by secondary carbonate (arrow) Scale bar 5 10 cm

3 km (Krapez 1984) In the north central part ofthe study area the formation consists entirely ofcoarse clastic sediments and minor shalewhereas to the south the predominate fill is lithicand feldspathic sandstone (Chan 1998) Theserocks provide the first clear evidence of streamactivity in the Pilbara succession (Krapez 1984Van Kranendonk and Collins 1998 Van Kra-nendonk 2000)

THE PILBARA SUPRACRUSTAL SUCCESSION

The late Archean and early Paleoproterozoicbasins that rest upon the ancient cratonic blocksof Western Australia (Fig 1) form a time seriesassociated with the formation and ultimate dis-assembly of one of Earthrsquos earliest major conti-nental masses This was followed by the suturingof the Pilbara and Yilgarn Cratons along theCapricorn Orogen as a new supercontinentevolved in the Paleoproterozoic (Rogers and San-tosh 2002) These basins contain an importantsedimentary record of the early Earth includingsome of Earthrsquos earliest stromatolites and plat-form carbonate deposits (Simonson et al1993ab) organic-rich black shales (Brocks et al1999) and BIF (Trendall 1983 1990) The strati-graphic and structural framework is well con-strained by radiometric ages (Arndt et al 1991Barley et al 1997 Trendall et al 1998) providinga record at a critical time in Earth history whenthe atmosphere was first becoming oxygen-rich(Lindsay and Brasier 2002ab)

The Late Archean-Early PaleoproterozoicHamersley Basin which lies close to the southernmargin of the Pilbara Block (Trendall 1983 1990Blake and Barley 1992) overlies a normal crustalthickness (30ndash40 km) (Drummond 1981) Thebasin began to subside at approximately 28 Gafollowing an episode of crustal extension The ar-chitecture of the basin fill is complex andpolyphase with major erosional surfaces separat-ing areally extensive megasequencessuperse-quences (Trendall 1990 Krapez 1996) DuringFortescue and Hamersley Group time (Fig 4)sedimentation began in a rift setting in a silici-clastic-starved environment The later stages ofHamersley Basin sedimentation (Turee CreekGroup times) record the early transition from apassive margin to a foreland basin setting andwith it the establishment of a terrigenous sedi-

ment supply (Tyler and Thorne 1990) The suc-cession is largely marine Evidence of eustasy ap-pears for the first time in the sedimentary suc-cession of the Hamersley Basin (Lindsay andBrasier 2002ab)

The geometry and sequence stacking pattern ofthe succession are both very similar to youngerintracratonic basins (see Lindsay et al 1993) TheHamersley Basin is thus the first basinal settingpreserved on the Australian craton in which ma-rine sediments and in particular platform car-bonate rocks have been preserved in a responseto broad regional crustal subsidence and wheredeposition is controlled by eustasy

The Ashburton Basin (22ndash18 Ga) discon-formably overlies the southern margin of theHamersley Basin along its the contact with theCapricorn Orogen (Fig 1) (Thorne and Seymour1991) The basin forms the northern margin of theCapricorn Orogen a deformed zone that devel-oped during ocean closure Loading of the oceanfloor by the approaching Yilgarn Craton led tothe development of a westndashnorthwest-orientedforeland basin [McGrath Trough (Horowitz1982)] with an uplifted orogenic margin to thesouthwest (Blake and Groves 1987 Blake andBarley 1992) with the result that sedimentationwas dominated by an abundant supply of ter-rigenous clastic material (Mount McGrath For-mation) Continued loading of the crust ulti-mately led to the supply of terrigenous materialsbeing disrupted and producing a prograding car-bonate shelf (Duck Creek Dolomite) Basin sedi-mentation was accompanied by active-marginmafic volcanism (Cheela Springs Basalt)

THE CAPRICORN OROGEN ANDASSOCIATED BASINS

The Yerrida (22ndash19 Ga) Bryah (20 Ga)Padbury (20 Ga) and Earaheedy (19ndash165Ga) Basins lie to the southeast of the Hamersleyand Ashburton Basins and overlie the CapricornOrogen (Fig 1) (Occhipinti et al 1997 1998 Pi-rajno et al 1998 Pirajno and Adamides 2000)These now fragmentary basins formed along thenorthern margin of the Yilgarn Craton in a back-arc setting and record the convergence and colli-sion of the Archean Pilbara and Yilgarn Cratons(Tyler and Thorne 1990 Thorne and Seymour1991 Occhipinti et al 1997 1998 Pirajno et al1998) Because of their active margin settings the

LINDSAY ET AL750

fill of these small basins is dominated by clasticsediments However platform carbonate unitsare preserved in the Yerrida Basin (Gee and Grey1993 Pirajno and Adamides 2000) and a signif-icant thickness of carbonate rocks also occurs inthe Earaheedy Basin

THE ARCHEAN BIOSPHERE

The end of the Archean is formally defined at25 Ga (Plumb 1991) In reality the Archean worldended at some point in time closer to 28 or 30 Gawhen the first large continental blocks began to as-semble The stratigraphic record preserved on theancient Australian Craton reflects this major tran-sition The rock record from the Early Archean isvery different from that of the Late Archean in thatthe early record is predominantly volcanic and hy-drothermal in nature (ldquogreenstonerdquo) while thelater record is largely clastic and more readily com-pared with the Phanerozoic record

Structures and textures of possible biogenicorigin that are preserved in the stratigraphic suc-cession in the Early Archean study area are wellknown and are believed to provide the earliestdirect evidence of Earthrsquos biosphere Stromatoliticstructures have been encountered at two levelswithin the section The Dresser Formation con-tains structures preserved in chert and barite in-tervals that have been interpreted as stromatolitesand oncolites (Dunlop et al 1978 Walter et al1980 Buick et al 1981 Groves et al 1981 Wal-ter 1983) The stromatolitic structures occur forthe most part in layered barite units that devel-oped over swarms of synvolcanic hydrothermalfeeder dykes along listric growth faults (Nijmanet al 1998 Van Kranendonk 2004) Although ithas been argued that these structures are biogenic(Walter et al 1980 Schopf and Walter 1983 Wal-ter 1983) their direct association with the feederdykes leaves room for doubt (Buick et al 1981)The setting is complex and there is considerablescope for more detailed contextual research

The second stromatolitic horizon occurs in theoverlying Strelley Pool Chert a complex unit con-sisting of at least five members (Lowe 1980 1983Hofmann et al 1999) The main stromatolitic unitoccurs as an 8-m-thick interval in the middle ofthe formation and appears to have a consistentand widespread pattern of deposition The struc-tures have been the focus of considerable discus-sion and their biogenicity has been questioned

(Buick et al 1981 Lowe 1995) The structures arecomplex and varied (Fig 12) leading to a morerecent re-evaluation again suggesting that theyare biogenic (Hofmann et al 1999) However thelaminae are largely isopachous an indication thatthey could be abiotic precipitates (cf Pope andGrotzinger 2000) Given the intense hydrother-mal activity that appears to have taken placethroughout the depositional time interval thestructure may well be the product of hydrother-mal deposition (Lindsay et al 2003ab) In all ourobservations suggest that considerably morework needs to be done to establish the deposi-tional context of these stromatolitic structures be-fore they can be confidently ascribed as biogenicin origin

Structures resembling remarkably preservedbacterial and cyanobacterial microfossils havebeen found at several localities in and close to thestudy area (Awramik et al 1983 Schopf andPacker 1987 Schopf 1992 1993 Rasmussen 2000)A complex biota of 11 species of filamentousprokaryotes which are compared with cyanobac-teria (Schopf and Packer 1987 Schopf 1992 1993)have been described from the Apex chert close toMarble Bar (21deg105579S 119deg418789E) The taxo-nomic interpretations are important in that theyprovide the earliest evidence of morphological lifeon Earth and support the hypothesis of an earlybeginning for oxygenic photosynthesis (Schopf1999) The structures are almost a billion yearsolder than the oldest known putative cyanobac-terial biomarkers (Summons et al 1999) and oc-cur well before the earliest evidence for the risein oxygen in the atmosphere (for a summary seeKnoll and Canfield 1998 Lindsay and Brasier2000 2002ab) However in a recent re-evaluationof the original Apex chert material it has been ar-gued that the filaments are not biogenic but aresecondary artifacts formed during diagenesisfrom abiogenic organic compounds generated inhydrothermal chert veins (Brasier et al 2002 VanKranendonk 2004)

Filaments believed to be biogenic in originhave also been reported from the 324 Ga Kan-garoo Caves Formation (Rasmussen 2000) wherethey form part of the Sulphur Springs deposit as-sociated with a black smoker hydrothermal ventsystem (Vearncombe et al 1995 1998) Ras-mussen (2000) argued that the filaments are rem-nants of a hydrothermal biota of thermophilicchemotrophic prokaryotes Sulfide textures areremarkably well preserved in these complex de-

ARCHEAN GEOLOGIC REFERENCE COLLECTION 751

posits and are worthy of much more detailedanalysis as they may well preserve evidence ofthe early biosphere

Finally there is evidence that hydrocarbons aretrapped in these ancient sedimentary rocks eitheras fluid inclusions in hydrothermal precipitates oras bituminous residues in larger cavities in theKangaroo Caves Formation and in the Lalla RookhSandstone (Dutkiewicz et al 1998 Rasmussen andBuick 2000) Buick et al (1998) also reported car-bonaceous globules in cherts from the lower War-rawoona Group In the Kangaroo Caves Forma-tion the hydrocarbons occur in hydrothermalbarite or within the sulfides of the volcanogenichydrothermal massive sulfide deposits that weresyndepositional with the formation (Rasmussenand Buick 2000) Textural information suggestthat these deposits were precipitated as blacksmoker chimneys in a submarine hydrothermalsetting driven by heat released from the intrudingStrelley Granite at approximately 324 Ga (Vearn-combe et al 1995) Fluorescing fluid inclusions oc-cur in quartz overgrowths in sandstones from theLalla Rookh Sandstone (Dutkiewicz et al 1998)Rasmussen and Buick (2000) and Dutkiewicz et al(1998) have argued for a biogenic origin for the hy-drocarbons however an abiotic origin involvingfor example Fischer-Tropsch-type synthesis ap-pears possible (Brasier et al 2002)

The Late Archean (and Early Paleoproterozoic)record preserved in the Hamersley and relatedbasins is stratigraphically much more coherentthan that preserved in the earlier successions TheHamersley Basin in particular provides a verycomprehensive record of the early oxygenation ofthe atmosphere in the form of large eustaticallycontrolled upward-shallowing sequences that in-clude black shales BIFs and ultimately at theirtops biogenic platform carbonates (Lindsay andBrasier 2002ab) These depositional cycles con-tain organic-rich shales at their base that recordevidence for some of Earthrsquos earliest biosphere(Brocks et al 1999) BIFs (Trendall 1983 1990)that record the rise of oxygen in the atmosphereand carbonates that document some of Earthrsquosearliest reef-like carbonate structures (Simonsonand Hassler 1996 1997 Simonson et al 1993ab)

SAMPLING STRATEGY

The Archean rocks of the Pilbara Craton arethus part of a protocontinent about which the

nascent Australian Craton developed It consistsof granitoid complexes overlain by a carapace ofsynchronously deposited volcano-sedimentarywedges Resting upon the early Archean proto-continent are a series of supracrustal basins Inall the sedimentary record of the Pilbara extendsfrom rocks as old as 354 Ga to some as young asapproximately 19 Ga As summarized in the fol-lowing paragraphs the sedimentary record of thePilbara records evidence of the major tectonic andgeological events that accompanied the evolutionof the Archean biosphere

It is here proposed that we develop a carefullydocumented and curated sample collection fromthis unique setting The collection would form thebasis for early Earth studies and a focus for astro-biology The sample set would be designed to re-flect the evolutionary succession described abovewith emphasis placed on cherts lower in the suc-cession and on clastic sediments higher in the suc-cession Any comprehensive reference collectionmust reflect diversity of the succession and includerocks from each of the sedimentary environmentsas well as igneous rocks reflecting the frameworkin which the protocontinent assembled

The five megasequences at the North Shawstudy area (Fig 3) are very different and reflectthe evolving Early Archean protocontinent Theearliest sedimentary record is limited and domi-nated by hydrothermally generated laminatedcherts and BIFs which appear as thin interbedswithin massive basalts (Coonterunah Group) (Fig5) Gradually clastic rocks appear dominated atfirst by volcanoclastics followed later by the firstevidence of sediments containing detrital quartzderived from true continental granitic crust Hy-drothermally generated laminated chert carbon-ate and barite units occur throughout the sectionThe first evidence of erosion appears at the top ofthe Coonterunah Group in the form of an angu-lar unconformity (Buick et al 1995b)

The basins that form the supracrustal succes-sion on the Pilbara Craton offer important in-sights into the closing phases of the Archean andthe immediate post-Archean world Because oftheir economic importance the basins are wellmapped and drill core is readily available (seeLindsay and Brasier 2002a) The succession pro-vides vital information about the early oxygena-tion of the atmosphere and the rapid growth ofthe biosphere that resulted from the assembly ofthe first continents and the initiation of the platetectonic cycle Platform carbonates whose geom-

LINDSAY ET AL752

etry was determined by eustasy are widespreadand contain considerable information on earlyoxygenic photosynthesis and the development ofthe first reef-like structure that are similar inmany ways to modern reefs (Simonson et al1993ab Lindsay and Brasier 2002ab)

The collection would consist of three groupingsof samples stratigraphic samples site-specificsamples and framework samples

Stratigraphic samples

The stratigraphic sample set would be collectedfrom well-defined settings such as carefully mea-sured and documented sections or from drill coreSamples would be collected from each of the fivestratigraphic groups or megasequences Thesupracrustal successions preserved in the Hamer-sley and associated basins would where possiblebe taken from existing drill core or from drill coresobtained for special projects Where drill core isnot available samples would be collected fromcarefully selected and measured outcrop sectionsThe samples would concentrate on rocks mostlikely to host biogenic material Samples from theEarly Archean volcaniclastic carapace would bedominated by cherts that are most likely to pre-serve fine biogenic structures but would also in-clude other sedimentary hydrothermal units suchas barite and carbonate that would be used in sta-ble isotope and geochemical studies

Site-specific sample

This sample set would focus on sites where po-tential biogenic structures have been documentedsuch as the stromatolitic intervals in the DresserFormation and the Strelley Pool Chert It wouldalso include detailed sampling of sites such asChinaman Creek and the Strelley Pool Chertwhere purported microfossils have been docu-mented (Schopf 1993) and the sulfide bodies inthe Kangaroo Caves Formation that contain pos-sible microfossils as well as evidence of hydro-carbons (Rasmussen 2000 Rasmussen and Buick2000) This would involve a broad samplingaround the sites in measured sections and drillcore to establish the geological context

Framework samples

While the focus of the collecting would be onmaterial that would help define the character of

the Earthrsquos earliest biosphere it is important thatthe broad context of the samples be defined TheArchean is a period when the Earthrsquos crust wasevolving rapidly and there is a growing body ofevidence to suggest that the evolution of theplanet drives the evolution of the atmosphere andbiosphere (Lindsay and Brasier 2002ab) Theframework sample set would include suites ofsample from the extrusive volcanic rocks col-lected along well-documented sections Sectionsthrough the volcanoclastic succession would em-phasize the dominant volcanic units (basalts ko-matiites andesites felsites and rhyolites) assources of information on the mantle and evolv-ing crust This sample set would also include adetailed sampling of the granitoid complexeswhich ultimately drove the entire process as wellas the later intrusions that were important in dri-ving the hydrothermal systems that may wellhave been the cradle of early life

SAMPLE CURATIONCHARACTERIZATION AND

DISTRIBUTION

All samples would be fully documented in thefield Samples would be photographed and theirlocations determined using a Global PositioningSystem receiver For specialized samples such asthose thought to contain easily contaminated bio-geochemical information the field collectionwould be modeled on techniques developed bythe NASANational Science FoundationSmith-sonian Institution Antarctic Search for Meteoritesprogram (Harvey and Schutt 1998)

Characterization

The petrologic and petrographic characteristicsof each sample would be determined using thin-section optical microscopy The quantitative bulkchemical composition of each sample would bemeasured by x-ray fluorescence spectrometry Fi-nally the qualitative bulk mineralogy of eachsample would be measured by powder x-ray dif-fraction

Data management

An Archean Geologic Reference Materials Data-base would be produced to fully document eachsample Field documentation would include sam-pling rationale supporting references and field

ARCHEAN GEOLOGIC REFERENCE COLLECTION 753

photographs Documentation would include theresults of all analyses along with images of rep-resentative sample surfaces and optical micro-graphs The Database would be produced in elec-tronic format and posted on the web Subsamplesof the Archean Geologic Reference Materialswould then be made available at no cost to re-searchers in the astrobiology community

DISCUSSION AND CONCLUSIONS

The Early Archean environment was uniqueRecent studies indicate that the atmosphere waslargely anoxic with oxygen levels 1025 of thepresent atmospheric level (Pavlov and Kasting2002) There were no large continents no activeor passive margins and by default no intracra-tonic basins Significantly perhaps there are nosigns of eustasy The crust was evolving rapidlyand heat flow was high leading to the intrusionof large granitoid bodies and the development ofa volcanic carapace above them In turn thismeant that the marine environment was domi-nated by hydrothermal activity

In the absence of large continental masses de-trital materials other than volcanoclastics are rel-atively rare in the Early Archean Significant vol-umes of quartz sand do not appear in the PilbaraCraton sedimentary record until after 32 Ga pre-sumably as the intruding igneous bodies begin tobe unroofed and eroded There are thus no ma-jor accumulations of sedimentary rocks that wenormally expect associated with passive conti-nental margins

With the assembly of the earliest large conti-nents beginning at approximately 30ndash28 Gathe Earthrsquos atmosphere hydrosphere and bios-phere changed rapidly Intracratonic basinssuch as the Hamersley Basin began to developon the new crustal blocks opening new shal-low-water environments that provided an idealsetting for photosynthesizing organisms andthe development of the first carbonate reefstructures (Lindsay and Brasier 2002ab) Hy-drothermal processes while still importantwere thus no longer the only sources of energyavailable to the evolving biosphere

The Archean was thus an unfamiliar environ-ment and unravelling the nature of the earliestbiosphere will require development of new ap-proaches and new techniques Without large con-tinental masses subareal exposures are likely to

be rare and short-lived There is no evidence forstream activity until the De Grey group at ap-proximately 29 Ga at which time braided streamgravels were preserved Thus models involvingearly biospheric evolution in ldquoa warm little pondrdquo(Darwin 1888) or in fresh water seem unlikelyInstead the focus in the search for the origins oflife on Earth should be directed to hydrothermalmarine settings involving high temperatures andwhat would today be hostile geochemical envi-ronments with significant concentrations of metal-lic ions and a largely reducing environment Therisks for the misidentification of abiotic structuresand geochemical signatures as signs of ancient lifeare significant as seen in recent discussions (egBrasier et al 2002 Fedo and Whitehouse 2002Schopf et al 2002)

These difficulties suggest that much could begained by the development of a well-documentedcollection of Archean rocks that could be madeavailable to all qualified investigators Until weunderstand the earliest stages of the evolution oflife on Earth we cannot hope to make significantprogress in the search for life on other planetssuch as Mars Until there is a sample return mis-sion to Mars we must make do with meteoritesthat come to us largely out of context An under-standing of the Archean Earth can help placethose limited samples in a planetary context

ACKNOWLEDGMENTS

Franco Pirajno Kath Grey and Martin VanKranendonk provided generous advice and as-sistance and the Geological Survey of WesternAustralia supported the field program We aregrateful to Peter Morant of Sipa Resources andAdrian Black of Newexco Perth for their adviceand for access to drill-core materials

ABBREVIATION

BIF banded iron formation

REFERENCES

Arndt NT Nelson DR Compston W Trendall AFand Thorne AM (1991) The age of the FortescueGroup Hamersley Basin Western Australia from ionmicroprobe zircon U-Pb results Aust J Earth Sci 38262ndash281

LINDSAY ET AL754

Awramik SM Schopf JW and Walter MR (1983) Fil-amentous fossil bacteria from the Archaean of WesternAustralia Precambrian Res 20 357ndash374

Barley ME Pickard AL and Sylvester PJ (1997) Em-placement of a large igneous province as a possiblecause of banded iron formation 245 billion years agoNature 385 55ndash58

Bettenay LF Bickle MJ Boulter CA Groves DIMorant P Blake TS and James BA (1981) Evolu-tion of the Shaw Batholithmdashan Archaean granitoid-gneiss dome in the eastern Pilbara Western AustraliaIn Special Publication 7 The Archaean edited by JEGlover and DI Groves Geological Society of WesternAustralia Perth pp 361ndash372

Blake TS and Barley ME (1992) Tectonic evolution ofthe Late Archaean to Early Proterozoic Mount BruceMegasequence Set Western Australia Tectonics 11 1415ndash1425

Blake TS and Groves DI (1987) Continental rifting andthe Archaean-Proterozoic transition Geology 15 229ndash232

Brasier MD and Lindsay JF (1998) A billion years ofenvironmental stability and the emergence of eukary-otes new data from Northern Australia Geology 26555ndash558

Brasier MD Green OR Jephcoat AP Kleppe AKVan Kranendonk MJ Lindsay JF Steele A andGrassineau NV (2002) Questioning the evidence forEarthrsquos oldest fossils Nature 416 76ndash81

Brauhart C (1999) Regional alteration systems associatedwith Archaean volcanogenic massive sulphide depositsat Panorama Pilbara Western Australia [PhD Thesis]University of Western Australia Perth

Brauhart C Groves DI and Morant P (1998) Regionalalteration systems associated with volcanogenic mas-sive sulfide mineralization at Panorama Pilbara West-ern Australia Econ Geol 93 292ndash302

Brocks J Logan GA Buick R and Summons RE(1999) Archean molecular fossils and the early rise ofeukaryotes Science 285 1033ndash1036

Buick R Dunlop JSR and Groves DI (1981) Stroma-tolite recognition in ancient rocks an appraisal of ir-regular laminated structures in an early Archaeanchert-barite unit from North Pole Western AustraliaAlcheringa 5 161ndash181

Buick R Des Marais DJ and Knoll AH (1995a) Sta-ble isotope compositions of carbonates from the Meso-proterozoic Bangemall Group northwestern AustraliaChem Geol 123 153ndash171

Buick R Thornett JR McNaughton NJ Smith JBBarley ME and Savage M (1995b) Record of emer-gent continental crust 35 billion years ago in the Pil-bara Craton of Australia Nature 375 574ndash577

Buick R Rasmussen B and Krapez B (1998) Archaeanoil evidence for extensive hydrocarbon generation andmigration 25ndash35 Ga Am Assoc Petroleum Geol Bull82 50ndash69

Chan M (1998) Sedimentology and structure of threesmall congolmeratic basins in the Lalla Rookh-WesternShaw structural corridor Pilbara Craton Western Aus-

tralia [BSc Honours Thesis] University of WesternAustralia Perth

Cullers RL DiMarco MJ Lowe DR and Stone J(1993) Geochemistry of a silicified felsic volcaniclasticsuite from the Early Archaean Panorama FormationPilbara Block Western Australiamdashan evaluation of de-positional and post-depositional processes with specialemphasis on the rare-Earth elements Precambrian Res60 99ndash116

Darwin F ed (1888) The Life and Letters of Charles Dar-win Vol 3 John Murray London

de Wit MJ (1998) On Archaean granites greenstonescratons and tectonics does the evidence demand a ver-dict Precambrian Geol 91 181ndash226

Drummond BJ (1981) Crustal structure of the Precam-brian terrains of northwest Australia from seismic re-fraction data BMR J Geol Geophys 6 123ndash135

Dunlop JSR Muir M Milne VA and Groves DI(1978) A new microfossil assemblage from the Ar-chaean of Western Australia Nature 274 676ndash678

Dutkiewicz A Rasmussen B and Buick R (1998) Oilpreserved in fluid inclusions in Archaean sandstonesNature 395 885ndash888

Fedo CM and Whitehouse MJ (2002) Metasomatic ori-gin of quartz-pyroxene rock Akilia Greenland and im-plications for Earthrsquos earliest life Science 296 1448ndash1452

Gee RD and Grey K (1993) Report 41 Proterozoic Rockson the Glengarry 1250 000 SheetmdashStratigraphy Structureand Stromatolite Biostratigraphy Geological Survey ofWestern Australia Perth

Glikson AY and Hickman AH (1981a) Chemicalstratigraphy and petrogenesis of Archaean basic-ultra-basic volcanic units eastern Pilbara Block WesternAustralia In Special Publication 7 Archaean Geologyedited by JE Groves and DI Groves Geological Soci-ety of Western Australia Perth pp 287ndash300

Glikson AY and Hickman AH (1981b) Record 198136Geochemistry of Archaean Volcanic Successions EasternPilbara Block Western Australia Australian Bureau ofMineral Resources Canberra

Glikson AY Davy R and Hickman AH (1991) Record199146 Trace Metal Distribution Patterns in Basalts Pil-bara Craton Western Australia with Stratigraphic-Geo-chemical Implications Australian Bureau of Mineral Re-sources Canberra

Groves DI Dunlop JSR and Buick R (1981) An earlyhabitat of life Sci Am 245 64ndash73

Harvey RP and Schutt JW (1998) Meteorite recoveryand reconnaissance in the Allan Hills-David Glacierand Darwin Glacier regions 1996ndash1997 Antarctic J US 32 25ndash27

Hickman AH (1975) Annual Report 1974 PrecambrianStructural Geology of Part of the Pilbara Region Geologi-cal Survey of Western Australia Perth pp 68ndash73

Hickman AH (1983) Bulletin 127 Geology of the PilbaraBlock and Its Environs Geological Survey of WesternAustralia Perth

Hickman AH (1984) Archaean diapirism in the PilbaraBlock Western Australia In Precambrian Tectonics Il-lustrated edited by A Kroner and R Greiling E

ARCHEAN GEOLOGIC REFERENCE COLLECTION 755

The Leilira Formation represents a significantchange in depositional patterns as it is the oldestunit in which clastic sediments are the dominantlithology It reflects the growing volcanic cara-pace and in particular the exposure of the grani-toid core to weathering and erosion the first signsthat a true crust was developing

Kunagunarrina Formation The KunagunarrinaFormation reaches a maximum of 24 km inthickness and consists predominantly of high-Mg basalts and komatiites Locally andesitic andfelsic units are present especially near the top ofthe formation A thick chert unit (10 m) in themiddle of the formation separates lower Mg

ARCHEAN GEOLOGIC REFERENCE COLLECTION 747

FIG 8 Stromatolitic structures in theDresser Formation (locality B Fig 2)The original barite has been largely re-placed by chert Scale bar 5 10 cm

FIG 9 Exposures of the Dresser Formation in the North Pole Dome area (locality B Fig 2) showing large chert-barite feeder dykes The dykes consist of black chert and coarse bladed barite Note the vehicle for scale

basalts at the base of the formation from ko-matiites towards the top

Kangaroo Caves Formation The Kangaroo CavesFormation which reaches a maximum thicknessof 15 km is best exposed towards the center ofthe study area The formation is complex and con-sists of a succession of andesitic volcanics anddacitic to rhyolitic sills and extrusives A chert in-terval up to 100 m thick forms a distinctivemarker at the top of the formation Close to thecenter of the study area at Sulphur Springs themassive chert is overlain by a polymict megabrec-cia that rests upon a sharp erosional contact (Hill1997 Vearncombe et al 1998)

Hosted within a 200-m dacite unit immediatelybeneath the marker chert are six sulfide ore bodiesspaced at regular intervals of 5ndash7 km around themargins of the Strelley Granite (Morant 1998) Theyconsist of pyrite sphalerite chalcopyrite and galenawith smaller amounts of other sulfides (Vearncombeet al 1995 1998 Vearncombe 1996 Morant 1998)The sulfides which were deposited in a blacksmoker hydrothermal setting occur in associationwith growth faults developed around the StrelleyGranite the intrusion of which was syndepositionalwith the Kangaroo Caves Formation (Vearncombe1996 Brauhart et al 1998 Brauhart 1999)

Megasequence 4mdashGorge Creek Group

The Gorge Creek Group consists of five for-mations (Fig 3) that show much greater litho-logic diversity than earlier megasequences Inparticular this interval includes for the firsttime a significant thickness of quartzose sand-stone as well as a variety of other clastic sedi-mentary rocks

Pincunah Hill Formation In the western part ofthe study area the basal Pincunah Hill Formationconsists of thinly bedded BIF with small amountsof laminated chert red to gray shale and silt-stone This basal unit is up to 1 km thick in thewestern part of the area but wedges out and dis-appears in a short distance eastward The upper

LINDSAY ET AL748

FIG 11 A measured section through the Strelley PoolChert showing the location of stromatolitic structuresand the carbonate interval (adapted from Van Kranen-donk 2000) (locality C Fig 2) The gamma log to theright shows continuity across the chert-carbonate

FIG 10 ldquoMicrofossilsrdquo from the Apex chert near Mar-ble Bar a computer-generated digital automontage ofputative beggiatoan Eoleptonema apex Holotype (Schopf1993) combining the most-sharply focused images fromsuccessive focal planes b and c Original manual pho-tomontage and interpretative sketch (Schopf 1993) whichomits lower structure at arrow in (b) d and e New sin-gle images showing continuity of original and newly im-aged and automontaged structures Reprinted with per-mission from Nature (Brasier et al 2002) Copyright 2002Macmillan Publishers Ltd

part of the formation consists largely of red orpurple shale (Van Kranendonk 2000)

Corboy Formation At its thickest in the easternpart of the study area the Corboy Formation in-cludes a number of distinct facies developedacross major growth faults (Wilhelmij and Dun-lop 1984 Wilhelmij 1986) Here the formationconsists of a basal quartzose sandstone abruptlyoverlain by conglomerate interbedded withgraded sandstone units red shale and coarsesandstone The formation is capped by massivesandstone and conglomerate The unit was de-posited as a series of small prograding shallowmarine fan deltas (Wilhelmij 1986 Hill 1997 VanKranendonk 2000)

Paddy Market Formation In the western part ofthe study area the Paddy Market Formation isdominantly cherty BIF with minor ferruginousshale Further east the formation becomes moreshale-rich although much of the shale has been re-placed by chert Sandstone is present in smallamounts in beds up to 10 m thick Towards thetop of the formation in the east the cherts containfelsic volcanic material that is overlain by up to 200m of massive dacite (Van Kranendonk 2000)

Honeyeater Basalt The Honeyeater Basaltwhich consists entirely of basalt and dolerite con-formably overlies the Paddy Market FormationThe lower part of the formation consists of high-Mg basalt interbedded with dolerite whereashigher in the formation the basalts are largelytholeiitic and interbedded with dolerite (Gliksonand Hickman 1981ab Glikson et al 1991)

Pyramid Hill Formation The Pyramid Hill For-mation is limited in areal extent It is found in thenorth central part of the study area east of theStrelley Granite The formation consists of redand black shales that when heavily silicifiedgrade into laminated black and white chert (VanKranendonk 2000) The Gorge Creek Group iscapped by a well-bedded BIF

Megasequence 5mdashDe Grey Group

The De Grey Group consists of a single for-mation the Lalla Rookh Sandstone (Fig 3)which is dominated by clastic sedimentary rocksthat contain a significant quartzose componentThe Lalla Rookh Sandstone which rests on awell-defined unconformity above the GorgeCreek Group reaches a maximum thickness of

ARCHEAN GEOLOGIC REFERENCE COLLECTION 749

FIG 12 Stromatolitic carbonates in the Strelley Pool Chert (locality C Fig 2) Laminae are isopachous and theapex of the cone is ruptured and veined by secondary carbonate (arrow) Scale bar 5 10 cm

3 km (Krapez 1984) In the north central part ofthe study area the formation consists entirely ofcoarse clastic sediments and minor shalewhereas to the south the predominate fill is lithicand feldspathic sandstone (Chan 1998) Theserocks provide the first clear evidence of streamactivity in the Pilbara succession (Krapez 1984Van Kranendonk and Collins 1998 Van Kra-nendonk 2000)

THE PILBARA SUPRACRUSTAL SUCCESSION

The late Archean and early Paleoproterozoicbasins that rest upon the ancient cratonic blocksof Western Australia (Fig 1) form a time seriesassociated with the formation and ultimate dis-assembly of one of Earthrsquos earliest major conti-nental masses This was followed by the suturingof the Pilbara and Yilgarn Cratons along theCapricorn Orogen as a new supercontinentevolved in the Paleoproterozoic (Rogers and San-tosh 2002) These basins contain an importantsedimentary record of the early Earth includingsome of Earthrsquos earliest stromatolites and plat-form carbonate deposits (Simonson et al1993ab) organic-rich black shales (Brocks et al1999) and BIF (Trendall 1983 1990) The strati-graphic and structural framework is well con-strained by radiometric ages (Arndt et al 1991Barley et al 1997 Trendall et al 1998) providinga record at a critical time in Earth history whenthe atmosphere was first becoming oxygen-rich(Lindsay and Brasier 2002ab)

The Late Archean-Early PaleoproterozoicHamersley Basin which lies close to the southernmargin of the Pilbara Block (Trendall 1983 1990Blake and Barley 1992) overlies a normal crustalthickness (30ndash40 km) (Drummond 1981) Thebasin began to subside at approximately 28 Gafollowing an episode of crustal extension The ar-chitecture of the basin fill is complex andpolyphase with major erosional surfaces separat-ing areally extensive megasequencessuperse-quences (Trendall 1990 Krapez 1996) DuringFortescue and Hamersley Group time (Fig 4)sedimentation began in a rift setting in a silici-clastic-starved environment The later stages ofHamersley Basin sedimentation (Turee CreekGroup times) record the early transition from apassive margin to a foreland basin setting andwith it the establishment of a terrigenous sedi-

ment supply (Tyler and Thorne 1990) The suc-cession is largely marine Evidence of eustasy ap-pears for the first time in the sedimentary suc-cession of the Hamersley Basin (Lindsay andBrasier 2002ab)

The geometry and sequence stacking pattern ofthe succession are both very similar to youngerintracratonic basins (see Lindsay et al 1993) TheHamersley Basin is thus the first basinal settingpreserved on the Australian craton in which ma-rine sediments and in particular platform car-bonate rocks have been preserved in a responseto broad regional crustal subsidence and wheredeposition is controlled by eustasy

The Ashburton Basin (22ndash18 Ga) discon-formably overlies the southern margin of theHamersley Basin along its the contact with theCapricorn Orogen (Fig 1) (Thorne and Seymour1991) The basin forms the northern margin of theCapricorn Orogen a deformed zone that devel-oped during ocean closure Loading of the oceanfloor by the approaching Yilgarn Craton led tothe development of a westndashnorthwest-orientedforeland basin [McGrath Trough (Horowitz1982)] with an uplifted orogenic margin to thesouthwest (Blake and Groves 1987 Blake andBarley 1992) with the result that sedimentationwas dominated by an abundant supply of ter-rigenous clastic material (Mount McGrath For-mation) Continued loading of the crust ulti-mately led to the supply of terrigenous materialsbeing disrupted and producing a prograding car-bonate shelf (Duck Creek Dolomite) Basin sedi-mentation was accompanied by active-marginmafic volcanism (Cheela Springs Basalt)

THE CAPRICORN OROGEN ANDASSOCIATED BASINS

The Yerrida (22ndash19 Ga) Bryah (20 Ga)Padbury (20 Ga) and Earaheedy (19ndash165Ga) Basins lie to the southeast of the Hamersleyand Ashburton Basins and overlie the CapricornOrogen (Fig 1) (Occhipinti et al 1997 1998 Pi-rajno et al 1998 Pirajno and Adamides 2000)These now fragmentary basins formed along thenorthern margin of the Yilgarn Craton in a back-arc setting and record the convergence and colli-sion of the Archean Pilbara and Yilgarn Cratons(Tyler and Thorne 1990 Thorne and Seymour1991 Occhipinti et al 1997 1998 Pirajno et al1998) Because of their active margin settings the

LINDSAY ET AL750

fill of these small basins is dominated by clasticsediments However platform carbonate unitsare preserved in the Yerrida Basin (Gee and Grey1993 Pirajno and Adamides 2000) and a signif-icant thickness of carbonate rocks also occurs inthe Earaheedy Basin

THE ARCHEAN BIOSPHERE

The end of the Archean is formally defined at25 Ga (Plumb 1991) In reality the Archean worldended at some point in time closer to 28 or 30 Gawhen the first large continental blocks began to as-semble The stratigraphic record preserved on theancient Australian Craton reflects this major tran-sition The rock record from the Early Archean isvery different from that of the Late Archean in thatthe early record is predominantly volcanic and hy-drothermal in nature (ldquogreenstonerdquo) while thelater record is largely clastic and more readily com-pared with the Phanerozoic record

Structures and textures of possible biogenicorigin that are preserved in the stratigraphic suc-cession in the Early Archean study area are wellknown and are believed to provide the earliestdirect evidence of Earthrsquos biosphere Stromatoliticstructures have been encountered at two levelswithin the section The Dresser Formation con-tains structures preserved in chert and barite in-tervals that have been interpreted as stromatolitesand oncolites (Dunlop et al 1978 Walter et al1980 Buick et al 1981 Groves et al 1981 Wal-ter 1983) The stromatolitic structures occur forthe most part in layered barite units that devel-oped over swarms of synvolcanic hydrothermalfeeder dykes along listric growth faults (Nijmanet al 1998 Van Kranendonk 2004) Although ithas been argued that these structures are biogenic(Walter et al 1980 Schopf and Walter 1983 Wal-ter 1983) their direct association with the feederdykes leaves room for doubt (Buick et al 1981)The setting is complex and there is considerablescope for more detailed contextual research

The second stromatolitic horizon occurs in theoverlying Strelley Pool Chert a complex unit con-sisting of at least five members (Lowe 1980 1983Hofmann et al 1999) The main stromatolitic unitoccurs as an 8-m-thick interval in the middle ofthe formation and appears to have a consistentand widespread pattern of deposition The struc-tures have been the focus of considerable discus-sion and their biogenicity has been questioned

(Buick et al 1981 Lowe 1995) The structures arecomplex and varied (Fig 12) leading to a morerecent re-evaluation again suggesting that theyare biogenic (Hofmann et al 1999) However thelaminae are largely isopachous an indication thatthey could be abiotic precipitates (cf Pope andGrotzinger 2000) Given the intense hydrother-mal activity that appears to have taken placethroughout the depositional time interval thestructure may well be the product of hydrother-mal deposition (Lindsay et al 2003ab) In all ourobservations suggest that considerably morework needs to be done to establish the deposi-tional context of these stromatolitic structures be-fore they can be confidently ascribed as biogenicin origin

Structures resembling remarkably preservedbacterial and cyanobacterial microfossils havebeen found at several localities in and close to thestudy area (Awramik et al 1983 Schopf andPacker 1987 Schopf 1992 1993 Rasmussen 2000)A complex biota of 11 species of filamentousprokaryotes which are compared with cyanobac-teria (Schopf and Packer 1987 Schopf 1992 1993)have been described from the Apex chert close toMarble Bar (21deg105579S 119deg418789E) The taxo-nomic interpretations are important in that theyprovide the earliest evidence of morphological lifeon Earth and support the hypothesis of an earlybeginning for oxygenic photosynthesis (Schopf1999) The structures are almost a billion yearsolder than the oldest known putative cyanobac-terial biomarkers (Summons et al 1999) and oc-cur well before the earliest evidence for the risein oxygen in the atmosphere (for a summary seeKnoll and Canfield 1998 Lindsay and Brasier2000 2002ab) However in a recent re-evaluationof the original Apex chert material it has been ar-gued that the filaments are not biogenic but aresecondary artifacts formed during diagenesisfrom abiogenic organic compounds generated inhydrothermal chert veins (Brasier et al 2002 VanKranendonk 2004)

Filaments believed to be biogenic in originhave also been reported from the 324 Ga Kan-garoo Caves Formation (Rasmussen 2000) wherethey form part of the Sulphur Springs deposit as-sociated with a black smoker hydrothermal ventsystem (Vearncombe et al 1995 1998) Ras-mussen (2000) argued that the filaments are rem-nants of a hydrothermal biota of thermophilicchemotrophic prokaryotes Sulfide textures areremarkably well preserved in these complex de-

ARCHEAN GEOLOGIC REFERENCE COLLECTION 751

posits and are worthy of much more detailedanalysis as they may well preserve evidence ofthe early biosphere

Finally there is evidence that hydrocarbons aretrapped in these ancient sedimentary rocks eitheras fluid inclusions in hydrothermal precipitates oras bituminous residues in larger cavities in theKangaroo Caves Formation and in the Lalla RookhSandstone (Dutkiewicz et al 1998 Rasmussen andBuick 2000) Buick et al (1998) also reported car-bonaceous globules in cherts from the lower War-rawoona Group In the Kangaroo Caves Forma-tion the hydrocarbons occur in hydrothermalbarite or within the sulfides of the volcanogenichydrothermal massive sulfide deposits that weresyndepositional with the formation (Rasmussenand Buick 2000) Textural information suggestthat these deposits were precipitated as blacksmoker chimneys in a submarine hydrothermalsetting driven by heat released from the intrudingStrelley Granite at approximately 324 Ga (Vearn-combe et al 1995) Fluorescing fluid inclusions oc-cur in quartz overgrowths in sandstones from theLalla Rookh Sandstone (Dutkiewicz et al 1998)Rasmussen and Buick (2000) and Dutkiewicz et al(1998) have argued for a biogenic origin for the hy-drocarbons however an abiotic origin involvingfor example Fischer-Tropsch-type synthesis ap-pears possible (Brasier et al 2002)

The Late Archean (and Early Paleoproterozoic)record preserved in the Hamersley and relatedbasins is stratigraphically much more coherentthan that preserved in the earlier successions TheHamersley Basin in particular provides a verycomprehensive record of the early oxygenation ofthe atmosphere in the form of large eustaticallycontrolled upward-shallowing sequences that in-clude black shales BIFs and ultimately at theirtops biogenic platform carbonates (Lindsay andBrasier 2002ab) These depositional cycles con-tain organic-rich shales at their base that recordevidence for some of Earthrsquos earliest biosphere(Brocks et al 1999) BIFs (Trendall 1983 1990)that record the rise of oxygen in the atmosphereand carbonates that document some of Earthrsquosearliest reef-like carbonate structures (Simonsonand Hassler 1996 1997 Simonson et al 1993ab)

SAMPLING STRATEGY

The Archean rocks of the Pilbara Craton arethus part of a protocontinent about which the

nascent Australian Craton developed It consistsof granitoid complexes overlain by a carapace ofsynchronously deposited volcano-sedimentarywedges Resting upon the early Archean proto-continent are a series of supracrustal basins Inall the sedimentary record of the Pilbara extendsfrom rocks as old as 354 Ga to some as young asapproximately 19 Ga As summarized in the fol-lowing paragraphs the sedimentary record of thePilbara records evidence of the major tectonic andgeological events that accompanied the evolutionof the Archean biosphere

It is here proposed that we develop a carefullydocumented and curated sample collection fromthis unique setting The collection would form thebasis for early Earth studies and a focus for astro-biology The sample set would be designed to re-flect the evolutionary succession described abovewith emphasis placed on cherts lower in the suc-cession and on clastic sediments higher in the suc-cession Any comprehensive reference collectionmust reflect diversity of the succession and includerocks from each of the sedimentary environmentsas well as igneous rocks reflecting the frameworkin which the protocontinent assembled

The five megasequences at the North Shawstudy area (Fig 3) are very different and reflectthe evolving Early Archean protocontinent Theearliest sedimentary record is limited and domi-nated by hydrothermally generated laminatedcherts and BIFs which appear as thin interbedswithin massive basalts (Coonterunah Group) (Fig5) Gradually clastic rocks appear dominated atfirst by volcanoclastics followed later by the firstevidence of sediments containing detrital quartzderived from true continental granitic crust Hy-drothermally generated laminated chert carbon-ate and barite units occur throughout the sectionThe first evidence of erosion appears at the top ofthe Coonterunah Group in the form of an angu-lar unconformity (Buick et al 1995b)

The basins that form the supracrustal succes-sion on the Pilbara Craton offer important in-sights into the closing phases of the Archean andthe immediate post-Archean world Because oftheir economic importance the basins are wellmapped and drill core is readily available (seeLindsay and Brasier 2002a) The succession pro-vides vital information about the early oxygena-tion of the atmosphere and the rapid growth ofthe biosphere that resulted from the assembly ofthe first continents and the initiation of the platetectonic cycle Platform carbonates whose geom-

LINDSAY ET AL752

etry was determined by eustasy are widespreadand contain considerable information on earlyoxygenic photosynthesis and the development ofthe first reef-like structure that are similar inmany ways to modern reefs (Simonson et al1993ab Lindsay and Brasier 2002ab)

The collection would consist of three groupingsof samples stratigraphic samples site-specificsamples and framework samples

Stratigraphic samples

The stratigraphic sample set would be collectedfrom well-defined settings such as carefully mea-sured and documented sections or from drill coreSamples would be collected from each of the fivestratigraphic groups or megasequences Thesupracrustal successions preserved in the Hamer-sley and associated basins would where possiblebe taken from existing drill core or from drill coresobtained for special projects Where drill core isnot available samples would be collected fromcarefully selected and measured outcrop sectionsThe samples would concentrate on rocks mostlikely to host biogenic material Samples from theEarly Archean volcaniclastic carapace would bedominated by cherts that are most likely to pre-serve fine biogenic structures but would also in-clude other sedimentary hydrothermal units suchas barite and carbonate that would be used in sta-ble isotope and geochemical studies

Site-specific sample

This sample set would focus on sites where po-tential biogenic structures have been documentedsuch as the stromatolitic intervals in the DresserFormation and the Strelley Pool Chert It wouldalso include detailed sampling of sites such asChinaman Creek and the Strelley Pool Chertwhere purported microfossils have been docu-mented (Schopf 1993) and the sulfide bodies inthe Kangaroo Caves Formation that contain pos-sible microfossils as well as evidence of hydro-carbons (Rasmussen 2000 Rasmussen and Buick2000) This would involve a broad samplingaround the sites in measured sections and drillcore to establish the geological context

Framework samples

While the focus of the collecting would be onmaterial that would help define the character of

the Earthrsquos earliest biosphere it is important thatthe broad context of the samples be defined TheArchean is a period when the Earthrsquos crust wasevolving rapidly and there is a growing body ofevidence to suggest that the evolution of theplanet drives the evolution of the atmosphere andbiosphere (Lindsay and Brasier 2002ab) Theframework sample set would include suites ofsample from the extrusive volcanic rocks col-lected along well-documented sections Sectionsthrough the volcanoclastic succession would em-phasize the dominant volcanic units (basalts ko-matiites andesites felsites and rhyolites) assources of information on the mantle and evolv-ing crust This sample set would also include adetailed sampling of the granitoid complexeswhich ultimately drove the entire process as wellas the later intrusions that were important in dri-ving the hydrothermal systems that may wellhave been the cradle of early life

SAMPLE CURATIONCHARACTERIZATION AND

DISTRIBUTION

All samples would be fully documented in thefield Samples would be photographed and theirlocations determined using a Global PositioningSystem receiver For specialized samples such asthose thought to contain easily contaminated bio-geochemical information the field collectionwould be modeled on techniques developed bythe NASANational Science FoundationSmith-sonian Institution Antarctic Search for Meteoritesprogram (Harvey and Schutt 1998)

Characterization

The petrologic and petrographic characteristicsof each sample would be determined using thin-section optical microscopy The quantitative bulkchemical composition of each sample would bemeasured by x-ray fluorescence spectrometry Fi-nally the qualitative bulk mineralogy of eachsample would be measured by powder x-ray dif-fraction

Data management

An Archean Geologic Reference Materials Data-base would be produced to fully document eachsample Field documentation would include sam-pling rationale supporting references and field

ARCHEAN GEOLOGIC REFERENCE COLLECTION 753

photographs Documentation would include theresults of all analyses along with images of rep-resentative sample surfaces and optical micro-graphs The Database would be produced in elec-tronic format and posted on the web Subsamplesof the Archean Geologic Reference Materialswould then be made available at no cost to re-searchers in the astrobiology community

DISCUSSION AND CONCLUSIONS

The Early Archean environment was uniqueRecent studies indicate that the atmosphere waslargely anoxic with oxygen levels 1025 of thepresent atmospheric level (Pavlov and Kasting2002) There were no large continents no activeor passive margins and by default no intracra-tonic basins Significantly perhaps there are nosigns of eustasy The crust was evolving rapidlyand heat flow was high leading to the intrusionof large granitoid bodies and the development ofa volcanic carapace above them In turn thismeant that the marine environment was domi-nated by hydrothermal activity

In the absence of large continental masses de-trital materials other than volcanoclastics are rel-atively rare in the Early Archean Significant vol-umes of quartz sand do not appear in the PilbaraCraton sedimentary record until after 32 Ga pre-sumably as the intruding igneous bodies begin tobe unroofed and eroded There are thus no ma-jor accumulations of sedimentary rocks that wenormally expect associated with passive conti-nental margins

With the assembly of the earliest large conti-nents beginning at approximately 30ndash28 Gathe Earthrsquos atmosphere hydrosphere and bios-phere changed rapidly Intracratonic basinssuch as the Hamersley Basin began to developon the new crustal blocks opening new shal-low-water environments that provided an idealsetting for photosynthesizing organisms andthe development of the first carbonate reefstructures (Lindsay and Brasier 2002ab) Hy-drothermal processes while still importantwere thus no longer the only sources of energyavailable to the evolving biosphere

The Archean was thus an unfamiliar environ-ment and unravelling the nature of the earliestbiosphere will require development of new ap-proaches and new techniques Without large con-tinental masses subareal exposures are likely to

be rare and short-lived There is no evidence forstream activity until the De Grey group at ap-proximately 29 Ga at which time braided streamgravels were preserved Thus models involvingearly biospheric evolution in ldquoa warm little pondrdquo(Darwin 1888) or in fresh water seem unlikelyInstead the focus in the search for the origins oflife on Earth should be directed to hydrothermalmarine settings involving high temperatures andwhat would today be hostile geochemical envi-ronments with significant concentrations of metal-lic ions and a largely reducing environment Therisks for the misidentification of abiotic structuresand geochemical signatures as signs of ancient lifeare significant as seen in recent discussions (egBrasier et al 2002 Fedo and Whitehouse 2002Schopf et al 2002)

These difficulties suggest that much could begained by the development of a well-documentedcollection of Archean rocks that could be madeavailable to all qualified investigators Until weunderstand the earliest stages of the evolution oflife on Earth we cannot hope to make significantprogress in the search for life on other planetssuch as Mars Until there is a sample return mis-sion to Mars we must make do with meteoritesthat come to us largely out of context An under-standing of the Archean Earth can help placethose limited samples in a planetary context

ACKNOWLEDGMENTS

Franco Pirajno Kath Grey and Martin VanKranendonk provided generous advice and as-sistance and the Geological Survey of WesternAustralia supported the field program We aregrateful to Peter Morant of Sipa Resources andAdrian Black of Newexco Perth for their adviceand for access to drill-core materials

ABBREVIATION

BIF banded iron formation

REFERENCES

Arndt NT Nelson DR Compston W Trendall AFand Thorne AM (1991) The age of the FortescueGroup Hamersley Basin Western Australia from ionmicroprobe zircon U-Pb results Aust J Earth Sci 38262ndash281

LINDSAY ET AL754

Awramik SM Schopf JW and Walter MR (1983) Fil-amentous fossil bacteria from the Archaean of WesternAustralia Precambrian Res 20 357ndash374

Barley ME Pickard AL and Sylvester PJ (1997) Em-placement of a large igneous province as a possiblecause of banded iron formation 245 billion years agoNature 385 55ndash58

Bettenay LF Bickle MJ Boulter CA Groves DIMorant P Blake TS and James BA (1981) Evolu-tion of the Shaw Batholithmdashan Archaean granitoid-gneiss dome in the eastern Pilbara Western AustraliaIn Special Publication 7 The Archaean edited by JEGlover and DI Groves Geological Society of WesternAustralia Perth pp 361ndash372

Blake TS and Barley ME (1992) Tectonic evolution ofthe Late Archaean to Early Proterozoic Mount BruceMegasequence Set Western Australia Tectonics 11 1415ndash1425

Blake TS and Groves DI (1987) Continental rifting andthe Archaean-Proterozoic transition Geology 15 229ndash232

Brasier MD and Lindsay JF (1998) A billion years ofenvironmental stability and the emergence of eukary-otes new data from Northern Australia Geology 26555ndash558

Brasier MD Green OR Jephcoat AP Kleppe AKVan Kranendonk MJ Lindsay JF Steele A andGrassineau NV (2002) Questioning the evidence forEarthrsquos oldest fossils Nature 416 76ndash81

Brauhart C (1999) Regional alteration systems associatedwith Archaean volcanogenic massive sulphide depositsat Panorama Pilbara Western Australia [PhD Thesis]University of Western Australia Perth

Brauhart C Groves DI and Morant P (1998) Regionalalteration systems associated with volcanogenic mas-sive sulfide mineralization at Panorama Pilbara West-ern Australia Econ Geol 93 292ndash302

Brocks J Logan GA Buick R and Summons RE(1999) Archean molecular fossils and the early rise ofeukaryotes Science 285 1033ndash1036

Buick R Dunlop JSR and Groves DI (1981) Stroma-tolite recognition in ancient rocks an appraisal of ir-regular laminated structures in an early Archaeanchert-barite unit from North Pole Western AustraliaAlcheringa 5 161ndash181

Buick R Des Marais DJ and Knoll AH (1995a) Sta-ble isotope compositions of carbonates from the Meso-proterozoic Bangemall Group northwestern AustraliaChem Geol 123 153ndash171

Buick R Thornett JR McNaughton NJ Smith JBBarley ME and Savage M (1995b) Record of emer-gent continental crust 35 billion years ago in the Pil-bara Craton of Australia Nature 375 574ndash577

Buick R Rasmussen B and Krapez B (1998) Archaeanoil evidence for extensive hydrocarbon generation andmigration 25ndash35 Ga Am Assoc Petroleum Geol Bull82 50ndash69

Chan M (1998) Sedimentology and structure of threesmall congolmeratic basins in the Lalla Rookh-WesternShaw structural corridor Pilbara Craton Western Aus-

tralia [BSc Honours Thesis] University of WesternAustralia Perth

Cullers RL DiMarco MJ Lowe DR and Stone J(1993) Geochemistry of a silicified felsic volcaniclasticsuite from the Early Archaean Panorama FormationPilbara Block Western Australiamdashan evaluation of de-positional and post-depositional processes with specialemphasis on the rare-Earth elements Precambrian Res60 99ndash116

Darwin F ed (1888) The Life and Letters of Charles Dar-win Vol 3 John Murray London

de Wit MJ (1998) On Archaean granites greenstonescratons and tectonics does the evidence demand a ver-dict Precambrian Geol 91 181ndash226

Drummond BJ (1981) Crustal structure of the Precam-brian terrains of northwest Australia from seismic re-fraction data BMR J Geol Geophys 6 123ndash135

Dunlop JSR Muir M Milne VA and Groves DI(1978) A new microfossil assemblage from the Ar-chaean of Western Australia Nature 274 676ndash678

Dutkiewicz A Rasmussen B and Buick R (1998) Oilpreserved in fluid inclusions in Archaean sandstonesNature 395 885ndash888

Fedo CM and Whitehouse MJ (2002) Metasomatic ori-gin of quartz-pyroxene rock Akilia Greenland and im-plications for Earthrsquos earliest life Science 296 1448ndash1452

Gee RD and Grey K (1993) Report 41 Proterozoic Rockson the Glengarry 1250 000 SheetmdashStratigraphy Structureand Stromatolite Biostratigraphy Geological Survey ofWestern Australia Perth

Glikson AY and Hickman AH (1981a) Chemicalstratigraphy and petrogenesis of Archaean basic-ultra-basic volcanic units eastern Pilbara Block WesternAustralia In Special Publication 7 Archaean Geologyedited by JE Groves and DI Groves Geological Soci-ety of Western Australia Perth pp 287ndash300

Glikson AY and Hickman AH (1981b) Record 198136Geochemistry of Archaean Volcanic Successions EasternPilbara Block Western Australia Australian Bureau ofMineral Resources Canberra

Glikson AY Davy R and Hickman AH (1991) Record199146 Trace Metal Distribution Patterns in Basalts Pil-bara Craton Western Australia with Stratigraphic-Geo-chemical Implications Australian Bureau of Mineral Re-sources Canberra

Groves DI Dunlop JSR and Buick R (1981) An earlyhabitat of life Sci Am 245 64ndash73

Harvey RP and Schutt JW (1998) Meteorite recoveryand reconnaissance in the Allan Hills-David Glacierand Darwin Glacier regions 1996ndash1997 Antarctic J US 32 25ndash27

Hickman AH (1975) Annual Report 1974 PrecambrianStructural Geology of Part of the Pilbara Region Geologi-cal Survey of Western Australia Perth pp 68ndash73

Hickman AH (1983) Bulletin 127 Geology of the PilbaraBlock and Its Environs Geological Survey of WesternAustralia Perth

Hickman AH (1984) Archaean diapirism in the PilbaraBlock Western Australia In Precambrian Tectonics Il-lustrated edited by A Kroner and R Greiling E

ARCHEAN GEOLOGIC REFERENCE COLLECTION 755

basalts at the base of the formation from ko-matiites towards the top

Kangaroo Caves Formation The Kangaroo CavesFormation which reaches a maximum thicknessof 15 km is best exposed towards the center ofthe study area The formation is complex and con-sists of a succession of andesitic volcanics anddacitic to rhyolitic sills and extrusives A chert in-terval up to 100 m thick forms a distinctivemarker at the top of the formation Close to thecenter of the study area at Sulphur Springs themassive chert is overlain by a polymict megabrec-cia that rests upon a sharp erosional contact (Hill1997 Vearncombe et al 1998)

Hosted within a 200-m dacite unit immediatelybeneath the marker chert are six sulfide ore bodiesspaced at regular intervals of 5ndash7 km around themargins of the Strelley Granite (Morant 1998) Theyconsist of pyrite sphalerite chalcopyrite and galenawith smaller amounts of other sulfides (Vearncombeet al 1995 1998 Vearncombe 1996 Morant 1998)The sulfides which were deposited in a blacksmoker hydrothermal setting occur in associationwith growth faults developed around the StrelleyGranite the intrusion of which was syndepositionalwith the Kangaroo Caves Formation (Vearncombe1996 Brauhart et al 1998 Brauhart 1999)

Megasequence 4mdashGorge Creek Group

The Gorge Creek Group consists of five for-mations (Fig 3) that show much greater litho-logic diversity than earlier megasequences Inparticular this interval includes for the firsttime a significant thickness of quartzose sand-stone as well as a variety of other clastic sedi-mentary rocks

Pincunah Hill Formation In the western part ofthe study area the basal Pincunah Hill Formationconsists of thinly bedded BIF with small amountsof laminated chert red to gray shale and silt-stone This basal unit is up to 1 km thick in thewestern part of the area but wedges out and dis-appears in a short distance eastward The upper

LINDSAY ET AL748

FIG 11 A measured section through the Strelley PoolChert showing the location of stromatolitic structuresand the carbonate interval (adapted from Van Kranen-donk 2000) (locality C Fig 2) The gamma log to theright shows continuity across the chert-carbonate

FIG 10 ldquoMicrofossilsrdquo from the Apex chert near Mar-ble Bar a computer-generated digital automontage ofputative beggiatoan Eoleptonema apex Holotype (Schopf1993) combining the most-sharply focused images fromsuccessive focal planes b and c Original manual pho-tomontage and interpretative sketch (Schopf 1993) whichomits lower structure at arrow in (b) d and e New sin-gle images showing continuity of original and newly im-aged and automontaged structures Reprinted with per-mission from Nature (Brasier et al 2002) Copyright 2002Macmillan Publishers Ltd

part of the formation consists largely of red orpurple shale (Van Kranendonk 2000)

Corboy Formation At its thickest in the easternpart of the study area the Corboy Formation in-cludes a number of distinct facies developedacross major growth faults (Wilhelmij and Dun-lop 1984 Wilhelmij 1986) Here the formationconsists of a basal quartzose sandstone abruptlyoverlain by conglomerate interbedded withgraded sandstone units red shale and coarsesandstone The formation is capped by massivesandstone and conglomerate The unit was de-posited as a series of small prograding shallowmarine fan deltas (Wilhelmij 1986 Hill 1997 VanKranendonk 2000)

Paddy Market Formation In the western part ofthe study area the Paddy Market Formation isdominantly cherty BIF with minor ferruginousshale Further east the formation becomes moreshale-rich although much of the shale has been re-placed by chert Sandstone is present in smallamounts in beds up to 10 m thick Towards thetop of the formation in the east the cherts containfelsic volcanic material that is overlain by up to 200m of massive dacite (Van Kranendonk 2000)

Honeyeater Basalt The Honeyeater Basaltwhich consists entirely of basalt and dolerite con-formably overlies the Paddy Market FormationThe lower part of the formation consists of high-Mg basalt interbedded with dolerite whereashigher in the formation the basalts are largelytholeiitic and interbedded with dolerite (Gliksonand Hickman 1981ab Glikson et al 1991)

Pyramid Hill Formation The Pyramid Hill For-mation is limited in areal extent It is found in thenorth central part of the study area east of theStrelley Granite The formation consists of redand black shales that when heavily silicifiedgrade into laminated black and white chert (VanKranendonk 2000) The Gorge Creek Group iscapped by a well-bedded BIF

Megasequence 5mdashDe Grey Group

The De Grey Group consists of a single for-mation the Lalla Rookh Sandstone (Fig 3)which is dominated by clastic sedimentary rocksthat contain a significant quartzose componentThe Lalla Rookh Sandstone which rests on awell-defined unconformity above the GorgeCreek Group reaches a maximum thickness of

ARCHEAN GEOLOGIC REFERENCE COLLECTION 749

FIG 12 Stromatolitic carbonates in the Strelley Pool Chert (locality C Fig 2) Laminae are isopachous and theapex of the cone is ruptured and veined by secondary carbonate (arrow) Scale bar 5 10 cm

3 km (Krapez 1984) In the north central part ofthe study area the formation consists entirely ofcoarse clastic sediments and minor shalewhereas to the south the predominate fill is lithicand feldspathic sandstone (Chan 1998) Theserocks provide the first clear evidence of streamactivity in the Pilbara succession (Krapez 1984Van Kranendonk and Collins 1998 Van Kra-nendonk 2000)

THE PILBARA SUPRACRUSTAL SUCCESSION

The late Archean and early Paleoproterozoicbasins that rest upon the ancient cratonic blocksof Western Australia (Fig 1) form a time seriesassociated with the formation and ultimate dis-assembly of one of Earthrsquos earliest major conti-nental masses This was followed by the suturingof the Pilbara and Yilgarn Cratons along theCapricorn Orogen as a new supercontinentevolved in the Paleoproterozoic (Rogers and San-tosh 2002) These basins contain an importantsedimentary record of the early Earth includingsome of Earthrsquos earliest stromatolites and plat-form carbonate deposits (Simonson et al1993ab) organic-rich black shales (Brocks et al1999) and BIF (Trendall 1983 1990) The strati-graphic and structural framework is well con-strained by radiometric ages (Arndt et al 1991Barley et al 1997 Trendall et al 1998) providinga record at a critical time in Earth history whenthe atmosphere was first becoming oxygen-rich(Lindsay and Brasier 2002ab)

The Late Archean-Early PaleoproterozoicHamersley Basin which lies close to the southernmargin of the Pilbara Block (Trendall 1983 1990Blake and Barley 1992) overlies a normal crustalthickness (30ndash40 km) (Drummond 1981) Thebasin began to subside at approximately 28 Gafollowing an episode of crustal extension The ar-chitecture of the basin fill is complex andpolyphase with major erosional surfaces separat-ing areally extensive megasequencessuperse-quences (Trendall 1990 Krapez 1996) DuringFortescue and Hamersley Group time (Fig 4)sedimentation began in a rift setting in a silici-clastic-starved environment The later stages ofHamersley Basin sedimentation (Turee CreekGroup times) record the early transition from apassive margin to a foreland basin setting andwith it the establishment of a terrigenous sedi-

ment supply (Tyler and Thorne 1990) The suc-cession is largely marine Evidence of eustasy ap-pears for the first time in the sedimentary suc-cession of the Hamersley Basin (Lindsay andBrasier 2002ab)

The geometry and sequence stacking pattern ofthe succession are both very similar to youngerintracratonic basins (see Lindsay et al 1993) TheHamersley Basin is thus the first basinal settingpreserved on the Australian craton in which ma-rine sediments and in particular platform car-bonate rocks have been preserved in a responseto broad regional crustal subsidence and wheredeposition is controlled by eustasy

The Ashburton Basin (22ndash18 Ga) discon-formably overlies the southern margin of theHamersley Basin along its the contact with theCapricorn Orogen (Fig 1) (Thorne and Seymour1991) The basin forms the northern margin of theCapricorn Orogen a deformed zone that devel-oped during ocean closure Loading of the oceanfloor by the approaching Yilgarn Craton led tothe development of a westndashnorthwest-orientedforeland basin [McGrath Trough (Horowitz1982)] with an uplifted orogenic margin to thesouthwest (Blake and Groves 1987 Blake andBarley 1992) with the result that sedimentationwas dominated by an abundant supply of ter-rigenous clastic material (Mount McGrath For-mation) Continued loading of the crust ulti-mately led to the supply of terrigenous materialsbeing disrupted and producing a prograding car-bonate shelf (Duck Creek Dolomite) Basin sedi-mentation was accompanied by active-marginmafic volcanism (Cheela Springs Basalt)

THE CAPRICORN OROGEN ANDASSOCIATED BASINS

The Yerrida (22ndash19 Ga) Bryah (20 Ga)Padbury (20 Ga) and Earaheedy (19ndash165Ga) Basins lie to the southeast of the Hamersleyand Ashburton Basins and overlie the CapricornOrogen (Fig 1) (Occhipinti et al 1997 1998 Pi-rajno et al 1998 Pirajno and Adamides 2000)These now fragmentary basins formed along thenorthern margin of the Yilgarn Craton in a back-arc setting and record the convergence and colli-sion of the Archean Pilbara and Yilgarn Cratons(Tyler and Thorne 1990 Thorne and Seymour1991 Occhipinti et al 1997 1998 Pirajno et al1998) Because of their active margin settings the

LINDSAY ET AL750

fill of these small basins is dominated by clasticsediments However platform carbonate unitsare preserved in the Yerrida Basin (Gee and Grey1993 Pirajno and Adamides 2000) and a signif-icant thickness of carbonate rocks also occurs inthe Earaheedy Basin

THE ARCHEAN BIOSPHERE

The end of the Archean is formally defined at25 Ga (Plumb 1991) In reality the Archean worldended at some point in time closer to 28 or 30 Gawhen the first large continental blocks began to as-semble The stratigraphic record preserved on theancient Australian Craton reflects this major tran-sition The rock record from the Early Archean isvery different from that of the Late Archean in thatthe early record is predominantly volcanic and hy-drothermal in nature (ldquogreenstonerdquo) while thelater record is largely clastic and more readily com-pared with the Phanerozoic record

Structures and textures of possible biogenicorigin that are preserved in the stratigraphic suc-cession in the Early Archean study area are wellknown and are believed to provide the earliestdirect evidence of Earthrsquos biosphere Stromatoliticstructures have been encountered at two levelswithin the section The Dresser Formation con-tains structures preserved in chert and barite in-tervals that have been interpreted as stromatolitesand oncolites (Dunlop et al 1978 Walter et al1980 Buick et al 1981 Groves et al 1981 Wal-ter 1983) The stromatolitic structures occur forthe most part in layered barite units that devel-oped over swarms of synvolcanic hydrothermalfeeder dykes along listric growth faults (Nijmanet al 1998 Van Kranendonk 2004) Although ithas been argued that these structures are biogenic(Walter et al 1980 Schopf and Walter 1983 Wal-ter 1983) their direct association with the feederdykes leaves room for doubt (Buick et al 1981)The setting is complex and there is considerablescope for more detailed contextual research

The second stromatolitic horizon occurs in theoverlying Strelley Pool Chert a complex unit con-sisting of at least five members (Lowe 1980 1983Hofmann et al 1999) The main stromatolitic unitoccurs as an 8-m-thick interval in the middle ofthe formation and appears to have a consistentand widespread pattern of deposition The struc-tures have been the focus of considerable discus-sion and their biogenicity has been questioned

(Buick et al 1981 Lowe 1995) The structures arecomplex and varied (Fig 12) leading to a morerecent re-evaluation again suggesting that theyare biogenic (Hofmann et al 1999) However thelaminae are largely isopachous an indication thatthey could be abiotic precipitates (cf Pope andGrotzinger 2000) Given the intense hydrother-mal activity that appears to have taken placethroughout the depositional time interval thestructure may well be the product of hydrother-mal deposition (Lindsay et al 2003ab) In all ourobservations suggest that considerably morework needs to be done to establish the deposi-tional context of these stromatolitic structures be-fore they can be confidently ascribed as biogenicin origin

Structures resembling remarkably preservedbacterial and cyanobacterial microfossils havebeen found at several localities in and close to thestudy area (Awramik et al 1983 Schopf andPacker 1987 Schopf 1992 1993 Rasmussen 2000)A complex biota of 11 species of filamentousprokaryotes which are compared with cyanobac-teria (Schopf and Packer 1987 Schopf 1992 1993)have been described from the Apex chert close toMarble Bar (21deg105579S 119deg418789E) The taxo-nomic interpretations are important in that theyprovide the earliest evidence of morphological lifeon Earth and support the hypothesis of an earlybeginning for oxygenic photosynthesis (Schopf1999) The structures are almost a billion yearsolder than the oldest known putative cyanobac-terial biomarkers (Summons et al 1999) and oc-cur well before the earliest evidence for the risein oxygen in the atmosphere (for a summary seeKnoll and Canfield 1998 Lindsay and Brasier2000 2002ab) However in a recent re-evaluationof the original Apex chert material it has been ar-gued that the filaments are not biogenic but aresecondary artifacts formed during diagenesisfrom abiogenic organic compounds generated inhydrothermal chert veins (Brasier et al 2002 VanKranendonk 2004)

Filaments believed to be biogenic in originhave also been reported from the 324 Ga Kan-garoo Caves Formation (Rasmussen 2000) wherethey form part of the Sulphur Springs deposit as-sociated with a black smoker hydrothermal ventsystem (Vearncombe et al 1995 1998) Ras-mussen (2000) argued that the filaments are rem-nants of a hydrothermal biota of thermophilicchemotrophic prokaryotes Sulfide textures areremarkably well preserved in these complex de-

ARCHEAN GEOLOGIC REFERENCE COLLECTION 751

posits and are worthy of much more detailedanalysis as they may well preserve evidence ofthe early biosphere

Finally there is evidence that hydrocarbons aretrapped in these ancient sedimentary rocks eitheras fluid inclusions in hydrothermal precipitates oras bituminous residues in larger cavities in theKangaroo Caves Formation and in the Lalla RookhSandstone (Dutkiewicz et al 1998 Rasmussen andBuick 2000) Buick et al (1998) also reported car-bonaceous globules in cherts from the lower War-rawoona Group In the Kangaroo Caves Forma-tion the hydrocarbons occur in hydrothermalbarite or within the sulfides of the volcanogenichydrothermal massive sulfide deposits that weresyndepositional with the formation (Rasmussenand Buick 2000) Textural information suggestthat these deposits were precipitated as blacksmoker chimneys in a submarine hydrothermalsetting driven by heat released from the intrudingStrelley Granite at approximately 324 Ga (Vearn-combe et al 1995) Fluorescing fluid inclusions oc-cur in quartz overgrowths in sandstones from theLalla Rookh Sandstone (Dutkiewicz et al 1998)Rasmussen and Buick (2000) and Dutkiewicz et al(1998) have argued for a biogenic origin for the hy-drocarbons however an abiotic origin involvingfor example Fischer-Tropsch-type synthesis ap-pears possible (Brasier et al 2002)

The Late Archean (and Early Paleoproterozoic)record preserved in the Hamersley and relatedbasins is stratigraphically much more coherentthan that preserved in the earlier successions TheHamersley Basin in particular provides a verycomprehensive record of the early oxygenation ofthe atmosphere in the form of large eustaticallycontrolled upward-shallowing sequences that in-clude black shales BIFs and ultimately at theirtops biogenic platform carbonates (Lindsay andBrasier 2002ab) These depositional cycles con-tain organic-rich shales at their base that recordevidence for some of Earthrsquos earliest biosphere(Brocks et al 1999) BIFs (Trendall 1983 1990)that record the rise of oxygen in the atmosphereand carbonates that document some of Earthrsquosearliest reef-like carbonate structures (Simonsonand Hassler 1996 1997 Simonson et al 1993ab)

SAMPLING STRATEGY

The Archean rocks of the Pilbara Craton arethus part of a protocontinent about which the

nascent Australian Craton developed It consistsof granitoid complexes overlain by a carapace ofsynchronously deposited volcano-sedimentarywedges Resting upon the early Archean proto-continent are a series of supracrustal basins Inall the sedimentary record of the Pilbara extendsfrom rocks as old as 354 Ga to some as young asapproximately 19 Ga As summarized in the fol-lowing paragraphs the sedimentary record of thePilbara records evidence of the major tectonic andgeological events that accompanied the evolutionof the Archean biosphere

It is here proposed that we develop a carefullydocumented and curated sample collection fromthis unique setting The collection would form thebasis for early Earth studies and a focus for astro-biology The sample set would be designed to re-flect the evolutionary succession described abovewith emphasis placed on cherts lower in the suc-cession and on clastic sediments higher in the suc-cession Any comprehensive reference collectionmust reflect diversity of the succession and includerocks from each of the sedimentary environmentsas well as igneous rocks reflecting the frameworkin which the protocontinent assembled

The five megasequences at the North Shawstudy area (Fig 3) are very different and reflectthe evolving Early Archean protocontinent Theearliest sedimentary record is limited and domi-nated by hydrothermally generated laminatedcherts and BIFs which appear as thin interbedswithin massive basalts (Coonterunah Group) (Fig5) Gradually clastic rocks appear dominated atfirst by volcanoclastics followed later by the firstevidence of sediments containing detrital quartzderived from true continental granitic crust Hy-drothermally generated laminated chert carbon-ate and barite units occur throughout the sectionThe first evidence of erosion appears at the top ofthe Coonterunah Group in the form of an angu-lar unconformity (Buick et al 1995b)

The basins that form the supracrustal succes-sion on the Pilbara Craton offer important in-sights into the closing phases of the Archean andthe immediate post-Archean world Because oftheir economic importance the basins are wellmapped and drill core is readily available (seeLindsay and Brasier 2002a) The succession pro-vides vital information about the early oxygena-tion of the atmosphere and the rapid growth ofthe biosphere that resulted from the assembly ofthe first continents and the initiation of the platetectonic cycle Platform carbonates whose geom-

LINDSAY ET AL752

etry was determined by eustasy are widespreadand contain considerable information on earlyoxygenic photosynthesis and the development ofthe first reef-like structure that are similar inmany ways to modern reefs (Simonson et al1993ab Lindsay and Brasier 2002ab)

The collection would consist of three groupingsof samples stratigraphic samples site-specificsamples and framework samples

Stratigraphic samples

The stratigraphic sample set would be collectedfrom well-defined settings such as carefully mea-sured and documented sections or from drill coreSamples would be collected from each of the fivestratigraphic groups or megasequences Thesupracrustal successions preserved in the Hamer-sley and associated basins would where possiblebe taken from existing drill core or from drill coresobtained for special projects Where drill core isnot available samples would be collected fromcarefully selected and measured outcrop sectionsThe samples would concentrate on rocks mostlikely to host biogenic material Samples from theEarly Archean volcaniclastic carapace would bedominated by cherts that are most likely to pre-serve fine biogenic structures but would also in-clude other sedimentary hydrothermal units suchas barite and carbonate that would be used in sta-ble isotope and geochemical studies

Site-specific sample

This sample set would focus on sites where po-tential biogenic structures have been documentedsuch as the stromatolitic intervals in the DresserFormation and the Strelley Pool Chert It wouldalso include detailed sampling of sites such asChinaman Creek and the Strelley Pool Chertwhere purported microfossils have been docu-mented (Schopf 1993) and the sulfide bodies inthe Kangaroo Caves Formation that contain pos-sible microfossils as well as evidence of hydro-carbons (Rasmussen 2000 Rasmussen and Buick2000) This would involve a broad samplingaround the sites in measured sections and drillcore to establish the geological context

Framework samples

While the focus of the collecting would be onmaterial that would help define the character of

the Earthrsquos earliest biosphere it is important thatthe broad context of the samples be defined TheArchean is a period when the Earthrsquos crust wasevolving rapidly and there is a growing body ofevidence to suggest that the evolution of theplanet drives the evolution of the atmosphere andbiosphere (Lindsay and Brasier 2002ab) Theframework sample set would include suites ofsample from the extrusive volcanic rocks col-lected along well-documented sections Sectionsthrough the volcanoclastic succession would em-phasize the dominant volcanic units (basalts ko-matiites andesites felsites and rhyolites) assources of information on the mantle and evolv-ing crust This sample set would also include adetailed sampling of the granitoid complexeswhich ultimately drove the entire process as wellas the later intrusions that were important in dri-ving the hydrothermal systems that may wellhave been the cradle of early life

SAMPLE CURATIONCHARACTERIZATION AND

DISTRIBUTION

All samples would be fully documented in thefield Samples would be photographed and theirlocations determined using a Global PositioningSystem receiver For specialized samples such asthose thought to contain easily contaminated bio-geochemical information the field collectionwould be modeled on techniques developed bythe NASANational Science FoundationSmith-sonian Institution Antarctic Search for Meteoritesprogram (Harvey and Schutt 1998)

Characterization

The petrologic and petrographic characteristicsof each sample would be determined using thin-section optical microscopy The quantitative bulkchemical composition of each sample would bemeasured by x-ray fluorescence spectrometry Fi-nally the qualitative bulk mineralogy of eachsample would be measured by powder x-ray dif-fraction

Data management

An Archean Geologic Reference Materials Data-base would be produced to fully document eachsample Field documentation would include sam-pling rationale supporting references and field

ARCHEAN GEOLOGIC REFERENCE COLLECTION 753

photographs Documentation would include theresults of all analyses along with images of rep-resentative sample surfaces and optical micro-graphs The Database would be produced in elec-tronic format and posted on the web Subsamplesof the Archean Geologic Reference Materialswould then be made available at no cost to re-searchers in the astrobiology community

DISCUSSION AND CONCLUSIONS

The Early Archean environment was uniqueRecent studies indicate that the atmosphere waslargely anoxic with oxygen levels 1025 of thepresent atmospheric level (Pavlov and Kasting2002) There were no large continents no activeor passive margins and by default no intracra-tonic basins Significantly perhaps there are nosigns of eustasy The crust was evolving rapidlyand heat flow was high leading to the intrusionof large granitoid bodies and the development ofa volcanic carapace above them In turn thismeant that the marine environment was domi-nated by hydrothermal activity

In the absence of large continental masses de-trital materials other than volcanoclastics are rel-atively rare in the Early Archean Significant vol-umes of quartz sand do not appear in the PilbaraCraton sedimentary record until after 32 Ga pre-sumably as the intruding igneous bodies begin tobe unroofed and eroded There are thus no ma-jor accumulations of sedimentary rocks that wenormally expect associated with passive conti-nental margins

With the assembly of the earliest large conti-nents beginning at approximately 30ndash28 Gathe Earthrsquos atmosphere hydrosphere and bios-phere changed rapidly Intracratonic basinssuch as the Hamersley Basin began to developon the new crustal blocks opening new shal-low-water environments that provided an idealsetting for photosynthesizing organisms andthe development of the first carbonate reefstructures (Lindsay and Brasier 2002ab) Hy-drothermal processes while still importantwere thus no longer the only sources of energyavailable to the evolving biosphere

The Archean was thus an unfamiliar environ-ment and unravelling the nature of the earliestbiosphere will require development of new ap-proaches and new techniques Without large con-tinental masses subareal exposures are likely to

be rare and short-lived There is no evidence forstream activity until the De Grey group at ap-proximately 29 Ga at which time braided streamgravels were preserved Thus models involvingearly biospheric evolution in ldquoa warm little pondrdquo(Darwin 1888) or in fresh water seem unlikelyInstead the focus in the search for the origins oflife on Earth should be directed to hydrothermalmarine settings involving high temperatures andwhat would today be hostile geochemical envi-ronments with significant concentrations of metal-lic ions and a largely reducing environment Therisks for the misidentification of abiotic structuresand geochemical signatures as signs of ancient lifeare significant as seen in recent discussions (egBrasier et al 2002 Fedo and Whitehouse 2002Schopf et al 2002)

These difficulties suggest that much could begained by the development of a well-documentedcollection of Archean rocks that could be madeavailable to all qualified investigators Until weunderstand the earliest stages of the evolution oflife on Earth we cannot hope to make significantprogress in the search for life on other planetssuch as Mars Until there is a sample return mis-sion to Mars we must make do with meteoritesthat come to us largely out of context An under-standing of the Archean Earth can help placethose limited samples in a planetary context

ACKNOWLEDGMENTS

Franco Pirajno Kath Grey and Martin VanKranendonk provided generous advice and as-sistance and the Geological Survey of WesternAustralia supported the field program We aregrateful to Peter Morant of Sipa Resources andAdrian Black of Newexco Perth for their adviceand for access to drill-core materials

ABBREVIATION

BIF banded iron formation

REFERENCES

Arndt NT Nelson DR Compston W Trendall AFand Thorne AM (1991) The age of the FortescueGroup Hamersley Basin Western Australia from ionmicroprobe zircon U-Pb results Aust J Earth Sci 38262ndash281

LINDSAY ET AL754

Awramik SM Schopf JW and Walter MR (1983) Fil-amentous fossil bacteria from the Archaean of WesternAustralia Precambrian Res 20 357ndash374

Barley ME Pickard AL and Sylvester PJ (1997) Em-placement of a large igneous province as a possiblecause of banded iron formation 245 billion years agoNature 385 55ndash58

Bettenay LF Bickle MJ Boulter CA Groves DIMorant P Blake TS and James BA (1981) Evolu-tion of the Shaw Batholithmdashan Archaean granitoid-gneiss dome in the eastern Pilbara Western AustraliaIn Special Publication 7 The Archaean edited by JEGlover and DI Groves Geological Society of WesternAustralia Perth pp 361ndash372

Blake TS and Barley ME (1992) Tectonic evolution ofthe Late Archaean to Early Proterozoic Mount BruceMegasequence Set Western Australia Tectonics 11 1415ndash1425

Blake TS and Groves DI (1987) Continental rifting andthe Archaean-Proterozoic transition Geology 15 229ndash232

Brasier MD and Lindsay JF (1998) A billion years ofenvironmental stability and the emergence of eukary-otes new data from Northern Australia Geology 26555ndash558

Brasier MD Green OR Jephcoat AP Kleppe AKVan Kranendonk MJ Lindsay JF Steele A andGrassineau NV (2002) Questioning the evidence forEarthrsquos oldest fossils Nature 416 76ndash81

Brauhart C (1999) Regional alteration systems associatedwith Archaean volcanogenic massive sulphide depositsat Panorama Pilbara Western Australia [PhD Thesis]University of Western Australia Perth

Brauhart C Groves DI and Morant P (1998) Regionalalteration systems associated with volcanogenic mas-sive sulfide mineralization at Panorama Pilbara West-ern Australia Econ Geol 93 292ndash302

Brocks J Logan GA Buick R and Summons RE(1999) Archean molecular fossils and the early rise ofeukaryotes Science 285 1033ndash1036

Buick R Dunlop JSR and Groves DI (1981) Stroma-tolite recognition in ancient rocks an appraisal of ir-regular laminated structures in an early Archaeanchert-barite unit from North Pole Western AustraliaAlcheringa 5 161ndash181

Buick R Des Marais DJ and Knoll AH (1995a) Sta-ble isotope compositions of carbonates from the Meso-proterozoic Bangemall Group northwestern AustraliaChem Geol 123 153ndash171

Buick R Thornett JR McNaughton NJ Smith JBBarley ME and Savage M (1995b) Record of emer-gent continental crust 35 billion years ago in the Pil-bara Craton of Australia Nature 375 574ndash577

Buick R Rasmussen B and Krapez B (1998) Archaeanoil evidence for extensive hydrocarbon generation andmigration 25ndash35 Ga Am Assoc Petroleum Geol Bull82 50ndash69

Chan M (1998) Sedimentology and structure of threesmall congolmeratic basins in the Lalla Rookh-WesternShaw structural corridor Pilbara Craton Western Aus-

tralia [BSc Honours Thesis] University of WesternAustralia Perth

Cullers RL DiMarco MJ Lowe DR and Stone J(1993) Geochemistry of a silicified felsic volcaniclasticsuite from the Early Archaean Panorama FormationPilbara Block Western Australiamdashan evaluation of de-positional and post-depositional processes with specialemphasis on the rare-Earth elements Precambrian Res60 99ndash116

Darwin F ed (1888) The Life and Letters of Charles Dar-win Vol 3 John Murray London

de Wit MJ (1998) On Archaean granites greenstonescratons and tectonics does the evidence demand a ver-dict Precambrian Geol 91 181ndash226

Drummond BJ (1981) Crustal structure of the Precam-brian terrains of northwest Australia from seismic re-fraction data BMR J Geol Geophys 6 123ndash135

Dunlop JSR Muir M Milne VA and Groves DI(1978) A new microfossil assemblage from the Ar-chaean of Western Australia Nature 274 676ndash678

Dutkiewicz A Rasmussen B and Buick R (1998) Oilpreserved in fluid inclusions in Archaean sandstonesNature 395 885ndash888

Fedo CM and Whitehouse MJ (2002) Metasomatic ori-gin of quartz-pyroxene rock Akilia Greenland and im-plications for Earthrsquos earliest life Science 296 1448ndash1452

Gee RD and Grey K (1993) Report 41 Proterozoic Rockson the Glengarry 1250 000 SheetmdashStratigraphy Structureand Stromatolite Biostratigraphy Geological Survey ofWestern Australia Perth

Glikson AY and Hickman AH (1981a) Chemicalstratigraphy and petrogenesis of Archaean basic-ultra-basic volcanic units eastern Pilbara Block WesternAustralia In Special Publication 7 Archaean Geologyedited by JE Groves and DI Groves Geological Soci-ety of Western Australia Perth pp 287ndash300

Glikson AY and Hickman AH (1981b) Record 198136Geochemistry of Archaean Volcanic Successions EasternPilbara Block Western Australia Australian Bureau ofMineral Resources Canberra

Glikson AY Davy R and Hickman AH (1991) Record199146 Trace Metal Distribution Patterns in Basalts Pil-bara Craton Western Australia with Stratigraphic-Geo-chemical Implications Australian Bureau of Mineral Re-sources Canberra

Groves DI Dunlop JSR and Buick R (1981) An earlyhabitat of life Sci Am 245 64ndash73

Harvey RP and Schutt JW (1998) Meteorite recoveryand reconnaissance in the Allan Hills-David Glacierand Darwin Glacier regions 1996ndash1997 Antarctic J US 32 25ndash27

Hickman AH (1975) Annual Report 1974 PrecambrianStructural Geology of Part of the Pilbara Region Geologi-cal Survey of Western Australia Perth pp 68ndash73

Hickman AH (1983) Bulletin 127 Geology of the PilbaraBlock and Its Environs Geological Survey of WesternAustralia Perth

Hickman AH (1984) Archaean diapirism in the PilbaraBlock Western Australia In Precambrian Tectonics Il-lustrated edited by A Kroner and R Greiling E

ARCHEAN GEOLOGIC REFERENCE COLLECTION 755

part of the formation consists largely of red orpurple shale (Van Kranendonk 2000)

Corboy Formation At its thickest in the easternpart of the study area the Corboy Formation in-cludes a number of distinct facies developedacross major growth faults (Wilhelmij and Dun-lop 1984 Wilhelmij 1986) Here the formationconsists of a basal quartzose sandstone abruptlyoverlain by conglomerate interbedded withgraded sandstone units red shale and coarsesandstone The formation is capped by massivesandstone and conglomerate The unit was de-posited as a series of small prograding shallowmarine fan deltas (Wilhelmij 1986 Hill 1997 VanKranendonk 2000)

Paddy Market Formation In the western part ofthe study area the Paddy Market Formation isdominantly cherty BIF with minor ferruginousshale Further east the formation becomes moreshale-rich although much of the shale has been re-placed by chert Sandstone is present in smallamounts in beds up to 10 m thick Towards thetop of the formation in the east the cherts containfelsic volcanic material that is overlain by up to 200m of massive dacite (Van Kranendonk 2000)

Honeyeater Basalt The Honeyeater Basaltwhich consists entirely of basalt and dolerite con-formably overlies the Paddy Market FormationThe lower part of the formation consists of high-Mg basalt interbedded with dolerite whereashigher in the formation the basalts are largelytholeiitic and interbedded with dolerite (Gliksonand Hickman 1981ab Glikson et al 1991)

Pyramid Hill Formation The Pyramid Hill For-mation is limited in areal extent It is found in thenorth central part of the study area east of theStrelley Granite The formation consists of redand black shales that when heavily silicifiedgrade into laminated black and white chert (VanKranendonk 2000) The Gorge Creek Group iscapped by a well-bedded BIF

Megasequence 5mdashDe Grey Group

The De Grey Group consists of a single for-mation the Lalla Rookh Sandstone (Fig 3)which is dominated by clastic sedimentary rocksthat contain a significant quartzose componentThe Lalla Rookh Sandstone which rests on awell-defined unconformity above the GorgeCreek Group reaches a maximum thickness of

ARCHEAN GEOLOGIC REFERENCE COLLECTION 749

FIG 12 Stromatolitic carbonates in the Strelley Pool Chert (locality C Fig 2) Laminae are isopachous and theapex of the cone is ruptured and veined by secondary carbonate (arrow) Scale bar 5 10 cm

3 km (Krapez 1984) In the north central part ofthe study area the formation consists entirely ofcoarse clastic sediments and minor shalewhereas to the south the predominate fill is lithicand feldspathic sandstone (Chan 1998) Theserocks provide the first clear evidence of streamactivity in the Pilbara succession (Krapez 1984Van Kranendonk and Collins 1998 Van Kra-nendonk 2000)

THE PILBARA SUPRACRUSTAL SUCCESSION

The late Archean and early Paleoproterozoicbasins that rest upon the ancient cratonic blocksof Western Australia (Fig 1) form a time seriesassociated with the formation and ultimate dis-assembly of one of Earthrsquos earliest major conti-nental masses This was followed by the suturingof the Pilbara and Yilgarn Cratons along theCapricorn Orogen as a new supercontinentevolved in the Paleoproterozoic (Rogers and San-tosh 2002) These basins contain an importantsedimentary record of the early Earth includingsome of Earthrsquos earliest stromatolites and plat-form carbonate deposits (Simonson et al1993ab) organic-rich black shales (Brocks et al1999) and BIF (Trendall 1983 1990) The strati-graphic and structural framework is well con-strained by radiometric ages (Arndt et al 1991Barley et al 1997 Trendall et al 1998) providinga record at a critical time in Earth history whenthe atmosphere was first becoming oxygen-rich(Lindsay and Brasier 2002ab)

The Late Archean-Early PaleoproterozoicHamersley Basin which lies close to the southernmargin of the Pilbara Block (Trendall 1983 1990Blake and Barley 1992) overlies a normal crustalthickness (30ndash40 km) (Drummond 1981) Thebasin began to subside at approximately 28 Gafollowing an episode of crustal extension The ar-chitecture of the basin fill is complex andpolyphase with major erosional surfaces separat-ing areally extensive megasequencessuperse-quences (Trendall 1990 Krapez 1996) DuringFortescue and Hamersley Group time (Fig 4)sedimentation began in a rift setting in a silici-clastic-starved environment The later stages ofHamersley Basin sedimentation (Turee CreekGroup times) record the early transition from apassive margin to a foreland basin setting andwith it the establishment of a terrigenous sedi-

ment supply (Tyler and Thorne 1990) The suc-cession is largely marine Evidence of eustasy ap-pears for the first time in the sedimentary suc-cession of the Hamersley Basin (Lindsay andBrasier 2002ab)

The geometry and sequence stacking pattern ofthe succession are both very similar to youngerintracratonic basins (see Lindsay et al 1993) TheHamersley Basin is thus the first basinal settingpreserved on the Australian craton in which ma-rine sediments and in particular platform car-bonate rocks have been preserved in a responseto broad regional crustal subsidence and wheredeposition is controlled by eustasy

The Ashburton Basin (22ndash18 Ga) discon-formably overlies the southern margin of theHamersley Basin along its the contact with theCapricorn Orogen (Fig 1) (Thorne and Seymour1991) The basin forms the northern margin of theCapricorn Orogen a deformed zone that devel-oped during ocean closure Loading of the oceanfloor by the approaching Yilgarn Craton led tothe development of a westndashnorthwest-orientedforeland basin [McGrath Trough (Horowitz1982)] with an uplifted orogenic margin to thesouthwest (Blake and Groves 1987 Blake andBarley 1992) with the result that sedimentationwas dominated by an abundant supply of ter-rigenous clastic material (Mount McGrath For-mation) Continued loading of the crust ulti-mately led to the supply of terrigenous materialsbeing disrupted and producing a prograding car-bonate shelf (Duck Creek Dolomite) Basin sedi-mentation was accompanied by active-marginmafic volcanism (Cheela Springs Basalt)

THE CAPRICORN OROGEN ANDASSOCIATED BASINS

The Yerrida (22ndash19 Ga) Bryah (20 Ga)Padbury (20 Ga) and Earaheedy (19ndash165Ga) Basins lie to the southeast of the Hamersleyand Ashburton Basins and overlie the CapricornOrogen (Fig 1) (Occhipinti et al 1997 1998 Pi-rajno et al 1998 Pirajno and Adamides 2000)These now fragmentary basins formed along thenorthern margin of the Yilgarn Craton in a back-arc setting and record the convergence and colli-sion of the Archean Pilbara and Yilgarn Cratons(Tyler and Thorne 1990 Thorne and Seymour1991 Occhipinti et al 1997 1998 Pirajno et al1998) Because of their active margin settings the

LINDSAY ET AL750

fill of these small basins is dominated by clasticsediments However platform carbonate unitsare preserved in the Yerrida Basin (Gee and Grey1993 Pirajno and Adamides 2000) and a signif-icant thickness of carbonate rocks also occurs inthe Earaheedy Basin

THE ARCHEAN BIOSPHERE

The end of the Archean is formally defined at25 Ga (Plumb 1991) In reality the Archean worldended at some point in time closer to 28 or 30 Gawhen the first large continental blocks began to as-semble The stratigraphic record preserved on theancient Australian Craton reflects this major tran-sition The rock record from the Early Archean isvery different from that of the Late Archean in thatthe early record is predominantly volcanic and hy-drothermal in nature (ldquogreenstonerdquo) while thelater record is largely clastic and more readily com-pared with the Phanerozoic record

Structures and textures of possible biogenicorigin that are preserved in the stratigraphic suc-cession in the Early Archean study area are wellknown and are believed to provide the earliestdirect evidence of Earthrsquos biosphere Stromatoliticstructures have been encountered at two levelswithin the section The Dresser Formation con-tains structures preserved in chert and barite in-tervals that have been interpreted as stromatolitesand oncolites (Dunlop et al 1978 Walter et al1980 Buick et al 1981 Groves et al 1981 Wal-ter 1983) The stromatolitic structures occur forthe most part in layered barite units that devel-oped over swarms of synvolcanic hydrothermalfeeder dykes along listric growth faults (Nijmanet al 1998 Van Kranendonk 2004) Although ithas been argued that these structures are biogenic(Walter et al 1980 Schopf and Walter 1983 Wal-ter 1983) their direct association with the feederdykes leaves room for doubt (Buick et al 1981)The setting is complex and there is considerablescope for more detailed contextual research

The second stromatolitic horizon occurs in theoverlying Strelley Pool Chert a complex unit con-sisting of at least five members (Lowe 1980 1983Hofmann et al 1999) The main stromatolitic unitoccurs as an 8-m-thick interval in the middle ofthe formation and appears to have a consistentand widespread pattern of deposition The struc-tures have been the focus of considerable discus-sion and their biogenicity has been questioned

(Buick et al 1981 Lowe 1995) The structures arecomplex and varied (Fig 12) leading to a morerecent re-evaluation again suggesting that theyare biogenic (Hofmann et al 1999) However thelaminae are largely isopachous an indication thatthey could be abiotic precipitates (cf Pope andGrotzinger 2000) Given the intense hydrother-mal activity that appears to have taken placethroughout the depositional time interval thestructure may well be the product of hydrother-mal deposition (Lindsay et al 2003ab) In all ourobservations suggest that considerably morework needs to be done to establish the deposi-tional context of these stromatolitic structures be-fore they can be confidently ascribed as biogenicin origin

Structures resembling remarkably preservedbacterial and cyanobacterial microfossils havebeen found at several localities in and close to thestudy area (Awramik et al 1983 Schopf andPacker 1987 Schopf 1992 1993 Rasmussen 2000)A complex biota of 11 species of filamentousprokaryotes which are compared with cyanobac-teria (Schopf and Packer 1987 Schopf 1992 1993)have been described from the Apex chert close toMarble Bar (21deg105579S 119deg418789E) The taxo-nomic interpretations are important in that theyprovide the earliest evidence of morphological lifeon Earth and support the hypothesis of an earlybeginning for oxygenic photosynthesis (Schopf1999) The structures are almost a billion yearsolder than the oldest known putative cyanobac-terial biomarkers (Summons et al 1999) and oc-cur well before the earliest evidence for the risein oxygen in the atmosphere (for a summary seeKnoll and Canfield 1998 Lindsay and Brasier2000 2002ab) However in a recent re-evaluationof the original Apex chert material it has been ar-gued that the filaments are not biogenic but aresecondary artifacts formed during diagenesisfrom abiogenic organic compounds generated inhydrothermal chert veins (Brasier et al 2002 VanKranendonk 2004)

Filaments believed to be biogenic in originhave also been reported from the 324 Ga Kan-garoo Caves Formation (Rasmussen 2000) wherethey form part of the Sulphur Springs deposit as-sociated with a black smoker hydrothermal ventsystem (Vearncombe et al 1995 1998) Ras-mussen (2000) argued that the filaments are rem-nants of a hydrothermal biota of thermophilicchemotrophic prokaryotes Sulfide textures areremarkably well preserved in these complex de-

ARCHEAN GEOLOGIC REFERENCE COLLECTION 751

posits and are worthy of much more detailedanalysis as they may well preserve evidence ofthe early biosphere

Finally there is evidence that hydrocarbons aretrapped in these ancient sedimentary rocks eitheras fluid inclusions in hydrothermal precipitates oras bituminous residues in larger cavities in theKangaroo Caves Formation and in the Lalla RookhSandstone (Dutkiewicz et al 1998 Rasmussen andBuick 2000) Buick et al (1998) also reported car-bonaceous globules in cherts from the lower War-rawoona Group In the Kangaroo Caves Forma-tion the hydrocarbons occur in hydrothermalbarite or within the sulfides of the volcanogenichydrothermal massive sulfide deposits that weresyndepositional with the formation (Rasmussenand Buick 2000) Textural information suggestthat these deposits were precipitated as blacksmoker chimneys in a submarine hydrothermalsetting driven by heat released from the intrudingStrelley Granite at approximately 324 Ga (Vearn-combe et al 1995) Fluorescing fluid inclusions oc-cur in quartz overgrowths in sandstones from theLalla Rookh Sandstone (Dutkiewicz et al 1998)Rasmussen and Buick (2000) and Dutkiewicz et al(1998) have argued for a biogenic origin for the hy-drocarbons however an abiotic origin involvingfor example Fischer-Tropsch-type synthesis ap-pears possible (Brasier et al 2002)

The Late Archean (and Early Paleoproterozoic)record preserved in the Hamersley and relatedbasins is stratigraphically much more coherentthan that preserved in the earlier successions TheHamersley Basin in particular provides a verycomprehensive record of the early oxygenation ofthe atmosphere in the form of large eustaticallycontrolled upward-shallowing sequences that in-clude black shales BIFs and ultimately at theirtops biogenic platform carbonates (Lindsay andBrasier 2002ab) These depositional cycles con-tain organic-rich shales at their base that recordevidence for some of Earthrsquos earliest biosphere(Brocks et al 1999) BIFs (Trendall 1983 1990)that record the rise of oxygen in the atmosphereand carbonates that document some of Earthrsquosearliest reef-like carbonate structures (Simonsonand Hassler 1996 1997 Simonson et al 1993ab)

SAMPLING STRATEGY

The Archean rocks of the Pilbara Craton arethus part of a protocontinent about which the

nascent Australian Craton developed It consistsof granitoid complexes overlain by a carapace ofsynchronously deposited volcano-sedimentarywedges Resting upon the early Archean proto-continent are a series of supracrustal basins Inall the sedimentary record of the Pilbara extendsfrom rocks as old as 354 Ga to some as young asapproximately 19 Ga As summarized in the fol-lowing paragraphs the sedimentary record of thePilbara records evidence of the major tectonic andgeological events that accompanied the evolutionof the Archean biosphere

It is here proposed that we develop a carefullydocumented and curated sample collection fromthis unique setting The collection would form thebasis for early Earth studies and a focus for astro-biology The sample set would be designed to re-flect the evolutionary succession described abovewith emphasis placed on cherts lower in the suc-cession and on clastic sediments higher in the suc-cession Any comprehensive reference collectionmust reflect diversity of the succession and includerocks from each of the sedimentary environmentsas well as igneous rocks reflecting the frameworkin which the protocontinent assembled

The five megasequences at the North Shawstudy area (Fig 3) are very different and reflectthe evolving Early Archean protocontinent Theearliest sedimentary record is limited and domi-nated by hydrothermally generated laminatedcherts and BIFs which appear as thin interbedswithin massive basalts (Coonterunah Group) (Fig5) Gradually clastic rocks appear dominated atfirst by volcanoclastics followed later by the firstevidence of sediments containing detrital quartzderived from true continental granitic crust Hy-drothermally generated laminated chert carbon-ate and barite units occur throughout the sectionThe first evidence of erosion appears at the top ofthe Coonterunah Group in the form of an angu-lar unconformity (Buick et al 1995b)

The basins that form the supracrustal succes-sion on the Pilbara Craton offer important in-sights into the closing phases of the Archean andthe immediate post-Archean world Because oftheir economic importance the basins are wellmapped and drill core is readily available (seeLindsay and Brasier 2002a) The succession pro-vides vital information about the early oxygena-tion of the atmosphere and the rapid growth ofthe biosphere that resulted from the assembly ofthe first continents and the initiation of the platetectonic cycle Platform carbonates whose geom-

LINDSAY ET AL752

etry was determined by eustasy are widespreadand contain considerable information on earlyoxygenic photosynthesis and the development ofthe first reef-like structure that are similar inmany ways to modern reefs (Simonson et al1993ab Lindsay and Brasier 2002ab)

The collection would consist of three groupingsof samples stratigraphic samples site-specificsamples and framework samples

Stratigraphic samples

The stratigraphic sample set would be collectedfrom well-defined settings such as carefully mea-sured and documented sections or from drill coreSamples would be collected from each of the fivestratigraphic groups or megasequences Thesupracrustal successions preserved in the Hamer-sley and associated basins would where possiblebe taken from existing drill core or from drill coresobtained for special projects Where drill core isnot available samples would be collected fromcarefully selected and measured outcrop sectionsThe samples would concentrate on rocks mostlikely to host biogenic material Samples from theEarly Archean volcaniclastic carapace would bedominated by cherts that are most likely to pre-serve fine biogenic structures but would also in-clude other sedimentary hydrothermal units suchas barite and carbonate that would be used in sta-ble isotope and geochemical studies

Site-specific sample

This sample set would focus on sites where po-tential biogenic structures have been documentedsuch as the stromatolitic intervals in the DresserFormation and the Strelley Pool Chert It wouldalso include detailed sampling of sites such asChinaman Creek and the Strelley Pool Chertwhere purported microfossils have been docu-mented (Schopf 1993) and the sulfide bodies inthe Kangaroo Caves Formation that contain pos-sible microfossils as well as evidence of hydro-carbons (Rasmussen 2000 Rasmussen and Buick2000) This would involve a broad samplingaround the sites in measured sections and drillcore to establish the geological context

Framework samples

While the focus of the collecting would be onmaterial that would help define the character of

the Earthrsquos earliest biosphere it is important thatthe broad context of the samples be defined TheArchean is a period when the Earthrsquos crust wasevolving rapidly and there is a growing body ofevidence to suggest that the evolution of theplanet drives the evolution of the atmosphere andbiosphere (Lindsay and Brasier 2002ab) Theframework sample set would include suites ofsample from the extrusive volcanic rocks col-lected along well-documented sections Sectionsthrough the volcanoclastic succession would em-phasize the dominant volcanic units (basalts ko-matiites andesites felsites and rhyolites) assources of information on the mantle and evolv-ing crust This sample set would also include adetailed sampling of the granitoid complexeswhich ultimately drove the entire process as wellas the later intrusions that were important in dri-ving the hydrothermal systems that may wellhave been the cradle of early life

SAMPLE CURATIONCHARACTERIZATION AND

DISTRIBUTION

All samples would be fully documented in thefield Samples would be photographed and theirlocations determined using a Global PositioningSystem receiver For specialized samples such asthose thought to contain easily contaminated bio-geochemical information the field collectionwould be modeled on techniques developed bythe NASANational Science FoundationSmith-sonian Institution Antarctic Search for Meteoritesprogram (Harvey and Schutt 1998)

Characterization

The petrologic and petrographic characteristicsof each sample would be determined using thin-section optical microscopy The quantitative bulkchemical composition of each sample would bemeasured by x-ray fluorescence spectrometry Fi-nally the qualitative bulk mineralogy of eachsample would be measured by powder x-ray dif-fraction

Data management

An Archean Geologic Reference Materials Data-base would be produced to fully document eachsample Field documentation would include sam-pling rationale supporting references and field

ARCHEAN GEOLOGIC REFERENCE COLLECTION 753

photographs Documentation would include theresults of all analyses along with images of rep-resentative sample surfaces and optical micro-graphs The Database would be produced in elec-tronic format and posted on the web Subsamplesof the Archean Geologic Reference Materialswould then be made available at no cost to re-searchers in the astrobiology community

DISCUSSION AND CONCLUSIONS

The Early Archean environment was uniqueRecent studies indicate that the atmosphere waslargely anoxic with oxygen levels 1025 of thepresent atmospheric level (Pavlov and Kasting2002) There were no large continents no activeor passive margins and by default no intracra-tonic basins Significantly perhaps there are nosigns of eustasy The crust was evolving rapidlyand heat flow was high leading to the intrusionof large granitoid bodies and the development ofa volcanic carapace above them In turn thismeant that the marine environment was domi-nated by hydrothermal activity

In the absence of large continental masses de-trital materials other than volcanoclastics are rel-atively rare in the Early Archean Significant vol-umes of quartz sand do not appear in the PilbaraCraton sedimentary record until after 32 Ga pre-sumably as the intruding igneous bodies begin tobe unroofed and eroded There are thus no ma-jor accumulations of sedimentary rocks that wenormally expect associated with passive conti-nental margins

With the assembly of the earliest large conti-nents beginning at approximately 30ndash28 Gathe Earthrsquos atmosphere hydrosphere and bios-phere changed rapidly Intracratonic basinssuch as the Hamersley Basin began to developon the new crustal blocks opening new shal-low-water environments that provided an idealsetting for photosynthesizing organisms andthe development of the first carbonate reefstructures (Lindsay and Brasier 2002ab) Hy-drothermal processes while still importantwere thus no longer the only sources of energyavailable to the evolving biosphere

The Archean was thus an unfamiliar environ-ment and unravelling the nature of the earliestbiosphere will require development of new ap-proaches and new techniques Without large con-tinental masses subareal exposures are likely to

be rare and short-lived There is no evidence forstream activity until the De Grey group at ap-proximately 29 Ga at which time braided streamgravels were preserved Thus models involvingearly biospheric evolution in ldquoa warm little pondrdquo(Darwin 1888) or in fresh water seem unlikelyInstead the focus in the search for the origins oflife on Earth should be directed to hydrothermalmarine settings involving high temperatures andwhat would today be hostile geochemical envi-ronments with significant concentrations of metal-lic ions and a largely reducing environment Therisks for the misidentification of abiotic structuresand geochemical signatures as signs of ancient lifeare significant as seen in recent discussions (egBrasier et al 2002 Fedo and Whitehouse 2002Schopf et al 2002)

These difficulties suggest that much could begained by the development of a well-documentedcollection of Archean rocks that could be madeavailable to all qualified investigators Until weunderstand the earliest stages of the evolution oflife on Earth we cannot hope to make significantprogress in the search for life on other planetssuch as Mars Until there is a sample return mis-sion to Mars we must make do with meteoritesthat come to us largely out of context An under-standing of the Archean Earth can help placethose limited samples in a planetary context

ACKNOWLEDGMENTS

Franco Pirajno Kath Grey and Martin VanKranendonk provided generous advice and as-sistance and the Geological Survey of WesternAustralia supported the field program We aregrateful to Peter Morant of Sipa Resources andAdrian Black of Newexco Perth for their adviceand for access to drill-core materials

ABBREVIATION

BIF banded iron formation

REFERENCES

Arndt NT Nelson DR Compston W Trendall AFand Thorne AM (1991) The age of the FortescueGroup Hamersley Basin Western Australia from ionmicroprobe zircon U-Pb results Aust J Earth Sci 38262ndash281

LINDSAY ET AL754

Awramik SM Schopf JW and Walter MR (1983) Fil-amentous fossil bacteria from the Archaean of WesternAustralia Precambrian Res 20 357ndash374

Barley ME Pickard AL and Sylvester PJ (1997) Em-placement of a large igneous province as a possiblecause of banded iron formation 245 billion years agoNature 385 55ndash58

Bettenay LF Bickle MJ Boulter CA Groves DIMorant P Blake TS and James BA (1981) Evolu-tion of the Shaw Batholithmdashan Archaean granitoid-gneiss dome in the eastern Pilbara Western AustraliaIn Special Publication 7 The Archaean edited by JEGlover and DI Groves Geological Society of WesternAustralia Perth pp 361ndash372

Blake TS and Barley ME (1992) Tectonic evolution ofthe Late Archaean to Early Proterozoic Mount BruceMegasequence Set Western Australia Tectonics 11 1415ndash1425

Blake TS and Groves DI (1987) Continental rifting andthe Archaean-Proterozoic transition Geology 15 229ndash232

Brasier MD and Lindsay JF (1998) A billion years ofenvironmental stability and the emergence of eukary-otes new data from Northern Australia Geology 26555ndash558

Brasier MD Green OR Jephcoat AP Kleppe AKVan Kranendonk MJ Lindsay JF Steele A andGrassineau NV (2002) Questioning the evidence forEarthrsquos oldest fossils Nature 416 76ndash81

Brauhart C (1999) Regional alteration systems associatedwith Archaean volcanogenic massive sulphide depositsat Panorama Pilbara Western Australia [PhD Thesis]University of Western Australia Perth

Brauhart C Groves DI and Morant P (1998) Regionalalteration systems associated with volcanogenic mas-sive sulfide mineralization at Panorama Pilbara West-ern Australia Econ Geol 93 292ndash302

Brocks J Logan GA Buick R and Summons RE(1999) Archean molecular fossils and the early rise ofeukaryotes Science 285 1033ndash1036

Buick R Dunlop JSR and Groves DI (1981) Stroma-tolite recognition in ancient rocks an appraisal of ir-regular laminated structures in an early Archaeanchert-barite unit from North Pole Western AustraliaAlcheringa 5 161ndash181

Buick R Des Marais DJ and Knoll AH (1995a) Sta-ble isotope compositions of carbonates from the Meso-proterozoic Bangemall Group northwestern AustraliaChem Geol 123 153ndash171

Buick R Thornett JR McNaughton NJ Smith JBBarley ME and Savage M (1995b) Record of emer-gent continental crust 35 billion years ago in the Pil-bara Craton of Australia Nature 375 574ndash577

Buick R Rasmussen B and Krapez B (1998) Archaeanoil evidence for extensive hydrocarbon generation andmigration 25ndash35 Ga Am Assoc Petroleum Geol Bull82 50ndash69

Chan M (1998) Sedimentology and structure of threesmall congolmeratic basins in the Lalla Rookh-WesternShaw structural corridor Pilbara Craton Western Aus-

tralia [BSc Honours Thesis] University of WesternAustralia Perth

Cullers RL DiMarco MJ Lowe DR and Stone J(1993) Geochemistry of a silicified felsic volcaniclasticsuite from the Early Archaean Panorama FormationPilbara Block Western Australiamdashan evaluation of de-positional and post-depositional processes with specialemphasis on the rare-Earth elements Precambrian Res60 99ndash116

Darwin F ed (1888) The Life and Letters of Charles Dar-win Vol 3 John Murray London

de Wit MJ (1998) On Archaean granites greenstonescratons and tectonics does the evidence demand a ver-dict Precambrian Geol 91 181ndash226

Drummond BJ (1981) Crustal structure of the Precam-brian terrains of northwest Australia from seismic re-fraction data BMR J Geol Geophys 6 123ndash135

Dunlop JSR Muir M Milne VA and Groves DI(1978) A new microfossil assemblage from the Ar-chaean of Western Australia Nature 274 676ndash678

Dutkiewicz A Rasmussen B and Buick R (1998) Oilpreserved in fluid inclusions in Archaean sandstonesNature 395 885ndash888

Fedo CM and Whitehouse MJ (2002) Metasomatic ori-gin of quartz-pyroxene rock Akilia Greenland and im-plications for Earthrsquos earliest life Science 296 1448ndash1452

Gee RD and Grey K (1993) Report 41 Proterozoic Rockson the Glengarry 1250 000 SheetmdashStratigraphy Structureand Stromatolite Biostratigraphy Geological Survey ofWestern Australia Perth

Glikson AY and Hickman AH (1981a) Chemicalstratigraphy and petrogenesis of Archaean basic-ultra-basic volcanic units eastern Pilbara Block WesternAustralia In Special Publication 7 Archaean Geologyedited by JE Groves and DI Groves Geological Soci-ety of Western Australia Perth pp 287ndash300

Glikson AY and Hickman AH (1981b) Record 198136Geochemistry of Archaean Volcanic Successions EasternPilbara Block Western Australia Australian Bureau ofMineral Resources Canberra

Glikson AY Davy R and Hickman AH (1991) Record199146 Trace Metal Distribution Patterns in Basalts Pil-bara Craton Western Australia with Stratigraphic-Geo-chemical Implications Australian Bureau of Mineral Re-sources Canberra

Groves DI Dunlop JSR and Buick R (1981) An earlyhabitat of life Sci Am 245 64ndash73

Harvey RP and Schutt JW (1998) Meteorite recoveryand reconnaissance in the Allan Hills-David Glacierand Darwin Glacier regions 1996ndash1997 Antarctic J US 32 25ndash27

Hickman AH (1975) Annual Report 1974 PrecambrianStructural Geology of Part of the Pilbara Region Geologi-cal Survey of Western Australia Perth pp 68ndash73

Hickman AH (1983) Bulletin 127 Geology of the PilbaraBlock and Its Environs Geological Survey of WesternAustralia Perth

Hickman AH (1984) Archaean diapirism in the PilbaraBlock Western Australia In Precambrian Tectonics Il-lustrated edited by A Kroner and R Greiling E

ARCHEAN GEOLOGIC REFERENCE COLLECTION 755

3 km (Krapez 1984) In the north central part ofthe study area the formation consists entirely ofcoarse clastic sediments and minor shalewhereas to the south the predominate fill is lithicand feldspathic sandstone (Chan 1998) Theserocks provide the first clear evidence of streamactivity in the Pilbara succession (Krapez 1984Van Kranendonk and Collins 1998 Van Kra-nendonk 2000)

THE PILBARA SUPRACRUSTAL SUCCESSION

The late Archean and early Paleoproterozoicbasins that rest upon the ancient cratonic blocksof Western Australia (Fig 1) form a time seriesassociated with the formation and ultimate dis-assembly of one of Earthrsquos earliest major conti-nental masses This was followed by the suturingof the Pilbara and Yilgarn Cratons along theCapricorn Orogen as a new supercontinentevolved in the Paleoproterozoic (Rogers and San-tosh 2002) These basins contain an importantsedimentary record of the early Earth includingsome of Earthrsquos earliest stromatolites and plat-form carbonate deposits (Simonson et al1993ab) organic-rich black shales (Brocks et al1999) and BIF (Trendall 1983 1990) The strati-graphic and structural framework is well con-strained by radiometric ages (Arndt et al 1991Barley et al 1997 Trendall et al 1998) providinga record at a critical time in Earth history whenthe atmosphere was first becoming oxygen-rich(Lindsay and Brasier 2002ab)

The Late Archean-Early PaleoproterozoicHamersley Basin which lies close to the southernmargin of the Pilbara Block (Trendall 1983 1990Blake and Barley 1992) overlies a normal crustalthickness (30ndash40 km) (Drummond 1981) Thebasin began to subside at approximately 28 Gafollowing an episode of crustal extension The ar-chitecture of the basin fill is complex andpolyphase with major erosional surfaces separat-ing areally extensive megasequencessuperse-quences (Trendall 1990 Krapez 1996) DuringFortescue and Hamersley Group time (Fig 4)sedimentation began in a rift setting in a silici-clastic-starved environment The later stages ofHamersley Basin sedimentation (Turee CreekGroup times) record the early transition from apassive margin to a foreland basin setting andwith it the establishment of a terrigenous sedi-

ment supply (Tyler and Thorne 1990) The suc-cession is largely marine Evidence of eustasy ap-pears for the first time in the sedimentary suc-cession of the Hamersley Basin (Lindsay andBrasier 2002ab)

The geometry and sequence stacking pattern ofthe succession are both very similar to youngerintracratonic basins (see Lindsay et al 1993) TheHamersley Basin is thus the first basinal settingpreserved on the Australian craton in which ma-rine sediments and in particular platform car-bonate rocks have been preserved in a responseto broad regional crustal subsidence and wheredeposition is controlled by eustasy

The Ashburton Basin (22ndash18 Ga) discon-formably overlies the southern margin of theHamersley Basin along its the contact with theCapricorn Orogen (Fig 1) (Thorne and Seymour1991) The basin forms the northern margin of theCapricorn Orogen a deformed zone that devel-oped during ocean closure Loading of the oceanfloor by the approaching Yilgarn Craton led tothe development of a westndashnorthwest-orientedforeland basin [McGrath Trough (Horowitz1982)] with an uplifted orogenic margin to thesouthwest (Blake and Groves 1987 Blake andBarley 1992) with the result that sedimentationwas dominated by an abundant supply of ter-rigenous clastic material (Mount McGrath For-mation) Continued loading of the crust ulti-mately led to the supply of terrigenous materialsbeing disrupted and producing a prograding car-bonate shelf (Duck Creek Dolomite) Basin sedi-mentation was accompanied by active-marginmafic volcanism (Cheela Springs Basalt)

THE CAPRICORN OROGEN ANDASSOCIATED BASINS

The Yerrida (22ndash19 Ga) Bryah (20 Ga)Padbury (20 Ga) and Earaheedy (19ndash165Ga) Basins lie to the southeast of the Hamersleyand Ashburton Basins and overlie the CapricornOrogen (Fig 1) (Occhipinti et al 1997 1998 Pi-rajno et al 1998 Pirajno and Adamides 2000)These now fragmentary basins formed along thenorthern margin of the Yilgarn Craton in a back-arc setting and record the convergence and colli-sion of the Archean Pilbara and Yilgarn Cratons(Tyler and Thorne 1990 Thorne and Seymour1991 Occhipinti et al 1997 1998 Pirajno et al1998) Because of their active margin settings the

LINDSAY ET AL750

fill of these small basins is dominated by clasticsediments However platform carbonate unitsare preserved in the Yerrida Basin (Gee and Grey1993 Pirajno and Adamides 2000) and a signif-icant thickness of carbonate rocks also occurs inthe Earaheedy Basin

THE ARCHEAN BIOSPHERE

The end of the Archean is formally defined at25 Ga (Plumb 1991) In reality the Archean worldended at some point in time closer to 28 or 30 Gawhen the first large continental blocks began to as-semble The stratigraphic record preserved on theancient Australian Craton reflects this major tran-sition The rock record from the Early Archean isvery different from that of the Late Archean in thatthe early record is predominantly volcanic and hy-drothermal in nature (ldquogreenstonerdquo) while thelater record is largely clastic and more readily com-pared with the Phanerozoic record

Structures and textures of possible biogenicorigin that are preserved in the stratigraphic suc-cession in the Early Archean study area are wellknown and are believed to provide the earliestdirect evidence of Earthrsquos biosphere Stromatoliticstructures have been encountered at two levelswithin the section The Dresser Formation con-tains structures preserved in chert and barite in-tervals that have been interpreted as stromatolitesand oncolites (Dunlop et al 1978 Walter et al1980 Buick et al 1981 Groves et al 1981 Wal-ter 1983) The stromatolitic structures occur forthe most part in layered barite units that devel-oped over swarms of synvolcanic hydrothermalfeeder dykes along listric growth faults (Nijmanet al 1998 Van Kranendonk 2004) Although ithas been argued that these structures are biogenic(Walter et al 1980 Schopf and Walter 1983 Wal-ter 1983) their direct association with the feederdykes leaves room for doubt (Buick et al 1981)The setting is complex and there is considerablescope for more detailed contextual research

The second stromatolitic horizon occurs in theoverlying Strelley Pool Chert a complex unit con-sisting of at least five members (Lowe 1980 1983Hofmann et al 1999) The main stromatolitic unitoccurs as an 8-m-thick interval in the middle ofthe formation and appears to have a consistentand widespread pattern of deposition The struc-tures have been the focus of considerable discus-sion and their biogenicity has been questioned

(Buick et al 1981 Lowe 1995) The structures arecomplex and varied (Fig 12) leading to a morerecent re-evaluation again suggesting that theyare biogenic (Hofmann et al 1999) However thelaminae are largely isopachous an indication thatthey could be abiotic precipitates (cf Pope andGrotzinger 2000) Given the intense hydrother-mal activity that appears to have taken placethroughout the depositional time interval thestructure may well be the product of hydrother-mal deposition (Lindsay et al 2003ab) In all ourobservations suggest that considerably morework needs to be done to establish the deposi-tional context of these stromatolitic structures be-fore they can be confidently ascribed as biogenicin origin

Structures resembling remarkably preservedbacterial and cyanobacterial microfossils havebeen found at several localities in and close to thestudy area (Awramik et al 1983 Schopf andPacker 1987 Schopf 1992 1993 Rasmussen 2000)A complex biota of 11 species of filamentousprokaryotes which are compared with cyanobac-teria (Schopf and Packer 1987 Schopf 1992 1993)have been described from the Apex chert close toMarble Bar (21deg105579S 119deg418789E) The taxo-nomic interpretations are important in that theyprovide the earliest evidence of morphological lifeon Earth and support the hypothesis of an earlybeginning for oxygenic photosynthesis (Schopf1999) The structures are almost a billion yearsolder than the oldest known putative cyanobac-terial biomarkers (Summons et al 1999) and oc-cur well before the earliest evidence for the risein oxygen in the atmosphere (for a summary seeKnoll and Canfield 1998 Lindsay and Brasier2000 2002ab) However in a recent re-evaluationof the original Apex chert material it has been ar-gued that the filaments are not biogenic but aresecondary artifacts formed during diagenesisfrom abiogenic organic compounds generated inhydrothermal chert veins (Brasier et al 2002 VanKranendonk 2004)

Filaments believed to be biogenic in originhave also been reported from the 324 Ga Kan-garoo Caves Formation (Rasmussen 2000) wherethey form part of the Sulphur Springs deposit as-sociated with a black smoker hydrothermal ventsystem (Vearncombe et al 1995 1998) Ras-mussen (2000) argued that the filaments are rem-nants of a hydrothermal biota of thermophilicchemotrophic prokaryotes Sulfide textures areremarkably well preserved in these complex de-

ARCHEAN GEOLOGIC REFERENCE COLLECTION 751

posits and are worthy of much more detailedanalysis as they may well preserve evidence ofthe early biosphere

Finally there is evidence that hydrocarbons aretrapped in these ancient sedimentary rocks eitheras fluid inclusions in hydrothermal precipitates oras bituminous residues in larger cavities in theKangaroo Caves Formation and in the Lalla RookhSandstone (Dutkiewicz et al 1998 Rasmussen andBuick 2000) Buick et al (1998) also reported car-bonaceous globules in cherts from the lower War-rawoona Group In the Kangaroo Caves Forma-tion the hydrocarbons occur in hydrothermalbarite or within the sulfides of the volcanogenichydrothermal massive sulfide deposits that weresyndepositional with the formation (Rasmussenand Buick 2000) Textural information suggestthat these deposits were precipitated as blacksmoker chimneys in a submarine hydrothermalsetting driven by heat released from the intrudingStrelley Granite at approximately 324 Ga (Vearn-combe et al 1995) Fluorescing fluid inclusions oc-cur in quartz overgrowths in sandstones from theLalla Rookh Sandstone (Dutkiewicz et al 1998)Rasmussen and Buick (2000) and Dutkiewicz et al(1998) have argued for a biogenic origin for the hy-drocarbons however an abiotic origin involvingfor example Fischer-Tropsch-type synthesis ap-pears possible (Brasier et al 2002)

The Late Archean (and Early Paleoproterozoic)record preserved in the Hamersley and relatedbasins is stratigraphically much more coherentthan that preserved in the earlier successions TheHamersley Basin in particular provides a verycomprehensive record of the early oxygenation ofthe atmosphere in the form of large eustaticallycontrolled upward-shallowing sequences that in-clude black shales BIFs and ultimately at theirtops biogenic platform carbonates (Lindsay andBrasier 2002ab) These depositional cycles con-tain organic-rich shales at their base that recordevidence for some of Earthrsquos earliest biosphere(Brocks et al 1999) BIFs (Trendall 1983 1990)that record the rise of oxygen in the atmosphereand carbonates that document some of Earthrsquosearliest reef-like carbonate structures (Simonsonand Hassler 1996 1997 Simonson et al 1993ab)

SAMPLING STRATEGY

The Archean rocks of the Pilbara Craton arethus part of a protocontinent about which the

nascent Australian Craton developed It consistsof granitoid complexes overlain by a carapace ofsynchronously deposited volcano-sedimentarywedges Resting upon the early Archean proto-continent are a series of supracrustal basins Inall the sedimentary record of the Pilbara extendsfrom rocks as old as 354 Ga to some as young asapproximately 19 Ga As summarized in the fol-lowing paragraphs the sedimentary record of thePilbara records evidence of the major tectonic andgeological events that accompanied the evolutionof the Archean biosphere

It is here proposed that we develop a carefullydocumented and curated sample collection fromthis unique setting The collection would form thebasis for early Earth studies and a focus for astro-biology The sample set would be designed to re-flect the evolutionary succession described abovewith emphasis placed on cherts lower in the suc-cession and on clastic sediments higher in the suc-cession Any comprehensive reference collectionmust reflect diversity of the succession and includerocks from each of the sedimentary environmentsas well as igneous rocks reflecting the frameworkin which the protocontinent assembled

The five megasequences at the North Shawstudy area (Fig 3) are very different and reflectthe evolving Early Archean protocontinent Theearliest sedimentary record is limited and domi-nated by hydrothermally generated laminatedcherts and BIFs which appear as thin interbedswithin massive basalts (Coonterunah Group) (Fig5) Gradually clastic rocks appear dominated atfirst by volcanoclastics followed later by the firstevidence of sediments containing detrital quartzderived from true continental granitic crust Hy-drothermally generated laminated chert carbon-ate and barite units occur throughout the sectionThe first evidence of erosion appears at the top ofthe Coonterunah Group in the form of an angu-lar unconformity (Buick et al 1995b)

The basins that form the supracrustal succes-sion on the Pilbara Craton offer important in-sights into the closing phases of the Archean andthe immediate post-Archean world Because oftheir economic importance the basins are wellmapped and drill core is readily available (seeLindsay and Brasier 2002a) The succession pro-vides vital information about the early oxygena-tion of the atmosphere and the rapid growth ofthe biosphere that resulted from the assembly ofthe first continents and the initiation of the platetectonic cycle Platform carbonates whose geom-

LINDSAY ET AL752

etry was determined by eustasy are widespreadand contain considerable information on earlyoxygenic photosynthesis and the development ofthe first reef-like structure that are similar inmany ways to modern reefs (Simonson et al1993ab Lindsay and Brasier 2002ab)

The collection would consist of three groupingsof samples stratigraphic samples site-specificsamples and framework samples

Stratigraphic samples

The stratigraphic sample set would be collectedfrom well-defined settings such as carefully mea-sured and documented sections or from drill coreSamples would be collected from each of the fivestratigraphic groups or megasequences Thesupracrustal successions preserved in the Hamer-sley and associated basins would where possiblebe taken from existing drill core or from drill coresobtained for special projects Where drill core isnot available samples would be collected fromcarefully selected and measured outcrop sectionsThe samples would concentrate on rocks mostlikely to host biogenic material Samples from theEarly Archean volcaniclastic carapace would bedominated by cherts that are most likely to pre-serve fine biogenic structures but would also in-clude other sedimentary hydrothermal units suchas barite and carbonate that would be used in sta-ble isotope and geochemical studies

Site-specific sample

This sample set would focus on sites where po-tential biogenic structures have been documentedsuch as the stromatolitic intervals in the DresserFormation and the Strelley Pool Chert It wouldalso include detailed sampling of sites such asChinaman Creek and the Strelley Pool Chertwhere purported microfossils have been docu-mented (Schopf 1993) and the sulfide bodies inthe Kangaroo Caves Formation that contain pos-sible microfossils as well as evidence of hydro-carbons (Rasmussen 2000 Rasmussen and Buick2000) This would involve a broad samplingaround the sites in measured sections and drillcore to establish the geological context

Framework samples

While the focus of the collecting would be onmaterial that would help define the character of

the Earthrsquos earliest biosphere it is important thatthe broad context of the samples be defined TheArchean is a period when the Earthrsquos crust wasevolving rapidly and there is a growing body ofevidence to suggest that the evolution of theplanet drives the evolution of the atmosphere andbiosphere (Lindsay and Brasier 2002ab) Theframework sample set would include suites ofsample from the extrusive volcanic rocks col-lected along well-documented sections Sectionsthrough the volcanoclastic succession would em-phasize the dominant volcanic units (basalts ko-matiites andesites felsites and rhyolites) assources of information on the mantle and evolv-ing crust This sample set would also include adetailed sampling of the granitoid complexeswhich ultimately drove the entire process as wellas the later intrusions that were important in dri-ving the hydrothermal systems that may wellhave been the cradle of early life

SAMPLE CURATIONCHARACTERIZATION AND

DISTRIBUTION

All samples would be fully documented in thefield Samples would be photographed and theirlocations determined using a Global PositioningSystem receiver For specialized samples such asthose thought to contain easily contaminated bio-geochemical information the field collectionwould be modeled on techniques developed bythe NASANational Science FoundationSmith-sonian Institution Antarctic Search for Meteoritesprogram (Harvey and Schutt 1998)

Characterization

The petrologic and petrographic characteristicsof each sample would be determined using thin-section optical microscopy The quantitative bulkchemical composition of each sample would bemeasured by x-ray fluorescence spectrometry Fi-nally the qualitative bulk mineralogy of eachsample would be measured by powder x-ray dif-fraction

Data management

An Archean Geologic Reference Materials Data-base would be produced to fully document eachsample Field documentation would include sam-pling rationale supporting references and field

ARCHEAN GEOLOGIC REFERENCE COLLECTION 753

photographs Documentation would include theresults of all analyses along with images of rep-resentative sample surfaces and optical micro-graphs The Database would be produced in elec-tronic format and posted on the web Subsamplesof the Archean Geologic Reference Materialswould then be made available at no cost to re-searchers in the astrobiology community

DISCUSSION AND CONCLUSIONS

The Early Archean environment was uniqueRecent studies indicate that the atmosphere waslargely anoxic with oxygen levels 1025 of thepresent atmospheric level (Pavlov and Kasting2002) There were no large continents no activeor passive margins and by default no intracra-tonic basins Significantly perhaps there are nosigns of eustasy The crust was evolving rapidlyand heat flow was high leading to the intrusionof large granitoid bodies and the development ofa volcanic carapace above them In turn thismeant that the marine environment was domi-nated by hydrothermal activity

In the absence of large continental masses de-trital materials other than volcanoclastics are rel-atively rare in the Early Archean Significant vol-umes of quartz sand do not appear in the PilbaraCraton sedimentary record until after 32 Ga pre-sumably as the intruding igneous bodies begin tobe unroofed and eroded There are thus no ma-jor accumulations of sedimentary rocks that wenormally expect associated with passive conti-nental margins

With the assembly of the earliest large conti-nents beginning at approximately 30ndash28 Gathe Earthrsquos atmosphere hydrosphere and bios-phere changed rapidly Intracratonic basinssuch as the Hamersley Basin began to developon the new crustal blocks opening new shal-low-water environments that provided an idealsetting for photosynthesizing organisms andthe development of the first carbonate reefstructures (Lindsay and Brasier 2002ab) Hy-drothermal processes while still importantwere thus no longer the only sources of energyavailable to the evolving biosphere

The Archean was thus an unfamiliar environ-ment and unravelling the nature of the earliestbiosphere will require development of new ap-proaches and new techniques Without large con-tinental masses subareal exposures are likely to

be rare and short-lived There is no evidence forstream activity until the De Grey group at ap-proximately 29 Ga at which time braided streamgravels were preserved Thus models involvingearly biospheric evolution in ldquoa warm little pondrdquo(Darwin 1888) or in fresh water seem unlikelyInstead the focus in the search for the origins oflife on Earth should be directed to hydrothermalmarine settings involving high temperatures andwhat would today be hostile geochemical envi-ronments with significant concentrations of metal-lic ions and a largely reducing environment Therisks for the misidentification of abiotic structuresand geochemical signatures as signs of ancient lifeare significant as seen in recent discussions (egBrasier et al 2002 Fedo and Whitehouse 2002Schopf et al 2002)

These difficulties suggest that much could begained by the development of a well-documentedcollection of Archean rocks that could be madeavailable to all qualified investigators Until weunderstand the earliest stages of the evolution oflife on Earth we cannot hope to make significantprogress in the search for life on other planetssuch as Mars Until there is a sample return mis-sion to Mars we must make do with meteoritesthat come to us largely out of context An under-standing of the Archean Earth can help placethose limited samples in a planetary context

ACKNOWLEDGMENTS

Franco Pirajno Kath Grey and Martin VanKranendonk provided generous advice and as-sistance and the Geological Survey of WesternAustralia supported the field program We aregrateful to Peter Morant of Sipa Resources andAdrian Black of Newexco Perth for their adviceand for access to drill-core materials

ABBREVIATION

BIF banded iron formation

REFERENCES

Arndt NT Nelson DR Compston W Trendall AFand Thorne AM (1991) The age of the FortescueGroup Hamersley Basin Western Australia from ionmicroprobe zircon U-Pb results Aust J Earth Sci 38262ndash281

LINDSAY ET AL754

Awramik SM Schopf JW and Walter MR (1983) Fil-amentous fossil bacteria from the Archaean of WesternAustralia Precambrian Res 20 357ndash374

Barley ME Pickard AL and Sylvester PJ (1997) Em-placement of a large igneous province as a possiblecause of banded iron formation 245 billion years agoNature 385 55ndash58

Bettenay LF Bickle MJ Boulter CA Groves DIMorant P Blake TS and James BA (1981) Evolu-tion of the Shaw Batholithmdashan Archaean granitoid-gneiss dome in the eastern Pilbara Western AustraliaIn Special Publication 7 The Archaean edited by JEGlover and DI Groves Geological Society of WesternAustralia Perth pp 361ndash372

Blake TS and Barley ME (1992) Tectonic evolution ofthe Late Archaean to Early Proterozoic Mount BruceMegasequence Set Western Australia Tectonics 11 1415ndash1425

Blake TS and Groves DI (1987) Continental rifting andthe Archaean-Proterozoic transition Geology 15 229ndash232

Brasier MD and Lindsay JF (1998) A billion years ofenvironmental stability and the emergence of eukary-otes new data from Northern Australia Geology 26555ndash558

Brasier MD Green OR Jephcoat AP Kleppe AKVan Kranendonk MJ Lindsay JF Steele A andGrassineau NV (2002) Questioning the evidence forEarthrsquos oldest fossils Nature 416 76ndash81

Brauhart C (1999) Regional alteration systems associatedwith Archaean volcanogenic massive sulphide depositsat Panorama Pilbara Western Australia [PhD Thesis]University of Western Australia Perth

Brauhart C Groves DI and Morant P (1998) Regionalalteration systems associated with volcanogenic mas-sive sulfide mineralization at Panorama Pilbara West-ern Australia Econ Geol 93 292ndash302

Brocks J Logan GA Buick R and Summons RE(1999) Archean molecular fossils and the early rise ofeukaryotes Science 285 1033ndash1036

Buick R Dunlop JSR and Groves DI (1981) Stroma-tolite recognition in ancient rocks an appraisal of ir-regular laminated structures in an early Archaeanchert-barite unit from North Pole Western AustraliaAlcheringa 5 161ndash181

Buick R Des Marais DJ and Knoll AH (1995a) Sta-ble isotope compositions of carbonates from the Meso-proterozoic Bangemall Group northwestern AustraliaChem Geol 123 153ndash171

Buick R Thornett JR McNaughton NJ Smith JBBarley ME and Savage M (1995b) Record of emer-gent continental crust 35 billion years ago in the Pil-bara Craton of Australia Nature 375 574ndash577

Buick R Rasmussen B and Krapez B (1998) Archaeanoil evidence for extensive hydrocarbon generation andmigration 25ndash35 Ga Am Assoc Petroleum Geol Bull82 50ndash69

Chan M (1998) Sedimentology and structure of threesmall congolmeratic basins in the Lalla Rookh-WesternShaw structural corridor Pilbara Craton Western Aus-

tralia [BSc Honours Thesis] University of WesternAustralia Perth

Cullers RL DiMarco MJ Lowe DR and Stone J(1993) Geochemistry of a silicified felsic volcaniclasticsuite from the Early Archaean Panorama FormationPilbara Block Western Australiamdashan evaluation of de-positional and post-depositional processes with specialemphasis on the rare-Earth elements Precambrian Res60 99ndash116

Darwin F ed (1888) The Life and Letters of Charles Dar-win Vol 3 John Murray London

de Wit MJ (1998) On Archaean granites greenstonescratons and tectonics does the evidence demand a ver-dict Precambrian Geol 91 181ndash226

Drummond BJ (1981) Crustal structure of the Precam-brian terrains of northwest Australia from seismic re-fraction data BMR J Geol Geophys 6 123ndash135

Dunlop JSR Muir M Milne VA and Groves DI(1978) A new microfossil assemblage from the Ar-chaean of Western Australia Nature 274 676ndash678

Dutkiewicz A Rasmussen B and Buick R (1998) Oilpreserved in fluid inclusions in Archaean sandstonesNature 395 885ndash888

Fedo CM and Whitehouse MJ (2002) Metasomatic ori-gin of quartz-pyroxene rock Akilia Greenland and im-plications for Earthrsquos earliest life Science 296 1448ndash1452

Gee RD and Grey K (1993) Report 41 Proterozoic Rockson the Glengarry 1250 000 SheetmdashStratigraphy Structureand Stromatolite Biostratigraphy Geological Survey ofWestern Australia Perth

Glikson AY and Hickman AH (1981a) Chemicalstratigraphy and petrogenesis of Archaean basic-ultra-basic volcanic units eastern Pilbara Block WesternAustralia In Special Publication 7 Archaean Geologyedited by JE Groves and DI Groves Geological Soci-ety of Western Australia Perth pp 287ndash300

Glikson AY and Hickman AH (1981b) Record 198136Geochemistry of Archaean Volcanic Successions EasternPilbara Block Western Australia Australian Bureau ofMineral Resources Canberra

Glikson AY Davy R and Hickman AH (1991) Record199146 Trace Metal Distribution Patterns in Basalts Pil-bara Craton Western Australia with Stratigraphic-Geo-chemical Implications Australian Bureau of Mineral Re-sources Canberra

Groves DI Dunlop JSR and Buick R (1981) An earlyhabitat of life Sci Am 245 64ndash73

Harvey RP and Schutt JW (1998) Meteorite recoveryand reconnaissance in the Allan Hills-David Glacierand Darwin Glacier regions 1996ndash1997 Antarctic J US 32 25ndash27

Hickman AH (1975) Annual Report 1974 PrecambrianStructural Geology of Part of the Pilbara Region Geologi-cal Survey of Western Australia Perth pp 68ndash73

Hickman AH (1983) Bulletin 127 Geology of the PilbaraBlock and Its Environs Geological Survey of WesternAustralia Perth

Hickman AH (1984) Archaean diapirism in the PilbaraBlock Western Australia In Precambrian Tectonics Il-lustrated edited by A Kroner and R Greiling E

ARCHEAN GEOLOGIC REFERENCE COLLECTION 755

fill of these small basins is dominated by clasticsediments However platform carbonate unitsare preserved in the Yerrida Basin (Gee and Grey1993 Pirajno and Adamides 2000) and a signif-icant thickness of carbonate rocks also occurs inthe Earaheedy Basin

THE ARCHEAN BIOSPHERE

The end of the Archean is formally defined at25 Ga (Plumb 1991) In reality the Archean worldended at some point in time closer to 28 or 30 Gawhen the first large continental blocks began to as-semble The stratigraphic record preserved on theancient Australian Craton reflects this major tran-sition The rock record from the Early Archean isvery different from that of the Late Archean in thatthe early record is predominantly volcanic and hy-drothermal in nature (ldquogreenstonerdquo) while thelater record is largely clastic and more readily com-pared with the Phanerozoic record

Structures and textures of possible biogenicorigin that are preserved in the stratigraphic suc-cession in the Early Archean study area are wellknown and are believed to provide the earliestdirect evidence of Earthrsquos biosphere Stromatoliticstructures have been encountered at two levelswithin the section The Dresser Formation con-tains structures preserved in chert and barite in-tervals that have been interpreted as stromatolitesand oncolites (Dunlop et al 1978 Walter et al1980 Buick et al 1981 Groves et al 1981 Wal-ter 1983) The stromatolitic structures occur forthe most part in layered barite units that devel-oped over swarms of synvolcanic hydrothermalfeeder dykes along listric growth faults (Nijmanet al 1998 Van Kranendonk 2004) Although ithas been argued that these structures are biogenic(Walter et al 1980 Schopf and Walter 1983 Wal-ter 1983) their direct association with the feederdykes leaves room for doubt (Buick et al 1981)The setting is complex and there is considerablescope for more detailed contextual research

The second stromatolitic horizon occurs in theoverlying Strelley Pool Chert a complex unit con-sisting of at least five members (Lowe 1980 1983Hofmann et al 1999) The main stromatolitic unitoccurs as an 8-m-thick interval in the middle ofthe formation and appears to have a consistentand widespread pattern of deposition The struc-tures have been the focus of considerable discus-sion and their biogenicity has been questioned

(Buick et al 1981 Lowe 1995) The structures arecomplex and varied (Fig 12) leading to a morerecent re-evaluation again suggesting that theyare biogenic (Hofmann et al 1999) However thelaminae are largely isopachous an indication thatthey could be abiotic precipitates (cf Pope andGrotzinger 2000) Given the intense hydrother-mal activity that appears to have taken placethroughout the depositional time interval thestructure may well be the product of hydrother-mal deposition (Lindsay et al 2003ab) In all ourobservations suggest that considerably morework needs to be done to establish the deposi-tional context of these stromatolitic structures be-fore they can be confidently ascribed as biogenicin origin

Structures resembling remarkably preservedbacterial and cyanobacterial microfossils havebeen found at several localities in and close to thestudy area (Awramik et al 1983 Schopf andPacker 1987 Schopf 1992 1993 Rasmussen 2000)A complex biota of 11 species of filamentousprokaryotes which are compared with cyanobac-teria (Schopf and Packer 1987 Schopf 1992 1993)have been described from the Apex chert close toMarble Bar (21deg105579S 119deg418789E) The taxo-nomic interpretations are important in that theyprovide the earliest evidence of morphological lifeon Earth and support the hypothesis of an earlybeginning for oxygenic photosynthesis (Schopf1999) The structures are almost a billion yearsolder than the oldest known putative cyanobac-terial biomarkers (Summons et al 1999) and oc-cur well before the earliest evidence for the risein oxygen in the atmosphere (for a summary seeKnoll and Canfield 1998 Lindsay and Brasier2000 2002ab) However in a recent re-evaluationof the original Apex chert material it has been ar-gued that the filaments are not biogenic but aresecondary artifacts formed during diagenesisfrom abiogenic organic compounds generated inhydrothermal chert veins (Brasier et al 2002 VanKranendonk 2004)

Filaments believed to be biogenic in originhave also been reported from the 324 Ga Kan-garoo Caves Formation (Rasmussen 2000) wherethey form part of the Sulphur Springs deposit as-sociated with a black smoker hydrothermal ventsystem (Vearncombe et al 1995 1998) Ras-mussen (2000) argued that the filaments are rem-nants of a hydrothermal biota of thermophilicchemotrophic prokaryotes Sulfide textures areremarkably well preserved in these complex de-

ARCHEAN GEOLOGIC REFERENCE COLLECTION 751

posits and are worthy of much more detailedanalysis as they may well preserve evidence ofthe early biosphere

Finally there is evidence that hydrocarbons aretrapped in these ancient sedimentary rocks eitheras fluid inclusions in hydrothermal precipitates oras bituminous residues in larger cavities in theKangaroo Caves Formation and in the Lalla RookhSandstone (Dutkiewicz et al 1998 Rasmussen andBuick 2000) Buick et al (1998) also reported car-bonaceous globules in cherts from the lower War-rawoona Group In the Kangaroo Caves Forma-tion the hydrocarbons occur in hydrothermalbarite or within the sulfides of the volcanogenichydrothermal massive sulfide deposits that weresyndepositional with the formation (Rasmussenand Buick 2000) Textural information suggestthat these deposits were precipitated as blacksmoker chimneys in a submarine hydrothermalsetting driven by heat released from the intrudingStrelley Granite at approximately 324 Ga (Vearn-combe et al 1995) Fluorescing fluid inclusions oc-cur in quartz overgrowths in sandstones from theLalla Rookh Sandstone (Dutkiewicz et al 1998)Rasmussen and Buick (2000) and Dutkiewicz et al(1998) have argued for a biogenic origin for the hy-drocarbons however an abiotic origin involvingfor example Fischer-Tropsch-type synthesis ap-pears possible (Brasier et al 2002)

The Late Archean (and Early Paleoproterozoic)record preserved in the Hamersley and relatedbasins is stratigraphically much more coherentthan that preserved in the earlier successions TheHamersley Basin in particular provides a verycomprehensive record of the early oxygenation ofthe atmosphere in the form of large eustaticallycontrolled upward-shallowing sequences that in-clude black shales BIFs and ultimately at theirtops biogenic platform carbonates (Lindsay andBrasier 2002ab) These depositional cycles con-tain organic-rich shales at their base that recordevidence for some of Earthrsquos earliest biosphere(Brocks et al 1999) BIFs (Trendall 1983 1990)that record the rise of oxygen in the atmosphereand carbonates that document some of Earthrsquosearliest reef-like carbonate structures (Simonsonand Hassler 1996 1997 Simonson et al 1993ab)

SAMPLING STRATEGY

The Archean rocks of the Pilbara Craton arethus part of a protocontinent about which the

nascent Australian Craton developed It consistsof granitoid complexes overlain by a carapace ofsynchronously deposited volcano-sedimentarywedges Resting upon the early Archean proto-continent are a series of supracrustal basins Inall the sedimentary record of the Pilbara extendsfrom rocks as old as 354 Ga to some as young asapproximately 19 Ga As summarized in the fol-lowing paragraphs the sedimentary record of thePilbara records evidence of the major tectonic andgeological events that accompanied the evolutionof the Archean biosphere

It is here proposed that we develop a carefullydocumented and curated sample collection fromthis unique setting The collection would form thebasis for early Earth studies and a focus for astro-biology The sample set would be designed to re-flect the evolutionary succession described abovewith emphasis placed on cherts lower in the suc-cession and on clastic sediments higher in the suc-cession Any comprehensive reference collectionmust reflect diversity of the succession and includerocks from each of the sedimentary environmentsas well as igneous rocks reflecting the frameworkin which the protocontinent assembled

The five megasequences at the North Shawstudy area (Fig 3) are very different and reflectthe evolving Early Archean protocontinent Theearliest sedimentary record is limited and domi-nated by hydrothermally generated laminatedcherts and BIFs which appear as thin interbedswithin massive basalts (Coonterunah Group) (Fig5) Gradually clastic rocks appear dominated atfirst by volcanoclastics followed later by the firstevidence of sediments containing detrital quartzderived from true continental granitic crust Hy-drothermally generated laminated chert carbon-ate and barite units occur throughout the sectionThe first evidence of erosion appears at the top ofthe Coonterunah Group in the form of an angu-lar unconformity (Buick et al 1995b)

The basins that form the supracrustal succes-sion on the Pilbara Craton offer important in-sights into the closing phases of the Archean andthe immediate post-Archean world Because oftheir economic importance the basins are wellmapped and drill core is readily available (seeLindsay and Brasier 2002a) The succession pro-vides vital information about the early oxygena-tion of the atmosphere and the rapid growth ofthe biosphere that resulted from the assembly ofthe first continents and the initiation of the platetectonic cycle Platform carbonates whose geom-

LINDSAY ET AL752

etry was determined by eustasy are widespreadand contain considerable information on earlyoxygenic photosynthesis and the development ofthe first reef-like structure that are similar inmany ways to modern reefs (Simonson et al1993ab Lindsay and Brasier 2002ab)

The collection would consist of three groupingsof samples stratigraphic samples site-specificsamples and framework samples

Stratigraphic samples

The stratigraphic sample set would be collectedfrom well-defined settings such as carefully mea-sured and documented sections or from drill coreSamples would be collected from each of the fivestratigraphic groups or megasequences Thesupracrustal successions preserved in the Hamer-sley and associated basins would where possiblebe taken from existing drill core or from drill coresobtained for special projects Where drill core isnot available samples would be collected fromcarefully selected and measured outcrop sectionsThe samples would concentrate on rocks mostlikely to host biogenic material Samples from theEarly Archean volcaniclastic carapace would bedominated by cherts that are most likely to pre-serve fine biogenic structures but would also in-clude other sedimentary hydrothermal units suchas barite and carbonate that would be used in sta-ble isotope and geochemical studies

Site-specific sample

This sample set would focus on sites where po-tential biogenic structures have been documentedsuch as the stromatolitic intervals in the DresserFormation and the Strelley Pool Chert It wouldalso include detailed sampling of sites such asChinaman Creek and the Strelley Pool Chertwhere purported microfossils have been docu-mented (Schopf 1993) and the sulfide bodies inthe Kangaroo Caves Formation that contain pos-sible microfossils as well as evidence of hydro-carbons (Rasmussen 2000 Rasmussen and Buick2000) This would involve a broad samplingaround the sites in measured sections and drillcore to establish the geological context

Framework samples

While the focus of the collecting would be onmaterial that would help define the character of

the Earthrsquos earliest biosphere it is important thatthe broad context of the samples be defined TheArchean is a period when the Earthrsquos crust wasevolving rapidly and there is a growing body ofevidence to suggest that the evolution of theplanet drives the evolution of the atmosphere andbiosphere (Lindsay and Brasier 2002ab) Theframework sample set would include suites ofsample from the extrusive volcanic rocks col-lected along well-documented sections Sectionsthrough the volcanoclastic succession would em-phasize the dominant volcanic units (basalts ko-matiites andesites felsites and rhyolites) assources of information on the mantle and evolv-ing crust This sample set would also include adetailed sampling of the granitoid complexeswhich ultimately drove the entire process as wellas the later intrusions that were important in dri-ving the hydrothermal systems that may wellhave been the cradle of early life

SAMPLE CURATIONCHARACTERIZATION AND

DISTRIBUTION

All samples would be fully documented in thefield Samples would be photographed and theirlocations determined using a Global PositioningSystem receiver For specialized samples such asthose thought to contain easily contaminated bio-geochemical information the field collectionwould be modeled on techniques developed bythe NASANational Science FoundationSmith-sonian Institution Antarctic Search for Meteoritesprogram (Harvey and Schutt 1998)

Characterization

The petrologic and petrographic characteristicsof each sample would be determined using thin-section optical microscopy The quantitative bulkchemical composition of each sample would bemeasured by x-ray fluorescence spectrometry Fi-nally the qualitative bulk mineralogy of eachsample would be measured by powder x-ray dif-fraction

Data management

An Archean Geologic Reference Materials Data-base would be produced to fully document eachsample Field documentation would include sam-pling rationale supporting references and field

ARCHEAN GEOLOGIC REFERENCE COLLECTION 753

photographs Documentation would include theresults of all analyses along with images of rep-resentative sample surfaces and optical micro-graphs The Database would be produced in elec-tronic format and posted on the web Subsamplesof the Archean Geologic Reference Materialswould then be made available at no cost to re-searchers in the astrobiology community

DISCUSSION AND CONCLUSIONS

The Early Archean environment was uniqueRecent studies indicate that the atmosphere waslargely anoxic with oxygen levels 1025 of thepresent atmospheric level (Pavlov and Kasting2002) There were no large continents no activeor passive margins and by default no intracra-tonic basins Significantly perhaps there are nosigns of eustasy The crust was evolving rapidlyand heat flow was high leading to the intrusionof large granitoid bodies and the development ofa volcanic carapace above them In turn thismeant that the marine environment was domi-nated by hydrothermal activity

In the absence of large continental masses de-trital materials other than volcanoclastics are rel-atively rare in the Early Archean Significant vol-umes of quartz sand do not appear in the PilbaraCraton sedimentary record until after 32 Ga pre-sumably as the intruding igneous bodies begin tobe unroofed and eroded There are thus no ma-jor accumulations of sedimentary rocks that wenormally expect associated with passive conti-nental margins

With the assembly of the earliest large conti-nents beginning at approximately 30ndash28 Gathe Earthrsquos atmosphere hydrosphere and bios-phere changed rapidly Intracratonic basinssuch as the Hamersley Basin began to developon the new crustal blocks opening new shal-low-water environments that provided an idealsetting for photosynthesizing organisms andthe development of the first carbonate reefstructures (Lindsay and Brasier 2002ab) Hy-drothermal processes while still importantwere thus no longer the only sources of energyavailable to the evolving biosphere

The Archean was thus an unfamiliar environ-ment and unravelling the nature of the earliestbiosphere will require development of new ap-proaches and new techniques Without large con-tinental masses subareal exposures are likely to

be rare and short-lived There is no evidence forstream activity until the De Grey group at ap-proximately 29 Ga at which time braided streamgravels were preserved Thus models involvingearly biospheric evolution in ldquoa warm little pondrdquo(Darwin 1888) or in fresh water seem unlikelyInstead the focus in the search for the origins oflife on Earth should be directed to hydrothermalmarine settings involving high temperatures andwhat would today be hostile geochemical envi-ronments with significant concentrations of metal-lic ions and a largely reducing environment Therisks for the misidentification of abiotic structuresand geochemical signatures as signs of ancient lifeare significant as seen in recent discussions (egBrasier et al 2002 Fedo and Whitehouse 2002Schopf et al 2002)

These difficulties suggest that much could begained by the development of a well-documentedcollection of Archean rocks that could be madeavailable to all qualified investigators Until weunderstand the earliest stages of the evolution oflife on Earth we cannot hope to make significantprogress in the search for life on other planetssuch as Mars Until there is a sample return mis-sion to Mars we must make do with meteoritesthat come to us largely out of context An under-standing of the Archean Earth can help placethose limited samples in a planetary context

ACKNOWLEDGMENTS

Franco Pirajno Kath Grey and Martin VanKranendonk provided generous advice and as-sistance and the Geological Survey of WesternAustralia supported the field program We aregrateful to Peter Morant of Sipa Resources andAdrian Black of Newexco Perth for their adviceand for access to drill-core materials

ABBREVIATION

BIF banded iron formation

REFERENCES

Arndt NT Nelson DR Compston W Trendall AFand Thorne AM (1991) The age of the FortescueGroup Hamersley Basin Western Australia from ionmicroprobe zircon U-Pb results Aust J Earth Sci 38262ndash281

LINDSAY ET AL754

Awramik SM Schopf JW and Walter MR (1983) Fil-amentous fossil bacteria from the Archaean of WesternAustralia Precambrian Res 20 357ndash374

Barley ME Pickard AL and Sylvester PJ (1997) Em-placement of a large igneous province as a possiblecause of banded iron formation 245 billion years agoNature 385 55ndash58

Bettenay LF Bickle MJ Boulter CA Groves DIMorant P Blake TS and James BA (1981) Evolu-tion of the Shaw Batholithmdashan Archaean granitoid-gneiss dome in the eastern Pilbara Western AustraliaIn Special Publication 7 The Archaean edited by JEGlover and DI Groves Geological Society of WesternAustralia Perth pp 361ndash372

Blake TS and Barley ME (1992) Tectonic evolution ofthe Late Archaean to Early Proterozoic Mount BruceMegasequence Set Western Australia Tectonics 11 1415ndash1425

Blake TS and Groves DI (1987) Continental rifting andthe Archaean-Proterozoic transition Geology 15 229ndash232

Brasier MD and Lindsay JF (1998) A billion years ofenvironmental stability and the emergence of eukary-otes new data from Northern Australia Geology 26555ndash558

Brasier MD Green OR Jephcoat AP Kleppe AKVan Kranendonk MJ Lindsay JF Steele A andGrassineau NV (2002) Questioning the evidence forEarthrsquos oldest fossils Nature 416 76ndash81

Brauhart C (1999) Regional alteration systems associatedwith Archaean volcanogenic massive sulphide depositsat Panorama Pilbara Western Australia [PhD Thesis]University of Western Australia Perth

Brauhart C Groves DI and Morant P (1998) Regionalalteration systems associated with volcanogenic mas-sive sulfide mineralization at Panorama Pilbara West-ern Australia Econ Geol 93 292ndash302

Brocks J Logan GA Buick R and Summons RE(1999) Archean molecular fossils and the early rise ofeukaryotes Science 285 1033ndash1036

Buick R Dunlop JSR and Groves DI (1981) Stroma-tolite recognition in ancient rocks an appraisal of ir-regular laminated structures in an early Archaeanchert-barite unit from North Pole Western AustraliaAlcheringa 5 161ndash181

Buick R Des Marais DJ and Knoll AH (1995a) Sta-ble isotope compositions of carbonates from the Meso-proterozoic Bangemall Group northwestern AustraliaChem Geol 123 153ndash171

Buick R Thornett JR McNaughton NJ Smith JBBarley ME and Savage M (1995b) Record of emer-gent continental crust 35 billion years ago in the Pil-bara Craton of Australia Nature 375 574ndash577

Buick R Rasmussen B and Krapez B (1998) Archaeanoil evidence for extensive hydrocarbon generation andmigration 25ndash35 Ga Am Assoc Petroleum Geol Bull82 50ndash69

Chan M (1998) Sedimentology and structure of threesmall congolmeratic basins in the Lalla Rookh-WesternShaw structural corridor Pilbara Craton Western Aus-

tralia [BSc Honours Thesis] University of WesternAustralia Perth

Cullers RL DiMarco MJ Lowe DR and Stone J(1993) Geochemistry of a silicified felsic volcaniclasticsuite from the Early Archaean Panorama FormationPilbara Block Western Australiamdashan evaluation of de-positional and post-depositional processes with specialemphasis on the rare-Earth elements Precambrian Res60 99ndash116

Darwin F ed (1888) The Life and Letters of Charles Dar-win Vol 3 John Murray London

de Wit MJ (1998) On Archaean granites greenstonescratons and tectonics does the evidence demand a ver-dict Precambrian Geol 91 181ndash226

Drummond BJ (1981) Crustal structure of the Precam-brian terrains of northwest Australia from seismic re-fraction data BMR J Geol Geophys 6 123ndash135

Dunlop JSR Muir M Milne VA and Groves DI(1978) A new microfossil assemblage from the Ar-chaean of Western Australia Nature 274 676ndash678

Dutkiewicz A Rasmussen B and Buick R (1998) Oilpreserved in fluid inclusions in Archaean sandstonesNature 395 885ndash888

Fedo CM and Whitehouse MJ (2002) Metasomatic ori-gin of quartz-pyroxene rock Akilia Greenland and im-plications for Earthrsquos earliest life Science 296 1448ndash1452

Gee RD and Grey K (1993) Report 41 Proterozoic Rockson the Glengarry 1250 000 SheetmdashStratigraphy Structureand Stromatolite Biostratigraphy Geological Survey ofWestern Australia Perth

Glikson AY and Hickman AH (1981a) Chemicalstratigraphy and petrogenesis of Archaean basic-ultra-basic volcanic units eastern Pilbara Block WesternAustralia In Special Publication 7 Archaean Geologyedited by JE Groves and DI Groves Geological Soci-ety of Western Australia Perth pp 287ndash300

Glikson AY and Hickman AH (1981b) Record 198136Geochemistry of Archaean Volcanic Successions EasternPilbara Block Western Australia Australian Bureau ofMineral Resources Canberra

Glikson AY Davy R and Hickman AH (1991) Record199146 Trace Metal Distribution Patterns in Basalts Pil-bara Craton Western Australia with Stratigraphic-Geo-chemical Implications Australian Bureau of Mineral Re-sources Canberra

Groves DI Dunlop JSR and Buick R (1981) An earlyhabitat of life Sci Am 245 64ndash73

Harvey RP and Schutt JW (1998) Meteorite recoveryand reconnaissance in the Allan Hills-David Glacierand Darwin Glacier regions 1996ndash1997 Antarctic J US 32 25ndash27

Hickman AH (1975) Annual Report 1974 PrecambrianStructural Geology of Part of the Pilbara Region Geologi-cal Survey of Western Australia Perth pp 68ndash73

Hickman AH (1983) Bulletin 127 Geology of the PilbaraBlock and Its Environs Geological Survey of WesternAustralia Perth

Hickman AH (1984) Archaean diapirism in the PilbaraBlock Western Australia In Precambrian Tectonics Il-lustrated edited by A Kroner and R Greiling E

ARCHEAN GEOLOGIC REFERENCE COLLECTION 755

posits and are worthy of much more detailedanalysis as they may well preserve evidence ofthe early biosphere

Finally there is evidence that hydrocarbons aretrapped in these ancient sedimentary rocks eitheras fluid inclusions in hydrothermal precipitates oras bituminous residues in larger cavities in theKangaroo Caves Formation and in the Lalla RookhSandstone (Dutkiewicz et al 1998 Rasmussen andBuick 2000) Buick et al (1998) also reported car-bonaceous globules in cherts from the lower War-rawoona Group In the Kangaroo Caves Forma-tion the hydrocarbons occur in hydrothermalbarite or within the sulfides of the volcanogenichydrothermal massive sulfide deposits that weresyndepositional with the formation (Rasmussenand Buick 2000) Textural information suggestthat these deposits were precipitated as blacksmoker chimneys in a submarine hydrothermalsetting driven by heat released from the intrudingStrelley Granite at approximately 324 Ga (Vearn-combe et al 1995) Fluorescing fluid inclusions oc-cur in quartz overgrowths in sandstones from theLalla Rookh Sandstone (Dutkiewicz et al 1998)Rasmussen and Buick (2000) and Dutkiewicz et al(1998) have argued for a biogenic origin for the hy-drocarbons however an abiotic origin involvingfor example Fischer-Tropsch-type synthesis ap-pears possible (Brasier et al 2002)

The Late Archean (and Early Paleoproterozoic)record preserved in the Hamersley and relatedbasins is stratigraphically much more coherentthan that preserved in the earlier successions TheHamersley Basin in particular provides a verycomprehensive record of the early oxygenation ofthe atmosphere in the form of large eustaticallycontrolled upward-shallowing sequences that in-clude black shales BIFs and ultimately at theirtops biogenic platform carbonates (Lindsay andBrasier 2002ab) These depositional cycles con-tain organic-rich shales at their base that recordevidence for some of Earthrsquos earliest biosphere(Brocks et al 1999) BIFs (Trendall 1983 1990)that record the rise of oxygen in the atmosphereand carbonates that document some of Earthrsquosearliest reef-like carbonate structures (Simonsonand Hassler 1996 1997 Simonson et al 1993ab)

SAMPLING STRATEGY

The Archean rocks of the Pilbara Craton arethus part of a protocontinent about which the

nascent Australian Craton developed It consistsof granitoid complexes overlain by a carapace ofsynchronously deposited volcano-sedimentarywedges Resting upon the early Archean proto-continent are a series of supracrustal basins Inall the sedimentary record of the Pilbara extendsfrom rocks as old as 354 Ga to some as young asapproximately 19 Ga As summarized in the fol-lowing paragraphs the sedimentary record of thePilbara records evidence of the major tectonic andgeological events that accompanied the evolutionof the Archean biosphere

It is here proposed that we develop a carefullydocumented and curated sample collection fromthis unique setting The collection would form thebasis for early Earth studies and a focus for astro-biology The sample set would be designed to re-flect the evolutionary succession described abovewith emphasis placed on cherts lower in the suc-cession and on clastic sediments higher in the suc-cession Any comprehensive reference collectionmust reflect diversity of the succession and includerocks from each of the sedimentary environmentsas well as igneous rocks reflecting the frameworkin which the protocontinent assembled

The five megasequences at the North Shawstudy area (Fig 3) are very different and reflectthe evolving Early Archean protocontinent Theearliest sedimentary record is limited and domi-nated by hydrothermally generated laminatedcherts and BIFs which appear as thin interbedswithin massive basalts (Coonterunah Group) (Fig5) Gradually clastic rocks appear dominated atfirst by volcanoclastics followed later by the firstevidence of sediments containing detrital quartzderived from true continental granitic crust Hy-drothermally generated laminated chert carbon-ate and barite units occur throughout the sectionThe first evidence of erosion appears at the top ofthe Coonterunah Group in the form of an angu-lar unconformity (Buick et al 1995b)

The basins that form the supracrustal succes-sion on the Pilbara Craton offer important in-sights into the closing phases of the Archean andthe immediate post-Archean world Because oftheir economic importance the basins are wellmapped and drill core is readily available (seeLindsay and Brasier 2002a) The succession pro-vides vital information about the early oxygena-tion of the atmosphere and the rapid growth ofthe biosphere that resulted from the assembly ofthe first continents and the initiation of the platetectonic cycle Platform carbonates whose geom-

LINDSAY ET AL752

etry was determined by eustasy are widespreadand contain considerable information on earlyoxygenic photosynthesis and the development ofthe first reef-like structure that are similar inmany ways to modern reefs (Simonson et al1993ab Lindsay and Brasier 2002ab)

The collection would consist of three groupingsof samples stratigraphic samples site-specificsamples and framework samples

Stratigraphic samples

The stratigraphic sample set would be collectedfrom well-defined settings such as carefully mea-sured and documented sections or from drill coreSamples would be collected from each of the fivestratigraphic groups or megasequences Thesupracrustal successions preserved in the Hamer-sley and associated basins would where possiblebe taken from existing drill core or from drill coresobtained for special projects Where drill core isnot available samples would be collected fromcarefully selected and measured outcrop sectionsThe samples would concentrate on rocks mostlikely to host biogenic material Samples from theEarly Archean volcaniclastic carapace would bedominated by cherts that are most likely to pre-serve fine biogenic structures but would also in-clude other sedimentary hydrothermal units suchas barite and carbonate that would be used in sta-ble isotope and geochemical studies

Site-specific sample

This sample set would focus on sites where po-tential biogenic structures have been documentedsuch as the stromatolitic intervals in the DresserFormation and the Strelley Pool Chert It wouldalso include detailed sampling of sites such asChinaman Creek and the Strelley Pool Chertwhere purported microfossils have been docu-mented (Schopf 1993) and the sulfide bodies inthe Kangaroo Caves Formation that contain pos-sible microfossils as well as evidence of hydro-carbons (Rasmussen 2000 Rasmussen and Buick2000) This would involve a broad samplingaround the sites in measured sections and drillcore to establish the geological context

Framework samples

While the focus of the collecting would be onmaterial that would help define the character of

the Earthrsquos earliest biosphere it is important thatthe broad context of the samples be defined TheArchean is a period when the Earthrsquos crust wasevolving rapidly and there is a growing body ofevidence to suggest that the evolution of theplanet drives the evolution of the atmosphere andbiosphere (Lindsay and Brasier 2002ab) Theframework sample set would include suites ofsample from the extrusive volcanic rocks col-lected along well-documented sections Sectionsthrough the volcanoclastic succession would em-phasize the dominant volcanic units (basalts ko-matiites andesites felsites and rhyolites) assources of information on the mantle and evolv-ing crust This sample set would also include adetailed sampling of the granitoid complexeswhich ultimately drove the entire process as wellas the later intrusions that were important in dri-ving the hydrothermal systems that may wellhave been the cradle of early life

SAMPLE CURATIONCHARACTERIZATION AND

DISTRIBUTION

All samples would be fully documented in thefield Samples would be photographed and theirlocations determined using a Global PositioningSystem receiver For specialized samples such asthose thought to contain easily contaminated bio-geochemical information the field collectionwould be modeled on techniques developed bythe NASANational Science FoundationSmith-sonian Institution Antarctic Search for Meteoritesprogram (Harvey and Schutt 1998)

Characterization

The petrologic and petrographic characteristicsof each sample would be determined using thin-section optical microscopy The quantitative bulkchemical composition of each sample would bemeasured by x-ray fluorescence spectrometry Fi-nally the qualitative bulk mineralogy of eachsample would be measured by powder x-ray dif-fraction

Data management

An Archean Geologic Reference Materials Data-base would be produced to fully document eachsample Field documentation would include sam-pling rationale supporting references and field

ARCHEAN GEOLOGIC REFERENCE COLLECTION 753

photographs Documentation would include theresults of all analyses along with images of rep-resentative sample surfaces and optical micro-graphs The Database would be produced in elec-tronic format and posted on the web Subsamplesof the Archean Geologic Reference Materialswould then be made available at no cost to re-searchers in the astrobiology community

DISCUSSION AND CONCLUSIONS

The Early Archean environment was uniqueRecent studies indicate that the atmosphere waslargely anoxic with oxygen levels 1025 of thepresent atmospheric level (Pavlov and Kasting2002) There were no large continents no activeor passive margins and by default no intracra-tonic basins Significantly perhaps there are nosigns of eustasy The crust was evolving rapidlyand heat flow was high leading to the intrusionof large granitoid bodies and the development ofa volcanic carapace above them In turn thismeant that the marine environment was domi-nated by hydrothermal activity

In the absence of large continental masses de-trital materials other than volcanoclastics are rel-atively rare in the Early Archean Significant vol-umes of quartz sand do not appear in the PilbaraCraton sedimentary record until after 32 Ga pre-sumably as the intruding igneous bodies begin tobe unroofed and eroded There are thus no ma-jor accumulations of sedimentary rocks that wenormally expect associated with passive conti-nental margins

With the assembly of the earliest large conti-nents beginning at approximately 30ndash28 Gathe Earthrsquos atmosphere hydrosphere and bios-phere changed rapidly Intracratonic basinssuch as the Hamersley Basin began to developon the new crustal blocks opening new shal-low-water environments that provided an idealsetting for photosynthesizing organisms andthe development of the first carbonate reefstructures (Lindsay and Brasier 2002ab) Hy-drothermal processes while still importantwere thus no longer the only sources of energyavailable to the evolving biosphere

The Archean was thus an unfamiliar environ-ment and unravelling the nature of the earliestbiosphere will require development of new ap-proaches and new techniques Without large con-tinental masses subareal exposures are likely to

be rare and short-lived There is no evidence forstream activity until the De Grey group at ap-proximately 29 Ga at which time braided streamgravels were preserved Thus models involvingearly biospheric evolution in ldquoa warm little pondrdquo(Darwin 1888) or in fresh water seem unlikelyInstead the focus in the search for the origins oflife on Earth should be directed to hydrothermalmarine settings involving high temperatures andwhat would today be hostile geochemical envi-ronments with significant concentrations of metal-lic ions and a largely reducing environment Therisks for the misidentification of abiotic structuresand geochemical signatures as signs of ancient lifeare significant as seen in recent discussions (egBrasier et al 2002 Fedo and Whitehouse 2002Schopf et al 2002)

These difficulties suggest that much could begained by the development of a well-documentedcollection of Archean rocks that could be madeavailable to all qualified investigators Until weunderstand the earliest stages of the evolution oflife on Earth we cannot hope to make significantprogress in the search for life on other planetssuch as Mars Until there is a sample return mis-sion to Mars we must make do with meteoritesthat come to us largely out of context An under-standing of the Archean Earth can help placethose limited samples in a planetary context

ACKNOWLEDGMENTS

Franco Pirajno Kath Grey and Martin VanKranendonk provided generous advice and as-sistance and the Geological Survey of WesternAustralia supported the field program We aregrateful to Peter Morant of Sipa Resources andAdrian Black of Newexco Perth for their adviceand for access to drill-core materials

ABBREVIATION

BIF banded iron formation

REFERENCES

Arndt NT Nelson DR Compston W Trendall AFand Thorne AM (1991) The age of the FortescueGroup Hamersley Basin Western Australia from ionmicroprobe zircon U-Pb results Aust J Earth Sci 38262ndash281

LINDSAY ET AL754

Awramik SM Schopf JW and Walter MR (1983) Fil-amentous fossil bacteria from the Archaean of WesternAustralia Precambrian Res 20 357ndash374

Barley ME Pickard AL and Sylvester PJ (1997) Em-placement of a large igneous province as a possiblecause of banded iron formation 245 billion years agoNature 385 55ndash58

Bettenay LF Bickle MJ Boulter CA Groves DIMorant P Blake TS and James BA (1981) Evolu-tion of the Shaw Batholithmdashan Archaean granitoid-gneiss dome in the eastern Pilbara Western AustraliaIn Special Publication 7 The Archaean edited by JEGlover and DI Groves Geological Society of WesternAustralia Perth pp 361ndash372

Blake TS and Barley ME (1992) Tectonic evolution ofthe Late Archaean to Early Proterozoic Mount BruceMegasequence Set Western Australia Tectonics 11 1415ndash1425

Blake TS and Groves DI (1987) Continental rifting andthe Archaean-Proterozoic transition Geology 15 229ndash232

Brasier MD and Lindsay JF (1998) A billion years ofenvironmental stability and the emergence of eukary-otes new data from Northern Australia Geology 26555ndash558

Brasier MD Green OR Jephcoat AP Kleppe AKVan Kranendonk MJ Lindsay JF Steele A andGrassineau NV (2002) Questioning the evidence forEarthrsquos oldest fossils Nature 416 76ndash81

Brauhart C (1999) Regional alteration systems associatedwith Archaean volcanogenic massive sulphide depositsat Panorama Pilbara Western Australia [PhD Thesis]University of Western Australia Perth

Brauhart C Groves DI and Morant P (1998) Regionalalteration systems associated with volcanogenic mas-sive sulfide mineralization at Panorama Pilbara West-ern Australia Econ Geol 93 292ndash302

Brocks J Logan GA Buick R and Summons RE(1999) Archean molecular fossils and the early rise ofeukaryotes Science 285 1033ndash1036

Buick R Dunlop JSR and Groves DI (1981) Stroma-tolite recognition in ancient rocks an appraisal of ir-regular laminated structures in an early Archaeanchert-barite unit from North Pole Western AustraliaAlcheringa 5 161ndash181

Buick R Des Marais DJ and Knoll AH (1995a) Sta-ble isotope compositions of carbonates from the Meso-proterozoic Bangemall Group northwestern AustraliaChem Geol 123 153ndash171

Buick R Thornett JR McNaughton NJ Smith JBBarley ME and Savage M (1995b) Record of emer-gent continental crust 35 billion years ago in the Pil-bara Craton of Australia Nature 375 574ndash577

Buick R Rasmussen B and Krapez B (1998) Archaeanoil evidence for extensive hydrocarbon generation andmigration 25ndash35 Ga Am Assoc Petroleum Geol Bull82 50ndash69

Chan M (1998) Sedimentology and structure of threesmall congolmeratic basins in the Lalla Rookh-WesternShaw structural corridor Pilbara Craton Western Aus-

tralia [BSc Honours Thesis] University of WesternAustralia Perth

Cullers RL DiMarco MJ Lowe DR and Stone J(1993) Geochemistry of a silicified felsic volcaniclasticsuite from the Early Archaean Panorama FormationPilbara Block Western Australiamdashan evaluation of de-positional and post-depositional processes with specialemphasis on the rare-Earth elements Precambrian Res60 99ndash116

Darwin F ed (1888) The Life and Letters of Charles Dar-win Vol 3 John Murray London

de Wit MJ (1998) On Archaean granites greenstonescratons and tectonics does the evidence demand a ver-dict Precambrian Geol 91 181ndash226

Drummond BJ (1981) Crustal structure of the Precam-brian terrains of northwest Australia from seismic re-fraction data BMR J Geol Geophys 6 123ndash135

Dunlop JSR Muir M Milne VA and Groves DI(1978) A new microfossil assemblage from the Ar-chaean of Western Australia Nature 274 676ndash678

Dutkiewicz A Rasmussen B and Buick R (1998) Oilpreserved in fluid inclusions in Archaean sandstonesNature 395 885ndash888

Fedo CM and Whitehouse MJ (2002) Metasomatic ori-gin of quartz-pyroxene rock Akilia Greenland and im-plications for Earthrsquos earliest life Science 296 1448ndash1452

Gee RD and Grey K (1993) Report 41 Proterozoic Rockson the Glengarry 1250 000 SheetmdashStratigraphy Structureand Stromatolite Biostratigraphy Geological Survey ofWestern Australia Perth

Glikson AY and Hickman AH (1981a) Chemicalstratigraphy and petrogenesis of Archaean basic-ultra-basic volcanic units eastern Pilbara Block WesternAustralia In Special Publication 7 Archaean Geologyedited by JE Groves and DI Groves Geological Soci-ety of Western Australia Perth pp 287ndash300

Glikson AY and Hickman AH (1981b) Record 198136Geochemistry of Archaean Volcanic Successions EasternPilbara Block Western Australia Australian Bureau ofMineral Resources Canberra

Glikson AY Davy R and Hickman AH (1991) Record199146 Trace Metal Distribution Patterns in Basalts Pil-bara Craton Western Australia with Stratigraphic-Geo-chemical Implications Australian Bureau of Mineral Re-sources Canberra

Groves DI Dunlop JSR and Buick R (1981) An earlyhabitat of life Sci Am 245 64ndash73

Harvey RP and Schutt JW (1998) Meteorite recoveryand reconnaissance in the Allan Hills-David Glacierand Darwin Glacier regions 1996ndash1997 Antarctic J US 32 25ndash27

Hickman AH (1975) Annual Report 1974 PrecambrianStructural Geology of Part of the Pilbara Region Geologi-cal Survey of Western Australia Perth pp 68ndash73

Hickman AH (1983) Bulletin 127 Geology of the PilbaraBlock and Its Environs Geological Survey of WesternAustralia Perth

Hickman AH (1984) Archaean diapirism in the PilbaraBlock Western Australia In Precambrian Tectonics Il-lustrated edited by A Kroner and R Greiling E

ARCHEAN GEOLOGIC REFERENCE COLLECTION 755

etry was determined by eustasy are widespreadand contain considerable information on earlyoxygenic photosynthesis and the development ofthe first reef-like structure that are similar inmany ways to modern reefs (Simonson et al1993ab Lindsay and Brasier 2002ab)

The collection would consist of three groupingsof samples stratigraphic samples site-specificsamples and framework samples

Stratigraphic samples

The stratigraphic sample set would be collectedfrom well-defined settings such as carefully mea-sured and documented sections or from drill coreSamples would be collected from each of the fivestratigraphic groups or megasequences Thesupracrustal successions preserved in the Hamer-sley and associated basins would where possiblebe taken from existing drill core or from drill coresobtained for special projects Where drill core isnot available samples would be collected fromcarefully selected and measured outcrop sectionsThe samples would concentrate on rocks mostlikely to host biogenic material Samples from theEarly Archean volcaniclastic carapace would bedominated by cherts that are most likely to pre-serve fine biogenic structures but would also in-clude other sedimentary hydrothermal units suchas barite and carbonate that would be used in sta-ble isotope and geochemical studies

Site-specific sample

This sample set would focus on sites where po-tential biogenic structures have been documentedsuch as the stromatolitic intervals in the DresserFormation and the Strelley Pool Chert It wouldalso include detailed sampling of sites such asChinaman Creek and the Strelley Pool Chertwhere purported microfossils have been docu-mented (Schopf 1993) and the sulfide bodies inthe Kangaroo Caves Formation that contain pos-sible microfossils as well as evidence of hydro-carbons (Rasmussen 2000 Rasmussen and Buick2000) This would involve a broad samplingaround the sites in measured sections and drillcore to establish the geological context

Framework samples

While the focus of the collecting would be onmaterial that would help define the character of

the Earthrsquos earliest biosphere it is important thatthe broad context of the samples be defined TheArchean is a period when the Earthrsquos crust wasevolving rapidly and there is a growing body ofevidence to suggest that the evolution of theplanet drives the evolution of the atmosphere andbiosphere (Lindsay and Brasier 2002ab) Theframework sample set would include suites ofsample from the extrusive volcanic rocks col-lected along well-documented sections Sectionsthrough the volcanoclastic succession would em-phasize the dominant volcanic units (basalts ko-matiites andesites felsites and rhyolites) assources of information on the mantle and evolv-ing crust This sample set would also include adetailed sampling of the granitoid complexeswhich ultimately drove the entire process as wellas the later intrusions that were important in dri-ving the hydrothermal systems that may wellhave been the cradle of early life

SAMPLE CURATIONCHARACTERIZATION AND

DISTRIBUTION

All samples would be fully documented in thefield Samples would be photographed and theirlocations determined using a Global PositioningSystem receiver For specialized samples such asthose thought to contain easily contaminated bio-geochemical information the field collectionwould be modeled on techniques developed bythe NASANational Science FoundationSmith-sonian Institution Antarctic Search for Meteoritesprogram (Harvey and Schutt 1998)

Characterization

The petrologic and petrographic characteristicsof each sample would be determined using thin-section optical microscopy The quantitative bulkchemical composition of each sample would bemeasured by x-ray fluorescence spectrometry Fi-nally the qualitative bulk mineralogy of eachsample would be measured by powder x-ray dif-fraction

Data management

An Archean Geologic Reference Materials Data-base would be produced to fully document eachsample Field documentation would include sam-pling rationale supporting references and field

ARCHEAN GEOLOGIC REFERENCE COLLECTION 753

photographs Documentation would include theresults of all analyses along with images of rep-resentative sample surfaces and optical micro-graphs The Database would be produced in elec-tronic format and posted on the web Subsamplesof the Archean Geologic Reference Materialswould then be made available at no cost to re-searchers in the astrobiology community

DISCUSSION AND CONCLUSIONS

The Early Archean environment was uniqueRecent studies indicate that the atmosphere waslargely anoxic with oxygen levels 1025 of thepresent atmospheric level (Pavlov and Kasting2002) There were no large continents no activeor passive margins and by default no intracra-tonic basins Significantly perhaps there are nosigns of eustasy The crust was evolving rapidlyand heat flow was high leading to the intrusionof large granitoid bodies and the development ofa volcanic carapace above them In turn thismeant that the marine environment was domi-nated by hydrothermal activity

In the absence of large continental masses de-trital materials other than volcanoclastics are rel-atively rare in the Early Archean Significant vol-umes of quartz sand do not appear in the PilbaraCraton sedimentary record until after 32 Ga pre-sumably as the intruding igneous bodies begin tobe unroofed and eroded There are thus no ma-jor accumulations of sedimentary rocks that wenormally expect associated with passive conti-nental margins

With the assembly of the earliest large conti-nents beginning at approximately 30ndash28 Gathe Earthrsquos atmosphere hydrosphere and bios-phere changed rapidly Intracratonic basinssuch as the Hamersley Basin began to developon the new crustal blocks opening new shal-low-water environments that provided an idealsetting for photosynthesizing organisms andthe development of the first carbonate reefstructures (Lindsay and Brasier 2002ab) Hy-drothermal processes while still importantwere thus no longer the only sources of energyavailable to the evolving biosphere

The Archean was thus an unfamiliar environ-ment and unravelling the nature of the earliestbiosphere will require development of new ap-proaches and new techniques Without large con-tinental masses subareal exposures are likely to

be rare and short-lived There is no evidence forstream activity until the De Grey group at ap-proximately 29 Ga at which time braided streamgravels were preserved Thus models involvingearly biospheric evolution in ldquoa warm little pondrdquo(Darwin 1888) or in fresh water seem unlikelyInstead the focus in the search for the origins oflife on Earth should be directed to hydrothermalmarine settings involving high temperatures andwhat would today be hostile geochemical envi-ronments with significant concentrations of metal-lic ions and a largely reducing environment Therisks for the misidentification of abiotic structuresand geochemical signatures as signs of ancient lifeare significant as seen in recent discussions (egBrasier et al 2002 Fedo and Whitehouse 2002Schopf et al 2002)

These difficulties suggest that much could begained by the development of a well-documentedcollection of Archean rocks that could be madeavailable to all qualified investigators Until weunderstand the earliest stages of the evolution oflife on Earth we cannot hope to make significantprogress in the search for life on other planetssuch as Mars Until there is a sample return mis-sion to Mars we must make do with meteoritesthat come to us largely out of context An under-standing of the Archean Earth can help placethose limited samples in a planetary context

ACKNOWLEDGMENTS

Franco Pirajno Kath Grey and Martin VanKranendonk provided generous advice and as-sistance and the Geological Survey of WesternAustralia supported the field program We aregrateful to Peter Morant of Sipa Resources andAdrian Black of Newexco Perth for their adviceand for access to drill-core materials

ABBREVIATION

BIF banded iron formation

REFERENCES

Arndt NT Nelson DR Compston W Trendall AFand Thorne AM (1991) The age of the FortescueGroup Hamersley Basin Western Australia from ionmicroprobe zircon U-Pb results Aust J Earth Sci 38262ndash281

LINDSAY ET AL754

Awramik SM Schopf JW and Walter MR (1983) Fil-amentous fossil bacteria from the Archaean of WesternAustralia Precambrian Res 20 357ndash374

Barley ME Pickard AL and Sylvester PJ (1997) Em-placement of a large igneous province as a possiblecause of banded iron formation 245 billion years agoNature 385 55ndash58

Bettenay LF Bickle MJ Boulter CA Groves DIMorant P Blake TS and James BA (1981) Evolu-tion of the Shaw Batholithmdashan Archaean granitoid-gneiss dome in the eastern Pilbara Western AustraliaIn Special Publication 7 The Archaean edited by JEGlover and DI Groves Geological Society of WesternAustralia Perth pp 361ndash372

Blake TS and Barley ME (1992) Tectonic evolution ofthe Late Archaean to Early Proterozoic Mount BruceMegasequence Set Western Australia Tectonics 11 1415ndash1425

Blake TS and Groves DI (1987) Continental rifting andthe Archaean-Proterozoic transition Geology 15 229ndash232

Brasier MD and Lindsay JF (1998) A billion years ofenvironmental stability and the emergence of eukary-otes new data from Northern Australia Geology 26555ndash558

Brasier MD Green OR Jephcoat AP Kleppe AKVan Kranendonk MJ Lindsay JF Steele A andGrassineau NV (2002) Questioning the evidence forEarthrsquos oldest fossils Nature 416 76ndash81

Brauhart C (1999) Regional alteration systems associatedwith Archaean volcanogenic massive sulphide depositsat Panorama Pilbara Western Australia [PhD Thesis]University of Western Australia Perth

Brauhart C Groves DI and Morant P (1998) Regionalalteration systems associated with volcanogenic mas-sive sulfide mineralization at Panorama Pilbara West-ern Australia Econ Geol 93 292ndash302

Brocks J Logan GA Buick R and Summons RE(1999) Archean molecular fossils and the early rise ofeukaryotes Science 285 1033ndash1036

Buick R Dunlop JSR and Groves DI (1981) Stroma-tolite recognition in ancient rocks an appraisal of ir-regular laminated structures in an early Archaeanchert-barite unit from North Pole Western AustraliaAlcheringa 5 161ndash181

Buick R Des Marais DJ and Knoll AH (1995a) Sta-ble isotope compositions of carbonates from the Meso-proterozoic Bangemall Group northwestern AustraliaChem Geol 123 153ndash171

Buick R Thornett JR McNaughton NJ Smith JBBarley ME and Savage M (1995b) Record of emer-gent continental crust 35 billion years ago in the Pil-bara Craton of Australia Nature 375 574ndash577

Buick R Rasmussen B and Krapez B (1998) Archaeanoil evidence for extensive hydrocarbon generation andmigration 25ndash35 Ga Am Assoc Petroleum Geol Bull82 50ndash69

Chan M (1998) Sedimentology and structure of threesmall congolmeratic basins in the Lalla Rookh-WesternShaw structural corridor Pilbara Craton Western Aus-

tralia [BSc Honours Thesis] University of WesternAustralia Perth

Cullers RL DiMarco MJ Lowe DR and Stone J(1993) Geochemistry of a silicified felsic volcaniclasticsuite from the Early Archaean Panorama FormationPilbara Block Western Australiamdashan evaluation of de-positional and post-depositional processes with specialemphasis on the rare-Earth elements Precambrian Res60 99ndash116

Darwin F ed (1888) The Life and Letters of Charles Dar-win Vol 3 John Murray London

de Wit MJ (1998) On Archaean granites greenstonescratons and tectonics does the evidence demand a ver-dict Precambrian Geol 91 181ndash226

Drummond BJ (1981) Crustal structure of the Precam-brian terrains of northwest Australia from seismic re-fraction data BMR J Geol Geophys 6 123ndash135

Dunlop JSR Muir M Milne VA and Groves DI(1978) A new microfossil assemblage from the Ar-chaean of Western Australia Nature 274 676ndash678

Dutkiewicz A Rasmussen B and Buick R (1998) Oilpreserved in fluid inclusions in Archaean sandstonesNature 395 885ndash888

Fedo CM and Whitehouse MJ (2002) Metasomatic ori-gin of quartz-pyroxene rock Akilia Greenland and im-plications for Earthrsquos earliest life Science 296 1448ndash1452

Gee RD and Grey K (1993) Report 41 Proterozoic Rockson the Glengarry 1250 000 SheetmdashStratigraphy Structureand Stromatolite Biostratigraphy Geological Survey ofWestern Australia Perth

Glikson AY and Hickman AH (1981a) Chemicalstratigraphy and petrogenesis of Archaean basic-ultra-basic volcanic units eastern Pilbara Block WesternAustralia In Special Publication 7 Archaean Geologyedited by JE Groves and DI Groves Geological Soci-ety of Western Australia Perth pp 287ndash300

Glikson AY and Hickman AH (1981b) Record 198136Geochemistry of Archaean Volcanic Successions EasternPilbara Block Western Australia Australian Bureau ofMineral Resources Canberra

Glikson AY Davy R and Hickman AH (1991) Record199146 Trace Metal Distribution Patterns in Basalts Pil-bara Craton Western Australia with Stratigraphic-Geo-chemical Implications Australian Bureau of Mineral Re-sources Canberra

Groves DI Dunlop JSR and Buick R (1981) An earlyhabitat of life Sci Am 245 64ndash73

Harvey RP and Schutt JW (1998) Meteorite recoveryand reconnaissance in the Allan Hills-David Glacierand Darwin Glacier regions 1996ndash1997 Antarctic J US 32 25ndash27

Hickman AH (1975) Annual Report 1974 PrecambrianStructural Geology of Part of the Pilbara Region Geologi-cal Survey of Western Australia Perth pp 68ndash73

Hickman AH (1983) Bulletin 127 Geology of the PilbaraBlock and Its Environs Geological Survey of WesternAustralia Perth

Hickman AH (1984) Archaean diapirism in the PilbaraBlock Western Australia In Precambrian Tectonics Il-lustrated edited by A Kroner and R Greiling E

ARCHEAN GEOLOGIC REFERENCE COLLECTION 755

photographs Documentation would include theresults of all analyses along with images of rep-resentative sample surfaces and optical micro-graphs The Database would be produced in elec-tronic format and posted on the web Subsamplesof the Archean Geologic Reference Materialswould then be made available at no cost to re-searchers in the astrobiology community

DISCUSSION AND CONCLUSIONS

The Early Archean environment was uniqueRecent studies indicate that the atmosphere waslargely anoxic with oxygen levels 1025 of thepresent atmospheric level (Pavlov and Kasting2002) There were no large continents no activeor passive margins and by default no intracra-tonic basins Significantly perhaps there are nosigns of eustasy The crust was evolving rapidlyand heat flow was high leading to the intrusionof large granitoid bodies and the development ofa volcanic carapace above them In turn thismeant that the marine environment was domi-nated by hydrothermal activity

In the absence of large continental masses de-trital materials other than volcanoclastics are rel-atively rare in the Early Archean Significant vol-umes of quartz sand do not appear in the PilbaraCraton sedimentary record until after 32 Ga pre-sumably as the intruding igneous bodies begin tobe unroofed and eroded There are thus no ma-jor accumulations of sedimentary rocks that wenormally expect associated with passive conti-nental margins

With the assembly of the earliest large conti-nents beginning at approximately 30ndash28 Gathe Earthrsquos atmosphere hydrosphere and bios-phere changed rapidly Intracratonic basinssuch as the Hamersley Basin began to developon the new crustal blocks opening new shal-low-water environments that provided an idealsetting for photosynthesizing organisms andthe development of the first carbonate reefstructures (Lindsay and Brasier 2002ab) Hy-drothermal processes while still importantwere thus no longer the only sources of energyavailable to the evolving biosphere

The Archean was thus an unfamiliar environ-ment and unravelling the nature of the earliestbiosphere will require development of new ap-proaches and new techniques Without large con-tinental masses subareal exposures are likely to

be rare and short-lived There is no evidence forstream activity until the De Grey group at ap-proximately 29 Ga at which time braided streamgravels were preserved Thus models involvingearly biospheric evolution in ldquoa warm little pondrdquo(Darwin 1888) or in fresh water seem unlikelyInstead the focus in the search for the origins oflife on Earth should be directed to hydrothermalmarine settings involving high temperatures andwhat would today be hostile geochemical envi-ronments with significant concentrations of metal-lic ions and a largely reducing environment Therisks for the misidentification of abiotic structuresand geochemical signatures as signs of ancient lifeare significant as seen in recent discussions (egBrasier et al 2002 Fedo and Whitehouse 2002Schopf et al 2002)

These difficulties suggest that much could begained by the development of a well-documentedcollection of Archean rocks that could be madeavailable to all qualified investigators Until weunderstand the earliest stages of the evolution oflife on Earth we cannot hope to make significantprogress in the search for life on other planetssuch as Mars Until there is a sample return mis-sion to Mars we must make do with meteoritesthat come to us largely out of context An under-standing of the Archean Earth can help placethose limited samples in a planetary context

ACKNOWLEDGMENTS

Franco Pirajno Kath Grey and Martin VanKranendonk provided generous advice and as-sistance and the Geological Survey of WesternAustralia supported the field program We aregrateful to Peter Morant of Sipa Resources andAdrian Black of Newexco Perth for their adviceand for access to drill-core materials

ABBREVIATION

BIF banded iron formation

REFERENCES

Arndt NT Nelson DR Compston W Trendall AFand Thorne AM (1991) The age of the FortescueGroup Hamersley Basin Western Australia from ionmicroprobe zircon U-Pb results Aust J Earth Sci 38262ndash281

LINDSAY ET AL754

Awramik SM Schopf JW and Walter MR (1983) Fil-amentous fossil bacteria from the Archaean of WesternAustralia Precambrian Res 20 357ndash374

Barley ME Pickard AL and Sylvester PJ (1997) Em-placement of a large igneous province as a possiblecause of banded iron formation 245 billion years agoNature 385 55ndash58

Bettenay LF Bickle MJ Boulter CA Groves DIMorant P Blake TS and James BA (1981) Evolu-tion of the Shaw Batholithmdashan Archaean granitoid-gneiss dome in the eastern Pilbara Western AustraliaIn Special Publication 7 The Archaean edited by JEGlover and DI Groves Geological Society of WesternAustralia Perth pp 361ndash372

Blake TS and Barley ME (1992) Tectonic evolution ofthe Late Archaean to Early Proterozoic Mount BruceMegasequence Set Western Australia Tectonics 11 1415ndash1425

Blake TS and Groves DI (1987) Continental rifting andthe Archaean-Proterozoic transition Geology 15 229ndash232

Brasier MD and Lindsay JF (1998) A billion years ofenvironmental stability and the emergence of eukary-otes new data from Northern Australia Geology 26555ndash558

Brasier MD Green OR Jephcoat AP Kleppe AKVan Kranendonk MJ Lindsay JF Steele A andGrassineau NV (2002) Questioning the evidence forEarthrsquos oldest fossils Nature 416 76ndash81

Brauhart C (1999) Regional alteration systems associatedwith Archaean volcanogenic massive sulphide depositsat Panorama Pilbara Western Australia [PhD Thesis]University of Western Australia Perth

Brauhart C Groves DI and Morant P (1998) Regionalalteration systems associated with volcanogenic mas-sive sulfide mineralization at Panorama Pilbara West-ern Australia Econ Geol 93 292ndash302

Brocks J Logan GA Buick R and Summons RE(1999) Archean molecular fossils and the early rise ofeukaryotes Science 285 1033ndash1036

Buick R Dunlop JSR and Groves DI (1981) Stroma-tolite recognition in ancient rocks an appraisal of ir-regular laminated structures in an early Archaeanchert-barite unit from North Pole Western AustraliaAlcheringa 5 161ndash181

Buick R Des Marais DJ and Knoll AH (1995a) Sta-ble isotope compositions of carbonates from the Meso-proterozoic Bangemall Group northwestern AustraliaChem Geol 123 153ndash171

Buick R Thornett JR McNaughton NJ Smith JBBarley ME and Savage M (1995b) Record of emer-gent continental crust 35 billion years ago in the Pil-bara Craton of Australia Nature 375 574ndash577

Buick R Rasmussen B and Krapez B (1998) Archaeanoil evidence for extensive hydrocarbon generation andmigration 25ndash35 Ga Am Assoc Petroleum Geol Bull82 50ndash69

Chan M (1998) Sedimentology and structure of threesmall congolmeratic basins in the Lalla Rookh-WesternShaw structural corridor Pilbara Craton Western Aus-

tralia [BSc Honours Thesis] University of WesternAustralia Perth

Cullers RL DiMarco MJ Lowe DR and Stone J(1993) Geochemistry of a silicified felsic volcaniclasticsuite from the Early Archaean Panorama FormationPilbara Block Western Australiamdashan evaluation of de-positional and post-depositional processes with specialemphasis on the rare-Earth elements Precambrian Res60 99ndash116

Darwin F ed (1888) The Life and Letters of Charles Dar-win Vol 3 John Murray London

de Wit MJ (1998) On Archaean granites greenstonescratons and tectonics does the evidence demand a ver-dict Precambrian Geol 91 181ndash226

Drummond BJ (1981) Crustal structure of the Precam-brian terrains of northwest Australia from seismic re-fraction data BMR J Geol Geophys 6 123ndash135

Dunlop JSR Muir M Milne VA and Groves DI(1978) A new microfossil assemblage from the Ar-chaean of Western Australia Nature 274 676ndash678

Dutkiewicz A Rasmussen B and Buick R (1998) Oilpreserved in fluid inclusions in Archaean sandstonesNature 395 885ndash888

Fedo CM and Whitehouse MJ (2002) Metasomatic ori-gin of quartz-pyroxene rock Akilia Greenland and im-plications for Earthrsquos earliest life Science 296 1448ndash1452

Gee RD and Grey K (1993) Report 41 Proterozoic Rockson the Glengarry 1250 000 SheetmdashStratigraphy Structureand Stromatolite Biostratigraphy Geological Survey ofWestern Australia Perth

Glikson AY and Hickman AH (1981a) Chemicalstratigraphy and petrogenesis of Archaean basic-ultra-basic volcanic units eastern Pilbara Block WesternAustralia In Special Publication 7 Archaean Geologyedited by JE Groves and DI Groves Geological Soci-ety of Western Australia Perth pp 287ndash300

Glikson AY and Hickman AH (1981b) Record 198136Geochemistry of Archaean Volcanic Successions EasternPilbara Block Western Australia Australian Bureau ofMineral Resources Canberra

Glikson AY Davy R and Hickman AH (1991) Record199146 Trace Metal Distribution Patterns in Basalts Pil-bara Craton Western Australia with Stratigraphic-Geo-chemical Implications Australian Bureau of Mineral Re-sources Canberra

Groves DI Dunlop JSR and Buick R (1981) An earlyhabitat of life Sci Am 245 64ndash73

Harvey RP and Schutt JW (1998) Meteorite recoveryand reconnaissance in the Allan Hills-David Glacierand Darwin Glacier regions 1996ndash1997 Antarctic J US 32 25ndash27

Hickman AH (1975) Annual Report 1974 PrecambrianStructural Geology of Part of the Pilbara Region Geologi-cal Survey of Western Australia Perth pp 68ndash73

Hickman AH (1983) Bulletin 127 Geology of the PilbaraBlock and Its Environs Geological Survey of WesternAustralia Perth

Hickman AH (1984) Archaean diapirism in the PilbaraBlock Western Australia In Precambrian Tectonics Il-lustrated edited by A Kroner and R Greiling E

ARCHEAN GEOLOGIC REFERENCE COLLECTION 755

Awramik SM Schopf JW and Walter MR (1983) Fil-amentous fossil bacteria from the Archaean of WesternAustralia Precambrian Res 20 357ndash374

Barley ME Pickard AL and Sylvester PJ (1997) Em-placement of a large igneous province as a possiblecause of banded iron formation 245 billion years agoNature 385 55ndash58

Bettenay LF Bickle MJ Boulter CA Groves DIMorant P Blake TS and James BA (1981) Evolu-tion of the Shaw Batholithmdashan Archaean granitoid-gneiss dome in the eastern Pilbara Western AustraliaIn Special Publication 7 The Archaean edited by JEGlover and DI Groves Geological Society of WesternAustralia Perth pp 361ndash372

Blake TS and Barley ME (1992) Tectonic evolution ofthe Late Archaean to Early Proterozoic Mount BruceMegasequence Set Western Australia Tectonics 11 1415ndash1425

Blake TS and Groves DI (1987) Continental rifting andthe Archaean-Proterozoic transition Geology 15 229ndash232

Brasier MD and Lindsay JF (1998) A billion years ofenvironmental stability and the emergence of eukary-otes new data from Northern Australia Geology 26555ndash558

Brasier MD Green OR Jephcoat AP Kleppe AKVan Kranendonk MJ Lindsay JF Steele A andGrassineau NV (2002) Questioning the evidence forEarthrsquos oldest fossils Nature 416 76ndash81

Brauhart C (1999) Regional alteration systems associatedwith Archaean volcanogenic massive sulphide depositsat Panorama Pilbara Western Australia [PhD Thesis]University of Western Australia Perth

Brauhart C Groves DI and Morant P (1998) Regionalalteration systems associated with volcanogenic mas-sive sulfide mineralization at Panorama Pilbara West-ern Australia Econ Geol 93 292ndash302

Brocks J Logan GA Buick R and Summons RE(1999) Archean molecular fossils and the early rise ofeukaryotes Science 285 1033ndash1036

Buick R Dunlop JSR and Groves DI (1981) Stroma-tolite recognition in ancient rocks an appraisal of ir-regular laminated structures in an early Archaeanchert-barite unit from North Pole Western AustraliaAlcheringa 5 161ndash181

Buick R Des Marais DJ and Knoll AH (1995a) Sta-ble isotope compositions of carbonates from the Meso-proterozoic Bangemall Group northwestern AustraliaChem Geol 123 153ndash171

Buick R Thornett JR McNaughton NJ Smith JBBarley ME and Savage M (1995b) Record of emer-gent continental crust 35 billion years ago in the Pil-bara Craton of Australia Nature 375 574ndash577

Buick R Rasmussen B and Krapez B (1998) Archaeanoil evidence for extensive hydrocarbon generation andmigration 25ndash35 Ga Am Assoc Petroleum Geol Bull82 50ndash69

Chan M (1998) Sedimentology and structure of threesmall congolmeratic basins in the Lalla Rookh-WesternShaw structural corridor Pilbara Craton Western Aus-

tralia [BSc Honours Thesis] University of WesternAustralia Perth

Cullers RL DiMarco MJ Lowe DR and Stone J(1993) Geochemistry of a silicified felsic volcaniclasticsuite from the Early Archaean Panorama FormationPilbara Block Western Australiamdashan evaluation of de-positional and post-depositional processes with specialemphasis on the rare-Earth elements Precambrian Res60 99ndash116

Darwin F ed (1888) The Life and Letters of Charles Dar-win Vol 3 John Murray London

de Wit MJ (1998) On Archaean granites greenstonescratons and tectonics does the evidence demand a ver-dict Precambrian Geol 91 181ndash226

Drummond BJ (1981) Crustal structure of the Precam-brian terrains of northwest Australia from seismic re-fraction data BMR J Geol Geophys 6 123ndash135

Dunlop JSR Muir M Milne VA and Groves DI(1978) A new microfossil assemblage from the Ar-chaean of Western Australia Nature 274 676ndash678

Dutkiewicz A Rasmussen B and Buick R (1998) Oilpreserved in fluid inclusions in Archaean sandstonesNature 395 885ndash888

Fedo CM and Whitehouse MJ (2002) Metasomatic ori-gin of quartz-pyroxene rock Akilia Greenland and im-plications for Earthrsquos earliest life Science 296 1448ndash1452

Gee RD and Grey K (1993) Report 41 Proterozoic Rockson the Glengarry 1250 000 SheetmdashStratigraphy Structureand Stromatolite Biostratigraphy Geological Survey ofWestern Australia Perth

Glikson AY and Hickman AH (1981a) Chemicalstratigraphy and petrogenesis of Archaean basic-ultra-basic volcanic units eastern Pilbara Block WesternAustralia In Special Publication 7 Archaean Geologyedited by JE Groves and DI Groves Geological Soci-ety of Western Australia Perth pp 287ndash300

Glikson AY and Hickman AH (1981b) Record 198136Geochemistry of Archaean Volcanic Successions EasternPilbara Block Western Australia Australian Bureau ofMineral Resources Canberra

Glikson AY Davy R and Hickman AH (1991) Record199146 Trace Metal Distribution Patterns in Basalts Pil-bara Craton Western Australia with Stratigraphic-Geo-chemical Implications Australian Bureau of Mineral Re-sources Canberra

Groves DI Dunlop JSR and Buick R (1981) An earlyhabitat of life Sci Am 245 64ndash73

Harvey RP and Schutt JW (1998) Meteorite recoveryand reconnaissance in the Allan Hills-David Glacierand Darwin Glacier regions 1996ndash1997 Antarctic J US 32 25ndash27

Hickman AH (1975) Annual Report 1974 PrecambrianStructural Geology of Part of the Pilbara Region Geologi-cal Survey of Western Australia Perth pp 68ndash73

Hickman AH (1983) Bulletin 127 Geology of the PilbaraBlock and Its Environs Geological Survey of WesternAustralia Perth

Hickman AH (1984) Archaean diapirism in the PilbaraBlock Western Australia In Precambrian Tectonics Il-lustrated edited by A Kroner and R Greiling E

ARCHEAN GEOLOGIC REFERENCE COLLECTION 755

Schweizerbartrsquosche Verlagbuchhandlung Stuttgart pp113ndash127

Hickman AH and Lipple SH (1978) 1250 000 Geologi-cal Series Marble Bar WA Sheet SF 50-8 2nd edit Ge-ological Survey of Western Australia Perth

Hill RM (1997) Stratigraphy structure and alteration ofhanging wall sedimentary rocks at the Sulphur Springsvolcanogenic massive sulfide (VMS) prospect east Pil-bara Craton [PhD Thesis] University of Western Aus-tralia Perth

Hofmann HJ Grey K Hickman AH and Thorpe R(1999) Origin of 345 Ga coniform stromatolites in theWarrawoona Group Western Australia Geol Soc AmBull 3 1256ndash1262

Horowitz RC (1982) Geological history of the early Pro-terozoic Paraburdoo hinge zone Western AustraliaPrecambrian Res 19 191ndash200

Knoll AH and Canfield DE (1998) Isotopic inferenceson early ecosystems In The Paleontological Society Papers4 Isotopic Paleobiology and Paleoecology edited by RDNorris and RM Corfield The Paleontological SocietyLawrence KS pp 212ndash243

Krapez B (1984) Sedimentation in a small fault-boundedbasinmdashLalla Rookh Sandstone East Pilbara Block InPublication 9 Archaean and Proterozoic Basins of the Pil-bara Western AustraliamdashEvolution and Mineralization Po-tential edited by JR Muhling DI Groves and TSBlake University of Western Australia Geology De-partment and University Extension Perth pp 89ndash110

Krapez B (1996) Sequence-stratigraphic concepts appliedto the identification of basin-filling rhythms in Pre-cambrian successions Aust J Earth Sci 43 355ndash380

Lindsay JF and Brasier MD (2000) A carbon isotopereference curve for c1700 to 1575 Ma Macarthur andMount Isa Basins northern Australia Precambrian Res99 271ndash308

Lindsay JF and Brasier MD (2002a) Did global tecton-ics drive early biosphere evolution Carbon isotoperecord from 26 to 19 Ga carbonates of Western Aus-tralian basins Precambrian Res 114 1ndash34

Lindsay JF and Brasier MD (2002b) A comment on tec-tonics and the future of terrestrial lifemdashreply Precam-brian Res 118 293ndash295

Lindsay JF Kennard JM and Southgate PN (1993)Application of sequence stratigraphy in an intracratonicsetting Amadeus Basin central Australia In SpecialPublication 18 Sequence Stratigraphy and Facies Associa-tions edited by HW Posamentier CP SummerhayesBU Haq and GP Allen International Association ofSedimentologists Blackwell Oxford pp 605ndash631

Lindsay JF Brasier MD McLaughlin N Green ORFogel M McNamara KM and Steele A (2003a) TheArchaean crucible of creation or cauldron of crud datafrom Western Australia [abstract] Astrobiology 2 455ndash456

Lindsay JF Brasier MD McLaughlin N Green ORFogel M McNamara KM Steele A and MertzmanSA (2003b) Abiotic Earthmdashestablishing a baseline forearliest life data from the Archean of Western Australia[abstract 1137] In Lunar and Planetary Science Conference

XXXIV [book on CD-ROM] Lunar and Planetary In-stitute Houston

Lowe DR (1980) Stromatolites 3400ndashMyr old from theArchaean of Western Australia Nature 284 441ndash443

Lowe DR (1983) Restricted shallow-water sedimenta-tion of early Archaean stromatolitic and evaporiticstrata of the Strelley Pool Chert Pilbara Block WesternAustralia Precambrian Res 19 239ndash283

Lowe DR (1994) Abiological origin of described stro-matolites older than 32 Ga Geology 22 387ndash390

Lowe DR (1995) Abiological origin of described stro-matolites older than 32 Ga Geology 23 191ndash192

McKay DS Gibson EK Thomas-Keprta KL Vali HRomanek CS Clemett SJ Chillier XDF Maech-ling CR and Zare RN (1996) Search for past life onMars possible relic biogenic activity in Martian mete-orite ALH84001 Science 273 942ndash930

McNaughton NJ Compston W and Barley ME (1993)Constraints on the age of the Warrawoona Group east-ern Pilbara Block Western Australia Precambrian Res60 69ndash98

Morant P (1998) Panorama zinc-copper deposits InMonograph 22 Geology of Australia and Papua NewGuinean Mineral Deposits edited by DA Berkman andDH MacKenzie Australian Institute of Mining andMetallurgy Melbourne pp 287ndash292

Murray CG Schreiber E and Walker RN (1989) Re-gional geological interpretation of a digital colouredresidual Bouguer gravity image of eastern Australiawith a wavelength cut-off of 250 km Aust J Earth Sci36 423ndash449

Nelson DR (1997) Record 19972 Compilation of SHRIMPU-Pb Zircon Geochronology Data 1996 Geological Sur-vey of Western Australia Perth

Nelson DR (1998) Record 19982 Compilation of SHRIMPU-Pb Zircon Geochronology Data 1997 Geological Sur-vey of Western Australia Perth

Nelson DR (1999) Record 19992 Compilation of Geochronol-ogy Data 1998 Geological Survey of Western AustraliaPerth

Nelson DR (2000) Record 20002 Compilation of Geochronol-ogy Data 1999 Geological Survey of Western AustraliaPerth

Nelson DR (2001) Record 20012 Compilation of Geochronol-ogy Data 2000 Geological Survey of Western AustraliaPerth

Nijman W de Bruijne KCH and Valkering ME (1998)Growth fault control of early Archaean cherts baritemounds and chert-barite veins North Pole Dome east-ern Pilbara Western Australia Precambrian Res 88 25ndash52

Occhipinti SA Grey K Pirajno F Adamides NGBagas L Dawes P and Le Blanc Smith G (1997)Record 19973 Stratigraphic Revision of PalaeoproterozoicRocks of the Yerrida Bryah and Padbury Basins (FormerGlengarry Basin) Geological Survey of Western Aus-tralia Perth

Occhipinti SA Swager CP and Pirajno F (1998) Struc-tural-metamorphic evolution of the Palaeoprotero-zoic Bryah and Padbury Groups during the Capricorn orogeny Western Australia Precambrian Res 90 141ndash158

LINDSAY ET AL756

Pavlov AA and Kasting JF (2002) Mass-independentfractionation of sulfur isotopes in Archean sedimentsstrong evidence for an anoxic Archean atmosphere As-trobiology 2 27ndash41

Pirajno F and Adamides NG (2000) Report 60 Geologyand Mineralisation of the Palaeoproterozoic Yerrida BasinWestern Australia Geological Survey of Western Aus-tralia Perth

Pirajno F Occhipinti SA and Swager CP (1998) Ge-ology and tectonic evolution of the PalaeoproterozoicBryah Padbury and Yerrida Basins (formerly Glen-garry Basin) Western Australia implications for thehistory of the south-central Capricorn Orogen Precam-brian Res 90 119ndash140

Plumb KA (1991) New Precambrian time scale Episodes14 139ndash140

Pope MC and Grotzinger JP (2000) Controls on fabricdevelopment and morphology of tufa and stromato-lites uppermost Pethei Group (18 Ga) Great SlaveLake Northwest Canada In Special Publication 67 Car-bonate Sedimentation and Diagenesis in the Evolving Pre-cambrian World edited by NP James and JP GrotzingerSEPM Tulsa OK pp 103ndash121

Rasmussen B (2000) Filamentous microfossils in a3240ndashmillion-year-old volcanogenic massive sulphidedeposit Nature 405 676ndash679

Rasmussen B and Buick R (2000) Oily old ores evidencefor hydrothermal petroleum generation in an Archaeanvolcanogenic massive sulfide deposit Geology 28 731ndash734

Rogers JJW and Santosh M (2002) Configuration of Co-lumbia a Mesoproterozoic continent Gondwana Res 55ndash22

Rutland RWR (1976) Orogenic evolution of AustraliaEarth-Sci Rev 12 161ndash196

Schopf JW (1992) Paleobiology of the Archean In TheProterozoic Biosphere A Multidisciplinary Study editedby JW Schopf and C Klein Cambridge UniversityPress Cambridge UK pp 25ndash39

Schopf JW (1993) Microfossils of the early ArchaeanApex chert new evidence of the antiquity of life Sci-ence 260 640ndash646

Schopf JW (1999) The Cradle of Life Princeton UniversityPress Princeton NJ

Schopf JW and Packer BM (1987) Early Archean (33billion to 35 billion-year-old) microfossils from Warra-woona Group Australia Science 237 70ndash73

Schopf JW and Walter MR (1983) Archaean microfos-sils new evidence of ancient microbes In Earthrsquos Earli-est BiospheremdashIts Origin and Evolution edited by JWSchopf Princeton University Press Princeton NJ pp214ndash239

Schopf JW Kudryavtsev AB Agresti DG WdowiakTJ and Czaja AD (2002) Laser-Raman imagery ofEarthrsquos earliest fossils Nature 416 73ndash76

Shaw RD Wellman P Gunn P Whitaker AJ Tar-lowski C and Morse M (1996) Record 199630 Guideto Using the Australian Crustal Elements Map AustralianGeological Survey Organisation Canberra

Simonson BM and Hassler SW (1996) Was the depo-

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Simonson BM and Hassler SW (1997) Revised corre-lations in the early Precambrian Hamersley Basin basedon a horizon of resedimented impact spherules AustJ Earth Sci 44 37ndash48

Simonson BM Hassler SW and Schubel KA (1993a)Lithology and proposed revision in stratigraphicnomenclature of the Wittenoom Formation (Dolomite)and overlying formations Hamersley Group WesternAustralia In Report 34 Professional Papers GeologicalSurvey of Western Australia Perth pp 65ndash79

Simonson BM Schubel KA and Hassler SW (1993b)Carbonate sedimentology of the early PrecambrianHamersley Group of Western Australia PrecambrianRes 60 287ndash335

Summons RE Jahnke LL Hope JM and Logan GA(1999) 2ndashMethylhopanoids as biomarkers for cyanobac-terial oxygenic photosynthesis Nature 400 554ndash557

Thorne AM and Seymour DD (1991) Bulletin 139 Ge-ology of the Ashburton Basin Geological Survey of West-ern Australia Perth

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Van Kranendonk MJ and Collins WJ (1998) Timingand tectonic significance of Late Archaean sinistralstrike-slip deformation in the Central Pilbara StructuralCorridor Pilbara Craton Western Australia Precam-brian Res 88 207ndash232

Van Kranendonk MJ and Morant P (1998) Revised Ar-

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Vearncombe S Barley ME Groves DI McNaughtonNJ Mikulki EJ and Vearncombe JR (1995) 326 Gablack smoker-type mineralisation in the Strelley BeltPilbara Craton Western Australia J Geol Soc Lond 152587ndash590

Vearncombe S Vearncombe JR and Barley ME (1998)Fault and stratigraphic controls on volcanogenic mas-sive sulfide deposits in the Strelley Belt Pilbara CratonWestern Australia Precambrian Res 88 67ndash82

Walter MR ed (1976) Stromatolites Elsevier Amster-dam

Walter MR (1983) Archaean stromatolites evidence ofEarthrsquos oldest benthos In Earthrsquos Earliest BiospheremdashItsOrigin and Evolution edited by JW Schopf PrincetonUniversity Press NJ pp 187ndash213

Walter MR Buick R and Dunlop JSR (1980) Stro-

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Wilhelmij HR (1986) Depositional history of the middleArchaean sedimentary sequences in the Pilbara BlockWestern Australiamdasha genetic stratigraphic analysis ofthe terrigenous rocks of the Pilgangoora syncline [PhDThesis] University of Western Australia Perth

Wilhelmij HR and Dunlop JSR (1984) A genetic strati-graphic investigation of the Gorge Creek Group in the Pil-gangoora Syncline In Publication 9 Archaean and Protero-zoic Basins of the Pilbara Western AustraliamdashEvolution andMineralization Potential edited by JR Muhling DI Grovesand TS Blake University of Western Australia GeologyDepartment and University Extension Perth pp 68ndash88

Williams IS and Collins WJ (1990) Granite-greenstoneterranes in the Pilbara Block Australia as coeval vol-cano-plutonic complexes evidence from U-Pb zircondating of Mount Edgar Batholith Earth Planet Sci Lett97 41ndash53

Address reprint requests toDr John F Lindsay

Lunar and Planetary Institute3600 Bay Area Boulevard

Houston TX 77058

E-mail johnflindsayjscnasagov

LINDSAY ET AL758