Prepared for the National Aeronautics and Space Administration

Geologic Map of the Northern Plains of Mars

By Kenneth L. Tanaka, James A. Skinner, Jr., and Trent M. Hare

Pamphlet to accompanyScientific Investigations Map 2888

2005

U.S. Department of the InteriorU.S. Geological Survey

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Contents

PageINTRODUCTION.............................................................................1PHYSIOGRAPHICSETTING....................................................................1DATA.......................................................................................2METHODOLOGY.............................................................................3 Unitdelineation............................................................................3 Unitnames................................................................................4 Unitgroupingsandsymbols..................................................................4 Unitcolors................................................................................4 Contacttypes..............................................................................4 Featuresymbols............................................................................4 GISapproachesandtools....................................................................5STRATIGRAPHY..............................................................................5 EarlyNoachianEpoch.......................................................................5 MiddleandLateNoachianEpochs.............................................................6 EarlyHesperianEpoch......................................................................7 LateHesperianEpoch.......................................................................8 EarlyAmazonianEpoch.....................................................................9 MiddleAmazonianEpoch....................................................................12 LateAmazonianEpoch......................................................................12STRUCTUREANDMODIFICATIONHISTORY....................................................14 Pre-Noachian..............................................................................14 EarlyNoachianEpoch.......................................................................14 MiddleandlateNoachianEpochs..............................................................14 EarlyHesperianEpoch......................................................................15 LateHesperianEpoch.......................................................................16 EarlyAmazonianEpoch.....................................................................16 MiddleAmazonianEpoch....................................................................17 LateAmazonianEpoch......................................................................18GEOLOGICSYNTHESIS.......................................................................18 Pre-NoachianandNoachian..................................................................18 Hesperian.................................................................................18 Amazonian................................................................................19ACKNOWLEDGMENTS.......................................................................19REFERENCESCITED..........................................................................19

Table1.......................................................................................26

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IntRoDUCtIonThenorthernplainsofMarscovernearlyathirdof

theplanetandconstitutetheplanet’sbroadestregionoflowlands.ApparentlyformedearlyinMars’history,thenorthern lowlandsservedasa repositoryboth for sedi-mentsshedfromtheadjacentancienthighlandsandforvolcanic flows and deposits from sources within and near the lowlands. Geomorphic evidence for extensivetectonicdeformationandreworkingofsurfacematerialsthroughreleaseofvolatilesoccursthroughoutthenorth-ernplains.Inthepolarregion,PlanumBoreumcontainsevidencefortheaccumulationoficeanddust,andsur-rounding dune fields suggest widespread aeolian trans-portanderosion.

The most recent regional- and global-scale mapsdescribingthegeologyofthenorthernplainsarelargelybasedonVikingOrbiterimagedata(Dial,1984;WitbeckandUnderwood,1984;ScottandTanaka,1986;Greeleyand Guest, 1987; Tanaka and Scott, 1987; Tanaka andothers, 1992a; Rotto and Tanaka, 1995; Crumpler andothers, 2001; McGill, 2002). These maps reveal high-land, plains, volcanic, and polar units based on mor-phologiccharacter,albedo,andrelativeagesusinglocalstratigraphicrelationsandcratercounts.

This geologic map of the northern plains is the first published map that covers a significant part of Mars using topographyand imagedata fromboth theMarsGlobalSurveyor and Mars Odyssey missions. The new dataprovideafreshperspectiveonthegeologyoftheregionthatrevealsmanypreviouslyunrecognizableunits,fea-tures,andtemporalrelations.Inaddition,weadaptedandinstituted terrestrialmappingmethodsandstratigraphicconventions that we think result in a clearer and moreobjectivemap.Wefocusonmappingwiththeintentofreconstructingthehistoryofgeologicactivitywithinthenorthern plains, including deposition, volcanism, ero-sion, tectonism, impact cratering, and other processeswith the aid of comprehensive crater-density determi-nations.MappedareasincludeallplainsregionswithinthenorthernhemisphereofMars,aswellasanapproxi-mately300-km-widestripofcrateredhighlandandvol-canicregions,whichbordertheplains.Notethatnotallof the contiguous northern plains are mapped, becausesomeminorpartsofElysiumandAmazonisPlanitiaeliesouthoftheequator.

PHYsIoGRAPHIC settInGThenorthernplainsofMarsarebroadlycharacter-

izedbyasmooth,gentlyslopingsurface(KreslavskyandHead, 2000) that covers much of the planet’s northernhemisphere below the mean planetary radius (fig. 1). We delineate three distinct regional topographic basins (fig. 1), informallynamedBorealisbasin(encompassingthepolar region), Utopia basin, and Isidis basin. Collec-

tively, the basin floors generally range in elevation from –3,000to–5,000m.Theplainsareboundedtothesouthbydensely crateredhighland terrains andby thebroadinformally named Tharsis rise (fig. 1), which includes the OlympusMonsandAlbaPaterashieldvolcanoes,aswellasthearcuate,broadridgecutbyAcheronFossaeandthegrooved aureole of Olympus Mons (Lycus Sulci). Theinformally named Elysium rise (fig. 1), which includes the Elysium Mons, Hecates Tholus, andAlbor Tholusvolcanoes, is the largest volcanic province that occursentirely within the lowlands. Southeast of the Elysiumrise,ElysiumPlanitiaconnectstoArcadiaandAmazonisPlanitiaebetweentheElysiumandTharsisrises.Acida-lia and Chryse Planitiae form an embayment betweenTempeandArabiaTerrae.SyrtisMajorPlanumconsistsofabroadshieldwestofIsidisPlanitia,whereasLunaePlanumisahighplainintheeasternpartoftheTharsisrise.The ruggedpeaksofLibyaMontes ring thesouthmarginofIsidisPlanitia.

Systemsofridges, troughs, fractures,andchannelsanddispersedcratersmarkthenorthernplains.Thelarg-estmountainousregioninthenorthernplainsisthenorth-trending, degraded ridges of Phlegra Montes locatednortheast of the Elysium rise. Gordii and EumenidesDorsa form broad ridges that extend from the TharsisriseintoAmazonisPlanitia.TantalusFossaearenarrow,linear,andshallowtroughs thatextend to thenortheastfromtheAlbaPaterashieldvolcanointoadjacentplains.Similarly,ElysiumFossaeformdeep,curvilinear,irregu-lar troughs mainly on the northwest flank of the Elysium rise. A system of deep, wide outflow channels (Kasei, Maja,Shalbatana,Simud,Tiu,Ares,andMawrthValles)emanate from chaotic terrains within Xanthe and Mar-garitiferTerrae, incise thehighlandplateau,andextendintoChrysePlanitia.OtherchannelfeaturesextendfromElysium Fossae (Hrad, Granicus, Apsus, and TinjarValles) to thenorthwest into the central part ofUtopiabasin.HephaestusFossaeandHebrusVallesareunusualdrainagesystemsthatconsistofpolygonaltroughs,col-lapse depressions, and channels in southeastern UtopiaPlanitia.AmenthesCavi�formabandofarcuatedepres-sionswithinsouthernUtopiaPlanitia.On thesoutheastflank of the Elysium rise, the curvilinear fractures of Cerberus Fossae appear to be the source ofAthabascaandGrjotá�Valles.These channel features, alongwithRahwayValles�,extendfromthesoutheastmarginoftheElysiumriseintoElysiumPlanitia.MarteVallisformsabroadchannelconnectingElysiumandAmazonisPlani-tiae to theeast.Networksofpolygonal troughsdisturbtheplainssurfaceatAdamasLabyrinthusinUtopiaPla-nitiaandatCydoniaLabyrinthusinsoutheasternAcida-liaPlanitia.Manylargecratersoccurwithinthenorthernplains, including thedouble-ringLyotcraternearDeu-

�ProvisionallyapprovedbyInternationalAstronomicalUnion

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teronilus Mensae and the elliptical Orcus Patera alongthenortheastmarginofElysiumPlanitia.

Ingeneral,thehighland/lowlandboundary(highlandnotmapped)isfragmentedintolargemesasandknobs,includingNepenthes,Deuteronilus,Protonilus,andCydo-niaMensaeandTartarusMontesand their surroundingdetritalplains.WithinnorthernAcidaliaPlanitia,AcidaliaMensaformsalargemesaadjoinedbyAcidaliaColles,an area of knobs formed within a shallow depression.Otherpatchesofknobsandknobbymountainrangesintheplains includeScandiaCollesnorthofAlbaPatera,GalaxiasCollesnorthoftheElysiumrise,TartarusCollesandTartarusandErebusMontesinandnearArcadiaPla-nitia,OrtygiaColleseastofAcidaliaColles,andAstapusColles inwesternUtopiaPlanitia.Irregulardepressionsareassociatedwithmanyoftheknobbyterrains.

Planum Boreum forms a 1- to 2-km-high, 1,000-km-diameternorthpolarplateaumarkedby a seriesoftroughs, Borealis and Gemini Scopuli�, as well as byChasmaBoreale,a100-km-wideand1.5-km-deepchasmmarked by two isolated depressions approximately 50km wide, Boreum and Tenuis Cavi�. West of ChasmaBoreale, the base of Planum Boreum is marked by anirregular scarp, RupesTenuis, that reaches as much as1,000minheight.BetweenPlanumBoreumandScandiaColles, irregular depressions form Scandia Cavi, whilelow,circular,irregulardomesformScandiaTholi.Dunefields surrounding the planum include Olympia, Abalos, andHyperboreaeUndae.

DAtAThebasemapimageisderivedfromtheMarsOrbiter

LaserAltimeter(MOLA)instrumentonboardtheNationalAeronautics and Space Administration (NASA) MarsGlobal Surveyor (MGS) spacecraft (Albee and others,2001; Smith and others, 1999, 2001). The image rep-resents a few hundred million measurements gatheredbetween 1999 and 2001 (Neumann and others, 2001).Themeasurementswereconverted intoadigital eleva-tionmodel(DEM;Neumannandothers,2001;Smithandothers,2001)witharesolutionof128pixels/degree.Inprojection,thepixelsare463.086minsizeattheequator.Dataaresparsenearthepole(abovelat87ºN.),becausetheareawassampledbyoff-nadiraltimetrytracks.Gapsbetweentracksof1–2kmarecommon,andsomegapsofasmuchas12kmoccurneartheequator.DEMpointslocated in these gaps in MOLA data were filled by inter-polation.

The polar stereographic projection is used for thehemisphericbaseandencompasseslat0ºto90ºN.withacentralmeridianoflat0ºN.Theadoptedequatorialradiusis 3,396.19 km (Seidelmann and others, 2002). Longi-tudeincreasestotheeastandlatitudeisplanetocentricasallowedbyIAU/IAG(InternationalAstronomicalUnion/

InternationalAssociation of Geodesy; Seidelmann andothers,2002) standardsand inaccordancewithcurrentNASAandU.S.GeologicalSurvey(USGS)standards.Asecondarygrid(printedinred)hasbeenaddedtothemapasareferencetothewestlongitude/planetographiclati-tudesystemthatisalsoallowedbyIAU/IAGstandards(Seidelmannandothers,2002)andhashistoricallybeenusedforMars.Calculationoftheplanetographicgridisbasedonanoblatespheroidwithanequatorialradiusof3,396.19kmandapolar radiusof3,376.2km(Seidel-mannandothers,2002).

To create the topographic base image, the originalDEM produced in simple cylindrical projection with aresolution of 128 pixels/degree was projected into thepolar stereographicprojectionof thehemisphericbase.This was mosaicked with another DEM produced inpolarstereographicprojectionthatcoveredlat70ºto90ºN.A shaded-relief base was generated from the com-binedDEMwithasunangleof45ºfromhorizontalandasunazimuthof225º(northwest),measuredclockwisefrom north, and a vertical exaggeration of 100 percent(the illuminationalgorithmassumesaplanar,Cartesiansurface). The file was then scaled to 1:15,000,000 at the northpolewitharesolutionof300dots/inch.

Geographic names are shown for features clearlyvisible at the scaleof thismap.Namesare adoptedbythe IAU,except for thosedenotedwithanasterisk (�),whichareprovisionallyapproved.ForacompletelistofIAU-approved nomenclature for Mars, see the Gazet-teer of Planetary Nomenclature at http://planetarynames.wr.usgs.gov.

OurprimarymappingdatasetwastheMOLADEMat128pixels/degree.Becauseofincreasingdatadensitypoleward, the lower latitude between-track (east-west)spacingcanlocallyexceedseveralkilometers,resultingininterpolatedelevationpoints.However,aboveapprox-imately lat 60° N. and lat 78° N., altimetry data-pointspacingpermitted the constructionof higher resolutionDEMs(respectively,256pixels/degreeor~230m/pixeland512pixels/degreeor115m/pixel).WegeneratedandreliedonnumerousderivedMOLAproductstoassistingeologic mapping and characterization, including cell-to-cellslope,aspect(slopedirection),localcontour,andcolorshaded-reliefmaps.SimilartothedetrendedMOLAmapsusedbyHeadandothers(2002),werana75-kmbounding-box median high-pass filter (MHPF) on the DEMtoremove(ordetrend)theregionalsurfaceslopeandenhancelocalrelief.Slopemapshelpedustochar-acterizesurfacetextureandlandformswhiletheMHPFDEMhelpedus to tracesubtle landformsandgeologicboundaries.

WealsousedtheVikingOrbiterMarsDigitalImageMosaic version 2.0, which provided both morphologicandalbedoinformationatasmuchas1/256°resolution(~230m/pixel),aswellasMarsOrbiterCamera(MOC)

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wide-angle images (~250 m/pixel) acquired by MGS.Thesedatawereparticularlyhelpfulinmappinglow-lati-tuderegionswhereMOLAtracksarewidelyspaced,andthe polar residual ice was identified from albedo. Though MGSMOCnarrow-angleimages(>1.4m/pixel)aretoodispersedtouseforgeologicmapping,theyprovedveryuseful for unit identification where diagnostic morpho-logic features could be identified. Finally, we used avail-able Mars Odyssey (MO) Thermal Emission ImagingSystem(THEMIS)images,consistingofinfrared(100m/pixel,daytimeandnighttime)andvisible(17–40m/pixel,1to5spectralbands)types,tolocatesubtlecontactsdif-ficult to follow in either MOLA DEM or Viking or MOC wide-angle images.TheTHEMISdaytimeinfraredandvisible images commonly reveal detailed morphologiccharacternotapparentinVikingimages.Atmiddleandlowerlatitudes,THEMISdaytimeandnighttimeinfraredimagesdisplayedcontrastsinthermophysicalpropertiesofsurfacematerialsthat,insomecases,assistedindif-ferentiatinggeologicunits.

MetHoDoLoGYWe implement comprehensive geologic mapping

methods that differ in many respects from previousapproaches common to planetary geologic mapping,with the goal of achieving a clearer and more objec-tive product. Our methodology involves carefully con-sidered schemes for (1) delineating, naming, grouping,andsymbolizingmapunits,(2)coloringunitsbyage,(3)identifyingcontact types,and (4)mappinggeomorphicfeatures.Ourmappingwasfacilitatedandenhancedbyusing Geographic Information Systems (GIS) softwaremappingtools.

UNITDELINEATION

Consistent with the approach that Hansen (2000)applied to Venus mapping, we identify and map rockunitsandsedimentarycoveronthebasisoftheirapparentgeologic uniqueness as defined by their primary physi-calfeatures,arealextent,relativeage,andgeologicasso-ciations. Primary features, including lobate flow scarps, layering, albedo, and thermophysical character, formedduringemplacement;whereassecondaryfeatures,includ-ing fault scarps, yardangs, valley networks, and windstreaks, are formed after emplacement. In many cases,delineatingprimaryandsecondaryfeaturesrequirescare-ful observation and may be difficult and uncertain where overprintingofsecondary featureshasmasked,obliter-ated,ormixedwithprimaryfeatures.

Map units require sufficient areal extent and geologic significance to be effectively portrayed at map scale and toreducetheclutterthatmappingsmalleronesimposes.Wechoseareasapproximately50kmacrossforthesmall-est mapped noncrater units and approximately 100 km

acrossforthesmallestmappedcraterunits.Thus,smalllocaloutcropsofwhatmaybeuniquegeologicmateri-alsareincludedinmoreextensiveunits.Whereextensiveoutcropsaretoosmalltomap,theybecomepartofcom-positeunits;forexample,smallknobsandmesasoftheNoachisTerraunitalongthehighland/lowlandboundaryareincludedwithadjacentplains-formingmaterialintheNepenthesMensaeunit.

Inmanyinstances,mapunitsmayconsistofdiverse,unknown, or uncertain lithologies, which make it difficult toimplementastrictrock-stratigraphicapproachprevi-ouslyencouragedinMarsgeologicmapping(Tanakaandothers, 1992b). We find that a more reasonable scheme for mapping complex units on Mars (as well as otherplanetarysurfaces)istouseallostratigraphicunits(NorthAmerican Commission on Stratigraphic Nomencla-ture,1983),alsoknownasunconformity-boundedunits(UBUs; Salvatore, 1994). This approach discriminatesgeologic units chiefly by relative age where a signifi-canthiatuscanbeinferredbetweenoverlyingandadja-centunits.Thedelineationofgeologicunitsbecomesacarefulstudyofagerelationsalongandacrossmappedcontacts. UBUs may commonly consist of multiplelithologies,which areusefulwhen an intimatemixtureofdiverse lithologies (common toMars) is impracticalto map but where all of these units have geologic andtemporal association. For example, though lobate andplains-forming materials of diverse, mixed morpholo-giesinUtopiaPlanitia(TinjarVallesaandbunits)mayinclude lava and ash flows, lahars, mudflows, and fluvial sedimentsofcomplexandvariablelithology,theyappeartoberelatedtothesameperiodofvolcanismanderosionof the west flank of the Elysium rise on the basis of strati-graphicrelationsandcratercounts.Anotherexampleistwo sets of lava flows in which an older set is faulted by grabens, whereas a younger set buries the grabens.WhileUBUshavebeenusedinpreviousMarsgeologicmaps,thelackoftheirformalrecognitionasaunittype,which includesadherence toguidelinesfor theirusage,has resulted in muddled unit definitions and character-izationsinsomecases.Weabandonformalstratigraphictermssuchas“formation”and“member”(or“synthem”and “synmember” for allostratigraphic units). We alsoavoidtheterm“material”because,mostoften,themate-rial characteristics are poorly known; instead, we useexclusivelythemoreinformalandgeneralterm“unit.”

Theseguidelineshelpedustodeterminewheremapunitsshouldbediscriminatedbasedongeologicobserva-tions and associations and to avoid mapping units thathave considerable overlap in both character and age.For example, we do not differentiate between MiddleandLateNoachianhighlandunitsasmappedpreviously(ScottandTanaka,1986).AlthoughmostNoachiansur-facesshowhighvariabilityincraterdensitiesandterrainruggedness, the variability is generally transitional and

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lacksmorphologicevidenceofembaymentrelationsthatsuggestdistinctiveepochs.However,theEarlyNoachianLibya Montes unit tends to be relatively high standinganddenselycrateredandappearstobeembayedbytheless ruggedandcrateredNoachisTerraunit.Generally,weselectedunitsonthebasisoftheirmorphologicchar-acteristicsandtheirassociationwithparticulargeologicstages,locales,andsources.

UNITNAMES

We did not use morphology, albedo, terrain type,oranyotherphysicalcharacteristicsinmap-unitnames,because these characteristics can be highly variableand suspect as definitive criteria for unit identification. Instead, units are primarily identified and delineated based on relative age and geologic relations, whichmakes themincongruentwithnamesbasedonphysicalcharacteristics.Wefollowacommonmethodemployedinterrestrialgeologicmapsinwhichunitsarenamedafterappropriate geographic terms, each of which includesapropernounandadescriptor-typeterm(forexample,Isidis Planitia unit). The exceptions are crater units,whichoccurthroughoutthestudyareaandhaveconsis-tentandwell-characterizedmorphologies.Thisapproachiscomparabletomappingterrestrialalluvium,aunitthatis generally not identified with a geographic place name. Becauseofour limitedknowledgeof the lithologiesofthemapunits,wedonotadopttheterrestrialapproachofusinga lithologic term(forexample,basalt,sandstone)inourunitnames.

UNITGROUPINGSANDSYMBOLS

PreviousMarsgeologicmappersgroupedunitsusingvarious schemes that incorporated terrain types (low-lands,highlands),volcanicregions,andunittypes(chan-nel, aeolian, and surficial materials). On Earth, groups of geologicunitsgenerallyhavelocalassociations,leavingstratigraphers to concern themselveswithhow isolatedsequences might correlate in both space and time. WefeelthatasimilarapproachissensibleformostunitsinthenorthernplainsofMars,whereinwedistinguishsixprovinces of geologic activity (fig. 1): Amazonis, Ely-sium, Tharsis, Isidis, Chryse, and Borealis provinces.Remaining units, referred to as widespread materials,have wide spatial occurrence and cannot be delineatedbyregion.

Using the informally named provinces to groupunits and our unit-naming approach, we propose asymbol scheme as follows: (1) the chronologic period(s) isshownbyalargecapitalletter(s)(A,Amazonian);(2)theprovince,whereapplicable,isdesignatedbyasmallcapitalletter(B,Borealisprovince);(3)theunitname,ageographicterm,isdenotedbyalower-caseletterofthepropernounpartofthename(v,VastitasBorealisunit);and(4)sequentialandfaciessubdivisionsareshownby

numberorlettersubscripts,respectively.Thus,thegeo-logicsymbolABvidenotestheVastitasBorealisinteriorunitofAmazonianageintheBorealisprovince.

UNITCOLORS

Mostgeologicmapsfollowacolorschemeinwhichunitsaregroupedbylithologyorphysicalcharacteristics.Weuseunitcolorstostressrelative-ageinformation;thenaturalviolettoredspectrumrepresentsoldesttoyoung-estunits.Relativelysharp,purecolorsareusedforunitsthat are specific to a particular epoch and (or) depositional province.Conversely,muted,mixedhuesareappliedtounitsofbroadtemporaland(or)spatialextent.Forexam-ple,theLateHesperianunitsoftheChryseprovincearerepresented in shades of vibrant blue while Noachianunitsofbroadarealextentarerepresentedinmixedhuesthatresultinshadesofmutedbrown.Asaresultofthiscolorscheme,visualinspectionprovidesageneralsenseoftheages,agerange,andarealextentofmapunitsandremovestheinterpretivebiasofcolorschemesbasedoninferredorigins.

CONTACTTYPES

We attempt to carefully define and delineate map contactsusingcertain,approximate,gradational,inferred,and inferred approximate contacts. A certain contactdenotesthemostprecisecontactbetweenwell-character-izedmaterialunits that contrast inappearanceand (or)relativeage.Alternatively,anapproximatecontactislesspreciselylocatedduetodataquality,subtletyofthecon-tact, and (or) secondary surface modification, where the existenceof the contact isgenerallynot inquestion.Agradationalcontactisusedaroundcompositeunitscon-sistingofmixturesofoldermaterialsandtheirapparenterosional products, such as older knobby material sur-roundedbyyoungerplainsmaterial;suchunitsgradewithadjacent,continuousoutcropsofboththeoldermaterialandyoungerplains-formingmaterial at thebaseof theknobs. An inferred contact is used when the unique-ness of two units is uncertain, for example the contactbetweentheVastitasBorealisinteriorandmarginalunits.The units may be of similar origin but modified in differ-entmanners,ortheymayhavefairlydistinctiveoriginsand (or) ages.An inferred approximate contact is usedwhereaninferredcontactcannotbeplacedprecisely.

FEATURESYMBOLS

Themapincludesavarietyofmorphologicfeaturesymbols, following precedents generally established inpreviousgeologicmapsofMars.Mappedlinearfeaturesaregenerally>150km long. In addition,we subdivideourmappingofsomefeaturesasindicatedintheExpla-nationofMapSymbols.Althoughconsistentmappingoffeatures isdesired, it is generallynotpossibleorprac-tical. Factors that complicate the consistent mapping

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of features include locally rugged terrain; variation infeature character, length, and relief; feature orientationversusilluminationdirection;andareasoflowdatareso-lution and quality. In addition, the features themselvescommonlygradeinappearance,andoverlapsinfeatureidentification occur (for example, troughs may be similar tochannels,whereasscarpsmayhavethesameappear-anceasasymmetricridges).Forridgesandtroughs,wedelineatedsubduedvarieties,whichtendedtolackdetailandwhoseboundingslopesweregenerallymodest(seealso Structure and Modification History section). Some of the features may have multiple origins and must beinterpretedwithcare.Wealsochosetoshowpolarresid-ualiceonandaroundPlanumBoreumasastipplepatternusingamosaicof springand summertimeMOCwide-angleimages,becausetheiceisonlymetersthick(andhasnoobvioustopographicexpressionatMOLAresolu-tion), is identified by albedo, and varies in extent from yeartoyear.Polygonalfractures(forexample,Adamasand Cydonia Labyrinthi) and thumbprint terrain (forexample,eastofDavies�crater)arerepresentedbypat-ternsinmostareas,becausethefracturesandtroughsandirregularridgesofthesefeaturesaregenerallytoosmalltomapindividually.Collectively,however,thestructurescoverextensiveexpansesoftheVastitasBorealisunits.

GISAPPROACHESANDTOOLS

OurmappingproceduresreliedheavilyonGISsoft-wareandtechniques,whichprovideduswithenhancedvisualization, mapping tools, and analytical capabili-ties.Wewereable to import incommonprojection theMOLA,MOC,andVikingdataandderiveMOLA-basedthematicmapsuponwhichtomapandtoreadilyperformcomparativeanalysisamongthedatasets.Somedatasetshad to be processed and (or) reprojected using USGSIntegratedSoftwareforImagersandSpectrometerssoft-ware. This software can be obtained online at http://isis.astrogeology.usgs.gov/.Westreamdigitizedouroriginallineworkwithavertexspacingof3kminmostcases.

stRAtIGRAPHYAlthough the geology of all or parts of the north-

ern plains region of Mars was mapped variously at1:15,000,000 or larger scale using Viking Orbiter images (Scott and Tanaka, 1986; Greeley and Guest, 1987;Tanaka and Scott, 1987), significant improvements in imageandtopographydataresolution,quality,andposi-tioning accuracy and in mapping techniques permit asubstantiallyrevisedandbetter-constrainedstratigraphy.Some initial results of our mapping were presented inTanaka and others (2003b); however, this map reflects many modifications and improvements over that prelimi-nary work.We attempt to limit the discussion of mor-phologicandotherphysicalproperties to those thoughtto be intrinsic to the unit when formed; unit modification

features are discussed in the Structure and Modification Historysection.

Stratigraphic relations among the map units arecomprehensivelysummarizedintable1,whichprovidesrelative-ageinformationdeterminedbycontactrelationsobservedinmappingdatasets.Althoughwearegenerallyconfident with these results, we caution that subtle rela-tions may be missed or incorrectly inferred, especiallywherethecontactisuncertain.Inaddition,someunitsarelikelytohaveformedoverconsiderableperiodsoftimeand (or) arepartlyburied.Asa result, someunitsmaynotbefullyand(or)properlycharacterized.Furthermore,someunitsthathaveundergonemodestresurfacingfol-lowingtheirinitialformationmayhaveobscureorevenreversedapparentstratigraphicagerelations;welargelyignore such modifications where the scale of the mapping precludespresentingsuchdetails.

Intable1,craterdensitieswerederivedusingcom-putedmapunitareasandBarlow’scraterdatabase,whichincludescratersize,location,andmorphologiccharacter-istics(N.G.Barlow,1988,andwrittencommun.,2003).MapunitswereassignedtotheeightMartianepochsonthebasisoftheschemeofTanaka(1986)forthenumberofcraters>5and>16kmindiameterpermillionsquarekilometers(N(5)andN(16),respectively),aswellasonstratigraphicrelationsandourunderstandingofthecraterretentionhistoryforgivenunitsurfaces.Asnotedintable1,someunitsmayberelativelythin(tensofmeterstoafew hundred meters thick) and have anomalously highcrater densities due to significant numbers of craters includedinthecountsthatareactuallyembayedorthinlyburiedbytheunit.Inaddition,agesreportedforpopula-tionsofsubkilometer-sizedcratersmaybegenerallysus-pectduetoapreponderanceofsecondarycratersinthissizerange(McEwenandothers,2005).

Wediscussthemapunitsinstratigraphicorderstart-ing with the oldest units, based on their age of onset(which is commonly only approximately understood).Many of the key relations are shown in the work ofTanaka and others (2003b). Although geologic crosssectionsaregenerallyhelpfulinelucidatingverticalandlateralrelationshipsbetweenmapunits,wehavenotpro-ducedsectionsforthismap,becausethesubsurfacegeol-ogyofmostofthemapareaispoorlyknownandlikelyhighlyvariable.AnexampleofaschematiccrosssectionacrosssouthernUtopiaPlanitiathatisconsistentwithourmappingresultsandinferencesisshowninTanakaandothers (2003a, fig. 9).

EARLYNOACHIANEPOCH

TheEarlyNoachianrepresentstheperiodofoldestexposedrocksonMars.Older,pre-Noachianrocksoftheprimordialcrustproducedduringearlyplanetarydiffer-entiationlikelyunderlieandmaybeconstituentsofNoa-chian rocks. The “quasi-circular depressions” that are

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tenstohundredsofkilometersindiameterinbothhigh-landandlowlandregionsaroundMarsmayrepresentapopulationofburiedimpactbasinsoccurringwithinsuchpre-Noachianrocks(Freyandothers,2002;Frey,2004;NimmoandTanaka,2005).

LibyaMontesunit

The Libya Montes unit (Nl) represents the oldestexposed rocks in the map area, forming the rugged,denselycrateredmassifsofLibyaandXanthe�MontesandruggedcrateredterrainnearNiliFossaeandAmen-thesRupesandinXanthe�andeasternMargaritiferTerra.WemaptheextentoftheunitwherecarvedbyKaseiandAresVallesbyassumingthattheunitsurfacedipsgentlyawayfromnondissectedunitsurfaces,whichisconsistentwithageometryofmaterialsraisedbyimpactand(or)tec-tonicprocesses.TheLibyaMontesunitappearsembayedby all adjacent materials, including the Noachis Terraunit (Nn) in most places (however, the contact is gen-erallyobscuredduetoresurfacing),andincludeshighlydegradedcraterfeatures.Therugged,high-standingchar-acteroftheLibyaMontesunitlikelymakesitsusceptibleto erosion, possibly accounting for its similar densitiesofobservedcratersrelativetothoseoftheNoachisTerraunit (table1).Also,partsof theunit likely extend intotheMiddleNoachian.Thesematerialsmayhaveformedfromupliftedancientcrustalmaterialswheretheyareinproximity to large,ancient impactbasins, includingtheIsidis and putative Chryse Planitia structures (Schultzandothers,1982);alternatively,XantheMontes�mayberelated toancient tectonismof theTharsis riseorothertectonism(FairénandDohm,2004).

NepenthesMensaeunit

The Nepenthes Mensae unit (HNn) includes (1)material formingknobs andmesasofNepenthes,Nilo-syrtis,andProtonilusMensae;Phlegra,Erebus,andTar-tarus Montes;Tartarus Colles; and Cydonia andAeolisMensae(outsidethemaparea)and(2)slopeandplainsdepositsbetweentheknobsandmesas.Unitoutcropsareasmuchasafewhundredkilometerswideandthousandsof kilometers long. Individual knobs and mesas rangefromlessthanakilometertoafewtensofkilometersinwidthandup toa fewkilometers in relief.Thebaseoftheunitoccursatelevationsof–2,000to–4,000mandhasareliefthatlocallyapproaches3,000to4,000m.ThematerialofthemesasandknobsincludeNoachianmate-rialsequivalent to theLibyaMontesandNoachisTerraunits (Nl and Nn), whereas the intervening slope- andplains-formingmaterialswereemplacedduringtheLateNoachianand into theEarlyHesperian.TheNepenthesMensae unit may be mass-wasted talus, slide, and flow materials.Depressionswithin theunit indicate thatcol-lapseoccurredduetodisplacementand(or)removalofsubsurface material (Tanaka and others, 2003b).Addi-

tionally, Parker and others (1989, 1993) hypothesizedthat ancient oceans filled the northern plains and eroded thecoastlines,producingthedenseknobsandinterveningdeposits.

MIDDLEANDLATENOACHIANEPOCHS

NoachisTerraunit

Many Viking-based geologic maps of Mars dis-tinguish various Middle and Late Noachian highlandmaterials,usingrelativecraterdensityandoccurrenceofmorphologicfeaturessuchasvalleys,fractures,andotherfeatures (Scott and Tanaka, 1986; Greeley and Guest,1987; Tanaka and Scott, 1987). We find, however, that in mostheavilycrateredhighlandregionsalongthelowlandmargin,theclearandconsistentdiscriminationofMiddleversusLateNoachianoutcropsisproblematicduetoalackofpotentialembaymentfeaturessuchasslightelevationdifferences,forexamplesubtlescarps,betweenareasofrelativelyhighandlowcraterdensity.Asaresult,wemapa singleunit, theNoachisTerraunit (Nn), to representbroadexpansesofheavilycrateredmaterial thatappar-ently embays thehigher reliefLibyaMontesunit (Nl).The timing of contacts between the Libya Montes andNoachisTerraunitscannotbepreciselydated,thereforeextensivetemporaloverlapbetweentheunitsispossibleandlikely.Degradedcratermaterialwithintheunitalsoisnotmapped separatelyas inpreviousmaps,becausetheunitisgenerallyinterpretedtoconsistofcratermate-rialinadditiontolocaldepositsofaeoliandeposits,allu-vium,volcanic rocks, andothermaterials that, inmostcases, cannot be confidently differentiated. In MOC narrow-angleimages,muchoftheunitdisplayslayering(Malin and Edgett, 2001).The unit occurs in highlandterrainsintheelevationrangeof–3,000to5,000maboveandsurroundingthenorthernplains;italsooccurswithinthenorthernplainsatOrcusPateraandAcidaliaMensa,between–2,000and–5,000minelevation.

LunaePlanumunit

Southern Lunae Planum (outside the map area)includes lobate flows and local rille structures that may bevolcanicchannels(Witbeckandothers,1991).WherecutbyCopratesChasma,aseriesofthicklayersthoughtto be lava flows form a sequence a few kilometers thick (McEwenandothers,1999).WeconsiderthattheLunaePlanumunit(HNTl)innorthernLunaePlanumalsocon-sists of a considerable thickness of lava flows and, per-haps,othervolcanic-relatedmaterials,wherelayeringhasalsobeendetectedinVikingimages(Scott,1991;Tanakaand Chapman, 1992). Such thick sequences of lava flows maypointtoaEurope-sized,NoachianbasinwithVallesMarinerisas itscentralpart(Dohmandothers,2001a).Thus, where Kasei Valles dissects the Lunae Planumunit, we show that the channel floor exposes a kilometer-

7

thicksequenceoftheunit(TanakaandChapman,1992)betweenabout–2,000and–1,000minelevation,partofwhichmaybeLateNoachianandolder.

ChrysePlanitia1unit

Chryse Planitia 1 unit (HNCc1) consists of high-elevation (mostly –3,500 to –2,000 m), plains-formingmaterialthatembaystheLibyaMontesandNoachisTerraunits(NlandNn)andkilometer-scaleremnantknobsandmesasofNoachianmaterialalongthe lowlandmarginsofChryseandAcidaliaPlanitiae.ChrysePlanitia1unitappears to result from mass wasting and local fluvial dis-sectionofnorthernXantheandMargaritiferTerraeduringtheLateNoachiantoEarlyHesperian.TheunitappearstobesimilarinageandorigintotheNepenthesMensaeunit (HNn), but consists of significantly less Noachian relictmaterialandfeaturesandpossiblyagreaterpropor-tion of fluvial material.

EARLYHESPERIANEPOCH

Crater and crater floor units

Throughoutthemaparea,impactingbolidesformedcraters whose rims and ejecta, including rampart mar-gins and secondary craters, presently remain relativelywell preserved. Presumably, such craters postdate theNoachian, when erosion rates across the entire planetmay have been significantly higher (Craddock and Max-well,1993;CraddockandHoward,2002).Thesecraters,therefore,donotbelongtotheNoachisTerraunit(Nn)andaremappedasthecraterunit(AHc).TheyrangeinagefromEarlyHesperiantoLateAmazonianandmostlyoverlieadjacentunits.Becausesomeadjacentunitsarethinner than crater ejecta rampart margins, as seen inMOCnarrow-angleimages,somewell-preservedcratersactually predate adjacent units.As a result, crater-den-sity data for such units should be viewed with caution(seetable1).Also,manydegradedcraterslargelywithinthe Libya Montes (Nl) and Noachis Terra units wereinfilled by plains-forming material mapped as the crater floor unit (AHcf)thatmayincludeaeoliandustandsand,volcanic ash, and fluvial deposits of post-Noachian age. MOCimagesindicatethatmanyexposuresofthecraterfloor unit are composed of finely layered, friable materi-als(MalinandEdgett,2001).TheoutcropswithinAramChaos are associated with hematite-rich material asdeterminedbytheMGSThermalEmissionSpectrometer(TES)(Christensenandothers,2001).

DeuteronilusMensae1unit

DeuteronilusMensae1unit(HBd1)formsthelow-lyingplainswithinthefrettedtroughsandbroaddepres-sionsboundingthelargemesasandknobsofDeuteronilusMensaealongthemarginofthehighland/lowlandbound-ary.ThisunitembaystheNoachisTerraunit(Nn)inthe

DeuteronilusMensaeregionandtheNepenthesMensaeunit(HNn)southeastofLyotcrater;theVastitasBorea-lis marginal and interior and Deuteronilus Mensae 2units(ABvm,ABvi,andABd2)embaytheDeuteronilusMensae1unit.TheseobservationsareconsistentwiththeEarly Hesperian age assigned to the formation of fret-tedchannelsbyMcGill(2000).Thechannelsandmesasappear to have formed by downslope, subsurface flow of ice-richmaterial,whichremovedthesupportivesubstrateandresultedincollapse(Sharp,1973;Lucchitta,1984).Spring-fedground-waterdischargeduringwarm-climateperiods may have also contributed to fretted channeldevelopment.

UtopiaPlanitia1unit

ThroughoutthesouthernandwesternmarginalzoneofUtopiaPlanitiaandinanareaofsouthwesternAma-zonisPlanitia,theNepenthesMensaeunit(HNn)gradesintoUtopiaPlanitia1unit(HBu1),whichisdominatedby (1) plains-forming material that appears rugged atten- to hundred-meter scales and (2) sporadic knobsgenerally smaller than those in the Nepenthes Mensaeunit.UtopiaPlanitia1unitoccursmostlyatelevationsof–3,000 to–2,000m.Smallpancake-shapedmesaswithinterior knobs and thin flows occur locally within the unitinsouthernUtopiaPlanitiaandmaybetheresultofmudormagmaticvolcanism(Tanakaandothers,2003a)thatmayrepresentminoractivitylateinunitformation.Locally, smoothdepositsembaycrateredandhilly sur-faces within the unit. Crater counts (table 1) indicatethatUtopiaPlanitia1unitisyoungerthantheNepenthesMensaeunit.UtopiaPlanitia1unitlikelyconsistsofthereworked, distal, coalesced debris shed from highland-boundary degradation associated with formation of theNepenthesMensaeunit.

AmenthesPlanumunit

Amenthes Planum�, a broad, northwest-trendingtroughonthewestsideofAmenthesRupes,iscoveredbytheAmenthesPlanumunit(HIa).Theunitrangesfromabout–2,000 to0melevationandembaysoutcropsoftheNoachisTerraandNepenthesMensaeunits(NnandHNn).Theunitgenerallylacksdistinctiveprimarymor-phologies except for local channels and scarps seen inTHEMIS visible images. The unit may be made up of flu-vial sediments and volcanic flows originating from local sources.ItscontactwithUtopiaPlanitia1unit(HBu1)isnot clearly defined, and the Amenthes Planum unit may extendfartherintoIsidisPlanitiathanmapped.

SyrtisMajorPlanumunit

Lobate flows mapped as the Syrtis Major Planum unit (HIs), some with pronounced medial ridges withcrestal fissures, cover Syrtis Major Planum and likely erupted from Nili and Meroe Paterae and local frac-

8

tures. The flows reach the west margin of Isidis Plani-tia,wheretheyappearlocallycrackedanderoded;somehaveabroadridge-likeformwithpitsandchannelsontheir crests (Ivanov and Head, 2003). Overall, flows of SyrtisMajorPlanumhaveanEarlyHesperiancraterage(Tanaka,1986;HiesingerandHead,2004),butalowerdensity of craters on the east flank of the planum (table 1) suggests that those flows are partly Late Hesperian, whichindicatesanextensivedurationofvolcanism.OurmappingobservationsandcratercountsindicatethattheSyrtisMajorPlanumunitpredatestheIsidisPlanitiaunit(AIi);however,IvanovandHead(2003)derivetheoppo-site result, suggesting that the Syrtis Major Planum flows wereemplacedontopoftheIsidisPlanitiaunit.

ArcadiaPlanitiaunit

In western Arcadia and Amazonis Planitiae, theArcadia Planitia unit (HAa) consists of plains materialwithsmallknobsandlobatemarginsinmanyareas.Theunit embays the Nepenthes Mensae and NoachisTerraunits(HNnandNn).TheArcadiaPlanitiaunitappearstobecomposedofamixtureofcolluviumfromlocalmasswasting of the Nepenthes Mensae unit, lava flows associ-atedwithElysiumriseand(or)localvents,andperhapsfluvial sediments from dissection of Marte and Mangala Valles(TanakaandChapman,1990).FlowsoftheEly-siumriseunit(AHEe)overlaptheArcadiaPlanitiaunitonitswesternmargins.

ChrysePlanitia2unit

Plains-forming outcrops of Chryse Planitia 2 unit(HCc2) in Chryse Planitia may result, in part, as col-luvium shed from older, adjacent units; however, mostof the unit probably consists of deposits derived fromscouring of Maja,Ares, Kasei (and associated fossae),Shalbatana, Bahram, Vedra, and Maumee Valles andperhapsfromearly,unpreservedscouringofSimudandTiuValles.Theunitalsoincludesrugged,high-standing,channel-floor deposits in northern Kasei Valles likely made up of channel material and perhaps debris shedfrombackwastingoftheLunaePlanumunit(HNTl)andNoachianwallrocks.

LATEHESPERIANEPOCH

SimudVallisunit

The Simud Vallis unit (HCs) consists of materialformingchaotic terrainwithinXantheandMargaritiferTerrae,includingHydraotesandAramChaos,aswellasthe chaotic and hummocky floors of Simud and Tiu Valles. Chaotic terrains include mosaics of large mesas andknobswithindepressionshundredstoafewthousandsofmetersdeep.ChaosdepressionsdisruptNoachianmateri-als and outcrops of the crater floor unit. Chaotic terrains aregenerallythoughttobethesourceofwater,waterorCO2-charged slurry, or ice that led to the formation of

the outflow channels (Sharp, 1973; Baker and Milton, 1974;Carr,1979;NummedalandPrior,1981;Witbeckandothers,1991;Tanaka,1997,1999;Hoffman,2000).Given this interpretation, theunitappears tobemainlyLateHesperianinage,coevalwithChrysePlanitia3and4andAresVallisunits(HCc3,HCc4,andHCa);somepartsmayalsobesynchronouswithearlierChrysePlani-tiaunits.Amazonianunitdevelopmentispossiblebuthasnotbeenclearlydemonstrated.

Elysiumriseunit

Extensive lava flows of the Elysium rise mapped as theElysiumriseunit (AHEe)eruptedfromElysiumMons,HecatesandAlborTholi,andotherlocalsources.The oldest exposed flows appear to predate Utopia Pla-nitia 2 unit (HBu2) in southern Elysium Planitia andnortheastofHecatesTholusandtounderlieTinjarVallesaandbunits(AEtaandAEtb)andtheVastitasBorealismarginalunit(ABvm)ineasternUtopiaPlanitia.Older,buriedvolcanicmaterialsoftheElysiumrisecouldpre-date the Late Hesperian (Tanaka and others, 1992a).Amazonian flows of the Elysium rise unit overlie the ArcadiaPlanitiaunit(HAa)inwesternArcadiaPlanitiaandtheVastitasBorealisinteriorunit(ABvi)insouthernUtopiaPlanitia.

UtopiaPlanitia2unit

AlongthesouthandwestmarginsofUtopiaPlani-tiaandthesouthmarginofElysiumPlanitia,theUtopiaPlanitia2unit(HBu2)consistsofplainsmaterialformedtopographicallylowerthanUtopiaPlanitia1unit(HBu1).UtopiaPlanitia2unitismarkedbydepressionsandscarpsuptotensofkilometersacrossandwithreliefofhundredsofmeters.Likely,collapse,degradation,anddownslopemovement of lower exposed parts of Utopia Planitia 1unitledtoemplacementofUtopiaPlanitia2unitascol-luvium(Tanakaandothers,2003a,b).InsouthernIsidisPlanitia,UtopiaPlanitia1unitisabsent,perhapsbecauseof thehigh reliefofLibyaMontesandoverprintingbyUtopiaPlanitia2unit,whichmayconsistofdetritusfromNoachianrocksmakingupLibyaMontes(CrumplerandTanaka, 2003). Where the outcrops of Utopia Planitia2 unit in southern Elysium Planitia are isolated withinyoungermaterials,theiroriginandsourceisuncertain.

AresVallisunit

TheAresVallisunit(HCa)formspitted,discontinu-ous,irregular,lowplateausabovesurroundingplainsandchannel floors. In THEMIS night-time infrared images, theoutcropsappeardistinctivelybright,indicatingrela-tivelywarmtemperatureandhigh thermal inertia.Out-cropsoftheunitextendfromIaniChaos(southofAresVallis, outside of the map area) into Ares Vallis andsouthern Chryse Planitia. The unit overlies Noachianmaterials,ChrysePlanitia1unit (HNCc1), and, inone

9

place, Chryse Planitia 2 unit (HCc2). The pitted mor-phologymaybeduetothermokarstprocesses(Lucchitta,1985;CostardandKargel,1995),whichactedonmate-rialthatmayhavehadarelativelyresistantsurfaceduetocementationorsomeotherprocess.Muchoftheunit,wherenotdustcovered,hasanolivine-richsignatureinTESspectraindicativeofanigneousorigin(Rogersandothers,2005).

ChrysePlanitia3unit

The Chryse Planitia 3 unit (HCc3) forms a broadplainatthemouthsofKaseiandMajaValles;plainssur-roundingthelower,distalreachofAresVallis;andscat-tered,relativelysmoothdepositsinlowerpartsofKaseiandAresValles.TheunitembaysChrysePlanitia2unit(HCc2)andtheAresVallisunit(HCa).Thelargerout-cropsmostlikelyconsistofsedimentsfromKasei,Maja,andAresValles.TherelativeagebetweensedimentsofKaseiandMajaVallesisuncertain;thehigherdensityofwrinkleridgesbelowMajaVallesmaymeanthattheunitin thatareaisolderorperhapsthinneror that itunder-wenthighercontractionalstrain.ThesmallpatcheswithinKasei and Ares Valles may be fluvial sediments and (or) lateraeoliandeposits(TanakaandChapman,1992).

ChrysePlanitia4unit

Chryse Planitia 4 unit (HCc4) forms the lowestplainsinChrysePlanitiaandincludesirregularfracturesandlow,convexmarginsasseeninVikingandTHEMISimages.Theunitoccurswithintherelativelylowestchan-nel floors that dissect Chryse Planitia 1–3 units (HNCc1,HCc2,andHCc3)andtheAresVallisunit(HCa)andiscutbychaosstructuresoftheSimudVallisunit(HCs).ThecontactwithChrysePlanitia3unitisuncertaineastof theKaseiVallesmouth,making the relativeagesofthe units uncertain in that area. Chryse Planitia 4 unitincludesnorth-trendingstreamlinedformsthatshowthatthedepositoriginatedfromtheSimud/TiuValleschan-nelsystem;tonguesoftheunitburypartoflowerAresValles.Thelobatemargins,fractures,andpossiblemud-volcanostructuresassociatedwiththeunitareconsistentwith an origin as sediment-laden flood or debris-flow deposits(Tanaka,1997,1999).

Chryse Planitia 3 and 4 units (HCc3 and HCc4)formedneartheendoftheHesperian,becausetheypre-datetheVastitasBorealismarginalunit(ABvm)accord-ingtoembaymentrelationsandcraterdensities.Tanakaandothers (2003b) suggested that theVastitasBorealisunitsresultedfromwaterandgasdischargesfromreser-voirsbeneaththeplains,aswellasfromlocalperiglacialprocesses acting on outflow channel sediments and other olderplainsmaterials.However,itremainspossiblethatearlyChrysePlanitiadepositseitherareburiedbeneaththeVastitasBorealisunitsorwerelaterreworkedtoformtheVastitasBorealisunits;thisinterpretationisconsistent

withthehypothesisthattheVastitasBorealisunitsrepre-sentocean sedimentsderived frommaterialdischargedfromtheChrysechannels(Parkerandothers,1989,1993;Bakerandothers,1991;KreslavskyandHead,2002).

AmazonisPlanitia1southunit

TheAmazonisPlanitia1southunit(AHAa1s)formsrelativelysmooth,locallylobateplainsinsouthernAma-zonisPlanitia.Itburies theArcadiaPlanitiaunit(HAa)and is superposed by youngerAmazonis Planitia unitsandtheMedusaeFossaeunit(AAm).Theunitmayrep-resent Late Hesperian to Early Amazonian lava flows and (or) sediment fromMangalaValles (Tanaka andChap-man,1990)orotherlocalsources.

AlbaPateraunit

Lobate shield flows on the north flank of Alba Patera aremappedasAlbaPateraunit(HTa). The flows embay theNoachisTerraandNepenthesMensaeunits(NnandHNn)andhaveaLateHesperiancraterdensity.Someofthe more pronounced, distal flows include medial chan-nels. One extensive flow underlies Milankovič craterejecta; the flow’s relative age to the Vastitas Borealis marginalunit(ABvm)isuncertain.

EARLYAMAZONIANEPOCH

We redefine the Hesperian and Amazonian bound-ary. Based on Mariner 9 geologic mapping, Scott andCarr (1978) assigned the beginning of theAmazonianto the base of their “cratered plains material” mappedacross much of the northern plains and less crateredpartsof theTharsis riseandsouthernhighlands.UsingViking-basedmapping(ScottandTanaka,1986;GreeleyandGuest,1987;andTanakaandScott,1987),Tanaka(1986) assigned the beginning of the Amazonian toemplacementof smoothplainsmaterial lying in south-ern Acidalia Planitia and defined their Vastitas Borealis Formation as Late Hesperian. We interpret the plainsmaterialinsouthernAcidaliaPlanitiaasanextensionoftheplainsmaterialinVastitasBorealis,whichwemapasVastitasBorealismarginalandinteriorunits(ABvmandABvi). We use these units to define the base of the Ama-zonian,andtheircraterdensitiesarestillfairlyconsistentwith the N(5) = 67 definition of Tanaka (1986) for the beginningoftheAmazonian.Becauseoftheirextentandapparentuniformage,theVastitasBorealisunitsseemtoexpress a broad, significant, yet brief episode of geologic activity for the planet and are better suited for defining such a significant stratigraphic boundary.

VastitasBorealisinteriorunit

TheVastitasBorealisinteriorunit(ABvi)consistsofrelativelybumpy,plains-formingmaterialintheMOLADEM and in 100-m/pixel-resolution THEMIS images.

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The unit covers most of the northern plains includingVastitas Borealis and Utopia, Acidalia, and northernArcadia Planitiae. Younger materials bury the unit ineastern and central Utopia Planitia, in Amazonis andsouthern Arcadia Planitiae, and in the northern polarregion.Theouter,higherpartsoftheinteriorunitincludesetsofparallel,arcuateridgescomposedofpittedcones,commonlyreferredtoas“thumbprintterrain,”aswellasrandomclustersofhillsmostly<1kmacross(FreyandJarosewich, 1982; Grizzaffi and Schultz, 1989; Scott and others, 1995). The cones were first interpreted as possible pseudocratersformedbyexplosionsassociatedwithlavaemplacement over wet ground (Frey and Jarosewich,1982;Carr,1986),cindercones(Scottandothers,1995),pingoes, ice-cored ridges (Lucchitta, 1981; RossbacherandJudson,1981),moraines(Lucchitta,1981;ScottandUnderwood,1991;KargelandStrom,1992;Lockwoodandothers,1992),ormoundsformedbythedisintegra-tion of stagnant ice covers (Grizzaffi and Schultz, 1989). More recently, theywere interpretedasmudvolcanoes(Tanaka,1997;Tanakaandothers,2003b).Withinafewhundredkilometersof theedgeof the interiorunit, thepitted cones become more widely spaced and tend tooccurinsmallgroupsorindividuallyuntiltheydisappearentirely.Here,polygonaltroughsystems(representativeonesaremappedasfreshtroughs)begintoappear,char-acterizing the surfaceof interior, apparentlyunmantledlower elevation regions of the interior facies such asAdamas Labyrinthus in central Utopia Planitia and inthe plains surrounding Planum Boreum as seen in the256-pixel/degreeMOLADEM.Also,thewell-developedpolygonaltroughsofCydoniaLabyrinthusoccurwithinashallowdepression.Thepolygonaltroughsmayformbydesiccationand(or)compactionofwetsediment (Luc-chittaandothers,1986;McGillandHills,1992).

VastitasBorealismarginalunit

TheVastitasBorealismarginalunit(ABvm)consistsof plains-forming material that appears smooth in theMOLA DEM and in 100-m/pixel-resolution THEMISimages.TheunitformspatchesmarginaltomostoftheVastitasBorealisinteriorunit(ABvi)inUtopiaPlanitia,alongArabiaTerra, insouthernAcidaliaPlanitia,northofAlbaPatera,and inwesternArcadiaPlanitia.WherethemarginalunitonlapsplainsunitssuchasUtopiaPla-nitia2unit(HBu2)andChrysePlanitia2–4units(HCc2,HCc3,andHCc4),theedgeoftheunithasabroad,lobateform with individual lobes commonly several kilome-tersacrossthatwraparoundsmallknobsinmanycases.Narrowridgesoutlinesomeofthemarginsofthelobes.At lower latitudes(<25°N.), themarginalunitappearsgenerallybrighterand,thus,warmerthanadjacentmate-rials in THEMIS daytime infrared images, which mayindicate a bulk surficial content of relatively fine-grained material compared to underlying or adjacent materials.

MaterialsoftheAmazonisandElysiumprovincesburypartsoftheunit.Themarginalunitincludeslocalsystemsof narrow sinuous troughs (larger ones are mapped assubduedtroughs)thatareafewkilometerswideandtensofkilometerslongandhavelowmedialridges;insomecases,thetroughsformintricatenetworkssuchasPyra-musFossaeinnorthwesternUtopiaPlanitiaandsimilarfeaturesinwesternArcadiaPlanitia(Tanakaandothers,2003b).Thesefeatureshavenoclearterrestrialanalogsandhavebeencompared toglacialmorainesandkamecomplexes (Lucchitta, 1982), glacial tunnel-valley andeskerfeatures(Kargelandothers,1995),orsoft-sedimentdeformationstructures(Tanaka,1997;Tanakaandothers,2003b).Stratigraphicandtopographicrelationsindicatethe features likely formedsoonafterunit emplacementandmayhavedeformedthesubstrateaswell.

The Vastitas Borealis marginal and interior unitsgenerallyhaveareadilymappablebutpoorlyunderstoodcontact and post-emplacement modification histories. We mapped it as an inferredcontactbecause the twounitspossiblyrepresentaoncecontinuousdepositthateither(1)wasreworkedintodistinctivematerialunitsor(2)hasretaineditsessentialstratigraphiccharacterandisasingleunit with variable and distinctive secondary modification. ThecollectiveoutermarginoftheVastitasBorealisunitsdisplaysafairlyconstantelevationthatmayindicatetheformerpresenceofastandingbodyofwater(Parkerandothers,1989,1993;Headandothers,1999;CliffordandParker,2001;CarrandHead,2003)oracommonground-watertable(Tanakaandothers,2003b).

IsidisPlanitiaunit

The Isidis Planitia unit (AIi) covers most of IsidisPlanitia, shares similar morphologic features with theVastitasBorealisunits(ABvmandABvi)intheBorealisprovince,andisseparatedfromtheBorealisprovincebyanarrow (<150km) stripof theUtopiaPlanitia 2unit(HBu2) that covers a low saddle between Utopia andIsidisbasins(Tanakaandothers,2003b).Wedonotdis-tinguishmarginalandinteriorunits,becausethefeaturesdistinguishingthemintheBorealisprovinceappearinti-matelymixed.Troughfeaturesoccuron thenorthwest-ernandsouthwesternmarginalareasoftheunit,whereastheremainderoftheunitincludesdistinctsetsofridgesof pitted cones of greater relief and steeper flanks than those in theVastitasBorealis interior unit (Tanaka andothers,2003b).AlongtheIsidisPlanitiaunitcontactwiththe Syrtis Major Planum unit (HIs), the Isidis Planitiaunit locally includes pitted cones and ridges occurringalongfractureswithintheSyrtisMajorPlanumunit.Thecraterdensityoftheunit indicatesanEarlyAmazonianage,anditmaybecoevalwiththeVastitasBorealisunits.The IsidisPlanitiaunit and its featuresgenerally sharethe same interpretations as the Vastitas Borealis unitswith the addition that Grizzaffi and Schultz (1989) inter-

11

preteditspittedconesasmoraineremnantsofanice-richdebrislayer.

MedusaeFossaeunit

The Medusae Fossae unit (AAm) covers parts ofsouthernElysiumandAmazonisPlanitiaeandtheThar-sis rise. This material is characterized by a sequenceof layers that appear tobeetchedby thewind to formstreamlinedyardangmounds;theconsistent,sometimescross-cuttingtrendsofthemoundsindicatethattheymayresultfrompreferentialerosionalongcracks(TanakaandGolombek, 1989). THEMIS daytime infrared imagesshow isolated patches of the unit within Elysium andsouthernAmazonis Planitiae. Dating the unit by cratercountsistenuousbecauseofthehighdegreeoferosion;stratigraphic relations with Tharsis flow materials indi-cate that the Medusae Fossae unit spans much of theAmazonian(ScottandTanaka,1982,1986;GreeleyandGuest,1987).Inthemaparea,theunitburiesAmazonisPlanitia1southunit(AHAa1s)andsomeNoachianandHesperianmaterials,anditappearstobothpredateandpostdateadjacentMiddleandLateAmazonianunits.Theunithasavarietyofinterpretations,includingexplosivevolcanism(ScottandTanaka,1982;Tanaka,2000;Hynekandothers,2003),ahypothesisconsistentwiththegeo-logicsettingoftheunit.Alternatively,theunitmaybeahighlyerodibleaeoliandeposit(ScottandTanaka,1986;Greeley and Guest, 1987) or other fine-grained deposits.

TinjarVallesaandbunits

SuperposedontheVastitasBorealisunitsinUtopiaPlanitia are lobate deposits of Tinjar Valles a and bunits (AEta and AEtb).TinjarValles a unit consists ofruggedmaterialforminglobesdissectedbyHrad,Apsus,Tinjar,andGranicusValles,whereasTinjarVallesbunitincludes relatively smooth material within and liningpartsofthechannels.TheTinjarVallesunitscollectivelyform a broad tongue of material extending >1,500 kmfrom the northwest flank of the Elysium rise into the central floor of Utopia basin. These Early Amazonian depositsareinterpretedtohavebeenemplacedaslaharsand (or) other volcanic or debris flows that resulted from the intermingling of magmatism and subsurface vola-tilereservoirsalongthedeeptroughsofElysiumFossae(Christiansen,1989;Tanakaandothers,1992a;RussellandHead2003).

Scandiaregionunit

DeepwithinBorealisbasinandontopoftheVasti-tasBorealisinteriorunit,theScandiaregionunit(ABs)forms(1)muchof thebaseofPlanumBoreumand(2)a complex of mesas, knobs, and circular and irregularpatches of material in Vastitas Borealis north of AlbaPatera.InPlanumBoreum,theunitoccursmostlywestofChasmaBorealeandrangesfromhundredsofmeters

tomorethan1,000minthickness.InMOCimages,theseoutcropsdisplaydark,crossbeddedlayersthaterodedintodebrisfansandmeter-sizebouldersandthatapparentlycontributed to the formation of dunes; underlying thecrossbeddedlayersarebrighter,platylayersthatextendfromscarpbases(MalinandEdgett,2001,2005;ByrneandMurray,2002;FishbaughandHead,2005).Theunitincludes the plateau on the floor of Chasma Boreale that isafewhundredmetersthickanddisplaysabout10layersinTHEMISimages;theselayersmayunderliethecross-beddedandplatylayersmakingupupperexposuresoftheunitinPlanumBoreum.ThePlanumBoreumexposuresaresuperposedbyafewcratersseveralkilometersacrossthatareshallowlyburiedand(or)exhumedfrombeneathPlanumBoreum1and2units(ABb1 andABb2).Thispartoftheunitmaybeasandseadeposit(ByrneandMurray,2002;Tanaka and others, 2003b; Fishbaugh and Head,2005),perhapsderivedfromerosionoftheVastitasBore-alisunits(ABvmandABvi;Wyattandothers,2004)and(or)theVastitasBorealisplainspartoftheScandiaregionunit.TheoutcropsinVastitasBorealisaretensofmetersto a couplehundredmeters thick and formadegradedcomplexofmesas,knobs(ScandiaColles),anddepres-sions thatoverlie theVastitasBorealisandAlbaPateraunits. Pancake-shaped mesas tens of kilometers across(ScandiaTholi)arepartlysurroundedbyshallowmoats.TwolargecomplexescontaintheirregulardepressionsofScandiaCavi;theseoutcropsalsoincludeirregularridgesand patches of relatively smooth, gently sloping flanks. ScandiaTholiandScandiaCavihavebeeninterpretedtobeformedbyeruptionofvolatile-richmaterial(Tanakaandothers,2003b);alternatively,someofthelargerout-crops have been suggested to be degraded, ice-coredglacialmoraines (FishbaughandHead,2001).Locally,terraces that likely represent layering can be observedin the plains outcrops in MOC and THEMIS images;however,muchoftheunitsurfaceisburiedbythethinmid-latitudemantlecoveringmuchofthenorthernplains(Mustard and others, 2001). Steep scarps commonlyappear tobemasswasted.Wealso include in theunit,southofthemouthofChasmaBoreale,irregularmesasandcrateredknobshundredsofmetershighthatweinter-prettobedegradedpedestalcraters,becausetheirheightscoincidewiththeapparentpre-erosionalthicknessoftheunit.Escorialcraterformedonapedestalasmuchas700mthickinthisarea(Tanakaandothers,2003b).Alterna-tively,thecrateredknobsalsohavebeeninterpretedtobevolcanoes(Garvinandothers,2000).

CerberusFossae1unit

TheCerberusFossae1unit(AEc1)formsahighlyfractured,ruggeddepositalongthesouthmarginofEly-siumPlanitia.Itappearstoresultfromintensedeforma-tionofthenorthmarginoftheMedusaeFossaeunit(AAm;Lanagan,2004)andpredatesMiddletoLateAmazonianCerberusFossae2and3units(AEc2andAEc3).

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AmazonisPlanitia1northunit

ExtendingacrossnorthernAmazonisPlanitia,ruggedflows form Amazonis Planitia 1 north unit (AAa1n).Theflows emanate from beneath Olympus Mons aureole material(theLycusSulciunit,ATl)andburytheVasti-tasBorealismarginalandinterior,ArcadiaPlanitia,andNepenthesMensaeunits(ABvm,ABvi,HAa,andHNn).The rugged character of the flows is unlike that of typical lava flows; rather, they may be pyroclastic flow deposits, lahar deposits, and (or) surficially modified lava flows. FullerandHead(2003)proposedthatthelavasrepresentaninitialstageofhigheffusionfortheincipientdevelop-mentofOlympusMons.ThecraterdensitysuggestsanEarlytoMiddleAmazonianage.

MIDDLEAMAZONIANEPOCH

LycusSulciunit

The Lycus Sulci unit (ATl) forms the rugged, cor-rugated lobes of the Olympus Mons aureole deposits,LycusSulci.TheunitoverliesAmazonisPlanitia1northunit (AAa1n) and is overlain by part of the MedusaeFossaeunit(AAm)andtheAmazonisPlanitia2northandsouthunits(AAa2nandAAa2s).Thesestratigraphicrela-tionssuggestthattheunitisMiddleAmazonian,whichis somewhat younger than previously thought (MorrisandTanaka,1994).Avarietyofideasfortheformationofthisunithavebeenproposed;rapidlandslides(Lopesandothers,1980;McGovernandothers,2004)orslowergravity spreading of a larger, perhaps ice-lubricated,proto-Olympus Mons edifice (Francis and Wadge, 1983; Tanaka,1985)aremechanismsincorporatingtheforma-tionoftheOlympusMonsbasalscarp.

DeuteronilusMensae2unit

DebrisapronsformingDeuteronilusMensae2unit(ABd2)occurintheDeuteronilusMensaeregion;similarapronsoccureastofHellasbasininthesouthernhemi-sphere(GreeleyandGuest,1987).Theapronsaresparselycratered and overlap fretted channel floors and the plains ofDeuteronilusMensae1andVastitasBorealismarginalunits (HBd1 and ABvm). Some outcrops include largeinliermesasofNoachianrocks,whicharenotincludedintheageassignmentfortheunit.TheapronslikelyresultfrommasswastingofthewallsoftheNoachisTerraunit(Nn),whichsuggeststhattheNoachisTerraunitisvola-tile rich in this area (Lucchitta, 1984).The young unitagemayindicatethatthemass-wastingprocessremainsrelativelyactive(Squyres,1978,1979a);initialdevelop-mentoftheunit,however,isuncertainandmayextendasfarbackintimeastheLateHesperian.

AmazonisPlanitia2southunit

TheAmazonisPlanitia2southunit(AAa2s)consistsof lobate flows that emanate from vents along the margin

ofsoutheasternAmazonisPlanitiaandextendhundredsofkilometerswestandnorthintotheplain.Theunitgen-erally appears to embay the Lycus Sulci and MedusaeFossaeunits(ATlandAAm)asseeninTHEMISimages.The northern contact of the unit grades intoAmazonisPlanitia2northunit(AAa2n).

AmazonisPlanitia2northunit

TheAmazonisPlanitia2northunit(AAa2n)consistsof lava flows that form an extremely flat and smooth (in theMOLADEM;KreslavskyandHead,2000) surfacein central Amazonis Planitia and that exhibit lava-flow surfacetexturesinMOCimagesandhighradarbackscat-ter (Fuller and Head, 2002). Broad, arcuate, low-reliefchannelsmigrateacross theunitasseen inMOLAandTHEMIS infrared images and may indicate late-stagewater floods (Fuller and Head, 2003) or lava channels. TheunitembaystheAmazonisPlanitia1northandtheLycus Sulci units (AAa1n and ATl) and large impactcratersthattheunitsurrounds.ThesourceoftheunitisuncertainandmayincludedistalextensionsofAmazonisPlanitia 2 south, Cerberus Fossae 2, and (or) CerberusFossae3units(AAa2s,AEc2,andAEc3).

CerberusFossae2unit

Cerberus Fossae 2 unit (AEc2) forms lobate andchannelized flows issuing from widely scattered parts of theCerberusFossaefracturesystemandcoveringpartsof eastern Elysium Planitia and terraces along MarteVallis.TheunitisassociatedwiththeGrjotáandRahwayValles�channelsystems(Plescia,2003).Topographicallysubdued, conical vents occur in theunit along fracturetracesinsoutheasternElysiumPlanitia.Thisunitlikelyconsists of lava flows and fluvial sediments associated withepisodesofvolcanismandwaterdischarge.TheunitembaystheCerberusFossae1unit(AEc1),Elysiumriseunit (AHEe), and other older units.The unit generallyappearstobelightlycratered,whichindicatesaMiddleto Late Amazonian age, but its crater count includesmanylargercratersthatappeardissectedbytheassoci-ated channels and, thus, are superposed on underlyingmaterial.Muchoftheunitwasdissectedandburiedbyfluvial discharges and lava flows associated with Cer-berusFossae3unit(AEc3;Lanagan,2004).

LATEAMAZONIANEPOCH

CerberusFossae3unit

A large field of lava flows making up Cerberus Fossae3unit(AEc3)extendsfromsitesalongsouthernCerberusFossaeandassociatedlowshieldsintoElysiumPlanitia and Marte Vallis. The flows have a platy, ridged texture resembling the historic Laki flood basalt in Ice-land(Keszthelyiandothers,2000).Theunitoverliesandembays most adjacent units including Cerberus Fossae

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1 and 2, Utopia Planitia 1 and 2, andArcadia Planitiaunits (AEc1, AEc2, HBu1, HBu2, and HAa; compareregionalmappingwiththatofLanagan,2004).Itgener-allyembaystheMedusaeFossaeunit(AAm),exceptforasmallpatchatlat~4°N.,long152°E.Densitiesofsub-kilometer-diametercratersindicatethattheunitmaybeafewto30Ma(HartmannandNeukum,2001).Wheretheflows originate from Cerberus Fossae, associated chan-neldissectionindicatesthatwaterdischargeoccurredinassociationwithlavaeruptions(Burrandothers,2002a,b;WilsonandHead,2002;Plescia,2003).

AstapusCollesunit

AlongthewestmarginofUtopiaPlanitiaatlat38°to48°N.,theAstapusCollesunit(ABa)formsathindeposit(tensofmetersthick)coveringtheVastitasBorealisandUtopiaPlanitiaunits (ABvm,ABvi,HBu1, andHBu2).Theunit forms amantle that appears tohave retreatedaround crater rims and ejecta deposits; this retreat hasbeeninterpretedasduetothermokarstformingpreferen-tiallyalongsuperposedcraterejectamargins(CostardandKargel,1995).Nearlyallcraters inMOCnarrow-angleandTHEMIS imagesunderlie theunit,which suggestsaLateAmazonianage.Craterburialrelationsindicateaunitthicknessof30to40m(McBrideandothers,2005).In THEMIS visible and MOC narrow-angle images,theunit appears tohavea relatively lowalbedoand iscrackedbypolygonalfracturesalongwhichnestedpitsarecommon;thesefeaturesappeartobeconsistentwithperiglacialandthermokarstorigins(CostardandKargel,1995).This unit occurs within the latitudeband wherea youthful, meters-thick, presumably ice-rich, debrismantleseeninMOCnarrow-angleimagesappearstobeeroding(Mustardandothers,2001).However,polygonalcracks tend tooccurnorthof lat55°N.where theMONeutron Spectrometer detected ice, indicating that theunitformedduringadifferentclimateepisodethatpre-datedthedebrismantle(Mangoldandothers,2004).

PlanumBoreum1and2units

Planum Boreum 1 and 2 units (ABb1 and ABb2)form the upper part of Planum Boreum, some nearbylow plateaus, and local crater fill. These units consist of stacks of layers that vary in albedo and (or) thickness,which ranges fromabout ameter to tensofmeters (asseen in MOC narrow-angle images). The units overliethe Scandia region and Vastitas Borealis interior units(ABsandABvi).Mostofthelayersappeartobenearlyflat lying, but locally some are folded or tilted and dis-play angular unconformities (Malin and Edgett, 2001).Surfacesof theplateau-cappingPlanumBoreum2unitare mantled by residual, water-rich polar ice. PlanumBoreumdisplays arcuate, swirling trough systems.ThetroughsdisplaylayersofPlanumBoreum1unitonequa-tor-facing slopes,whereaspole-facing slopes appear to

becoveredbyPlanumBoreum2unit.Locally,sequencesof layers inPlanumBoreum1unitare truncatedalongunconformities, indicating uneven deposition and (or)local erosion during accumulation of the unit. PlanumBoreumalsodisplaysamajorre-entrant,ChasmaBore-ale, containing several deep pits that expose the Scan-dia region unit below the Planum Boreum units. Twocraters approximately 330 and 415 m across (MOCimages r0100641 and m2000372, respectively) super-pose Planum Boreum 1 unit, and two craters approxi-mately42and90macross(MOCm2901628)occuronPlanum Boreum 2 unit. The paucity of craters on themore widely exposed Planum Boreum 2 unit indicatesasurfaceageof<100ka (HerkenhoffandPlaut,2000;Koutnikandothers,2002)andperhaps<20kasincethepreviouspolarinsolationmaximum(Laskarandothers,2002).Manystudiesofthelayers,troughs,andchasmasuggest a range of contrasting origins and timing rela-tionsfortheirdevelopment,involvingtheplanet’sobliq-uity and eccentricity variations; atmospheric volatileanddusttransportmechanisms;surfaceice;topography;wind activity; ice sublimation, deformation, and melt-ing;andmelt-waterdischarge(Murrayandothers,1972;Cutts,1973;Howard,1978;Squyres,1979b;Blasiusandothers,1982;Howardandothers,1982;DialandDohm,1994;Benitoandothers,1997;Fisher,2000;KolbandTanaka, 2001; Malin and Edgett, 2001; Fishbaugh andHead, 2001, 2002).The Planum Boreum deposits mayhaveformedprimarilyduringthelowobliquityandlowpolar insolation period of the past 5 m.y. (Laskar andothers,2002).

OlympiaUndaeunit

Surrounding Planum Boreum, dune fields and mod-eratetolowalbedomantlesformtheOlympiaUndaeunit(ABo). The 1/256°/pixel- and 1/512º/pixel-resolutionMOLA DEMs resolve many of the dunes individually,and dune fields display a distinctive, alternating bright and dark artificial illumination pattern in the shaded-relief images.Thispatternisaresultofthevaryingorientationof dune faces to the artificial illumination direction. High-resolutionVikingandMOC imagesof thedunes showbarchanand transverse forms (Breedandothers,1979;Tsoar and others, 1979; Malin and Edgett, 2001). Theduneseasmaybeatleastseveralmetersthickonaverage(LancasterandGreeley,1990).TheunitoverliesPlanumBoreum1and2,Scandia region, andVastitasBorealisinteriorunits(ABb1,ABb2,ABs,andABvi)andlocallyunderliesPlanumBoreum2unit(ABb2).Mostoutcropsof the unit form broad, continuous deposits, but someareas south and west of Olympia Undae include smallgroupsofdunes.TheduneshavealowalbedoandstrongType 2 TES spectral type (Bandfield and others, 2000) andappeartooriginatefromerosionofPlanumBoreum1unit(ByrneandMurray,2002;Wyattandothers,2004).

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Noevidence fordunemigrationover thepast20years(bycomparisonof>40m/pixelVikingOrbiterand<1.5m/pixel MOC images) has been discovered yet (MalinandEdgett,2001).Inaddition,weincludemantlemate-rialassociatedwiththedunesintheunit.IntheMOLADEM, the mantle material produces a subdued topog-raphy over the surface of theVastitas Borealis interiorunit.WithinMOCimages,themantlematerialislocallyrippledandappearstoembaytheduneforms.Bynature,this unit is transient and, therefore, young as defined by itspresentform.However,asadynamicunititmayhavebeeninexistencesincetheVastitasBorealisinteriorandScandiaregionunitsbeganerodingasearlyastheEarlyAmazonian.

Surficial material and properties

RemotelysensedpropertiesoftheMartiansurface,suchasalbedo, thermal inertia, topographic roughness,neutronabundance,gamma-ray emissions, and thermalemissivity, relate to intrinsic properties, compositions,and modification signatures of materials of the upper few micronstometersofthesurface.Inmanycases,surfaceproperties reflect recent geologic and climatic activity (Mustardandothers,2001;Headandothers,2003).Map-pingandinterpretingsuchpropertiesisgenerallybeyondthescopeofthismap.Oneexceptionisthatwehaveusedrelative albedo data from MOC wide-angle, defrostedimages to document the recent extent of the transientnorth polar residual ice, which may be meters to tensofmetersthick(MalinandEdgett,2001).Thedistribu-tionofthismaterial,coveringmuchofPlanumBoreum2unit(ABb2)andhigh-latitudecratersandoutcropsoftheScandiaregionunit(ABs),isshownonthemapasawhitestipplepatterncalledpolarice(seeExplanationofMapSymbols).

stRUCtURe AnD MoDIFICAtIonHIstoRY

Tectonism,periglacialreworking,collapse,erosion,andothergeologicprocessesthatpostdateunitemplace-ment modified materials in the northern plains of Mars. Evidenceof geologic processes are mappable and pro-vide a basis for determining age relations among geo-logicmaterialsandfeatures.

PRE-NOACHIAN

Thepre-Noachianperiodpredatestheoldestexposedrockrecord,whichbeginsinthemapareawiththeLibyaMontesunit(Nl;Frey,2004;NimmoandTanaka,2005).Oneoftheearlieststructuresinthemapareaisthenorth-ern plains basin. Postulated origins for the lowlandsinclude tectonism (Wise and others, 1979; Fairén andDohm, 2004), a mega-impact (Wilhelms and Squyres,1984), or a series of large impacts (Frey and Schultz,

1988).Inparticular,Utopiabasinhasacircularoutline,gravitysignature,andassociatedstructurethatsupportsanimpactorigin(McGill,1989;Smithandothers,1999)butseemstolackrelatedrockexposures.Free-airgravityanomalypatternssuggestapossibleburiedimpactbasinin northwestern Amazonis Planitia (Fuller and Head,2002). Smaller, quasi-circular depressions tens to hun-dredsofkilometersindiameterarescatteredthroughoutthelowlandsandmaybethesurfaceexpressionoflarge,buried impacts whose density exceeds that of exposedcraterstructures(Freyandothers,2002).Thus,manyofthesestructuresarelikelydeeplyburiedandtectonicallyrelaxedandmaydateback to thepre-Noachian.Estab-lishmentofthenorthernlowlandslikelyaffectedtheori-entation of the planet’s spin axis and greatly influenced subsequenttectonic,volcanic,andvolatile-relatedresur-facingactivity.

EARLYNOACHIANEPOCH

EarlyNoachiangeologicactivityaffectedtheLibyaMontes unit (Nl). Isidis basin is partly ringed by mas-sifs and heavily cratered terrain of the Libya Montesunit.A large,positivegravityanomaly iscenteredoverthebasin,substantiatingtheimpacthypothesisforIsidisbasin(Zuberandothers,2000).XantheMontes�formachainofmassifsthatoutlinetheroughlycircularChrysePlanitia and may define a large impact basin (Schultz and others,1982).However,thisinterpretationremainsspecu-lativeassupportingstructuralevidenceislimitedandtheplainlacksapronouncedgravityanomaly.Alternatively,XantheMontes�andthebroadridgetheyoccuronmaybe due to early Tharsis deformation or, perhaps, someothertectonicorigin(Bakerandothers,2002;FairénandDohm,2004).MOLAdataalsodemonstratemanysubtlebasin features tens to hundreds of kilometers acrossthroughoutmuchofthemaparea(Freyandothers,2002).Theseearliestfeaturesprovidetopographicandstructuralcontrolsaffectingsubsequentgeologicactivity.

MIDDLEANDLATENOACHIANEPOCHS

Noachian and perhaps Hesperian structures thatdissect parts of the Libya Montes, Noachis Terra, andNepenthesMensaeunits(Nl,Nn,andHNn)includethearcuategrabensofNiliandAmenthesFossae,whichareconcentricwiththeIsidisbasinrimandmayrelatetotec-tonicrelaxationoftheIsidisbasinstructure(Schultzandothers,1982)ortootherstructurallycontrolledprocesses.Arcuate grabens and normal faults ofAcheron Fossaetrend along the strike of the broad ridge that they cut.West-towest-northwest-trendinggrabenscuttheNepen-thes Mensae outcrops that comprise Phlegra Montes,TartarusMontes,andTartarusColleseastoftheElysiumrise.ScarpsthatdeformtheNoachisTerraunitthrough-out the map area may be contractional fold and thrustfeatures that may have developed contemporaneously

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with formationof theNoachisTerraunit as a resultofglobalcontractionduetoplanetarycooling(Tanakaandothers,1991;Watters,1993)and(or)lithosphericbend-ingcausedbyloadingofthenorthernlowlands(Watters,2003). Rugged terrain, degradation of structural land-forms,anddeformationalongpre-existingcrater-relatedstructures probably have largely obscured much of thecontractionaldeformationwithinNoachianrocks.

Scattered,sinuousvalleysystemscutmostexpansesofNoachianrocksincludingTempe,Xanthe,Margariti-fer, andArabia Terrae; Libya Montes; and Terra Cim-meria (Carr and Clow, 1981; Scott and Tanaka, 1986;GreeleyandGuest,1987;TanakaandScott,1987;Crad-dockandMaxwell,1993;Scottandothers,1995;HynekandPhillips,2001;CraddockandHoward,2002).Someof these valleys have few tributaries, which may indi-cate immaturedrainagesestablishedbysmalloutburstsofstandingwaterorbysapping(CarrandMalin,2000;Irwinandothers,2002)ratherthanbyextensiveprecipi-tation-driven overland flow. Relatively subtle drainages canbemappedinhigh-resolutiontopographicandimagedata, resulting in a high density of valley features thatmay indicate precipitation-fed runoff (Hynek and Phil-lips,2003),thoughconstrainingtheageofthesefeaturesis difficult in most cases. Overall, it appears that few, if any, of the valleys and their sediments either incise orburyHesperianunitsthattheyencounteralongthehigh-land-lowland boundary, though definitive observations remainincomplete.ChrysePlanitia1unit(HNCc1)maypartlyrepresentsedimentsfromthisdissection.

Accompanying the formation of the NepenthesMensaeunit(HNn),arcuateandlineartroughsandirreg-ularandcirculardepressionsdevelopedintheadjacent,plateau-forming Noachis Terra unit (Nn), producing aknobby,hummockyterrain.Thislandscapeappearstobeformedbyfracturing,grabenformation,andstructurallycontrolled collapse and mass wasting.The depressionsindicateremovalofsubsurfacematerial,particularlyalongimpact and other structures (Tanaka, 2004; Rodriguezandothers,2005),whichsuggeststhatthematerialwasvolatilerich.ThisdegradationapparentlycontinuedintotheEarlyHesperianandledtodepositionoftheUtopiaPlanitia1unit(HBu1;Tanakaandothers,2003a,b).TheNepenthes Mensae unit resulted from mass wasting ofmostexposuresoftheNoachisTerraandLibyaMontesunitsalongthehighland-lowlandboundary,exceptalongthemarginsofChrysePlanitia.EastoftheElysiumrise,the Nepenthes Mensae degradation affected low-lying(below–2,000melevation)crateredterrainthroughoutaregionthatisafewthousandkilometersacross.SomeofthedegradationinsouthernAmazonisPlanitiahasbeenattributed to catastrophic flooding (Dohm and others, 2001a), but the evidence remains obscure. Some plan-etary scientists have proposed that the northern plainscould have been submerged beneath an ocean during

the Noachian and into the Hesperian, perhaps due toground-water flow or precipitation-fed runoff (Clifford andParker,2001;Fairénandothers,2003).Thoughwedo not observe any definitive geomorphic evidence for earlyoceans, the intensityand longevityof resurfacingprocesses that occurred after ocean formation precluderulingouttheirpotentialexistence.

EARLYHESPERIANEPOCH

Contractional wrinkle ridge and scarp deformationcontinuedtodeformnearlyallNoachianunitsandmostEarly Hesperian units, including the Lunae Planum,ChrysePlanitia1,UtopiaPlanitia1,ArcadiaPlanitia,andNepenthesMensaeunits(HNTl,HNCc1,HBu1,HAa,andHNn).Ridgeswithinthenorthernplainsmayhavebeendevelopingduringthistime,perhapsdeformingmaterialsunderlyingtheVastitasBorealisunits(ABvmandABvi;Headandothers,2002).ManyoftheridgesintheEarlyHesperianunitsfollowmultipletrends,havesymmetriccross-sectional forms, seem to be enhanced within theunitsthemselves,andbarelyextendintonearbyoutcropsoftheNoachisTerraunit(Nn).Thismightindicatethatthestrengthcharacteristics in theyoungerunits lead toenhanced strain on fewer structures, whereas strain inolder units may be distributed along smaller but morenumerousstructures.Noachianunitsmaybeweakduetoimpact-relatedfracturing,grinding,andcomminutionofancienttargetmaterial(MacKinnonandTanaka,1989).

Linear, north- to northeast-trending complex gra-bens and normal faults dissect the Noachis Terra unitof Mareotis and Tempe Fossae, which ranges fromNoachiantomiddleHesperian(ScottandDohm,1990;Moore,2001)andmayrelatetoriftingalongtheTharsisMontesaxialtrend.

Fretted channels and troughs in the DeuteronilusMensae region of northernArabia Terra developed bysubsurface flow and withdrawal of presumably ice-rich material (Sharp,1973;Squyres,1978;Lucchitta,1984;McGill, 2000), although other hydrologic phenomenasuchas spring-fedactivity related toclimatechange inavolatile-richregionmayhavealsocontributedtotheirformation (Bakerandothers,1991;Barlowandothers,2001). Locally, some pitted hills within the frettedtroughsmaybevolcanoesresultingfrommagma/ground-iceinteractions(CarruthersandMcGill,1998),rootlesscones related to diapirism, mud volcanism related tocompactionofvolatile-richsediments(Tanakaandothers2003b),orsomeotherextrusivemechanism.

Within the Vastitas Borealis, Utopia Planitia, andolderChrysePlanitiaunits(ABvm,ABvi,HBu1,HBu2,HNCc1,andHCc2),addingthenumberofdegradedcra-ters having flat floors and low or absent rims (fig. 2) to the numberoffresh-appearingcratersyieldscraterdensitiesconsistentwithanEarlyHesperianburiedsurface(Head

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and others, 2002). Alternatively, the degraded craterrecordmaybeincompleteorthesubstratemaybecom-plex,andamixtureofNoachiantoLateHesperiansur-facesandmaterialsunderlietheVastitasBorealisunits.

LATEHESPERIANEPOCH

Narrow, en echelon grabens of Tantalus Fossaeextend hundreds of kilometers and cut theAlba Pateraunit (HTa) of northeastern Alba Patera. The Scandiaregion unit (ABs) buries some of these grabens. Someoftheparallel-trendinggrabensofTempeandMareotisFossae farther east may also have formed at this time.The deformation may involve rifting along theTharsisMontes axial trend, perhaps stimulated partly byAlbaPatera and Tempe Terra volcanism (Scott and Tanaka,1986;ScottandDohm,1990;Tanaka,1990).

Contractional, symmetric wrinkle-ridge and scarpdeformationcontinuedtodeformearlierunitsmarkedbyridges and scarps, as well as Late Hesperian materialsincludingtheArcadiaPlanitia,Elysiumrise,ChrysePla-nitia2and3,andUtopiaPlanitia2units(HAa,AHEe,HCc2, HCc3, and HBu2). Ridges in Utopia Planitia 2unit follow multiple trends and seem to be enhancedwithintheunitand,perhaps,inadjacentEarlyHesperianunits,buttheybarelyextendintonearbyoutcropsoftheNoachisTerraunit(Nn).Also,somewrinkleridgescon-centric toAlba Patera deform the southern part of theScandia region unit (ABs) and the outermost flows of theAlbaPateraunit(HTa).SuchdeformationisabsentwithintheinnerpartoftheAlbaPaterashieldandalongthe Tantalus Fossae fracture swarm and either may beburiedordidnotoccur.Someof thisdeformationmaybeAmazonian.

Collapse of lower parts of Utopia Planitia 1 unit(HBu1)insouthernUtopiaPlanitiaapparentlyledtofor-mationof the irregulardepressionsofAmenthesCavi�anddepositionofUtopiaPlanitia2unit (HBu2).Someofthedepressionsappearnestedandhaveraisedorfrac-tured rims and associated knobs, fractured hills (somewithmoats),andpitted topography inTHEMISvisibleimages.Thefracturedhillsmaybesitesofheaving.BothUtopiaPlanitiaunitslikelywerevolatilerich.Intensivemass wasting of Noachian, perhaps volatile-rich rocksofDeuteronilusMensaeresultedinknobbyplainsmate-rial of Deuteronilus Mensae 1 unit (HBd1; Mangoldand others, 2002). Some of the older unit surfaces arecutbynetworksofpolygonalfracturesspacedhundredsofmetersapart,whichindicatesperiglacialicewedging(Lucchitta,1985).

Major incision of the circum-Chryse outflow chan-nelsystemandchaosdevelopmentoccurredatthistime,possibly related to a significant pulse of Tharsis magma-tism(Dohmandothers,2001b).Thechannelandchaossystems demonstrate progressively deeper downcut-tingandcollapse(Witbeckandothers,1991;Rottoand

Tanaka,1995).Someof theearlierdischargesofKaseiVallesappear tohaveoriginatedfromtheSacraFossaetroughs (Tanaka and Chapman, 1992). The resultanttopographyrepresentstheaccumulatedhistoryofmanyindividualepisodesandevents,includingfeaturesrelatedtodischargeandrecharge,erosion,collapse,deposition,and settling of deposits (Carr, 1979; Nummedal andPrior, 1981; Lucchitta, 1985; MacKinnon and Tanaka,1989;HarrisonandGrimm,2004;Rodriguezandothers,2005).

EARLYAMAZONIANEPOCH

TheVastitasBorealisunits (ABvm andABvi) gen-erally have been considered to be the result of outflow channel sedimentation (Parker and others, 1989, 1993;Baker and others, 1991; Tanaka, 1997). However, thestratigraphicrelationsandourcratercountsindicatethatthe Vastitas Borealis units may postdate the youngestChrysePlanitiaunits(HCc3andHCc4).Ifanoceanwererapidly emplaced via outflow channel discharges and (or) springs fromwithin thenorthernplains (as representedby the Hephaestus/Hebrus fracture and valley system,butonabroaderscale),itwouldfreezeinapproximately104yearsandtheice,ifitwereinsulated,wouldremainin the subsurface (Kreslavsky and Head, 2002). Possi-bly,suchanice-richdepositwassubsequentlyreworkedperhaps due to one or more significant regional or global warmingeventsandthecontinuedintroductionofgroundwater from aquifers within the substrate and adjacentplains materials. This reworking could have involveddeformation, sedimentary volcanism, and (or) mass flow (toexplainthelobateedgesoftheVastitasBorealismar-ginal unit) that ultimately might have transformed andaugmentedapaleo-oceandeposit.

WithintheVastitasBorealisinteriorunit(ABvi),theformationofpolygonaltroughsandcirculardepressions(also called ghost craters) in Utopia andAcidalia Pla-nitiae and Vastitas Borealis may be related to desicca-tion,contraction,andrelaxationofawater-richdeposit>600m thick (McGillandHills,1992).ThepolygonaltroughsoccurmostlyinlowerelevationareasoftheunitwithinUtopiaPlanitiaandsurroundingPlanumBoreum.ThecirculardepressionsaremorewidelydistributedandlikelyareburiedimpactcraterswhosedensityindicatesthattheydateatleasttotheEarlyHesperian(KreslavskyandHead,2002).Thosehavingdouble rings inUtopiaPlanitia suggest that the wet sediment thickness mayhave been 1 to 2 km (Buczkowski and Cooke, 2004).Alternativelyor,perhaps,inaddition,polygonaltroughsin Utopia Planitia have been proposed to result fromrebound after removal of water (Hiesinger and Head,2000;ThomsonandHead,2001).

Long, sinuous troughs with medial ridges devel-opedinplaceswithintheVastitasBorealismarginalunit(ABvm).Thesefeatureshavebeeninterpretedvariously

17

ascoastalmarinespits(Parkerandothers,1993),glacialtunnel valleys with eskers (Kargel and others, 1995),and deformation of a partly frozen, wet debris deposit(Tanaka,1997).

Local regions of the northern lowlands and partsofnorthwesternArabiaTerraappeartohaveundergonecollapse and other volatile-related landscape modifica-tion following formation of the Vastitas Borealis units: (1) Scandia region, which includes Scandia Cavi andothershallower,irregulardepressionsandscarpsnorthofScandiaCollesandTantalusFossae;(2)northernAcida-lia Planitia, with irregular depressions and knobby ter-rainsintheAcidaliaandOrtygiaCollesregion;and(3)Cydoniaregion,includingknobbyterrain,subtledepres-sions,andhigh-standingmesassurroundedbymoatsinlowlandmaterialsanddeeppitswithinhighlandmaterial.Inpart, thecollapse featuresmaybecontrolledby ice-rich stratigraphy related to the putative ancient Chryseimpact basin (Tanaka, 2004). Collapse is likely due tothe removal of subsurface ice. The mesas with moatsmay representenhancedpumpingofgroundwater intozonesbeneaththemesasduetorisesinthetopographyof the permafrost/unfrozen ground interface. The pan-cake-shapedmesasand irregularhillsofScandiaTholimay representmudvolcanism,perhaps related topres-surizationofporewaterasthepermafrostzonedeepened.Alternatively,FishbaughandHead(2000)interpretedthefeatures as degraded moraines of a retreated, formerlyextensiveicecap.

ThetroughsofElysiumandHyblaeusFossae,whicharebothconcentricwithandradial(northwest-trending)totheElysiumrise,cuttheElysiumriseunit(AHEe)onthe flanks of the rise and served as sources of Tinjar Valles aandbunits(AEtaandAEtb)and,perhaps,someoftheElysium rise unit as a consequence of magma/ground-iceandwaterinteractions(Mouginis-Mark,1985;Chris-tiansen, 1989; Tanaka and others, 1992a; Russell andHead,2003).Possiblyassociatedwiththisactivity,theformation of Hephaestus Fossae and Hebrus Valles insoutheasternUtopiaPlanitiaincludediscontinuouschan-nel scour features originating from depressions and, insome cases, terminating at depressions downslope, aswellaspitsandpolygonaltroughs.Thesefeaturesseemtoindicatespringdischargeofwaterandbothsurfaceandsubsurface fluvial erosion along fracture systems. The featuresoriginate inUtopiaPlanitia2unit (HBu2)anddissecttheVastitasBorealisunits.Eastofthesefeatures,severalscatteredpatchesofelevated,fracturedtopogra-phyindicatesitesofpossibleheaving,perhapsresultingfromhighpore-volatilepressures.

SomeofthemajorgrabensofTantalusFossaewerereactivated,extendinghundredsofkilometersintoVasti-tas Borealis across Alba Patera, Scandia region, andVastitasBorealismarginalandinteriorunits(HTa,ABs,ABvm,andABvi).ThesouthcontactoftheVastitasBore-alismarginalunitrisesasmuchas650mabovethemean

elevation of this contact (~–3,800 m) where it crossesTantalusFossae,whichmaybeasignatureofupliftasso-ciatedwiththisregionofextension.

Contractionaldeformation in theEarlyAmazonianresultedinwidelyspaced,gentlyexpressedridgesasseeninMOLAdatabutgenerallywasnotdetectedinVikingandTHEMISimagesmarkingtheVastitasBorealismar-ginalandinteriorunits(ABvmandABvi),Elysiumriseunit (AHEe),ChrysePlanitia3and4units (HCc3 andHCc4), and surfaces scouredbyKasei andAresValles(Tanakaandothers,1991,2003a).OtherHesperianandNoachian surfaces likely experienced modest Amazo-niancontraction inplaces.Mostof the ridges inVasti-tasBorealishavesubduedorsymmetriccross-sectionalprofiles and generally lack crenulations. The subdued formsinVastitasBorealismaybeduetotheirdevelop-mentinweaksurfacerocks.Alternatively,ThomsonandHead(2001)andHeadandothers(2002)suggestedthatthewrinkleridgesinVastitasBorealisaresubduedfrommantlingbytheVastitasBorealisunitsandactuallyareformedonlyduringtheEarlyHesperian.

The northern plains, especially theVastitas Borea-lis units, provide a test area for northern plains crater-inganalysisbecauseof theirapparentuniformageandexpansiveness. As mentioned earlier, degraded cratersoccurwithinVastitasBorealis andotheroldernorthernplains units. In contrast, younger volcanic materials,includingTinjarVallesandAmazonisPlanitiaunits,donotdisplaytheimpressionsofcompletelyburiedcraters(fig. 2), which indicates a lack of differential compac-tionwithin thevolcanicunits.The largestcratersuper-posed on the Vastitas Borealis units is the double-ringcraterLyot,withconcentricrimsabout100and200kmindiameterandanEarlyAmazoniancraterdensity(N(5)=51±9).Itexhibitsafewchannelsdissectingitsejecta.TheVastitasBorealisunitsdisplayasteepsize-frequencydistributionofcraterssuchthattheN(5)craterdensityisabout ten times that of the N(16) density (table 1), reflect-ingan~D–2size-frequencyrelation.However,Hesperianunits such as the Lunae Planum and Utopia Planitia 1(HNTl and HBu1) units are consistent with shallowerpower-lawfunctions(slopesof–1.3to–1.7).Finally,theVastitasBorealisinteriorunit(ABvi)includesnumerouspedestal craters (fig. 2), which indicate that aeolian man-tlesprobablyhavecomeandgonewithinthelowerpartsofthenorthernlowlands.Thesemantlesmayberelatedto more active climate regimes and (or) availability ofeasilymobilizeddust.

MIDDLEAMAZONIANEPOCH

Relatively dense systems of crosscutting, mostlyasymmetric ridges developed across Isidis Planitia fol-lowingformationoftheIsidisPlanitiaunit(AIi)perhapsin response to sediment loading.Like those inVastitasBorealis,theseridgesystemshaveasubtletopographic

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expression. Also, linear, north-trending wrinkle ridgesdeformed flows of Amazonis Planitia 1 south and north units(AHAa1sandAAa1n).ThesamesystemofridgesappearswithinAmazonisPlanitia2northunit(AAa2n)and likely is embayedby that unit. Systemsof closelyspaced fractures (too small tomapat this scale), someforming conjugate sets, formed within the MedusaeFossaeunit;thesefeaturesmaybeerodedtensionjointsandshearfractures(TanakaandGolombek,1989).

Debris aprons of the Deuteronilus Mensae 2 unit(ABd2)thatformedaroundhigh-standingmesasofDeu-teronilus Mensae display flow structures that indicate ice-lubricated mass flow similar to that of rock glaciers. Theicemayhavebeeninthesubsurface,indicatingthattheupper1to2kmoftheNoachianmaterialsinthisareaareicerich(MangoldandAllemand,2001).ThisactivitymayhavebegunpriortotheMiddleAmazonianafterthemensaeformedintheLateHesperianandappearstohavecontinuedintotheLateAmazonian.

The ridge and trough structures of the OlympusMonsaureolematerial(LycusSulciunit,ATl)mayhavedevelopedasaresultoflandslidingorgravityspreadingof the lower flanks of the Olympus Mons shield (Lopes and others, 1980; Francis and Wadge, 1983; Tanaka,1985;McGovernandothers,2004).

The Scandia region unit (ABs) is deeply eroded;much of the erosion occurred sometime between unitemplacement in the EarlyAmazonian and its mantlingbyPlanumBoreum1and2units(ABb1andABb2)intheLateAmazonian.Thiserosion,whichappearstobeongo-ing,producesthedarkdunesoftheOlympiaUndaeunit(ABo;MalinandEdgett,2001;ByrneandMurray,2002;Wyattandothers,2004;FishbaughandHead,2005)

LATEAMAZONIANEPOCH

InsouthernAmazonisPlanitia,aconjugatesystemofnortheast-trendingscarpsandnorthwest-trendingridgesdeformtheAmazonisPlanitia2southandArcadiaPlani-tiaunits(AAa2sandHAa)andmaybestrike-slipfaultsand contractional wrinkle ridges, respectively (Tanakaandothers,2003b).CerberusFossaepropagatedthroughthe Elysium rise unit (AHEe), as both aqueous floods andlavasweredischargedfromsomelocationsalongthefracturesystem(Burrandothers,2002a,b).

TheScandiaregionandthePlanumBoreum1and2units(ABs,ABb1,andABb2)makingupPlanumBoreumunderwent significant erosion, resulting in local, steep scarps. Some investigators proposed that glacial-like flow of Planum Boreum (Clifford, 1987; Fisher, 2000) andsubglacial melt-water discharges from beneath PlanumBoreum(DialandDohm,1994;Benitoandothers,1997;FishbaughandHead,2002)createdthistopography,butothers suggest that suchprocessesdidnotoccur (KolbandTanaka,2001;MalinandEdgett,2001).

GeoLoGIC sYntHesIsThegeologichistoryofthenorthernplainsofMars

based on this geologic map must be regarded as pro-visional and interpretive, given uncertainties in rockcompositions and formational processes, character andtiming of landform modification, and unit and feature associations.

PRE-NOACHIANANDNOACHIAN

Thenorthernplainsbasinformedbyimpactand(or)tectonicprocessesduringheavybombardmentfollowingthe solidification of the planet’s crust. Next, large impacts produced quasi-circular depressions, including Utopiabasin,incrustalrocksunderlyingtheoldestexposedNoa-chianrocks.DuringtheNoachian,waningbombardmentproducedimpactstructures,includingIsidisbasin,insur-facerocks.Noachianrocksincludeamixtureofimpactbrecciaandmelt,volcanicrocks,andclasticsedimentaryrocksofaeolian,alluvial,mass-wasting,andotherunde-terminedorigins.Theimpactedcrustdevelopedaporous,fractured,andpoorlysortedmegaregolithinwhichicesand liquidswereemplacedbyvariousmeans.Muchofthevolcanicandpossiblyintrusivebuild-upofthebroadTharsis rise took place. Large impacts and early tecto-nism, perhaps Tharsis related, resulted in local upliftsandformationofmassifs.Relaxationof thelithospheresurroundingtheIsidisimpactledtodevelopmentofNiliFossae, and rifting radial to Syria Planum (outside ofthemaparea)producedTempeFossae.Planetarycool-ingresultedincontractionaldeformationinmostareas.Toward the end of the Noachian, sapping and surfacerunoff produced valley networks, likely fed by pre-cipitationofrainand(or)snowand,perhaps,sustainedby hydrothermal activity. Along the highland/lowlandboundary and across the Elysium-Arcadia-Amazonisplains, wholesale mass-wasting and subsurface volatilemovementsledtofracturing,collapse,disintegrationofNoachianrocksintomesasandknobs,andemplacementof mass flows and plains deposits. Potentially, significant dissection along outflow channels and highland valleys fromtheTharsisriseandnorthwesternArabiaTerramayhaveoccurredatthistime,butevidenceforthisisobscureduetolaterresurfacing.

HESPERIAN

Volcanic buildup of the Tharsis rise, formation ofTempe Fossae, widespread contractional deformation,and highland boundary degradation continued fromtheNoachianinto theHesperian.Volcanismalso ledtodevelopmentoftheElysiumrise,SyrtisMajorPlanum,and emplacement of lava flows throughout much of Arcadia andAmazonis Planitiae, generally after initialhighlandboundarydegradationceased.DuringtheLateHesperian,anepisodeofwidespreaderosionofthehigh-

19

land boundary resulted in (1) collapse and mass-wast-ingofEarlyHesperianlowlandandNoachianhighlandmaterial along margins of Utopia, Isidis, and ElysiumPlanitiae,(2)frettedterrainformationintheDeuteronilusMensae region, and (3) outflow channel dissection and chaos development on the eastern flank of the Tharsis rise and across Margaritifer Terra and Chryse Planitia.Eachof theseerosionalprocesses likely involved largeamountsofwaterand,perhaps,somecarbondioxidetoenablecollapse,disintegration,and transportofsurfacematerials; also,magmatic and impact eventsmayhavetriggeredtheactivity.Detailsofthedevelopmentofout-flow channel and chaotic terrain are uncertain, because muchoftheevidenceforearlierphasesofactivitylikelyhasbeendestroyedorburiedbyyoungeractivity.

AMAZONIAN

TheAmazonianbeganwithformationoftheVasti-tasBorealisunits.Theexposedmarginoftheunitsisatrelatively constant elevation, supporting the hypothesisthattheunitsresultedfromemplacementwithinabodyofwater.BecausetheVastitasBorealisunitsembaytheChrysechannelsystemandAcidaliaMensaat lowele-vationwithinAcidaliaPlanitiaandhavea lowercraterdensitythanChrysePlanitiaunits, theVastitasBorealisunits may have resulted from periglacial reworking ofplains deposits and (or) spring discharges from withinthe northern plains, rather than or in addition to outflow-channeldischarges.Suchactivityproducedmarginalandinterior troughandpittedconefeatures,aswellascol-lapsefeatures,inAcidaliaandCydoniaandintheScan-dia region unit. Fine particles produced by weatheringanderosionoftheScandiaregionandVastitasBorealisunitsand,perhaps,othermaterialsledtotheformationofdispersedpedestalcraters,anextantmantleinnorthwest-ern Utopia Planitia, and the basal platform of PlanumBoreum. Renewed volcanism at Syrtis Major Planummayhavebeen responsible for thegenerationofmate-rialcoveringIsidisPlanitia,whichunderwentreworkingandcontractionaldeformationsimilartothatseenintheVastitasBorealisunits.

The graben formation at Tantalus Fossae and theimpactofLyotcraterdeeplyfracturedthecrustalongthehighland/lowland boundary without significant associ-atedvolatiledischarges.However,dischargesonthewestflank of the Elysium rise led to collapse, which formed ElysiumFossae,carvedchannels,anddepositedvolcani-clastic flows in Utopia Planitia. Likely, magma/volatile interactionswereinvolved.Shieldvolcanismwanedonthe Tharsis and Elysium rises, but large flow fields were emplacedontoadjacentUtopia,Elysium,andAmazonisPlanitiaeinseparatepulsesofactivity.Episodicdischargesfrom fracture-controlled depressions in the lowlandssurrounding the Elysium rise carved lower HephaestusFossae, HebrusValles, MarteVallis, and other channel

systemsnearCerberusFossae.Along the southmarginofElysiumPlanitia,theMedusaeFossaeunitunderwentdeformationandreworking,perhapsrelatedtonear-sur-facevolatileactivity.Ice-andpossiblywater-lubricatedslope movements formed the Olympus Mons aureoledeposits anddebris aprons in theDeuteronilusMensaeregion. Some of the youngest fluvial discharges on Mars, perhapsjustmillionsofyearsold,originatedfromCer-berus Fossae and coincided with pulses of volcanismfrom the fossae and nearby vents. Additional, younglava flows erupted from vents in southeastern Amazonis Planitiaandweresubsequentlyfaultedbyconjugatecon-tractionaland,possibly,strike-slipstructures.ErosionofdarkbasalmaterialofPlanumBoreumformedthedark,circumpolardunes.WatericeanddustwererhythmicallylayeredatopPlanumBoreum,accumulatingtohundredsofmetersandmoreinthickness,andweresubsequentlyerodedoverthepastfewmillionsofyears.Severaladdi-tional layers of dust and the residual ice cap that mayrepresent polar dust and ice deposition during the pastfewmillionyearsnowcoatmuchofPlanumBoreumandpatchesofsurroundingterrain.

ACKnoWLeDGMentsWewishtothankMaryChapman(U.S.Geological

Survey) and Taylor Joyal andAlisa Wenker (NorthernArizonaUniversity)forassistancewithpreliminarygeo-logicmappingofthestudyregion.JennyBlue(U.S.Geo-logicalSurvey)andtheMarsTaskGroupoftheWorkingGroupforPlanetarySystemNomenclature(InternationalAstronomical Union) helped with updating geographicnomenclature to adapt to new topographic and imageobservations. Nadine Barlow (Northern Arizona Uni-versity) provided updated versions of her Mars craterdatabasetoassistwithourcratercounting.JamesDohm(UniversityofArizona)andLeslieBleamaster(PlanetaryScience Institute) provided insightful comments thatimproved, in particular, the map text. Jan Zigler (U.S.GeologicalSurvey)editedthemapandhelpedusincor-porateformatandstylestandardsusedinterrestrialgeo-logicmaps.

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table 1. Descriptions of map units in the northern plains of Mars: areas, crater densities,and superposition relations.

Unitname Unitlabel Area1 N(5)2 N(16)2 Superpositionrelations3

(106km2)

Amazonis province

MedusaeFossaeunit AAm 0.90 42.1±6.8 5.5±2.5 <HNn,HBu1,HBu2,HAa,AHc, AHAa1s,ATl,AEc3;~AAa2s; >AEc1, AEc3AmazonisPlanitia2northunit AAa2n 0.63 14.3±4.84 --- <HNn,HAa,AHAa1s,AAa1n,ATl; ~AAa2s;>AEc3AmazonisPlanitia2southunit AAa2s 0.45 18.0±6.44 --- <AHc,AHAa1s,ATl;~AAm,AAa2nAmazonisPlanitia1northunit AAa1n 1.37 21.1±3.9 2.9±1.5 <Nn,HNn,HAa,ABvi,ABvm; >AAa2n,ATlAmazonisPlanitia1southunit AHAa1s 0.60 73.1±11.0 --- <HNn,HAa;>AAa2s,AAa2n,AAmArcadiaPlanitiaunit HAa 1.27 137.4±10.4 21.3±4.1 <Nn,HNn;>AHc,AHEe,AHAa1s, AAa1n,AAa2n,ABvi,ABvm, AHAa1s,AAa2s,AAm,AEc2,AEc3

elysium province

CerberusFossae3unit AEc3 0.93 31.2±5.84 --- <Nl,Nn,HNn,HBu1,HBu2,HAa, AHEe,AEc1,AEc2,AAm,AAa2n; <AAmCerberusFossae2unit AEc2 0.42 81.6±14.04 24.0±7.6 <Nn,HNn,HAa,AHc,AHEe,AEc1; >AEc3CerberusFossae1unit AEc1 0.04 89.7±44.8 --- <AAm;>AEc2,AEc3TinjarVallesbunit AEtb 0.08 --- --- <HBu2,AHEe,AHc,ABvi,AEta; ~AEtaTinjarVallesaunit AEta 1.32 75.7±7.64 8.3±2.54 <AHc,AHEe,ABvi,ABvm;~AEtb; ~AEtbElysiumriseunit AHEe 3.47 92.7±5.2 10.7±1.8 <Nn,HNn,HBu1,HAa,ABvi;>HBu2, ABvm,AEc2,AEc3,AEta,AEtb

tharsis province

LycusSulciunit ATl 0.41 --- --- <Nn,AAa1n;>AAm,AAa2s,AAa2nAlbaPateraunit HTa 0.95 76.7±9.0 14.7±3.9 <Nn,HNn;>ABs,ABvmLunaePlanumunit HNTl 0.27 165.5±25.0 41.4±12.5 <Nl,Nn;~Nn;>HCc2,HCc3,AHc

Isidis province

IsidisPlanitiaunit AIi 0.64 59.5±9.7 --- <HNn,HBu1,HBu2,HIs;>AHcSyrtisMajorPlanumunit HIs 0.27 120.6±21.0 14.6±7.3 <Nl,Nn,HNn;>AIiAmenthesPlanumunit HIs 0.04 167.0±63.1 --- <Nl,Nn,HNn;~HBu1

Chryse province

SimudVallisunit HCs 0.35 93.9±16.3 28.4±9.0 <Nl,Nn,HNCc1,~HCc4;>AHc,AHcfChrysePlanitia4unit HCc4 0.34 93.2±16.5 --- <Nl,Nn,HNCc1,HCa,HCc3;~HCc3, HCs;>ABvm,AHcChrysePlanitia3unit HCc3 0.57 86.7±12.4 7.1±3.5 <Nl,Nn,HNTl,HNCc1,HCa,HCc2; ~HCs,HCc4;>ABvm,HCc4,AHcAresVallisunit HCa 0.11 52.7±21.5 --- <Nl,Nn,HNCc1,HCc2;>HCc3, HCc4,ABvmChrysePlanitia2unit HCc2 0.49 120.4±15.7 18.4±6.1 <Nl,Nn,HNCc1;>HCc3,HCc4,AHc, ABvmChrysePlanitia1unit HNCc1 0.43 227.3±23.0 46.4±10.4 <Nl,Nn;>HCc2,HCc3,HCc4,HCs, AHc

27

table 1. Descriptions of map units in the northern plains of Mars: areas, crater densities,and superposition relations—continued

Unitname Unitlabel Area1 N(5)2 N(16)2 Superpositionrelations3

(106km2)

Borealis province

OlympiaUndaeunit ABo 0.68 --- --- <ABvi,ABs,ABb1,ABb2;>ABb2PlanumBoreum2unit ABb2 0.88 --- --- <ABb1,ABo,ABs,ABvm,AHc;>ABoPlanumBoreum1unit ABb1 0.09 --- --- <ABvi,ABs;>ABb2,ABoAstapusCollesunit ABa 0.24 63.3±16.34 --- <HBu1,HBu2,ABvi,ABvmDeuteronilusMensae2unit ABd2 0.21 117.0±23.94 --- <Nn,HNn,HBd1,AHc,ABvmScandiaregionunit ABs 1.95 79.2±6.4 10.8±2.4 <ABvi,ABvm;>ABb1,ABb2,ABo, AHcVastitasBorealismarginalunit ABvm 2.19 63.0±5.4 7.3±1.8 <Nn,HNn,HNCc1,HBu1,HBu2, HBd1,HCc2,HAa,AHc,HCc3, HCc4,HTa,~ABvi;>AHEe,ABs, ABd2,ABa,AHc,AAa1n,AEtaVastitasBorealisinteriorunit ABvi 15.49 74.5±2.2 7.0±0.7 <Nn,HNn,HBu1,HBu2,AHc,AHEe, HCc2,~ABvm;>AHc,ABa,AAa1n, AEta,AEtb,ABb1,ABb2,ABs,ABoUtopiaPlanitia2unit HBu2 1.19 91.9±8.8 11.8±3.2 <Nl,Nn,HNn,HBu1;~AHEe;>AHc, AHEe,ABvi,ABvm,ABa,AIi,AAm, AEtb,AEc2,AEc3UtopiaPlanitia1unit HBu1 1.61 164.9±10.1 22.9±3.8 <Nl,Nn;~HNn,HIa;>HBu2,AHc, ABvi,ABvm,AHEe,AIi,ABa,AAm, AEc3DeuteronilusMensae1unit HBd1 0.14 176.1±36.0 51.4±19.4 <Nn;>AHc,ABvi,ABvm,ABd2

Widespread materials

Craterunit AHc 2.28 --- --- <Nl,Nn,HNn,HNTl,HNCc1,HBu1, HBu2,HTa,HBd1,HAa,HCc2, HCc3,HCc4,HCs,AHEe,ABvi, ABvm,ABs,HIs,AIi,AAm,AHAa1s; >AEta,AEtb,AAm,AAa2s,AAa2n, AEc3,ABb2,ABd2Crater floor unit AHcf 0.05 --- --- <Nl,Nn,HCsNepenthesMensaeunit HNn 2.80 261.8±9.7 77.9±5.3 <Nl,Nn;~Nn;>HIa,HBu1,HBu2, HBd1,HTa,HIs,AHEe,AHc,AIi, ABvi,ABvm,ABd2,AEc2,AEc3, HAa,AHAa1s,AAa1n,AAa2n,AAmNoachisTerraunit Nn 3.87 469.3±11.0 144.5±6.1 <Nl;~Nl,HNn;>HNTl,HNCc1, HBu1,HBd1,HIa,HTa,HIs,HCa, HCc2,HCc3,HCc4,HCs,AHEe, AHc,AHcf,ABs,AAa1n,ABd2, ABvi,ABvm,AEc2,AEc3,ATlLibyaMontesunit Nl 1.6546 413.4±15.8 146.9±9.4 ~Nn;>Nn,HNn,HNTl,HNCc1, HBu1,HBu2,HIa,HIs,HCs,HCa, HCc2,HCc3,HCc4,AHc,HAa, AEc31Areaincludesunitandcraters(AHc)occurringwithinunit.2N(x)=totalnumberofcraters>xkmindiameterper106km2;---,nodatashownwhere<4craterscountedandforcraterunits.Craterlocationsand

sizesprovidedbyN.G.Barlow(writtencommun.,2004).3<,youngerthan;~,overlapsintimewith;and>,olderthan.4Crater count includes significant number of embayed or buried craters within unit.