Sedimentary basins and global tectoniest

Harold G. Reading

READING, H. G. 1982. Sedimentary basins and global tectonics Proc. Geol. Ass., 93 (4)321-350. Concepts of geosynclines developed from the study of ancient orogenic belts andsedimentary basins. Now, however, geophysical data, derived mainly from oceanic areas andcontinental margins, have led to the theory of plate tectonics which has largely replacedgeosynclinal theory as the basis for understanding orogenic belts and sedimentary basins. Avariety of basins can now be distinguished which have developed (1) on continental crust eitheras large downwarps, such as the Chad basin, or as rift basins following long-Jived fault systems,such as the East African rift, (2) in association with ocean-floor spreading, as newly formedrifts, such as the Red Sea, at mid-oceanic spreading centres, as failed rifts, such as the Benuetrough, and at continental margins either rifted or transform, (3) at subduction zones either astrenches, outer-arc, slope or back-arc-basins, the best examples being along the Indonesianmargin, off New Zealand and Japan, (4) at collision zones such as the Himalayas (5) alongtransform/strike-slip belts, such as the onland and offshore Californian basins and the DeadSea. In addition, thick sedimentary accumulations may form on oceanic crust as largesubmarine fans, such as the Bengal Fan, as sedimentary swells due to thermohaline currents,such as the Outer Ridge off the Blake Plateau, and as tectonically accreted wedges in fore-arcregions. Postulated examples of such features in the British Isles include the rifted margins ofthe Iapetus ocean and present Atlantic, the Southern Uplands accretionary prism, MidlandValley fore-arc basin becoming a series of strike-slip basins in Devonian times, the LakeDistrict island arc and Welsh back-arc basin with sag basins and rift basins in the North Sea.The old geosynclinal terminology can now usually be abandoned provided it is realized thatmany sedimentary basins fall into more than one category and that interpretations of theancient are never more than working hypotheses.

Department of Geology and Mineralogy, University of Oxford, Parks Road, Oxford, UK

1. INTRODUCTION

Of all the factors that control sedimentation, tectonicsis the most fundamental. Directly it controlssedimentary thicknesses, facies and sequences, andfacies patterns. Indirectly it influences local climate,changes of sea-level, oceanic circulation and thechemistry and composition of sedimentary sourcematerial. Conversely sedimentation-and erosion­influence tectonic movements.

2. GEOSYNCLINES

The intimate relationship of tectonics to sedimentationwas recognised in the 19th Century in the con­troversies on the origin of geosynclines. Hall (1859)argued that geosynclinal subsidence was the result ofsedimentation while Dana (1873) considered thatlateral compression (i.e. tectonics) produced both thesubsidence of the 'geosynclinal belt' and the subse­quent orogeny. Another controversy which dominatedgeosynclinal theory was whether geosynclines wereasymmetrical and formed at continental margins pri­marily on continental crust, or were symmetrical andpartly oceanic. Most Americans, as they lookedoceanward from an older central craton towards suc­cessively younger orogenic belts, argued for asymmet­rical continental accretion. Conversely Europeans,

t Special Invited Lecture.

starting in the middle of the Alps and recognising theoceanic nature of radiolarian cherts and ophiolites,generally thought of geosynclines as being symmetricaland developing on oceanic crust (Fig. 1).

Geosynclinal theory developed almost entirely bythe study of older, now uplifted, mountain belts sincethe structure of present-day sedimentary basins wasquite unknown except for the limited gravity dataacquired by Vening Meinesz in the 1920's for theIndonesian arc and trench. Modern Indonesian basinswere described in geosynclinal terminology rather thangeosynclines being interpreted by analogies with mod­ern basins. For example, Aubouin (1965) fitted theIndonesian Sunda Arc into a model based onMediterranean Alpine chains. Admittedly analogieswere sometimes drawn with the Gulf of Mexico bysome Americans and with the Persian Gulf and theJava Trench by some Europeans, but it was not untilthe advent of seismic data that the structure of moderncontinental margins and offshore sedimentary basinscould be ascertained and they could be used as analo­gies for ancient geosynclines rather than the other wayround. The first people to do so were Drake, Ewing &Sutton (1959), who compared the Appalachiangeosyncline with the east coast of North America (Fig.2) an analogy which was given further stimulus by theconcept of ocean-floor spreading as the cause of thepresent Atlantic (Dietz & Holden, 1966). As theconcept of ocean-floor spreading developed into that

321

322 H. G. READING

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SEDIMENTARY BASINS AND GLOBAL TECTONICS 323

of plate tectonics and a firm basis grew for understand­ing modern oceanic basins and continental margins aspate of publications appeared (e.g. Mitchell & Read­ing, 1969; Dewey & Bird, 1970; Dickinson, 1971) inwhich ancient geosynclines and sedimentary basinswere fitted into a 'modern' framework. Mitchell &Reading developed an existing terminology which dis­tinguished between an Atlantic-type (tectonically inac­tive or passive) continental margin and a Pacific-type(tectonically active) continental margin into one basedon plate tectonics. They divided continental marginsinto three principal types: (1) Atlantic-type, wherecontinents ride on and are coupled to oceanic crust,(2) Andean-type, where ocean floor descends beneatha continent, and (3) Island-are-type, where oceaniccrust descends beneath oceanic crust. These threetypes of margin, together with the small ocean basinslying behind some island arcs (referred to as 'Japansea-type'), and small ocean basins between continents(e.g. the Mediterranean), were used as models forgeosynclines. Mitcheli & Reading also suggested thatthere were three types of orogeny: Andean and islandarc-type where ocean floor is going under continentand ocean respectively and Himalayan-type where twocontinents are colliding. Modifications and extensionsof this scheme have been developed during the 1970s.

3. PLATE TECTONICS

It is now possible to distinguish several types of basindepending on whether they are found within lithos­pheric plates (intraplate settings) or near the marginsof plates (interplate settings). Intraplate basins mayoccur either on continental crust or on oceanic crust.Interplate basins may occur at (1) divergent margins,where ocean floor is being created, (2) convergentmargins, where lithosphere is being subducted orwhere two continents are colliding, and (3) conserva­tive margins, where lithosphere is neither being cre­ated nor destroyed and where motion between the twoplates is transform or strike-slip. These divisionsshould not be drawn too rigidly since many areas areundergoing a combination of both extension andstrike-slip (e.g. Gulf of California) or of both converg­ence and strike-slip (e.g. Burma-Indonesia subductioncomplex and the North Island of New Zealand). Inaddition, it is in places impossible to ascertain whetherwe are dealing with intraplate or interplate phe­nomena because we cannot define all plate marginsespecially where, as in Iran and Afghanistan, thelithosphere is fragmented into so many microplates.With these reservations in mind a range ofsedimentary basins within the context of plate tecto­nics can be described.

(a) Basins on continental crust

Most of the sediment deposited on continents isephemeral, temporarily at rest on its way between

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Fig. 3. Map of West Africa to show (1) the large gentle sagbasins, such as the Chad basin (Fig. 4), which develop oncontinental crust, (2) the Benue trough, a failed rift oraulacogen of Cretaceous age (Fig. 7) and (3) the subsequentmiogeoclinal Tertiary Niger delta (Fig. 8) (based on Petters,1978). Depth to basin floor in metres.

denuding mountains and the sea. However, the pre­sence of huge accumulations of Old Red and New RedSandstone, and of similar 'molasse' in many parts ofthe world, testifies to the ability of much of thissediment to collect on continents in sedimentary orclastic traps. These are essentially tectonically con­trolled.

Some of these traps or basins are relatively slowlysubsiding broad sags on the earth's crust, such as thoseoff West Africa (Fig. 3). The Chad basin (Fig. 4) hasan area of about 600,000 km2 and has subsided atabout 2 cm/lOOO years (20 m/my) (Burke, 1976). Thesubsidence of such a basin is controlled not only bydeep-seated plate motion but also by the weight ofsediment and/or water, which varies according to ero­sion rates of the surrounding highlands and the rainfallof the area. Another basin of this type is the Michiganbasin where 3.5 km of sediment accumulated in about150 Ma during the Palaeozoic, with a sedimentation

324 H. G. READING

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Fig. 5. East African rift system showing relationship of lakesto rift tectonics (after King, 1970). The location of most ofthe lakes is governed by faults of the rift system and byvolcanoes (e.g. Lake Kivu). Lake Victoria contrasts in beinga broad shallow sag basin between the two main rifts. Drain­age is mostly away from the rift valleys to that they arestarved of clastic sediment. Sedimentation is biologically andchemically controlled. Most of the lakes are fresh (i.e. have asalinity < 5%0 of dissolved salts). Some are alkaline salinelakes (e.g. Lakes Magadi and Natron). Depths range fromabout 100 m for Lakes Albert and Rudolf to over 1400 m forLakes Tanganyika and Kivu (Beadle, 1974).

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Some of these large sag basins, such as the Michiganand Hudson Bay basins, lay under platform seas dur­ing deposition. Others were filled by fresh water andmay have been centres for internal drainage. A char­acteristic feature of the latter is the very wide fluctua­tions in area due to climatic changes; Lake Chad hasfluctuated between 25 and 10,000 km2 during the pre­sent century and has extended over at least 300,000km2 in the last 10,000 years (Servant & Servant, 1970).

Of a very different nature, and much smaller, arethe rift basins, such as those in the East African rifts,the Baikal rift and the Rhinegraben, Although theRhine now flows along the last, rift valleys are general-

Fig. 4. The Chad Basin. Map shows extent of present LakeChad, extent of Lake Megachad about 10,000 years ago, andthe area of the drainage basin (outlined by dashes). Stippledareas are peripheral uplifts. The cross-section shows theextent of Lake Megachad and thickness of sediment fill (afterBurke, 1976).

SEDIMENTARY BASINS AND GLOBAL TECTONICS 325

ly starved of sediment because the marginal lip is thehighest topographic feature and sediment is carriedaway from the graben itself. Sediment-starved basinsbecome lakes and hence lakes, both freshwater andsaline, are a characteristic feature of rifts, particularlywhere the rift is broken up by cross-faults, by offset­ting of the main rift-margin faults, or by volcanoes, asin the East African rift (Fig. 5). Sedimentary thick­nesses up to 2 km are known beneath some lakes inthe East African rift system; in the Baikal rift they areup to 5 km thick, and the present lake bottom is over1,600 m below sea-level.

The origin of these rifts is controversial. In the earlydays of plate tectonics the East African and Baikalrifts were taken as examples of the early stages ofsea-floor spreading before horizontal separation hadbegun (e.g. Drake, 1972). The great age of the EastAfrican rift and recent geological and geophysicalobservations suggest it can equally well be consideredto be a 5,000 km long taphrogenic lineament affectingthe whole of the lithosphere, dating from at least theArchaean 2,500 Ma ago, and which has been reacti­vated by later thermo-tectonic events (McConnell,1980). It is one of the major long-lived fundamentalsystems (Reading, 1980) which may form extensivedislocation zones in the Precambrian (Sutton &Watson, 1974; Watson, 1980). Many of these areconfined to continental crust and have nothing todo with ocean-floor spreading. The Baikal rift hasbeen looked upon as a result of early spreading. Ittoo is probably better interpreted as a purely continen­tal feature either resulting from the stresses induced bythe continental collision of the Himalayas (Molnar &Tapponnier, 1975), or by forces directly beneath therift, possibly in the mantle (Bahat, 1981). Similarly,

movements in the Rhinegraben may reflect stressesgenerated in the Alps. (lilies & Greiner, 1979).

(b) Basins associated with ocean-floor spreading,failed rifts and Atlantic-type continental margins

The clearest examples of basins associated with theearly stages of ocean-floor spreading are the Red Seaand Gulf of Aden. In these cases, simple extension,with thermal doming demonstrated by the initial out­pourings of basaltic volcanics, appears to have beenresponsible for the rifting. Being in a semi-arid tropic­al climate, evaporites are a dominant feature of thesediments. Within the basins, which develop on tiltedfault blocks (Fig. 6), alluvial fans and volcanics arealso prominent. Structurally similar basins formed inMesozoic times around the Atlantic, but some of thosedeveloped at higher latitudes in a humid climate;submarine fans were the main depositional environ­ment (Surlyk, 1978).

At oceanic spreading centres, particularly the slowlyspreading and highly fractured ones such as the Atlan­tic, there are many small basins acting as sedimenttraps parallel to the ridge crest. The lack of subaerialsource areas, apart from the occasional volcanic is­land, results in a relative lack of available sediment tofill these basins, nevertheless some basins only 100 kmfrom the rift have received 500 m of fine-grained turbi­dites (Van Andel & Komar, 1969). In addition, thereare basins along fracture zones transverse to the ridgesand yet smaller ones trending diagonally to the frac­ture zones (Figs. 24 & 25). As the ocean ridge sub­sides, during cooling of the newly created oceaniclithosphere, continentally-derived turbidites tend tooverlap the older fractured basins on to the highs, thus

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Fig. 6. Section across the Red Sea and Danakil depression showing how Mesozoic sandstones and limestones, which thickento the seaward, were block faulted and tilted landward in Tertiary times as the present Red Sea began to form. Fissure basaltsformed at the intersection of horsts and graben and up to 3000 m of volcanics and volcaniclastic rocks accumulated on theflanks of the Danakil Alps. These pass laterally into marine pyroclastics and then evaporites, which are mainly halite, in theOligo-Miocene of the eastern graben and gypsiferous in the Pliocene to Recent of the Danakil depression (after Hutchinson &Engels, 1970).

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101-..-:::::;;;;...;...:..-.....:..;.;"'--- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ---'Fig. 7. Cross-section through the Benue Trough failed rift or aulacogen and the Tertiary Niger delta (after Burke et al., 1972;Petters, 1978). Major folding in Santonian times, with folds parallel to the axis of the rift and over 2 km of sediment removed,separates the Cretaceous aulacogen phase from the Cretaceous to Present miogeoclinal phase.

o km 100

Fig. 8. Schematic cross-section of the miogeoclinal TertiaryNiger Delta. The present day facies changes from deep waterpelagic sediments passing landwards into submarine fans,slope shales with diapirs, deltaic sandstones and shales, tocoarser fluvial and interfluvial sediments are represented inthe 1()""12 km thick vertical succession of the trough. Theopen arrows indicate that there is seaward flowage of shales,producing diapirs, and downward sinking of deltaic sedimentsdue to the overburden of delta top and fluvial sediments(after Weber, 1971; Burke, 1972).

smoothing out the rugged oceanic topography andforming abyssal plain basins of very wide extent but nogreat thickness (Fig. 9).

At the margins of the Atlantic there are rifts whichbegan, probably at triple junctions, as the Atlanticopened, but where spreading and opening haveceased. These basins are the so-called failed rifts,sometimes referred to as aulacogens after the sup­posed ancient equivalents found in the USSR andCanada (Burke, 1977). The best known is the Benuetrough (Figs. 3 & 7) where a 1,000 km long and100 km wide trough of Cretaceous age, striking into

SWTIME - STRATIGRAPHICLINE

NECONTI NENTAL CRETACEOUS RISESANOSTONES OEPOSITS

CONTINENTALCRUST

the African continent from the NE shoulder of SouthAmerica (Burke, Dessauvagie & Whiteman, 1972),collected 5 km of fluvial, deltaic and marine sedi­ments. Seaward of the aulacogen a Tertiary delta builtout as the Atlantic opened to give a further 12 kmthick sedimentary succession (Fig. 8). The Tertiarysediments consist of sandstones, deposited in sub­marine fans, underlain by a thin oceanic pelagic layerand overlain by thick marine shales. The latter aresucceeded by deltaic sandstones and siltstones whichare overlain by coarser fluvial deposits. In detail thereare complex patterns of interfingering fluvial and tidalchannel sandstones, beach/barrier sandstones,offshore and interftuvial muds. Petroleum is generatedin the marine shales and trapped by growth faults andtheir associated roll-over anticlines to accumulate invarious types of fluvial and deltaic reservoir sand­stones (Weber, 1971; Weber & Daukoru, 1975).

Continental margins are large asymmetrical down­warps-the original American miogeoclinal andeugeoclinal couplet of Dietz & Holden (1966). Under­lying the margins are extensional rift valleys filled bythick continental sediments in the early stages of rift­ing. These may develop into linear troughs parallelwith the continental margin and filled by as much as500,000 knr' of shallow marine sediments (Fig. 10) asthe margins pass from an extensional rifting(taphrogenic) stage to one of simple basinal subsi­dence and miogeoclinal progradation of sediments(Kent, 1977). Some continental margins are modifiedby salt diapirism. Seaward dipping sediments drapeover the older rocks and change laterally from shelfthrough slope to continental rise sediments. Rates ofsedimentation and subsidence on the shelf may have

S E D IM EN T A RY BAS INS A ND GLO BAL TECTONICS 327

CONTINENTAL SHELF CONTINENTAL RISE ABYSSAL PLAIN OCEANIC RIDGE

Fig. 9. Generalized cross-section across the western Atlantic (after Dewey & Bird, 1970). Notice in part icular the smalloceanic ridge basins , the broad onlapping turbidite-filled abyssal plains, the fine-grained sedimentary build-up of contouritesforming the Outer Ridge (and also much of the continental rise) , the thick continental rise sedimentary wedge and thecontinental shelf wedge overlying rift sed iments.

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Fig. 10. Schematic cross section through the Baltimore Canyon trough which underlies the present coasta l plain, continentalshelf and upper continental slope off the eastern United States. The trough is about 500 km long, parallel with the presentshoreline, and lOO-Zoo km wide , with up to 14 km of mainly Mesozoic sediments (contrast Fig. 9). Triassic continentalrift-valley sandsto nes are followed by parti ally synchronous marginal marine evapo rites and then very thick transgressiveJurassic shelf limestones with carbonate reef build-up s over highs. Shorewards there is passage into marginal marinesandstones . A major early Cre taceous prograding wedge is followed by a late Cretaceous transgression . The Tertiaryconfiguration is one of delta progradat ion with minor transgression , passing seaward into slope deposits of uncertain age andthickne ss (after Poag, ]979; Schlee , 1981).

328 H. G. READING

Fig. 11. Location and thickness of Neogene slumps off SWAfrica, a typical Atlantic-type passive margin. Slumps rangeto over 450 m in thickness.

risen to as much as 10 emil ,000 years in the Jurassic ofthe Baltimore Canyon trough, but generally are of theorder of 1-4 emil ,000 years (Grow, Mattick & Schlee,

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Fig. 13. Generalized model of a subaqueous outer continen­tal margin rotational slump (after Dingle, 1977, 1980).

PROXIMAL ! DIS TAL1

1979). The major sedimentary processes on the shelfare waves, storms and tidal currents, and some bioche­mical accumulation of carbonates, occasionally de­veloping into reefs, or phosphates. On the slope andrise mass-flow is probably the most importantsedimentary process, in particular turbidity currentswhen sea-level was low. However, downslope slidingon a variety of scales is only now being appreciated tothe full as an almost ubiquitous process on 'passive'margins (Embley & Jacobi, 1977). Some of theseslipped masses are up to 70,000 km2 in area and maycontain 17,000 krrr' sediment (Dingle, 1980) (Figs. 11,12 & B),

Turbidity currents are the main transporting anddepositing mechanism for the large deep-sea fanswhich build out at the foot of continental slopes onpassive margins. For example, the Laurentian Fan offNova Scotia (Stow, 1981), which is relatively elongateand composed of fine-grained sediment, extends600 km from the base of the slope out on to theabyssal plain. Approximately 2-3 km of sediment haveaccumulated since the Miocene (Uchupi & Austin,

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6Fig. 12. Cross section through continental margin off SW Africa (Line A-B on Fig. 11) showing a Neogene slump underlainby Cretaceous slumps and intervening unslumped sediments. Palaeogene slumps are also recognized on this margin but do notoccur in this section. the Cretaceous slumps are thought to represent Mississippi delta-type down-slope sediment cascades withreverse faulting and diapirism. They formed when the Orange River was bringing down abundant sediment, in contrast to theTertiary slumps, which formed when terrigenous sediment input was lower (after Dingle, 1980). Vertical scale in secondsbelow sea-level datum.

SEDIME NTARY BASINS AND G LO BAL TECTO NICS 329

1979) with sedimentation rates of IG-30 cm/1,000years (Stow , 1981).

In addition much fine-grained sediment is trans­ported in nepheloid layers-slow-moving suspensioncurrents-which carry silt and mud from the shelf outon to the continental slope and rise. Of very greatimportance in the North Atl antic as an erodin g, trans­porting and depositing agent is the thermohaline con­tour-following current , which, in the northern hemis­phere , flows anticlockwise and carries silt and finesand southwards off the eastern seaboard of NorthAmerica (Hollister & Heezen , 1972). These contourcurrents are not only important in the development ofthe continental slope and rise but also have producedbroad sedimentary swells out in the oceans such as theOuter Ridge off the Blake Plateau (Fig. 9) and theGreater Antilles Outer Ridge . The latter is 1,800 kmlong, 1lG-220 km wide and is composed of more than100,000 krrr' of sediment up to 700 m thick and depo­sited at rate s up to 30 cm/1,000 years , more than 10times that in the surrounding abyssal plains (Tucholke,1975) .

(c) Subduction-related basins

At convergent margins there are 5 areas wheresedimentary successions accumulate-on the down­going oceanic plate well out in the ocean, in thetrench, in small basins on the slope of the inner wall ofthe trench, in the outer-arc or fore-arc basin seawardof the volcanic arc and in the back-arc basins behindthe volcanic arc. In addition, apparently thicksedimentary successions may be tectonically stacked inthe accretionary prism of the fore-arc (Fig. 14).

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~~Fig . 14. Model of an obliquely subducting margin, e.g. NorthIsland of New Zealand; Sumatra (after Walcott , 1978; Lewis,1980). Movement of the trench and accretionary prism isessentially convergent with a very minor strike-slip compo­nent. Movement in the frontal ridge , volcanic arc and in theback-arc basin (not shown) is essentially by strike-slip (hereshown to be dextral) , with some extens ion . Thicksedimentary sequences may develop on the subducting plate(e.g. Bengal fan) , in the trench (e.g. off Oregon) , in slopebasins (e.g . New Zealand) , in the fore-arc basin (e.g. offSumatra) , within the volcanic arc or in the back-arc basins(e.g . Anadaman Sea). Tectonic thickening of the accretion­ary prism gives an apparently thick succession .

The largest sedimentary pile forming on oceaniccrust out in an ocean today is the Bengal Fan (Moore ,Curray, Raitt & Emmel, 1974). This extends 3,000 kminto the Indian Ocean from the Ganges-Brahm arutradelta. The area of the fan is about 3,000,000 krn ; thethickness of sediments ranges up to about 15 km withan average of 7.5 km so that the volume of fan sedi­ments is about 20,000,000 krrr' (Graham, Dickinson &Ingersoll, 1975), and this does not include the volumealready accreted (see below) . The fan sediment s havebeen deposited since continental collision betweenInd ia and Asia started about 55 Ma ago giving a rateof sedimentation of 2- 10 cmll ,OOOyears.

Trench sediments are generally not very thick andfew have successions more than 500 m unless they areclose to continental margins in humid regions , wherethey may be completely filled (e.g. off Oregon, offCentral America , in the Mediterranean). In the Aleu­tian trench, Piper , von Huene & Duncan (1973) haveshown that tren ch sedimentation is by lateral slumpingdown the wall of the trench and longitudinal turbiditycurrents flowing in a 2.5-6 km wide channel.

Accretionary prisms are tectonicall y thickenedwedges which occur where sediments depo sited on theocean floor or in trenches are scraped off as a series ofslices as the subduction zone migrates seawards andthe fore-arc stack gradually rises upward with succes­sive steepening of the thrust surfaces (Figs. 14 & 19).Although the highly folded strata in each slice youngtowards the volcanic are, individual slices becomeyounger towards the ocean becau se they are remnantsof progressively younger slices of oceanic sedimentsand ocean floor created at the spreading centre . Thebest know examples of accreti onary prisms are offCentral America (Seely, Vail & Walton, 1974), offOregon (Kulm & Fowler , 1974) and off Japan (Okada,1980) where they are submerged. Off the North Islandof New Zealand (Walcott, 1978; Lewis, 1980; van derLingen , 1982), along the Barbados Ridge (Westbrook,1975), off southern Alaska in the eastern Aleutians(Dickinson & Seely, 1979; von Huene, 1979) andalong the Arakan Yoma-Andaman-Nicobar-Mentawairidge (Karig, Lawrence, Moore & Curray, 1980)which stretches from Burma to Sumatra they arepartl y exposed on land .

Sitting upon the accretionary prisms are smallsedimentary basins known as accretionary basins(Dickinson & Seely, 1979) inner slope basins or justslope basins. Off the North Island of New Z ealandthey are 5-30 km wide, 1G-60 km long and contain upto 2-3 km of sediment (Lewis , 1980) (Fig. 15). OnNias Island , near Sumatra (Moore & Karig, 1976;Moore, Billman , Hehanussa & Karig, 1980) and inNew Zealand the sediments are largely terrigenouswith turbidites, some quite coarse-grained, hemipela­gic muds , and debris flows as well as volcanic ash (vander Lingen & Petting a, 1980; Lewis, 1980) (Fig. 16).Slumps and slides also occur , some having a volume of

330 H. G READING

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.. ' . . " :" ' . .....:.... .

40 ·8

3J ·.•.i ,.·.····>.~ : "

Fig. 15. Map of North Island of New Zealand (after Lewis, 1980; van der Lingen & Pettinga, 1980) showing volcanic arc andforearc region, the Tonga-Kerrnadec island arc to the north and the strike-slip fault belt at the northeastern end of the AlpineFault. Notice the inner slope sedimentary basins of probable late Pliocene to Quaternary age. Lines represent faults. Lines of'V's represent major submarine canyons. Shelf edge shown by large dots. Stippled areas are sedimentary basins. Miocenebasins are not shown (e.g. Makara basin where section in Fig. 16 is shown by cross).

8I

Middle and UpperMiocene sediments

kmI

Plio·Pleistocene

oI

50g~~~~~~~~~~~/~~~ "''-'<<1'=1000

1500

metres

Fig. 16. Cross section through the Miocene inner slope Makara basin, North Island, New Zealand (after van der Lingen &Pettinga, 1980). Isoclinally folded Mesozoic-Palaeogene 'basement' rocks form an imbricate landward-dipping thrust zone withassociated melanges. Neogene basins form between the ridges, the Makara basin shown here being 30 x 20 km in area andfilled by 2200 m of turbidites, pebbly mudstones, rhyolitic tuff beds and hemipelagic mudstones The central anticline isprobably a late feature.

SED IMENTAR Y BA SINS AND GLOBAL TECTONICS 331

Fig. 17. Stru ctural profile of the Japan trench inner slope offnorthern Honshu (after Okada, 1980). There is not a substan­tial accreti on ary prism and off-scraping of oceanic sediment sis on a small scale . Neogene sediments tend to drape theinner slope and slumping is a dominant process. Notice theextensional horst and graben structure under the lower innerslope and the imbricate thrust structure higher up . Verticalscale is in seconds below sea level.

8 krrr' (Lewis, 1971). In the Quaternary of New Zea­land , uplift of hills may be as much as 170 cml1,000years and subsidence of the basins as much as 150 eml1,000 year s so that there might be 3 km of differentialmovement in a million years. In this time sedimentscan be tilted 30° (Lewis, 1980) so that sedimentation isboth rapid and synchronous with deformation. Cal­culations made for Nias Island (Moore et al., 1980) oncompacted Miocene sediments in slope basins suggestsedimentation rates of 24-53 cmll ,OOO years, whichwould be 50-100 cm11 ,000 yea rs of uncompacted sedi­ment. Uplift of the trench slope basins calculated overthe 20 my of the Miocene was at a rate of 20 emil ,000years. Off Japan the sediments are finer grained withcoarse turbidites absent and yet no significant pelagicmaterial. Here , the sediments tend to drape the accre­tionary prism rather than to accumulate in small basinsand large-scale slumping is more important than offNew Zealand (Okada, 1980) (Fig. 17).

Fore-arc basins (outer-arc troughs) are relativelylarge (50-100 km wide) basins lying between the vol­canic arc and the structural high (trench-slope break)at the top of the accretionary prism . In oceanic arcsthey may overlie oceanic crust, but the better knownfore-arc basins lie on older continental crust or on theoldest part of the accretion ary prism. In New Zea­land the fore-arc basin is not very obvious, but may berepresented by the Raukumara deep-sea plain in thenorth and the series of onland Quaternary basinswhich extend SW from Hawke Bay to Cook Strait(Fig. 15). These are considered by Lewis (1980) todevelop from the highest accretionary basin (Fig. 14).The most extensive fore-ar c basin system is that whichlies off Sumatra and runs discontinuously northwardsas far as onland Burma for about 3,000 km (Curray,Moore , Lawver , Emmel , Raitt , Henry & Kieckhefer,

11

34

5

6 (j)

7 -go

8 ()cJ)

9 (/)

1011

1979). Off Sumatra about 4 km of sediment haveaccumulated since late Oligocene times (Karig et al,1980) at a rate of about 15 cm11,000 yea rs. The sedi­ments include a high proportion of volcanically de­rived ash and montmorillonitic clays, and turbid itesgrading up into shallow water sediments. In detail thebasin shape and turbidite fill are controlled by right­lateral strike-slip faults that splay across the fore- arcbasin from the Sumatra fault zone (Fig. 18). The basinwidened with time as the subduction zone migratedsouthwestwards and the cont inental margin to the NEsubsided (Fig. 19). Off Jap an the fore-arc basins areup to 200 km long and 50 km wide and contain sedi­ment s up to 5 km thick off northern Honshu (Okada,1980). The best known exposed fore-arc basin is thelate Jurassic-Cretaceous Gre at Valley sequence of theSacramento Valley of California (Dickinson & Seely,1979).

Back-arc basins occur on continental crust , onoceanic crust which is older than that of the volcanicare, and on ocea nic crust which is younger than thevolcanic arc and has formed by sea-floor spreading andmigrat ion of the volcanic arc away from an oldercontinent.

Some back-arc areas, particularly those on the west­ern side of the Pacific are formed by extension . TheJapan Sea , behind the Japanese Island are, is a verylarge and complex basin filled by thick sediments asvaried as those to be found in major oceans. TheOkinawa trough behind the Ryukyu Island arc to thesouth of Japan is a smaller one where normal faultingallowed turbidites to be deposited and then deformedas extension proceeded (Herman, Anderson &Truchan , 1979).

The And aman Sea behind the Burma-Indonesia arcis a much more complex basin with substantial strike­slip and creation of new ocean floor . In these respect s,and in the fact that the cont inent faces west, it hassimilarities to the Gulf of California (Curray et al.,1979). The And aman Sea (Rodolfo, 1969a,b) is1,200 km long and 650 km wide with a 250 km wideshelf on the east (Fig. 18). A central volcanic arcseparates two troughs, a western one forming thefore-arc basin and an eastern one which is muchdeeper (down to over 4,000 m) and has narrow riftvalleys 5-8 km wide and 500 m deep trend ing ENE­WSW. These rift valleys are the spreading centres ofpull-apart basins associated with the major dextr alstrike-slip fault which divides a thin strip of BurmaPlate from the main China Plate (Curray et al., 1979).Sediments in this eastern trough are surprisinglythin-s-only 1.5 km-s-consider ing the huge input ofsediment from the Irrawaddy River delta , reflectingthe youthfulness of the basin. Sedimentation rates forunconsolidated sediments range from an average of15 cml1,000 years in the Central Andaman trough to200 cmll,OOO years in front of the Irrawaddy delta(Rodolpfo, 1969, b). The structural pattern can be

332 H. G. READING

Fig. 18. Burma-Indonesia arc-trench system (Cur ray et al.,1979; Karig et al., 1980) to show how an obliquely convergingmargin has elements of (1) convergence and compression inthe fore-arc zone (see also Fig. 19), (2) strike-slip within thevolcanic arc, in the fore-arc zone and in the back-arc area ofthe Andaman Sea and (3) extension in the back-arc basin

where spreading centres are forming as pull-apart basins.Sediments from the Bengal and Nicobar Fans are accreted onthe fore-arc. The whole complex extends for over 3000 kmbut is made up of several distinct belts. Because of theenormous input of sediment the features change along thebelts from those of a marine subduction complex to those ofa continental one.

followed northwards into Burma where the volcanicarc separates the western fore-arc basin from theeastern back-arc basin, though locally Irrawaddy deltasediments cover all structural zones. In Burma thesedimentary fill is perhaps 12 km in the fore-arc (west­ern) trough (Rodolfo, 1969, a) and 17 km in the back­arc trough (Mitchell & McKerrow, 1975), a thicknesswhich Curray et al., argue is indicative of underlyingoceanic crust. Thus this whole 'geosynclinal complex'extends from northern Burma to the Malacca Straitsoff northern Sumatra as a series of belts: trench,accretionary prism with slope basins, fore-arc trough,island arc and back-arc basin. Changes in sedimentarythickness and type of fill depend primarily on proxim­ity to the enormous sedimentary source resulting fromcontinental collision and uplift to the north.

(d) Continental collision basins

Continental collision occurs when two plates carryingcontinental crust, island arcs or even thicksedimentary piles converge either by subduction or bystrike-slip motion with a component of convergence.Since continental margins are always irregular andbecause convergence is seldom orthogonal, remnant­ocean basins (Graham, Dickinson & Ingersoll, 1975)occur within the collision belt and, because of upliftand erosion of the mountain belt, enormous quantitiesof sediment are available for any adjacent sedimentarytrough. Hence, one result of collision of India withAsia is to fill the various subduction-related troughsdescribed above in Burma and to turn part of theIndian Ocean into quasi-continental crust (Fig. 20). Inaddition, there is a major basin parallel to the moun­tain belt where the present Ganges-Brahmaputra runs.The sediments which fill this trough are known asSiwaliks, a continental molasse tectofacies of lateTertiary age. Similar sediments are found in the IndusValley, derived both longitudinally from the mainmountain chain and transversely from the westernPakistan orogenic belt, primarily a sinistral strike-slipzone with many complications in detail, formed bymovement of the Indian sub-continent northwards re­lative to the Iranian plate.

(e) Basins associated with transform/strike-slip faultzones

Strike-slip faults are those whose primary motion ishorizontal and parallel to the fault trace. They rangein size from plate boundaries, such as the San Andreas

GULFOF

THAILAND

Thru st faultStrike -sl ip fault

Spreading rid ge

Volc ano

100' E

EASTERN

BURMA

HIGHLANDS

400I

km

INDIANOCEAN

SEDIMENTARY BASINS AND GLOBAL TECTONICS 333

swLOWER TRENCH SLOPE.... TRENCH SLOPE BREAK ..FORE-ARC BASINBARISAN UPLIFT

D.PRESENT

NE

+~ t~~§~~~;:::;:;~,:;:~~§~~~~~~~~~~~~m~m!0~~m'frr.iij~;;:=-------=t2

km 4

6

+ + + + +

Oceanic sediments

Oceanic crust

5

Permo -Triassic gran ite+2

~~~~fiF~~~~@~~~~~m~~~=---I-1=km 4

6

+2o--J"-~"",,,"==,,,,----==~.,,-----------------~2--L",...."W-~"""',..;j

km 4

6

oI

kmI

50I

A. EARLY-MIOCENE21 Ma

Fig. 19. Development of Sumatran fore-arc system in the Neogene (from Karig et ai., 1980). Line of section shown on Fig. 18.Structure below the shelf is hypothetical to illustrate the kind of lithologies present. The earliest formed slope basins areoverlain by sediments of the fore-arc basin.

fault of California, the Alpine fault of New Zealandand those which border the Indian plate as it impingesupon the Asian plate (Fig. 20), through intra-platefaults and those which border micro-plates such as theGreat Glen fault and others in Asia (Fig. 20), tosmall-scale fractures with movement of only a fewhundred metres, such as the Sticklepath fault of De­von. In addition, on oceanic crust there are primarytransform faults occurring as fracture zones associatedwith spreading ridges.

However, fault regimes are seldom purely transcur­rent (Harland, 1971). Movement between blocks isnormally somewhat oblique and so strike-slip motionmay be either divergent (transtensile) or convergent(transpressive) (Harland, 1971; Wilcox, Harding &Seely, 1973). Divergent strike-slip increases the likeli-

hood of normal faulting, sedimentary basin formationand volcanicity. Convergent strike-slip leads to foldingand uplift with thrust and reverse faulting. The natureof many major strike-slip systems changes from timeto time. For example, both the San Andreas faultsystem and the Alpine fault have changed from domi­nantly transtensile systems in the Miocene to domi­nantly transpressive ones in the Pliocene (Nardin &Henyey, 1978; Norris, Carter & Turnbull, 1978).

Individual strike-slip faults are seldom straight.They tend to curve, to split into several brancheswhich may come together again and they are frequent­ly offset, side-stepping one another with successive enechelon faults taking up the regional movement. Thesecomplex patterns lead to localized zones of extensionand compression (Figs. 21 & 22). Sedimentary basins

334 H. G. READING

b.

Down~

a. DIVERGENCE

c. d.

30 ~~NVERGENCE

11040

1 ZAGROS THRUST2 HIMALAYAN FRONTAL THRUST3 INDONESIAN TRENCH ISUBOUCTION ZONEI

km 2000I I

Fig. 20. Map of Indian subcontinent and surrounding areasto show the major strike-slip faults and some of the majorsedimentary basins and accumulations resulting from thecollision of the Indian and Arabian plates with Asia (afterTapponnier & Molnar, 1975; Graham et al., 1975; Page,Bennett, Cameron, Bridge, Jeffery, Keats & Thaib, 1979).

form where there is extension. Where there is com­pression uplift leads to erosion and a source of sedi­ment for the adjacent basins.

The shape of the basins depends on the pattern offaulting. Curving faults and anastomosing faults resultin wedge-shaped or elliptical basins (Fig. 22 a & b).Side-stepping faults produce rectangular or rhomboid­al pull-apart basins e.g. the Dead Sea, Salton trough ofCalifornia (Fig. 22f). The detailed fold and fault pat­tern of strike-slip basins and the adjoining areas ofdeformation can frequently be understood by applyingsimple shear models to the zones of strike-slip (Wilcox

Fig. 22. Types of strike-slip fault pattern that produce adja­cent extensional sedimentary basins and compressional, up­lifted blocks (from Reading, 1980; after Kingma, 1958;Quennell, 1958; Crowell, 1974b). (a) divergent and conver­gent fault patterns, (b) anastomosing fault pattern with bothwedge-shaped highs, wedge-shaped lows and pull-apartbasins, (c) & (d) fault terminations, (e) & (f) side-steppingfaults.

et al., 1973) (Fig. 23). If it is known whether thedirection of movement is dextral or sinistral then theorientation of the folds and faults and the shape of thebasins can be predicted. Alternatively, if the structuralpattern is known, then the direction of movement of

Normalfault.

Direction of Extension &sediment transport. subsidence.

Fig. 21. Illustration of how the curvature of a dextral strike­slip fault may produce both an extensional basin and acompressional uplifted area from which sediments are eroded(after Crowell's 1974a model for the Ridge basin of Califor­nia). Superimposed on this model is the structural pattern tobe expected from a dextral strike-slip system (ef. Fig. 23)(after Wilcox et al., 1973; modified by Mitchell & Reading,1978). Sediment transport is to the right.

Fig. 23. Structural pattern resulting from simple shear (afterHarding, 1974). A NW-SE dextral shear couple producesN-S trending normal faults and E-W trending fold axis,reverse faults and thrusts.

SEDIMENTARY BASINS AND GLOBAL TECTONICS 335

the strike-slip zone is predictable. However, becauseof progressive rotation of earlier formed structures asmovement continues and because so many majorstrike-slip zones reflect fundamental faults, which mayhave moved in different ways in the past, it is impor­tant to date the structures and even then in olderzones it may not be possible to match orientation ofstructures to fault motion, because of the overprintingof earlier structures by later ones.

A point that cannot be stressed too strongly is thatwhile the overall movement across a fault is horizon­tal, at anyone place the main movement may bedip-slip. This vertical movement may be substantialand it is this that produces the main sedimentaryeffects of a strike-slip system. In older tectonic regim­es, where evidence for lateral motion is so difficult toobtain, vertical movements may be the only proveablefault motion.

Sedimentologically the most important features ofthese basins are the extreme lateral facies changes, thevery great thicknesses of rapidly deposited sediment,abundant sediment supply from multiple sources andthe evidence nearby for unconformities and contem­poraneous deformation, sometimes in the form ofextensive thrusting, even along the faulted basin mar­gins.

Normal acoustic echo sounders are quite inadequateto map deep rugged topographies on ocean floors.Hence, it is only recently that the use of submersibles,side-scan sonar and deeply towed instruments hasenabled the detailed morphology of oceanic ridges tobe discerned. These studies (Arcyana, 1975; Lonsdale,1978; Searle, 1979) have shown that, in addition tobasins running parallel to the ridge crest, there is acomplex series of basins controlled by the structuralpattern (Figs. 24 & 25). Transform valleys and ridgesrun between and perpendicular to the main spreadingcentres and parallel the direction of plate separationwhich is essentially that of a strike-slip fault. Majorescarpments occur with a relief of more than 1 km, anaverage inclination of 25-30° and including steppedcliffs with gradients up to 60° (Lonsdale, 1978). At thetoes of these cliffs are volcaniclastic talus breccias (cf.Francheteau, Choukroune, Hekinian, Le Pichon &Needham, 1976). In addition minor horsts and grabenare found within the transform valleys; these trendobliquely to both the main ridge and the fracture zonein the expected direction for normal faults in a sinistralstrike-slip zone (Lonsdale, 1978; cf. Searle, 1979).

The Gulf of California lies between the San Andreasfault and the East Pacific Rise in a transtensionalsetting resulting from dextral divergent transform mo­tion which began about 4 my ago. The gulf is 1,300 kmlong and 100--250 km wide with relatively small basinsdeepening from 600 m in the north to 3,000 m furthersouth, separated by fault-controlled sills and islands(for summary see Kelts, 1981). Some of these basinsare pull-aparts with high heat-flow, alkaline volcanics

and hydrothermal mineralization; they are spreadingcentres. Other basins are perched along the slopes.The dominant sediments are hemipelagic diato­maceous muds. Sedimentation rates range from 40-­120 cm/1,000 years. These rates are extraordinary con­sidering the surrounding land areas are arid and thereis very little sand.

In southern California, the Palaeogene subductionregime changed to strike-slip during the Miocene (e.g.Crouch, 1981) as the San Andreas fault system de­veloped, taking up northward movement of the Pacificplate relative to the American plate. The San Andreasfault is not a single feature: it is only the most impor­tant of a number of dextral strike-slip faults in a beltwhich is up to 500 km wide and extends offshore tothe Californian Continental Borderland (Fig. 26),where sedimentary basins, some over 2 km deep andup to 20 x 50 km in area, have developed during thelate Cainozoic. They are separated from each other bysills or islands formed of uplifted basement and oldersediments. Sedimentation at the present day is byturbidity currents, slides, slumps and debris flows andby near-continuous raining of fine-grained terrigenousand pelagic material (Fig. 27). Thicknesses of lateCainozoic sediments range from 8 km in basins adja­cent to the land, such as the petroleum producing LosAngeles basin, to less than 2 km in the relativelystarved basins away from the continent (Fig. 26).Sedimentation rates vary between 5 and 40 cm/1,000years. Although sediments in the basin centres aregenerally more or less horizontal, adjacent to thefaulted margins they may be very deformed with com­plex stratigraphical and structural relationships (Fig.28).

Another small marine basin associated with strike­slip is the Yallahs basin off Kingston, Jamaica (Burke,1967). Jamaica lies within a 200 km wide plate bound­ary zone which separates the North American andCaribbean plates (Burke, Grippi & Senger, 1980).Movement between the plates is taken up by E-Wsinistral strike-slip faults. Anticlines and thrusts, suchas those forming the Long Mountain anticline andthrusts in the Point Royal Mountains are alignedNW-SE or NNW-SSE, and normal faults, such asthose which bound the Yallahs basin as the westernand eastern scarps, trend NNE-SSW compatible withthe E-W sinistral fault trends (Burke et al., 1980). Thebasin is about 20 x 30 km in size with a 1,300 m deepportion, 100 km2 in area (Fig. 29). A range ofsedimentary facies is forming, with subaerial braidedalluvial fans, submarine fan-deltas (Westcott &Ethridge, 1980) and distal turbidites in the deep basin.These fan deltas form clastic wedges of conglomeraticsandstones, which pass without a break from subae­rial alluvial fan deposits, through a very narrow andbarely perceptible shelf, to deep water submarine fanand slope deposits. On the narrow shelf sandy spitsand bars pass laterally into carbonate reefs from which

336 H. G. READING

-3250- Bathymetry in metres

3500 I

km

.-- ---3250

~I3°30

1<1 Shoaier than 3000m

o·I

..FRACTURE RIDGE 3250

~

oABYSSAL HILLS PARALLEL

TO SPREADING AXIS

3250~._~--r_r--,

=:::C::::>:::><?::>RELIEF PARAllEL TO TRAN~FDRM VALLEY

Fig. 24. Sedimentary basins and ridges at the intersection of the N-S trending East Pacific Rise crest and the E-W trendingQuebrada transform fault zone, which passes eastwards into the currently active part of a major fracture zone (from Lonsdale,1978). Square shows area of Fig. 25.

mass-flow deposits fall off fault scarps into the basin(Fig. 29).

The classic on-land strike-slip basin is the Dead Sea(Quennell, 1958; Freund, 1965; Garfunkel, 1978),which has formed as the Palestinian plate movedsinistrally with respect to the Arabian plate and as theDead Sea fault side steps (Fig. 30). Although faultcurvature to the north, in the Lebanon, producesuplift, run off of detrital sediment from the mountainsis limited in this arid basin and sedimentation isdominated by evaporites and marginal alluvial fans.

In contrast, the Pliocene Ridge Basin of Californiadeveloped in a relatively humid climate and was filled

mainly from one end by turbidites and fluvial sedi­ments (Fig. 31). Marginal conglomerates, in particularthe Violin Breccia, a unit several km thick but whichextends laterally for no more than 1 km into the basin,pass into fine-grained marine or lacustrine sediments(Crowell, 1975; Link & Osborne, 1978). The apparentthickness of sediments as measured from SE to NW, inthe direction of dip is about 12 km. However, becausethe locus of sedimentation has moved progressivelyNW as the basin opened, the vertical depth to thebasin floor at anyone place may be no more than4 km. The sedimentation rate of the Ridge Basin isabout 100 cm/1,OOO years.

SEDIMENTARY BASINS AND GLOBAL TECTONICS 337

... , , .... .. .. '" ... .. .. .. .. .. .. .... .. .. .. .. .. .,

2I

N

t

Lava outcrop

Talus

Fault Breccia

Sediment veneer

Sediment pond

3500

L..4000

3750

.. .. .. .. .. .... .. .. .. ..:::::::::::05.:6:::'::::::.... , .

............ .. .. ..

...... vvvvvvvvvvvvv.. ·· .. ·· ..-::- -::- -:.V ~.v \ ~ ": ~ V.: V.: . .WEST FLANK OF EAST

PACIFIC RISE

rTT1 Fault scarp

«) 3750r _ 5 _ Isopachs in metres

.. .. .. .. , , :::::::::.::::::::',...-.::.. ::--:: .. ::--:: .. . .. a km....... '.," , ," , .. I I

;~~~#········s.,~~,~:~llim.u 2::;;;;;i;;;;;iljjljjj1ii::•..,<::':'~:~:~:.~:.:';-:-.;.>;-~.:.:"~.::.':~::_:.:'.;_::::..;:.:::-::..::.~:~..:.:..:.~ :.:.:: :.:.: ::.:.:: :.:.:: :.:.:::.:.'.' ..::.... :.::.~:...:' ....':.:.::.. ':-:.::.:'::::_ ~.::::' :.: '.:'.:.' :: :.~N~::o:::R·::T:;H<li:: v.:,:v.:::v::.:v,:'~:~.~..~...:~...'.~.' ..~: ..~.~.~::~.~.:v:v.. !.: v~:'~~.~.:'~~.':~..:'.,; .

.'~::;:::~:::::::::::::::::::::::::::::::::':::::<;:::::::::: ., eM''! "" .. :: .. :: .. : ..~

)·,P.::::::::::::·:::':::::::::::::::::::::::::::::::::::::::::::.::·::.:::::::.TERI1A~~. ::::::::: .... ::::::::::-:::::::::: ...

Fig. 25. Quebrada transform fault zone showing the transform valley and fault scarps, and NE-SW trending extensionalbasins resulting from the E-W sinistral strike-slip movement of the fault zone (from Lonsdale, 1978),

4. SOME SEDIMENTARY BASINS IN ANDAROUND BRITAIN

No attempt will be made to give an account of allsedimentary basins in the British Isles. Papers read atthe recent William Smith meeting of the LondonGeological Society (Harris, 1982) show only too wellhow difficult and controversial are interpretations ofthe evolution of the British Isles. Sedimentation is theresult of several factors of which the local tectonicregime may be important, but it is not the only factor.Eustatic sea-level changes are becoming increasingly

well-documented (Vail & Todd, 1981). During periodsof lower sea-level, gradients increase, rivers debouchat continental margins and there is an increase indetrital material to basins. When sea-level is high,clastic sediment supply is diminished and biochemicalsediment widespread.

In addition, sedimentary basins are not usually theproduct of one simple tectonic regime. Even in therelatively well known present day basins where anorigin has been postulated in terms of plate tectonicsthere is normally a combination of factors at work. Forexample, the Andaman Sea can be looked upon either

338 H. G. READING

a 600L--J

km't- Anticline

Fault, bar on downthrown side

(s2> Cainozoic basins

8·0 Maximum thickness of strata in km.

33°N

a km1 I

---IC'l

~~,~"

~"'~___1

Fig. 26. Map of Continental Borderland and onland southern California. The NW trending faults are dextral faults syntheticto the main San Andreas fault. The Santa Monica fault is sinistral. Basins are largely fault controlled. Notice the trend ofanticlines is mainly WNW, reflecting the dextral movement along the fault zone. Thickness of basin sediment is greatestadjacent to the coast (after Blake et al., 1978; Howell et al., 1980).

as an extending back-arc basin resulting from its posi­tion behind a subduction-related island arc or as aseries of deep basins lying along the line of a majorstrike-slip plate boundary, their orientation and shapethe result of strike-slip motion. The configuration ofthe Andaman Sea is the result of subduction, ofextension and of lateral motion. The Gulf of Califor­nia is another transtensional basin, resulting from acombination of strike-slip motion and extension. Thusit is seldom possible to categorize a basin as onesimple type.

There is then the question of scale. Both the Anda­man Sea and the Gulf of California are large basins,over 1,000 km in length. They have a generalsedimentary pattern and facies resulting from theirtectonics, oceanic chemistry and currents, and theclimate of the surrounding land areas. Within them,though, there are many smaller basins which, becauseof their position within the major basin, have many

common features. Yet these basins may have beenseparated by positive tectonic features and the age oftheir initiation, sedimentation and termination maynot have been the same. They may have developed atrather different times and have had different structuralpositions. Consequently the larger scale basins have tobe distinguished from the smaller ones into which theyare divided. The same problem arises with the Califor­nian Borderlands where a number of small 'sub-basins'occurs within the whole Borderland region. Similarsedimentary facies fill each of these sub-basins and thedifferences in ages of these basins may be too small tobe perceived with biostratigraphy. Consequently,when reconstructing an ancient 'geosyncline', it is alltoo easy to see just one, very large basin and not torealize that it may be composed of a complex ofsmaller basins of slightly different ages and separatedfrom each other by uplifted blocks.

A few examples of sedimentary basins will now be

SEDIMENTARY BASINS AND GLOBAL TECTONICS 339

o kmI

Fig. 27. Sedimentary pattern in three Californian basins (ct. Fig. 26) (after Gorsline, 1978). Active submarine canyons showedby arrows; inactive canyons shown by dashed lines.

discussed which illustrate how models developed inthe preceding sections can be used to clarify ourunderstanding of basin sedimentation. A recent sum­mary of sedimentary basins in northwest Europe hasbeen given by Ziegler (1981). The late Precambrianopening of the Iapetus ocean was followed by itsclosure in Lower Palaeozoic times, with subduction-

Fig. 28. Stratigraphical and sedimentological model for anoff-shore strike-slip basin in the southern Californian Border­land (after Howell et al., 1980). Marginal wedges of coarse­grained inner submarine fan and slope mass-flow depositspass basinward into sandstones deposited in submarine fans.Whether the main upper fill is sandstone or hemipelagicmudstone depends on availability of sediment sources. Noticecomplex structural and stratigraphical relationships onfaulted margin, due to synchronous faulting and sedimenta­tion.

related processes on either side. The Devonian sutur­ing of the Laurentian-Greenland and Fennoscandiancontinental plates was dominated by strike-slip andtranspressional tectonics. In Carboniferous times ex­tensional tectonics over most of Britain passed intocompressional or strike-slip tectonics in the south. Thetectonically quiescent Permian saw the beginning ofthe extensional rifting of the Triassic which continuedin the North Sea throughout Mesozoic and Tertiarytimes.

The Torridonian of NW Scotland is composed oflacustrine and fluvial clastic sediments which wereprobably deposited in extensional graben. These wererather like those which formed around the Atlantic inMesozoic times and represent an early pre-openingstage of oceanic rifting. The Torridonian is followedby the dominantly shallow marine and turbiditic clas­tics of the Dalradian Supergroup which includes somecarbonates and volcanics. The apparent stratigraphicalthickness deposited over 300 my from late Precam­brian to early Ordovician was 25 km (Harris, Baldwin,Bradbury, Johnson & Smith, 1978) giving a rate ofsedimentation of 8 cm/1,000 years. This is a compara­tively rapid overall rate for such Atlantic-type margins(cf. p 328) but sedimentation was in a number offault-bounded basins (e.g. Anderton, 1979) and therate was almost certainly much more rapid locally overshorter periods of time. There were several large-scalebasin-deepening and -shallowing sequences within theDalradian region with active faulting governing the

30km2010o

,~\t----~

Medium-fine f~',J"----~4 sandstone ~ 'f">.-,..-~~:

km

340 H . G . RE A DING

1. LONG MOUNTAIN ANTICLINE

Fig. 29. Sketch map of the Yallahs strike-slip basin offKingston , Jamaica lookin g north (after Bur ke, 1967).

t> ... 1>...

SAN GABRIEL FAULT~

NWDi'ection 01sedimenl 110: SEOire~l ion ofstratigraphica' \'lunging

km 5, !, ,

Marine sediments D Sandstone ~ Conglomerate~Shal e ~ .Ia22J Mudstone ~ Breccia

Fig. 31. Map, transverse cross-section and longitudinal cross­section through Pliocene Ridge Basin of California to showstructural and sedimentary patt erns (for locat ion see Fig. 26)(after Crowell, 1975; Link & Osborne , 1978; Reading, 1980).The active fault during sedimentation was the San Gabrielfault ; the San Andreas fault is later.

subside nce of indi vidual basins. Harris et al., (1978)compared the tectonic framework to that of a margin­al, fault-controlled basin similar to the North Sea inMesozoic and Tertiary times. As the Dalradi an de­veloped, in Cambrian times , it can be compared withAtl anti c-type continental shelves in the ea rly stages ofoceanic rifting.

Exa mples of ocean closure are well seen on eitherside of the Iapetus Ocean in Ordovician and Siluriantime s. Th e geology of the Southern Uplands is bestexplained by comparison with modern accre tionaryprisms (Fig. 32) . A series of sequences which , wherecomplete , pass from basalt throu gh chert and carbo­naceou s shales to turbidites , young towards the NW.The sequences are separated from each othe r by re­verse strike-faults. An important fea ture is that eachsequence is progressively younger toward s the SE, inthe opposite direction to that in which the slicesthemselves young (Mitchell & McKerr ow, 1975; Leg­gett, McKe rrow & Eales, 1979) (cf Figs. 14 & 19).During Lower Palaeozoic times there is as yet littledirect proof that strike-slip was an important factor inbasin developm ent , though the re see ms littl e doubtthat the two plates were collidin g oblique ly (Phillips,Stillman & Murp hy, 1976) , and basins like the SouthMayo Trough (Dewey, 1963; Ryan & Archer, 1977)have many similarities to pull-apart basins. However ,by Devon ian times the allu vial basins within the Mid­land Valley of Scotland cert ainly be explained asstrike-slip basins result ing from sinistral motion on theHighl and Boundary Fault and the South ern Upl andFaul t. (Bluck, 1978, 1980). Large volumes of alluvialfan conglomerates and sandstones pour ed off the NE­SW trend ing sinistra l faults into two major basins, the

10I

YALLAHS BASIN1300m

kmaI

Fig. 30. Block diagram to show origin of the Dead Sea bysinistral strike-slip movement and side-stepping of the fault(after Quennell, 1958).

SEDiMENTARY BASINS AND G LO B AL T ECTONICS 341

o km 50l.--J

which basin developm ent in th e Ordovician and Silu­rian of Wales can be understood in terms of strike-slipmot ion on the fundamental faults , or whether thefaults are just normal faults res ulting from the exte n­sion of a back- arc basin.

In Upper Carboniferous tim es, over most of Britainextensional basins formed (Leeder, 1976) while str ike­slip basins may have predom inated over much ofwestern Euro pe (e.g. Heward & Read ing, 1980). SWEngland is an enigma since the condensed sandstone­deficient Upper Devonian-Lower Car bonifero us sequ­ences of cherts, shales and limestones formed onblocks and in basins , and by indicatin g extensio na ltec to nics contras t with the postulated late Devon ian ­ea rly Carbonifero us tecton ic ph ase of southern Co rn­wall (Sanderso n & Dearman , 1973). One explanatio nof this juxtaposition of extensional and comp ressionaltectonics is that the region was affected by strike-slipmovements which caused both deformation and theformation of exte nsiona l sedimentary basins (Bad ha m,1976) .

Th e relat ionship of tectonics to sedi mentation inand aro und Britain in Mesozoic and Terti ary times isbe tte r understood from the North Sea (Fig . 35) than itis fro m onland Britain because of the seismic anddrilling data availab le from the oil industry . The tecto­nic history of the North Sea falls into two phases. Thefirst phase was one of taphrogen ic rift movementswhich started in the Permo-Triassic and end ed in theLower Cretaceous (Fig. 36) . Th e second phase wasone of more gradua l subsidence which has lasted tillthe present day thoug h, in some places, such as theSole Pit gas field area of the southern North Sea,

Fig. 33. Location of Scottish Old Red Sandstone Basins(shown stippled and of Southern Uplands accretionary prism(after Bluck, 1978). Both the Lanark Basin and the Strath­more Basin contain sub-basins. Continuous arrows indicatemain sediment dispersal directions in Lower ORS times.Dashed arrow indicates main sediment dispersal in UpperORS times.

Site ofSolway Fir th

Is,Sea level

CENTRAL BElTNORTHE RN BELT.. .. ...Fig. 32. The Southern Upland accretionary prism in Wen­lock times (after Leggett, McKerrow & Eales, 1979 (forlocation see Fig. 33). The oldest sequence is to the NW, andthe oldest three successions contain basalt and chert at thebase with shales and turbidites above. The central sequencescontain shales, occasional chert and turbidites. The youngest,to the SE, contains only turbidites the present level oferosion is arcuate to allow for probable post-Silurian over­steepening of the prism. Scale is approximate. Noticethat thesediments themselves young to the NW and, if the detailedstratigraphical palaeontology were unknown, a successionseveral tens of km thick might be estimated as the deposition­al thickness of sediments.

Strathmore and Lanark basins of the Lower Old RedSandston e (Fig. 33). There was also longitudinal fill,partly derived from normal faults within the basin(Fig. 34) .

On the south side of the closing Iapetu s Ocean , theevide nce for subduction is ra ther different. An accre­tionary prism is not known. Instead , the calc-alkalinevolcanics of the English Lake Distri ct and Balb rigganin Irel and are ta ken to rep rese nt an Ordovician islandarc passing SE into the more rhyolitic volcanics ofNorth Wales. Behind this main volcan ic arc is theWelsh basin , geographically a back-arc basin, withsome volcanic activity, mainly basaltic. In the Ordovi­cian and Silurian of the Welsh basin the re is a verycompl ex patt ern of turbidites shallow marine sedi­ments and volcanics, deposited in sma ller basins prob ­ably gove rne d by faults (James & James, 1969). Thesefaults had considera ble vertica l move me nt during sedi­ment at ion and there is some evidence that they alsohad a lat er al component. Th e anasto mosing fault pat­tern of faults shown by James & Ja mes (1969) trendingNE-SW with subsidiary E-W and ENE-WSW faults,together with the N-S and NNE-SSW trend ing anti­clines (Coward & Siddans, 1979) suggests some dex­tral movem ent in Lower Palaeozoic times which is thesame direction as that postulated by Phillips et al.(1976) . It would be interesting to know the degree to

UPPER SLOPE \ EMERGENT ILOWER TRENCH SLOPE ITRENCHBAS IN TRENC H SLOPE

, BREAK "" /, /

Site of Mid land : :Valley inl ier s : :

jSite of Southern

NW UPlOdFau lt

342 H . G . READIN G

N

tDc

E

a

d

Source 3.

a/

.i->: """dsandstone

'\II '. ' . _- - _......... . ~... >. ', ' .pebbly i " '-;' " ' ",': sandstone ..

-: : .. : ", ~. ", :'~.,' ... .,,' ..,~" Basin 3. . , '. ",: .'

debris flows &': .: " ':-. fz, , proximal braided stream

e " deposits · '" ' " ,', . .o Q 0 'd ' 0 '0 . ~ • '. . ' . . ' . ' . • .

o ~ OC I: 6' b ~ : ' • •

o()o ~:/ .• t :. . . . ' ."

~ not to sca le

b

?

Fig . 34, Model for Upper Old Red Sand stone strike-slip basins (after Bluck , 1980), The Highland Boundary Fault (F) isconside re d to be a sinistral strike-slip fault which extended the basem en t on the SE side . This extension results in Basins 1, 2,3 dev elop ing pro gressively to the SW as faults [), 12, f, are down- fault ed . Pet rographi c differences of the th ree ph ases of fillsuggest there were temporary sources of sediment from within the Midland Valley. There is also a sub stan tial input of sedimentfro m across the master fault to the NW .

inversion (r.e. uplift of an earlier basin) took placeduring the Cretaceous (Glennie & Boegner , 1981).

During the Jurrassic, intra -cratonic deformation ofthe northern North Sea led to crustal arching of acentral dome and radiating graben extending from it(Fig , 37). Within the graben , delt aic sands form themajor reservoirs of the Brent group of oilfields east ofthe Shetlands. In detail the sedimentary facies are very

complex (Budding & Inglin , 1981) with barrier sands,lagoons and tidal deltas super imposed on the simplerdeltaic model shown in Fig. 38.

By Upper Jurassic times , conditions had changedsomewhat and, in place of the relatively large MiddleJurassic delta s which filled the graben, smaller andmore localized fan-deltas, such as those that form theBrae and Piper fields, were der ived from active fault

SE D IM ENTARY BA SI NS A N D GLOBAL T E GrO NICS 343

560~.

I

-to~~..A~~

~~I

-a» <SIt!).Ul

I~

53°N I

I 0 100L......!..~

I \

3° 2° lOW 0° 3° 4° r- 8

I60 0~--I---t~.

Fig. 35. Struc tural units in the North Sea (afte r Day, Cooper, A ndersen , Burgers, Ronn evik & Schoneich, 1981). Major oiland gas fields B. = Brent ; Br. = Brae ; E . = Ekofisk ; Fa . = Fort ies; Fr. = Frigg; Ma. = Magnus ; Mo . = Montrose ; P. =Piper; Fine stipple = major plat forms and highs; Coarse stipple = smaller highs and horsts.

344 H . G . RE ADI N G

EHorda Platform

oo

o VOLCANICS VV UNCONFORMITY

" .

~ ANHYDRITE~HALITE

o

~MUDSTON E

QUATERNARY... ......... . .o

, .. - - -"~" '~~~~~.' ==: : '-== . ' . ' .=='. '.~~-. -- " ._--'-~~.----v~ ... -EOCENE-PLIOCENE - . .__...__...__... .._."

v v ·.. __---...- ...--- ..---...--- ... --- ... ... . ..--'

~~~~~' ••..••~~

1000

20 00

4000

30 00

Fig. 36. Schematic cross-section through the northern Viking Grabe n a pro bable 'failed rift'. Compare the lower , faultedgra ben with Fig. 6. of the Red Sea (from Je nkins & Twombley . 1981) .

EJ] Continental clastics

§ Paralic-delta ic

~ Shallow marine

fVVvl V I .~ oicarucs

Fig. 37. Simplified palaeogeograph y of northern Brit ain andthe North Sea in Middle Jurassic times (af te r Eynon , 1981) .T he volcanics occur at the junction of the three radiatingbasins, the Moray Firth Basin (MFB) , Viking Graben (VK)and Central Graben (CG) and sediments are derived bothlongitudinally from the upd om ed trilete jun ction and fromlateral, possibly faulted-bounded , margins . Notice the MinchBasin (MB), where sed iments are exposed in the InnerHebrides, and the Sole Pit Basin (SPB) , where they extendon to land in east Yorks hire .

Fig. 38. Detailed Middle Jurassic (Lo wer Bathonian ) ~elta ic

palaeogeography (fro m Eynon, 1981) . Sediment flow is-bothalong the Viking Graben from the south and from the EastShetland Platform to the west. Sedimen ts are also presumedto come from Scandinavia to the eas t. Tidal influenc e isprobably importa nt in the mar ine arm to the west.

SEDIMENTARY BASINS AND GLOBAL TECTONICS 345

scarps (e.g. the Helmsdale fault of NE Scotland).Opinions differ as to whether the conglomeratic sand­stone facies formed as submarine fans in relativelydeep water or as alluvial fans (Harms, Tackenberg,Pickles & Pollock, 1981). They were probably a mix­ture of the two, with some conglomeratic sandstonebodies forming under partly subaerial and partly sub­marine conditions, as with the fan-deltas of the Yal­lahs basin (Fig. 29). Understanding the role of tecto­nics in sedimentation at and near the Upper Jurassic­Lower Cretaceous boundary is complicated by theworld-wide drop in sea-level in the Berriasian (earlyCretaceous) (Vail & Todd, 1981), which led to emerg­ence of the highs and an increase in detrital sedimenta­tion.

A world-wide rise of sea-level during the Cretaceousled to a decrease in clastic sedimentation. In thePalaeocene, clastic sedimentation returned, primarily

Fig. 39. Palaeogeographic map of the Palaeocene MontroseGroup (after Rochow, 1981). Thick submarine fan sand­stones pour into the Viking and Central Grabens and theMoray Firth Basin from the west. Thicknesses of sandstonesare up to 6--800 m in the Moray Firth sub-basins either sideof the Halibut Horst and up to 5-600 m in the VikingGraben.

as submarine fans in rather broader troughs, but stillto some extent fault controlled (Fig. 39). It was duringthe Palaeocene and Eocene that the principal reser­voirs were formed in the Central Graben, such asthose of the Forties (Carman & Young, 1981) andMontrose fields, and also in the Viking Graben, espe­cially the Frigg Field (Heritier, Lossel & Wathne,1979) whose sediments were derived from the west.Sedimentation at this time was decidedly asymmetric­al, as it was in the Upper Jurassic, in contrast to theMiddle Jurassic when derivation was from both sidesof the graben.

5. VERTICAL MOVEMENTS AND RATES OFSEDIMENTATION

Differential vertical movements are an important pre­requisite for sedimentation and in the preceeding sec­tions many figures have been quoted of rate of subsidenceof a basin, rate of sedimentation and rate of uplift.Although these figures show significant differencesbetween some types of basins, the figures have to beapproached with caution.

One factor is that sediments become compactedand, therefore, a sedimentation rate measured onsurface sediments with a high porosity and watercontent may be less than half when measured 2-3 kmbelow the surface. Therefore, sedimentation rates me­asured on recent sediments appear to be much higherthan those measured on older sediments. Measure­ments of compaction are difficult (see, for example,Perrier & Quibler, 1974), but, in general, shales com­pact more than sandstones and, after the first fewhundred metres where the thickness is quickly reducedto half, tend to compact only slowly at greater depths.Sandstones generally compact less than shales, withquartzose sandstones perhaps decreasing by only 10­15%, but sandstones with a high proportion of lithicclasts, although retaining their porosity in the first fewhundred metres may compact to 60% of the originaland at depths greater than 1,000 m may compact fasterthan shale.

Thus, as a very rough estimate, sediments buriedbelow 2,000 m are reduced to about half their originalthickness, mostly in the first 100-200 m of burial. Inancient rocks there is not only the problem of allowingfor deformation effects but of determining what mea­sured stratigraphical interval indicates a 'true' thick­ness, in the sense that there was once such a verticalthickness of sediment above 'basement' at anyoneplace. It is all too easy to measure a series of stratig­raphically determined sections from place to placeover a wide area and to superimpose them to give asubstantial cumulative sedimentary thickness. Thewider the area one takes for the measurement ofsections the greater will be the total apparent thick­ness, because it is more likely that they were depositedin separate basins. For example, if one were to add the

346 H . G . R E ADING

thickness measured in a number of Californian basin swhich were filled at slightl y different times, one wouldgrossly over-estimate the tot al thickness. This is veryeasy to do in a region of ancient rocks where outcrops ofa particular system are widely separated. It is important,therefore, when calculating ancient sedimentary thick­nesses, to distinguish between the total 'stratigraph­ical' thickness over the whole region and the sedi­mentary thickness of an y single basin.

In addition. even with in one basin , a mea suredsection does not necessarily indicate the true depth ofthe basin , as can be seen from the Ridge Basin (Fig .31). Many structurally controlled basin s widen duringsedimenta tion and the locu s of sedimentation changes.Stratigraphical sections in onlapping delt as, such asthe Niger tc]. Fig. 8), also may give a misleading figurefor basin depth.

Nevertheless, (Schwab , 1976; Miall, 1978) intracra­tonic basins show accumulation rates of about 2 em/1,000 years comp ared to up to 10 cm/l,OOO years forpassive continental margins and about 100 cm/l ,OOOyears for subduction-related and strike-slip basins. Inareas of high sedimentation , such as delt as, sedimenta­tion rate s may be even greate r but , since sedimenta­tion is usuall y into an existing trough , the rates are notthose of basin subsidence. These accumul ation ratescompare with vertical movements which may be asmuch as 800 cm/l ,OOO years in California (Schumm,1977) and 700---1,000 cm/l ,OOO years for the late Pleis­tocene to Recent movement of the Alpine fault ofNew Zealand (Adams, 1981). It is important to notethat, while the se rates occur in extremely active tecto­nic regions across fau lt lines , post-glacial isostatic re­bound produces uplifts of 500 cmll ,OOO years for th eGreat Lakes region and up to 1,000 emil ,000 years forScandinavia in regions which are consid ered cratonic(Miall , 1978). The se rates compare with glacioeustaticfluctuations of sea-level of up to 1,000 emil ,000 years(Pitman, 1978) and Me sozoic eustatic fluctuations of4-10 cmll ,OOO years (H ardenbol, Vail & Ferrer,1981).

In ancient successions, sedimentation rates usuallyappear to be much less than their modern counterparts(Miall , 1978). This discrepancy may partly be due tomeasurements made on compacted sediments beingcompared with those made on uncompacted sedi­ments . However , it is probably mainl y due to thedifficulties of dating ancient sediments with precisionand thinking that sedimenta tion lasted longer than itdid , also including unrecognised phases of non­deposition and ero sion and migration of basin de­pocentres.

6. DISCUSSION

In this paper emphasis has been on sed iment at ion intectonically active areas. Nothing has been mentioned

about the contras ting widespread cratonic sedimenta­tion , as seen in the interior of North America in theProterozoic and in Cambro-Ordovician times, wherethin, but very extensive sheets of superficially monoto­nous orthoquartzites and carbonates, less than1,500 m thick, were deposited on a shallow shelf whichcovered thous ands of square km (Do tt & Byers, 1981).There is no easy explanation as to how sha llowmarin e , or even fluvial pro cesses could oper ate oversuch extensive she lf areas. Cratonic areas were notonly stable during sedimentation; most of them haveremained undisturbed ever since and the sedimentsare still flat-lying. In the are as described in this paper,tectonic movement was cont inuous with sed imenta­tion , slow in the case of some large basins , but veryfast in strike-slip zones.

The old geosynclinal terminology once served auseful purpose when we knew little about the present.It might now be abandoned in favour of a descriptionof sedimentary basins , the sub-basins and of the tecto­nic regime in which they developed . It should only beretained to simplify areas about which we kno w verylittle . However , to jump straight into a plate tectonicterminology is dangerous, since the categories todayare not clear cut and interpretations of ancient basinsare bound to be subjective and no more than workinghypotheses .

Thick 'geosynclina l' success ions are formed in man yways. They may develop (1) very slowly on continentalcrust with migrat ory facies belts extending for hun­dreds of km; (2) rather faster, but with equally exten­sive facies belts , in oceans as large submarine fansextending for thousands of km , large build-ups ofsediment from ocean bottom currents or in largetroughs on passive continental margins, where thefacies belts run parallel to the margin; (3) in largedeltas extending perpendicular to the continental mar­gin , where facies belts on a gross scale are veryextensive but, in detail , very variable; (4) in extension­al rifts, forming linear troughs some (e.g. aulacogens)perpendicular to continental margins, oth ers sub­parallel to them , such as the graben associated withthe opening of oceans. Sedimentary facies are exten­sive, parallel to the fault lines , but very variableperpendicular to them ; compressional deformationduring sedimentation is absent; (5) very rapidly indeedin small, strik e-slip or oceanic transform basins, zonesof which may be very extensive , but which indi viduallyare quite small. Facies belts are limited laterally,making lithofacies mapping a difficult task. Facies passlaterally into oth er , coeval , facies. The prox imity ofrising areas to sinking ones make s correlation by 'foldphases ' or by the linking of unconformities extremelydangerous. In addition , the fact that sedimentation isextremely rapid and that thousand s of metres of sedi­ment may be deposited in a very short space of time ,well within the finest of stra tigraphical sub-zones,

SED IMENTA R Y BASINS A ND G LOBAL T ECTONICS 347

makes it impossible to separate chronologically forma­tions which may have been deposited at differenttimes; (6) as tectonically accreted successions, divisionof which is only possible with good stratigraphicalpalaeontology or b~ th~ recogni~ion of the shear zoneswhich separate the individual slices.

ACKNOWLEDGEMENTS

The figures were drawn by Gillian Collins to whom theauthor is extremely grateful for her care and patien ce .Thank s are also extended to Carol Pudsey for readingand commenting on an early draft and to Diana Rel­ton for typing the manuscript.

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