1991 reading deep water system classification

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doi:10.1144/gsjgs.148.3.04271991; v. 148; p. 427-430Journal of the Geological Society

H. G. READING

feeder systemThe classification of deep-sea depositional systems by sediment caliber and

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Journal of the Geological Society , London, Vol. 148, 1991, pp . 427-430, 1 fig. Printed in Northern Ireland

S H O R T P A P E R

The classification of deep-seadepositional systems by sediment calibre

and feeder system

H . G . R E A D I N G

Department of Earth Sciences, University, Oxford

O X 1 3 P R , U K

Twomethodsofportraying a classification of deep-sea lasticdepositional ystemsare hown,based on sedimentcalibreand

theature ofhe feeder system,whetheringle or multiple

sourced.Mud-dominated ystems,particularlypoint-sourced ub-marine fans, ar e large in area, volume, radius and size of flow, withpersistent channel/levee systems hat end to meanderandhavesheet-lie sands in the associated basin plain. With increasing grain

size, there is an increase in slope, frequency of flow, impersistence

of channel system and channels tend to switch more frequently. As

the number of sources increases from a single point source through

multiple sources (e.g. submarine ramp) to an almost linear source

slope apron, so feeder channelstabilityand organization of the

depositional system decreases.

For 15 years a fter the discovery that graded beds could bedeposited by turbidity currents, geologists considered thatturbiditeswere eposited onde ep, relatively flat, basinfloors similar to the abyssal plains of the mod ern Atlanticocean. However, owards the end of the 1960s and duringthe 1970s, submarine fansecame the model forheinterpretation of turb idite systems, t o the virtual exclusionof all others . Thepeak of the fashion for submarine fans wasreached at heend of th edecade with the all embracingsingle fan model of Walker (1978) and he ubsequ entdiscussion by Nilsen et al . (1980). Most autho rs of this time

were attempting to find a single fan mode l. Norm ark (1974)however had suggested that there were two, and during the1980s aonsensus eveloped that two types could bedistinguished (e.g. Mutti 1979; Stow et a1 1985; Shanmugam& Moiola 1985). Although the particular fe atures that eachauthor stressed were not always the same, it was clear thatthere was (1) an elongate , mud -dom inated, high-efficiency,open basin fan that is relatively large and normally fed by amajorelta,nd (2) aadial, sand-dom inated, low-efficiency, restricted basin fan tha t is relatively small and fedfrom a shelf canyon. To these two types a third was addedby Stow (1986), butot by others; a fanelta or

short-headed delta ront anhat is coarse-grained andusually feeds into shallow water. The fan delta was explicitly

eliminated by Shanmugam & Moiola (1988) in theirclassification of submarine fans onhe grounds thatsubmarinean swere eposited in deepwater, usuallybeyond the continental shelf, and fan delt as generally werenot.

The importance of slope aprons was not fullyappreciateduring the 1970s, althoughhey had beenknown a t least since the work in Pacific basins of Gorsline &Emery (1959). Slope apronshadbeen considered tobelinear margins where sand-carrying turbidity curr ents weremissing and sedimentation was dominated by hemipelagicmud, possibly deposited by diluteurbidity currents or

nepheloidayers, nd by mass flows, especially slumps.More recently it hasbeen realized (e.g. Stow 1981) tha t,

during periods of lower sea level, when the coast reachedthe shelf edge, sands may have been deposited at the base of

slopes, or even part way down the lope, by turbiditycurrents forming small-scale submarine fans and , perh aps,coalescing sandy systems.

Meanwhile the concept of a fan, as a single point-sourcedepositional system, hadbeen hallenged.Work on heclassic Eo cen e submarine an of th e Tyee Formation of

Oregon led first to an interpretation of it as a ‘line-sourcefan’ fed by multiple channels from a delt a (Chan & Dott1983) and then to a proposal for a ‘submarine ra mp m odel’(Heller & Dickinson 1985), the ramp being fed by multipleshallow gullies or delta sloperoughseading toodistinction betweenchannel and overbank or interchanneldeposits and hence no lobes. T he ramp model differs fromthe submarine anmodel n lacking a single feed er slopechannel or canyon.

Problems of classification. In sedimentary systems, differ-ent methods of classification have been used. Schumm(1981) used a simple spectrum of grain sizes and sedi-ment load along the horizontal axis and channel pattern

along the vertical axis. Galloway (1975) lassified delt ason a riangular diagram to show the interaction of fluvialand reworking basinal processes (wave or tidal) at the deltafront, This triangle was extend ed in the third dimension byOrton (1988) to ta ke into account the grain size of differentdelta systems and to emphasize the distinctions between theeffectiveness of processes on different types of sedimentinput, in the same way as Johnson & Baldwin (1986) haddone for shallow marine clastic systems.

Recently there haveeen two majo r syntheses of

deep-sea depositional systems. In ne, Shanm ugam &

Moiola (1988) set out a classification based on he mega-tectonic setting of submarine ans.This is unsatisfactorybecause their ‘imma ture passive margin’ fans have charac-

teristics similar to ‘active’ margin fans nd the B engal,Indusan dOrinocohave to be placed in a ‘mixed‘ cate-gory. In he othe r, by Pickering et al . (1989), no attem ptwas made to formulate a general model or to classify sub-marine fans. Instea d, they took an entirely descriptive ap-proach. Th ere is, oweve r, consensus thathere remud-rich and sand-rich submarine fans, and also fan del-tas, and if systems can be com pared and contraste d, andmodels formulated, the relationshipsbetween facies andprocesses and the significance of the controlling factorscan be clarified.

Although the nature of deep-sea systems is controlled

by sea level, tectonics and sediment supply, and sea-levelchanges and local tectonics are critical in controlling thegrowth patterns of deep-sea depositional systems, sea leveland mega-tectonics are unsatisfactory asmeans of class-ification because hey neit her affect the dee p-sea systemdirectly, nor are easily observable n ancient systems. Th einput rom the eeder system, on heotherhand , is thefundamental ontrol on henature of deep-sea systems.Sediment calibre or grain size, and sedim ent volume , gov-ern he type of sediment gravity flow that delivers sedi-ment, the distance that flows can trave l, the size and gra-dient of the depositionalystem, andhe attern of

valleys, channels, levees and lobes. A secondary control isthe pattern of delivery to th e deep-sea system. These arethe two essential controls that need to be included in anysystem of classification.

427



428 H . C. R E A D I N G

Alternative classifications of deep-seaepositional

systems. In n ttempt to link alluvial, shoreline/delta,shelf and eep water environmentsnto largermacro-systems, Reading & Orton (1991) used ediment alibreand sedimentvolume s the main control and link be-tween systems and ther efore the principal feature in class-ification of all depositional systems.

For deep-sea systems they adopted a Schumm-like dia-

gram putting grain size along the horizon tal axis to form aspectrum from mud-dominated through sand-dominated to

gravel-dominated systems (Fig. la).The vertical axis canthen be used to por t ray the type of delivery system froma point ource, typical of fans, to a multiple source, in-cluding theubmar ineamps of Heller & Dickinson

DEEP SEA DEPOSITIONAL SYSTEMS

MUD-DOMINATED SAND-DOMINATED GRAVEL-DOMINATED

DELGADA LA JoLLA VA,RINDUS

VALENCIA N~GER AVON CALABAR

MISSISSIP$AE::ENA LyArGSAVY YALLAHS

LAURENTIAN MONTEREV FORTES FM

KONGSFJORD FM PERA CAVA FM CAP W A G E FM

KONGSFJORD FM (TRANS) TVEEFM"ILEMCURAVACAS FM

MOZAMBIOUE EBRO MWEELREA OR

MATULJA FM

I KNIGHTS INLET

GULL ISLAND FM

NOVA SCOTIA $$vES*CLOET*EL WOLLASTONORLAND GHIGH SEA LEVEL

SARDHIA-TVRRHENIAN FM

, HAREELV FM (TRANS)

S.W.AFRICA

hcreasmg size of sowce area, size of dep osli nal system, radius, sue ofRows. tendency for mabr sbmps. persistence and size of fan charnels,

charmelllevee systems, tendency to meande r, sheet-like sands in lower fan

27hannel systems. tendency for channels to migrate laterally

hcreasmg wain size, slope. frequency of flows. inpersistence of

Fig.1. Two alternative classifications of modern and ancient

deep-sea systemsbased on grain-size and feeder system.FM, Formation; G R , Group; LONG, Longitudinal; TRANS,Transverse. (a) is modified from Reading & Orton (1991).

(1985), to he extrem e multiple or linear ource of theclassical slope apron. Whilst it is not possible todifferentiate systems with mixed grain sizes in this form of

classification, it can be directly linked to the feeder fluvialan ddeltaic systems that have been classified with grainsize on the horizontal axis. Boundariesbetween systemsare eliminated and em phasis is put o n the gradations fromone category to another.An alternative is to construct atriangulardiagram,putting grain size at he apices andthen extending the figure along the axis to account for thespectrum of supply systems from a single point sourcethrough increasingly multiplepoint ources to an essen-tially linear slope apron (Fig. lb ). This has the advantageof being abl e o include a mixed grain size distributionsuch as gravel and mud or a complete mixture of gravel,sand ndmud within the triangle. Th e disadvantage isthat t differs fundamentally rom the systems used forother sedimentary systems where grain su e runs along thehorizontal axis and processes ar e eit he r within the triangleor o n the vertical axis.

In both classifications, the placing of individual systemsis qualitative since quantitative data are not available an d

the positioning of any system is debatable. However theapproximate position of any system is correct, and placingit in position focuses attentionon he controlling factorsand comparison with other systems.

Point source submarine fans

Mud-dominated submarine fans include all the classic elon-gate, high-efficiency fans that form themajor submarine

fans at the surface and in the subsurface of the presentoceans. They stem rom a large source area hat funnelssediment into a feeder system of limited width and haveturbidity currents hat penetra te far nto the basin, build-ing large stabledepositional an obes, uperimposed on

onenother (e.g.Weimer 1990). Onhepper andmiddle fan, largeersistenthannel-levee or channel-overbank systems m eande r and avulse. T he lower fan haslarge subtleobes ma de from sheet-like sands, thatpenetra te into the associated basin plain.

Sand-dominated submarine fans are epresented by theclassic Californian continentalBorderlandsubmarinefans,Delgada, La Jollaand Navy. However hey may includeany fan forming on a passive margin that is fed by re-worked shelf sands unneled down canyons no t directlyconnected to a majordelta, or example the Avon andCalabar fans either side of the Niger delta (Burke 1972).Th e relatively small-scale sandy urbidity cu rre nts do not

run very far; they are ' inefficient'. O n the mid-fan, supra-fan lobes develop. O n he lower fan, he sandsform dis-tinct lobes. In theory, a radial pattern shoulddevelop.How ever, since they requently form in active, rifted or

strike-slip basins, local tectonics and synsedimentary tec-tonics, due either o compaction or growth folding withinthe basin, may govern the shape of the an .The basinplain is filled largely by pelagic sediments.

Gravel-dominated submarine fans are the deep water por-tion of fan deltas where mass flow processes predominate.Th e Yallahs F an of Jamaica passes into 2000m of water(Wescott & Ethrid ge 1980). They have een roperlystudied only in fjords (Prior & Bornho ld 1989) where theyhave been examined down to 400-500m depth . They have



S H O R T P A P E R 4 2 9

steep slopes, flows are frequen t but small, and channels o rchutes switch and migrate easily.

Slopeaprons. Thesehavean almost inear source, withmultiple feeders, and are he most aterallyextensive ba-sin margin system.

Mud-dominated lopeaprons are common at hepresentday and duringperiods of relatively high sea level. They

are ed by low density urbidity currentsand continuousbasin currentshat can both deposit and rode.Theybuild up slowly, but are unst able and subject to slumping.Where ands are nterbedde d with the mudstones,he yoccur in gullies and have abun dant soft-sediment deforma-tion structures s in theHareelvFormation transversemarginal facies (Surlyk 1987).

Sand-dominatedlope prons were robably a commonfeatureat times of lower sea level, when alluvial plainsreached to he shelf breakand many small rivers coulddischarge directly to he op of the slope. Theyare seenon faulted margins as n thepresent Sardinia-TyrrhenianSea (Wezel et al. 1981).

Gravel-dominated slopeaprons are apparently rare today.This may be because deep water systems off tectonicallyactive islands havenot been extensively studied, andouronly information abou t them comes fro m studies in fjords.Yet s much riverine ediment enters he ea rom hetectonically active western Pacific islands, including Japan,Taiwan,ndonesia,hilippines, New Guin eand NewZealand, as fromheontinents of Northnd outhAmericaombined, and Taiwan alone produces onlyslightly less sediment than the US A without Alaska (Mill-iman & Meade 1983). Much of this edime nt is coarsegrained and is supplied fro m linear alluvial braid plains, asoff western Taiwan. Th e Wollaston Forlan d Grou p (Surlyk

1984) may be an ancient example.

Multipleeeder/intermediateeep-seaystems. Thesesystems havecharacteristics of both ubmarine fans andslope prons. The y were first separa ted as a group byReading & Orton (1991) to include the ‘line-source fans’of Chan & Do tt (1983) and ‘submarine amps’ of Heller& Dickinson (1985), represente d by the Tye e and MatilijaFormations. At the coarser end of the grain-size spectrumare he many examples of ‘coalescing fans’ such as arefound in theNor thSea , or example the conglomeratic

Braeormation (Turner et al. 1986) andhe sandyMontrose Group Heritier et al. 1979). Athe resentday, he almost inear contine ntal margin of the southern

Natal coast off SE Africa, which is dissected by some85canyons, fee ds coalescing continen tal rise lobes (Dingle &

Robson 1985).

Summarynd discussion. Any form of classification isbound to oversimplify and this one is no exception. Cer-tain elected para me ters re overemphasized andothersunderplayed or even ignored. Comparison is made be-tween complex systems that have scales spread over seve-ral orders of magnitude, sizes ranging from radii of lessthan 1km to more than loo0 km and time periods rangingfrom a few years to several million years.Comparisonbetween systems with such enorm ous size ranges is sure tobeangerous, andatural systems will have many

exceptions.

Neverthe less, grain-size is a unifymg factor etweendepositional systems, linking the deep-sea system to itsfeeder system. If the type of deep-sea system is known,the nature of th e upcurre nt feeding delta, shelf and allu-vial system, its limate, tectonics and processes of sedi-mentation , can be predicted. If th e type of source area is

known, then thenature of the deep-sea system can bepredicted. For exam ple, large-scale, low-gradien t, elongate

fans require a large fluvial deltaic feeder system whetherderived from an immense area of stable continent, as theMississippi and Amazon fans,or a continental collisionalmountain belt, as the Indus and Bengal fans. Passive mar-gin fans, such as the Avon and Calabar fans, marginal tothe Niger delta, deriving their sediment by longshoredrifted shelf sediments via canyons (Burke 1972) are likelyto be short-radius, sand-dominated fans similar to those ofactive, faulted basins. It is also possible that some of

these and-dominatedans aremo re active during sea-level high stands, ncontrast to delta-fedmud-dominatedfans which are mo re likely to be active during falling andlow stand sea levels (see also Dingus & Galloway 1990).

The type of feeder system, ranging from a single point

source, typical of submarineans, through multiplesources to he linearslope apro n, governs the shape, sta-bility andorganization of the deep-sea ystem. A singlepoint ource tend s o build a coherent system with wellorganized channel-leveeequence s tha t extend parallelwith current flow an d are clearly broken by th e effects of

the relatively rare switching or by sea-level changes. Mul-tiple sources give less coherent systems with disorganizedsequences thatendo xtend parallel with the basinmargin.

Up o now,slope aprons have usually been separatedfrom deep-sea fans and placed in a separate category. He rethey are differentiated on he basis of grain size, ndan intermediate division with elements of both ansan d

slope aprons is created . Th e existence of this intermediatedivision raises the questionas to why presentda ydeltasseem to feed into deep-sea systems categorized as sub-marine fans, yet so many ancient systems appear o bemultiple sourced. The answer is perhaps in the scale, bothspatial and em pora l, t which we look at he system.Present day mud-dominatedsubma rine ans have radii of

102 to >103 km. A delta source w ith a width of 200 kmmay look,on a global scale, like a pointsource, and , inthe shortime scale that we have seen t, he feedingdeltaha sprobably had only one main position. Yet onthe scale that we see ancient systems, a 200 m widesource may app ear obe multiple rath er than single be-cause we can examine itover a longer ime scale andtherefore more easily recognize multiple, changingsources. It is significant tha t he Mississippi, which s theonly modern fan that has been studied in depth , has beenshown to becomposed of 17 temporarilystablechannel-levee systems superimposed a nd overlapping one ano ther ,and fed from a source that switched and migrated about250 km along the shelf (W eimer 1990). Ea ch channe l-leveesystem is a point-sourced submarine fan.Yet he whole4 km thick Pliocene to Pleistocene Mississippi subma rinefan , deposited ver a period of-4 M a, has the ap-pearance of a multiple source/interm ediate deep-sea

system.Whilst an nterm ediate division certainlyenhancesun -

derstanding of some systems, especially submarineramps,



430 H . C . R E A D I N G

it does leadoome possibly dan gero us simplificationof the relationshipsbetwee n the grain-size divisions anddependent eatures. In addition , he inverse relationsh ipbetweengrain-size and size of source area nd su e ofdepo sitional system may be rue orsubmarine ans,butnot for slope apron s. The absenc e of a majo r point source

where a large volume of sediment can accumulate to pro-

duce hemajor flowsof a typical fine-grained subm arine

fan may limit the istance hat flow can travel ndpenetrate into a basin. U nfortunately, we know very littleabouthe possibleoarse-grained systems thatccuraroun d he islands of the SW Pacific and he volum es ofsediment that they can contain.

Throughout this p aper, the emphasis has be en on sub-marine fans and slope aprons. Yet flat basin plains are alocus of turbiditedeposition.Although probably confinedto he finer-grain ed half of the spectrum , hese areas areimp ortant today and clearly were in the past. Basin plainscan occur on all scalesPilkey et al. 1980) fromhe660 OOO km2 Sohm Basin Plain in the Atlan tic to the small

strike-slip basins such as the 390 km2 Santa Monica of theCalifornian Borderlandsand many smaller ones n akes,

slope basins and rench es e.g.Navidad, 200 km’). Thedepositional pattern, size of flow and penetration into thebasin here oo is controlle d by the supply of sedimen t,which s aconse quen ce of the relative size of source or

drainag e area to basinal area (Pilkey et al . 1980), and thefrequency of tec tonic mov eme nt, relativelyquiescent bas-ins receiving the larger flows.

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Received 22 October 1990; revised typescript accepted 1 February 1991

1991 reading deep water system classification

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