johnson et al 2001 ecca basin floor to slope tudbid (4)

Anatomy, geometry and sequence stratigraphy of basin floorto slope turbidite systems, Tanqua Karoo, South Africa

STEPHEN DAVID JOHNSON*1, STEPHEN FLINT*, DAVID HINDS*and H. DE VILLE WICKENS*STRAT Group, Department of Earth Sciences, University of Liverpool, Brownlow Street,Liverpool L69 3BX, UKStatoil Exploration and Petroleum Technology Centre, Rotvoll, N-7005, Trondheim, NorwayDepartment of Geology and Petroleum Geology, Kings College, Aberdeen AB24 4UE, UKDepartment of Geology, University of Stellenbosch, Matieland, South Africa

ABSTRACT

The Tanqua area of the Karoo basin, South Africa, contains five Permian deep-

water turbidite fan systems, almost completely exposed over some 640 km2.

Reconstruction of the basin-fill and fan distributions indicates a progradational

trend in the 450 m+ thick succession, from distal basin floor (fan 1) through

basin-floor subenvironments (fans 2, 3 and 4) to a slope setting (fan 5). Fans are

up to 65 m thick with gradational to sharp bases and tops. Facies associations

include basin plain claystone and distal turbidite siltstone/claystone and a

range of fine-grained sandstone associations, including low- and high-density

turbidite current deposits and proportionally minor debris/slurry flows.

Architectural elements include sheets of amalgamated and layered styles and

channels of five types. Each fan is interpreted as a low-frequency lowstand

systems tract with the shaly interfan intervals representing transgressive and

highstand systems tracts. All fans show complex internal facies distributions

but exhibit a high-frequency internal stratigraphy based on fan-wide zones of

relative sediment starvation. These zones are interpreted as transgressive and

highstand systems tracts of higher order sequences. Sandy packages between

these fine-grained intervals are interpreted as high-frequency lowstand systems

tracts and exhibit dominantly progradational stacking patterns, resulting in

subtle downdip clinoform geometries. Bases of fans and intrafan packages are

interpreted as low- and high-frequency sequence boundaries respectively.

Facies juxtapositions across these sequence boundaries are variable and may be

gradational, sharp or erosive. In all cases, criteria for a basinward shift of facies

are met, but there is no standard motif for sequence boundaries in this system.

High-frequency sequences represent the dominant mechanism of active fan

growth in the Tanqua deep-water system.

Keywords Architectural elements, correlation, Karoo Basin, sequencestratigraphy, turbidites.

INTRODUCTION

There has been a renaissance of research intodeep-water clastic depositional systems in recentyears as a result of an increasing emphasis inthe oil industry on the exploration for, and

1Present address: Statoil International, Exploration andProduction West Africa, B217, Grenseveien 21, N-4035,Stavanger, Norway (E-mail: [email protected]).

Sedimentology (2001) 48, 9871023

2001 International Association of Sedimentologists 987

exploitation of, turbidite-hosted reservoirs. Theneed to develop accurate predictive geologicalmodels for deep-water clastic systems requiresdata sets that span seismic-scale geometries tocore-scale lithofacies distributions. This paperdescribes and interprets the sedimentology andarchitecture of just such an area, the Tanquaturbidite systems of South Africa. The paper alsoassesses how the concepts of high-resolutionsequence stratigraphy may be applied to deep-water clastic systems. Finally, a model is devel-oped to predict the distribution and stratigraphicevolution of the Tanqua submarine fans.

GEOLOGICAL SETTING

The study area is located in the south-westernKaroo foreland basin and is bounded by thewestern and southern branches of the Cape FoldBelt (CFB; Fig. 1A). The southern margin of theKaroo Basin is underlain by the Cape Supergroup,a lower Palaeozoic passive margin clastic wedgeup to 8 km thick (Tankard et al., 1982). Duringthe Permian and Triassic, the Cape Fold Beltdeveloped as a retro-arc thrust belt (De Wit &Ransome, 1992). Major contractional/upliftevents dated at 278, 258, 247 and 230 Ma frommica growth in cleavage (Halbich et al., 1983)supplement the paucity of other chronostrati-graphic information (Turner, 1999). The CFBdeveloped two limbs (Fig. 1B) with a centralsyntaxis, the growth of which controlled thedevelopment of the separate Tanqua and Laings-burg subbasins (De Beer, 1990; Wickens, 1994).Previous sedimentological studies on the Tanquafan complex reported by Wickens (1994), Bouma& Wickens (1991, 1994), Bouma et al. (1995) andWickens and Bouma (2000) define five majorlithofacies for the sand-rich fan systems, but donot present detailed analysis of the fine-grainedintervals between the fans. Specific studies onindividual fans and elements of fans are presen-ted by Basu & Bouma (2000), Rozman (2000) andKirschner & Bouma (2000), leaving this paper asthe first detailed treatment of the entire basin-fillsuccession using an integrated sedimentological/sequence stratigraphic approach.

Stratigraphy of the Tanqua subbasin

The Karoo Supergroup in the south-western Karoobasin is divided into the Dwyka Group (Westpha-lian to early Permian), the Ecca Group (Permian)and the Beaufort Group (Permo-Triassic; Fig. 2A).

The Dwyka Group consists of glacial deposits. Asubsequent sea-level rise as a result of melting icesheets (Visser, 1991) established an extensiveshallow sea, represented by basal Ecca Groupmarine shales of the Prince Albert and WhitehillFormations (Visser, 1992). Overlying distal turbi-dites and ashes of the Collingham Formationprovide evidence of active arc volcanism to thesouth. The Collingham Formation is overlain bydark basinal shales of the Tierberg Formation,succeeded by the Skoorsteenberg Formation, a450 m+ thick succession of five major deep-water,sand-rich submarine fan complexes separated byfine-grained intervals (Bouma & Wickens, 1991;Wickens, 1994; Bouma, 1997), which forms thebasis of this study. The absence of body fossils ledsome workers to propose a lacustrine setting forthe turbidites (see Scott et al., 2000), but thetrace fossils identified in this study suggest abasin with marine salinity, as proposed byWickens (1994). The turbidite succession isoverlain by shoreface and tidal sandstones, whichalso support a marine basin setting. This upperEcca Group shallow-marine succession is overlainby fluvial deposits of the Beaufort Group(Wickens, 1994).

Data set and methodology

The Skoorsteenberg Formation crops out over640 km2 (Fig. 2B) and provides areas of near-continuous exposure up to 60 km long. Verticalexposure allows measurement of 200500 m con-tinuous vertical sections through the completestratigraphy (over 3 km of section was logged atcentimetre-scale in fans 15 and the overlyingdeltaic succession). This permits lithofaciesdistributions, geometries and depositional envi-ronments to be interpreted at scales from theindividual architectural element to the completefan. The vertical sections (Fig. 2B), combinedwith field mapping and interpretation of land andaerial photomontages, constrain the large-scalearchitecture and depositional style. Post-deposi-tional tectonic disturbance is restricted to rarelow-angle thrust and normal faults.

In this paper, lithofacies associations are des-cribed separately from geometrical arrangementsof facies associations, thus allowing an initialprocess-based interpretation. The geometricalcontext of the facies associations is then presen-ted, allowing a direct comparison with seismicinterpretation from subsurface data sets. Faciesassociations and their geometries are thenassigned to architectural elements, as described

988 S. D. Johnson et al.

2001 International Association of Sedimentologists, Sedimentology, 48, 9871023

by Pickering et al. (1995). An interpretation of thedepositional setting is then provided; this proce-dure allows objective analysis of complex sys-tems in which similar processes and productsmay be generated in different deep-water envi-ronments, with different resultant geometries.The second part of the paper interprets keysurfaces, stacking patterns and finally presents asequence stratigraphic model for the evolution ofthe fan systems.

LITHOFACIES AND FACIESASSOCIATIONS

The general characteristics of the individuallithofacies are described in Table 1, based on adescriptive scheme cross-referenced to Pickeringet al. (1986, 1989) and Mutti & Ricci Lucci (1972)with examples shown in Figs 3 and 4.

Cape Town

Tanqua Basin FloorFan Complex

Laingsburg Basin FloorFan Complex

Swartberg Branch

Direction of sedimenttransport

Direction of compression

Karoo Supergroup

Cape Fold Belt

Scale

N

100 km

Klaasstroom

Syntaxisanticlinoria

B

NAMIBIA BOTSWANA

ZIMBABWE

SOUTHAFRICA

KAROO BASINSTUDY AREA

AFRICA

CAPETOWN

A

Laingsburg

160 E 200 E 240 E 280 E 320 E

200 S

240 S

280 S

240 E220 E200

E

Fig. 1. (A) Location map of thestudy area. (B) General tectonicsetting of the south-western Karoo(after Wickens, 1994).

Anatomy and stratigraphy of Karoo turbidite systems 989


24 Logged section

2a

1a

4

7

8

1113

14

16 17

6

0 5 Km

18

B

Fig. 8C

Fig. 8F

Fig. 8E

Fig. 8D

Fig. 8G

Fig. 8A

Figs 8Band 8H

ISOLATED RIPPLES

RIPPLE TO MASSIVE (Tc-e)

WAVY LAMINATION

RIPPLE TO PARALLEL LAMINATION (Tc-d)

MASSIVE (Ta)

DISH STRUCTURES

PARALLEL LAMINATION (Td to Tb)

PARALLEL TO MASSIVE (Tb-Ta)

WOOD AND LEAFFRAGMENTS

ORGANIC FRAGMENTS

MASSIVE TO PARALLEL LAMINATION (Ta-Tb)

CURRENT RIPPLE LAMINATION (Tc)

SCOUR AND FILL STRUCTURE

MASSIVE TO RIPPLELAMINATION (Ta-Tc with no Tb)

LOADSTRUCTURE

STARVEDRIPPLES

KEY TO LOGGED SECTIONS

LOADED BASE TO BED

RIP-UP CLASTS

CHAOTIC GEOMETRIES

CONCRETION

C

TRIASSIC

PERMIAN

CARBONIFEROUS

DEVONIAN

ORDOVICIAN

PRECAMBRIAN

POORTJIE SST

KOEDESBERG FM.

KOOKFONTEIN FM.

TIERBERG FM.

WHITEHALL FM.

PRINCE ALBERT FM

SKOORSTEENBERG FM.

BEAUFORTGROUP

ECCAGROUP

DWYKA GROUP

WITTEBERGGROUP

BOKKEVELDGROUP

CAPE GRANITE

A

Ongeluks Rivier

Gemsbok Rivier

Skoorsteenberg



Facies association 1: Hemipelagic suspensiondeposits

Description

This facies association comprises claystone(lithofacies 1), siltstone (lithofacies 2; Fig. 3G),volcanic ash (lithofacies 9; Fig. 3I) and concre-tionary horizons (lithofacies 10; Fig. 3H). Thesedeposits are usually structureless or parallellaminated, and claystone beds commonly coarsenupwards into silty claystone or siltstone to form

Table 1. Characteristics of lithofacies defined in this paper.

Lithofaciesnumber Lithology

Sedimentarystructures

Boumadivisions

Boundingsurfaces Thickness Geometry

Trace fossilsand othernotable features

Process ofdeposition

Facies code ofother workers

1 Claystone/siltyclaystone

Generallymassive.Rare parallellamination

None inclaystone.Tdce insilt

Gradational 2-cm to>10-mpackages

Oftenlaterallyextensivesheets

Rare to present.Chondrites.Commonconcretionaryhorizons(lithofacies 10)

Depositionfromhemipelagicsuspensionand low-concentrationturbiditycurrents

P:E2 M:G

2 Claystone/silty clay-stone withmm to cmsilt/vfslaminae

Generallymassive toparallel lam.Starved ripplelamination

Tde rareTcde

Laminaehave sharpbases andgradationaltops

Laminae011 cm.Units10 cm to4 m

Laminaetabular tolenticular.Unitsoftensheet-like

Rare to moderate.Chondrites,Gordia sp.

Hemipelagicand low-concentrationturbiditycurrentdeposition

P:D2/E M:D,E

3 Siltvfs/vfs clay-stonecouplets

Silt to vfs havestarvedripples,parallellaminationand massivenature. Prodsand grooveson base

Variable.CommonTcde, Tdeand Tbc

Sands havesharp bases,tops aregradationalto sharp

1 cm to5 cm

Individualbedstabular attheoutcropscale.Unitsdisplay asheetgeometry

Common.Chondrites,Helminthopsis,Helminthoida,Gordia sp.Lorenzinia,LophocteniumCosmorhaphe,Palaeodyctyon

Generallylow-concentrationturbiditycurrent andminorhemipelagicdeposition

P:C2/D M:E-D

4 Vfsfs/siltcouplets

Variable.Climbingripples,parallellamination,massive.Localsigmoidgeometryand pinchand swell

Tcde TabTae(Tcdemostcommon)

Sharp basesto sands.Sharp togradationaltops. Upperfine layer(Te) can beabsent

Sandsusuallyform unitsdominatedby beddingof 510 cm,1020 cmand>20 cm

Beds tabular.Sheetgeometryto units

Lowhighconcentrationturbiditycurrents

P:C/D M:C(B)

992

S.

D.

Joh

nso

net

al.

2001

Inte

rnatio

nal

Asso

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tion

of

Sed

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tolo

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Sed

imen

tolo

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48,

9871023

Table 1. Continued.

Lithofaciesnumber Lithology

Sedimentarystructures

Boumadivisions

Boundingsurfaces Thickness Geometry

Trace fossilsand othernotable features

Process ofdeposition

Facies code ofother workers

5 Medium- tothick-beddedsandstone

Commonlymassive,parallellamination,climbingripples. Somescour and fill.Dewatering

Tab andTaccommon

Bases sharpand rarelyerosional

2060 cm Tabular tolocallylenticular.Unitsform sheetandchannelgeometries

Rare.Helmenthoides

High-concentrationturbiditycurrents.Variety offlow types butdominated bydepletivesteady flow

P:B1B11M:(B)

6 Massivesandstone

Massive, localparallellamination.Local scourand fill.Dewatering

Ta Sharp toerosivebases andsharp tops

2080 cm10-mpackagescausedby amal-gamation

Channel fillsand somesheets

Woodfragmentsoccur

High-concentrationturbiditycurrents withdepletivesteady andunsteady flows

P:B M:B

7 Intraclast-richconglo-merate

Chaotic Sharp andsometimeserosive

can be common at the tops of beds (Fig. 4D andE). Upper parts of beds may contain rare ripplelamination. Thicker beds (>70 cm thick) arecommonly formed as a result of the amalgamationof thinner beds. Amalgamation surfaces are com-monly expressed as rip-up clast horizons appar-ently floating within a bed, but can often betraced laterally into a siltstone parting betweentwo separate sandstone beds (Fig. 4A).

Greater occurrence of sedimentary structurescharacterizes the second bedding style. Sandgrade varies between very fine and fine sand(rarely medium grained; Fig. 5), and some normalgrading is visible. Bed bases are sharp, commonlyloaded and can show localized scour and prodand groove marks into interbedded shale. Internalstructures are variable, and complete Boumasequences are not observed. Tac sequences withdish structures are common; alternatively, bedsare completely dominated by Tc lamination,which varies laterally along beds. Rare troughcross-bedding and scour-and-fill structures areobserved (Fig. 3F). The scour-and-fill structure isconsidered to represent occasional turbulentbursts that create local scours on the sedimentwater interface that are subsequently filled byparallel lamination. Rip-up clasts and woodfragments are commonly found in the upper partof an individual bed (Fig. 4E). Trace fossilsincluding Helminthoida are primarily associatedwith the silty shale interbeds.

Interpretation

Massive sandstones are interpreted as high con-centrations of grains that flow rapidly on a slopeand are deposited in the upper flow regime (Lowe,1982). A rapid decrease in flow velocity results inabrupt fall-out of sediment, which suppressesdevelopment of tractional structures or sorting ofgrains at the base. The presence of floating rip-upclasts and clast-rich tops to beds suggest possiblyaggrading flows of a high-density nature that formin a depletive steady flow (Kneller, 1995). Exten-sive climbing current ripples (bedding style 2)suggest rapid deposition from high rates of sus-pension fall-out from sustained flows.

Facies association 4: Sigmoidalripple-dominated deposits

Description

This distinctive facies association is confined tothe section between fans 4 and 5. It compriseslithofacies 4 and 3 in small-scale (

packages that show either thickening/slight coar-sening-upward, fining-upward or more symmet-rical motifs (Fig. 5, 110175 m on log section).Key features are the dominance of sharp-basedand sharp-topped, very fine to fine sandstonebeds that contain current ripple lamination withsiltstone drapes. The ripples are 15 cm in height(examples up to 10 cm are interpreted as minordunes) and show low to moderate angles of climb(015). Beds exhibit a distinct pinch and swell toa sigmoidal geometry (Fig. 3D). Concretionaryhorizons are common. Trace fossils includeChondrites, Helminthopsis, Helminthoida, Gor-dia sp., Lorenzinia, Lophoctenium, Granulana sp.and Cosmorhaphe.

Interpretation

The ripples, small dunes and parallel laminationindicate tractional processes of the lower flow

regime. The finer couplet of siltstone settles as aresult of suspension from the dilute tail of anindividual turbidity current. The diverse tracefossil assemblage suggests relatively unstressedopen marine conditions, in contrast to previouslacustrine interpretations (Scott et al., 2000). Theripple-dominated beds differ from rippled bedstypical of overbank deposition lower in thesuccession on account of their sharp bases andtops, the absence of grading and the pinch andswell/sigmoidal bed geometries. These beds mayrepresent low mud content turbidites with relat-ively high velocities that produced traction-dominated structures before rapid cessation offlow to give the sharp tops. The pinch and swellnature of the bedding reflects the preservation oforiginal depositional topography created by thethree-dimensional ripple form. The sigmoidgeometry (Fig. 3D) to some beds may be a result

Fig. 3. (A) Amalgamated sandstones of the thick- to massive-bedded turbidite facies association showing massivestyle and faint diffuse parallel lamination in places. Vertical view is 7 m. (B) Thin-bedded turbidite facies asso-ciation with a thickening-upward signature. Note the tabular nature at scale of outcrop. (C) Ripple-dominated thin-bedded turbidites with pinch and swell bedding geometry (spade is 60 cm long). (D) Sigmoidal rippled bedding stylethat characterizes slope sedimentation in the study area (pencil 15 cm). (E) Current ripple (Tc)-dominated bedding(pencil 15 cm). (F) Finger points to the base of a local scour and migrating bedform within a generally thick-beddedto massive turbidite facies association. (G) Siltstone facies characterized by starved ripples deposited from slowmoving, very dilute turbidity current (pencil, 6 cm in view). (H) Concretionary horizon (pencil 15 cm). (I) Tuffa-ceous horizon (arrowed) picked out by lighter grey horizontal band (total view 50 cm wide).



of minor compensation effects, in which laterflows deposit preferentially in the hollowscreated by the depositional topography fromearlier flows. Deposits with these characteristicshave been interpreted as deposition on a slopewith the ripple forms produced by supercriticalflow (Prave & Duke, 1990). The sedimentologicalevidence for variations in the mud content of theflow and rate of sedimentation, combined withthe stratigraphic restriction of these deposits andthe character of the overlying fan 5 (see below),combine to indicate a partially aggradational,stable slope depositional setting for this faciesassociation.

Fig. 4. (A) Internal bedding amalgamation marked by rip-up clasts to the left of the pencil but not visible to the rightof the pencil. (B) Large-scale dewatering and ball and pillow structure from the axis of a depositional channel in fan 3(25 cm of spade for scale). (C) Channels in upper fan areas show marked erosive scour (arrowed) and abundant rip-upclasts (r). Vertical view 5 m. (D) Tops of thick-bedded to massive turbidites characterized by abundant organicmaterial, silt and rip-up clasts. (E) Wood fragments on bedding plane from the top of a thick- to massive-beddedturbidite (pencil 15 cm). (F) Rip-up clasts (pencil 15 cm). (G) Slumped heterolithic sandstones preserved insome channel scours exhibit a brecciated nature and could be misinterpreted as rip-up clasts from core. Thisexample could represent a locally slumped channel margin/levee and is only recognized in two places in the studyarea (hammer 35 cm). (H) Outsize rip-up clasts are common and can appear to be floating in the fine-grainedmatrix. Examples such as this can be traced laterally to a surface of bed amalgamation and therefore relate to either anaggrading sedimentwater interface or an erosive bed amalgamation surface (pencil 15 cm). (I) Locally reverse-faulted and fractured heterolithic sandstone and shale, possibly related to a localized slumped margin/levee(pencil 15 cm).

Fig. 5. Example of the vertical arrangement of thefacies associations and their relation to differentdepositional settings. For key, see Fig. 2C. In this sec-tion (Fig. 2B; log section 4), fans 1 and 2 are not pre-sent, because this section is located basinwards of theirpinchouts. Fans 3 and 4 represent a mid- to outer fanenvironment dominated by sheets. The finer intervalsbelow fans 3 and 4 are interpreted to represent basin-floor deposition of turbidites and hemipelagic deposits.There is a clear change in the fine interval above fan 4(110 m), where the section becomes more sand rich anddominated by the sigmoidally bedded facies associ-ation. This is thought to reflect overall progradation ofthe depositional system and development of a slopeabove fan 4. Fan 5 is a slope fan deposited in anintraslope basin comprising, in the lower part, chann-elized, stacked, thick- to massive-bedded turbiditesand, in the upper part, heterolithic thin-beddedchannelized turbidites.



Facies association 5: Debris flow/slurry faciesassociation

Description

Sandstone beds are usually thin (

60

45

50

55

1

5

10

15

10

25

20

15

AXIAL CHANNELFACIES

PROXIMAL CHANNELMARGIN FACIES

DISTAL CHANNEL

MARGIN FACIES

FLOW DIRECTION

CHANNEL AXIS

PROXIMAL CHANNEL MARGIN

DISTAL CHANNEL MARGINDISTAL CHANNEL MARGIN

PROXIMAL CHANNEL MARGIN

schematicchannel margin

dips (x 10)Outcrop location of measured sections

Log 2 Log 3 Log 4

2 3 4

SCHEMATIC18 m

log locations

0 m

3 m

(i). Erosive, multiple-event channel with a complex fill: Significant erosion and truncation of underlying stratais followed by thick-bedded turbidity current deposits in axial and marginal areas. Subsequently, the upperparts of the channel fill contain components of thin-bedded turbidites in channel margins and axial thickbedded turbidites. Finally a drape fill of thin-bedded turbidites occurs over the full channel width

(iv). Erosional channel with a heterolithic thin-bedded fill. Significant erosion followed by a fillof alternating thin-bedded turbidites, siltstones and minor amounts of thick-bedded turbidites

(v). Channel complex. Variable architecture consisting of channel fills which stack in an offset lateral andvertical arrangement. Slumped material is observed together with channel types i) to iv) above. Mostcommon are complex erosional and depositional channel types (i and iii)

(iii). Depositional and minor erosional channel. Clear internal partitioning of sand-richaxial facies and heterolithic channel margin facies

(ii). Erosional multiple-event channel with a simple fill. Significant erosion and truncation of underlying stratafollowed by multiple-event fill of thick bedded turbidites. Zero to minor amounts of thin-bedded channelmargin facies

B.

A.

Variable lateralscale from 300 m to

> 1Km



Erosional, multiple-event channelwith simple fill

These channels occur in the upper fan and atthe transition zone between the upper and mid-fan and also characterize the lower part of thefan 5 slope system. Initial erosion was followedby a fill of thick- to massive-bedded turbidites(Figs 6A (ii) and 8C). N:G can be as high as90% across the complete width of the channelfill.

Depositional and minor erosional channel

Depositional and minor erosional channels arecommon in the mid-fan settings of fans 14(Figs 6A (iii) and 8D). There can be minor erosionat the channel base, but the predominant depo-sitional style is of aggradation through time.Internal fills can be complex, and the overallgeometry comprises a sand-rich axial zone withextensive heterolithic wings (Fig. 6B). Thesechannels pass laterally into sandy overbankdeposits that have a sheet geometry. Extensivelevees appear to be absent, which suggests thatthe fill of the depositional channels and laterallyequivalent overbank sheets are approximatelytime equivalent (Zelt & Rossen, 1995). This mid-fan depositional environment may represent adeep marine equivalent of an aggrading braided-fluvial setting. In the axis of this channel type,N:G can reach 80% (Fig. 6B; log section 3),although an abrupt reduction to 30% is observedat the margins.

Erosional channel with heterolithic,thin-bedded fill

These channels (Figs 6A (iv) and 8C) occur in theupper parts of the slope fan system (fan 5). Theyare erosive and contain alternations of thin-bedded, rippled to parallel-laminated thin-bed-ded turbidites that fill the entire eroded conduit.The channels commonly stack laterally andvertically to define a large-scale sheet geometrywith N:G of 50% or less.

Channel complex

Channel complexes (Figs 6A (v) and 8B) areconfined to the proximal upper fan sections ofthe basin-floor fans and the slope fan (fan 5). Basin-floor fan channel complexes are dominated byerosional channels and some depositional channeltypes. Abundant rip-up clast material and locali-zed slumps (Fig. 4G and I) are interpreted as smalllevees that have slumped locally into the channelaxes. In the slope fan (fan 5), channel complexesin the lower part of the fan are dominated byerosive simple fill channels (Fig. 6A), and theupper half of the fan is characterized by erosive,heterolith-filled channels (Fig. 6A).

Transitional depositional styles

Transitional depositional styles are characterizedby units with an overall tabular sheet geometry,mappable over several kilometres, with internalscours and very minor channelization that definesa complex internal heterogeneity (Figs 7A and8E). This architectural style is common in areason the fan where flow changes from a confined toan unconfined nature, and has been recognized atthe downdip termination of channels and inoverbank areas downstream of interpreted chan-nel bends. The most common facies associationsare thick-bedded turbidite and hemipelagicdeposits. Debris and slurry flows (Lowe & Guy,2000) sometimes characterize bed tops. Internalbedding is very complex and dominated byabrupt lateral pinchouts and complex migratingbed- and bar-forms, and is similar to that des-cribed in the Black Flysch of northern Spain byVincente Bravo & Robles (1995). N:G is 6085%.

Sheets

Sheets have a tabular geometry with planar upperand lower surfaces and can be divided intoamalgamated sheets and layered sheets (Fig. 7B).The deposits are similar in style and architectureto those described for the Ross Formation ofWestern Ireland by Chapin et al. (1994).

Amalgamated sheets

Amalgamated sheets contain thin-bedded (40 cm thick)turbidites, shown schematically in Fig. 7B. Inboth types of sheet, compensation bedding stylesare common. The sheets contain

giving a massive to blocky appearance on photo-montages (Fig. 8F) and in sedimentary logs.Amalgamated sheets occur as thick (up to 15 m)units in the mid- to outer fan region, downdip oftransitional architectural styles and main feedersystems. They also occur in the areas betweenchannels in a mid-fan setting, where they arethinner (maximum 8 m), less extensive and dom-inated by ripple-laminated sandstone beds thatalternate with massive sandstone.

Layered sheets

Layered sheets (Figs 7B, 8G and 8H) have anexternal geometry similar to amalgamated sheetsand exhibit planar upper and lower surfaces withan overall tabular geometry. They are distin-guished by the presence of siltstone and claystone

(40% or more) beds. Layered sheets commonly liestratigraphically between amalgamated sheets,forming the less prominent, heterolithic horizonsvisible on photomontages (Fig. 8G and H).Layered sheets have been divided into thosecomposed of turbidite beds >40 cm thick andthose composed of turbidite beds

concentration flow deposits and are dominated byripple forms (Tc) throughout the beds. In theseareas, they represent interchannel sheets and passlaterally across depositional strike into deposi-tional channel systems. N:G can reach 60%.

Fines

Claystone-dominated sheets

Claystone-dominated sheets occur downdip ofthe siltstone pinchouts of each fan system, as thefiner intervals between sand-rich fans, and as thin(>4 m thick) but extensive intrafan fines (Fig. 7C).The claystones are predominantly hemipelagicdeposits (lithofacies 1); however, in updip local-ities, a component of turbiditic claystone andsiltstone is also present.

Siltstone-dominated sheets

These sheets occur immediately downdip of thesandstone pinchouts of the fans and have beenmapped intermittently for over 15 km in a basin-

ward direction. They reach a maximum thicknessof 3 m, contain parallel lamination and some rare,starved ripple lamination and are interpreted asthe product of very low concentration turbiditycurrents.

Distribution of architectural elements

Maps of the main architectural elements have beenmade for fans 15, and a single genetic fan modelhas been generated for a basin-floor fan (Fig. 9).The outcrop window for each fan is illustrated inFig. 10 in relation to the preserved architecture.The maps can be used to appreciate the plan-viewgeometries and distribution of architecturalelements within successive fans. The downdipevolution of channels, from erosive/bypass todepositional types (summarized in Fig. 6), isshown in Fig. 11. In fan 3, erosive, nested channels(Fig. 11A) pass basinwards through erosive, mul-tiple-event (Fig. 11C) and depositional (Fig. 11B)styles into unconfined, depositional sheets(Fig. 11E) over a distance of 25 km. Channel to

Fig. 8. Plate of main architectural styles observed in the study area. (A) Erosive, multiple-event channel withcomplex fill (scale bar 25 m, truncation arrowed). (B) Channel complex (individual channels numbered 14; scalebar 20 m). (C) Erosive, multiple-event channel with simple fill (labelled 1 and 2) and erosional channel withheterolithic, thin-bedded fill (labelled 3). Scale bar 10 m. (D) Depositional and minor erosive channel (labelled 1).Note the clearly thinner wings (w) away from the axis (a). Scale bar 15 m. (E) Transitional architectural stylecharacterized by overall sheet geometry with internal scour (arrowed). Scale bar 17 m. (F) Amalgamated sheets infan 4 (a) and layered sheets in fan 3 (b). Scale bar 20 m. (G) Alternation of amalgamated sheets in overbankenvironment (fan 3 mid-fan, labelled a) and layered sheets in a mid-fan environment interchannel setting labelled (b).Scale bar 10 m. (H) Example of succession dominated by layered sheets (thick-bedded labelled a, thin beddedlabelled b) in interchannel setting in fan 3, upper to mid-fan transition. Scale bar 20 m.



Erosive channel Depositional channel Massive filled broadthin channel

Heterolith-filledbroad thin channel

Transitional depositional style

Amalgamated thick- andthin-bedded sheet

Layered thin-bedded sheetLayered thick-bedded sheet Fines (silts and claystones)

ZONE 1 ZONE 2 ZONE 3 ZONE 4This area is dominated bymulti-storey and multi-lateral channel complexes.Channels are erosive andrarely depositional withextensive rip-up clasts,local slumped depositsand many erosivecontacts. In early fandevelopment sedimentsmainly bypass this area.Channel fill isaggradational toretrogradational in laterstages of fan development.

This zone is a mixture oferosive channels andlaterally-inextensive interchannel sheets.Channels have massive tothick-bedded fills and overbanksare rippled, thin- to thick-beddedamalgamated sheets.

A complex zone with architecturecontrolled by geographic positionand up-dip to down-diprelationships.Largest proportion of interchannel deposition in this zone ischaracterised by extensivetabular sheets of rippled beddedsandstone. Dominantsedimentation occurs indepositional channelsand associated interchannel areas.In the down-dip areas of thiszone extensivetabular sheets can developwith a massive-bedded style.

In this zone down-dip of extensivesheet deposits, deposition ischaracterised by isolated broad,thin channels and laterallyinextensive- to moderately-extensive thin-bedded sheets.Fan deposition in the pinch outarea is ultimately only representedby a thin silt unit.

Sequence boundary iserosive to sharp withchannels overlyingthe sequence boundary.

Sequence boundary is erosiveto sharp. Channels and somesheet like deposits overlie thesequence boundary.

Variable sequence boundaryexpression in this zone:Erosive below channels;Sharp below sheets;Gradational below thin-beddedturbidites and silts.

Sequence boundary expressionis dominated by gradational andsharp styles in this zone.Rare sharp to erosive sequenceboundary expression at the baseof localised channels.

Schematic map viewand expectedcross-sectional profiles atspecific points on the fan

Fig. 9. Model for basin-floor fan systems developed from fans 14.



sheet transitional styles (Fig. 7A) are well exposedin fan 2 (Fig. 11D), and amalgamated sheets of themid-fan area of fan 4 are shown in Fig. 11F.Layered sheets (Fig. 7B), developed within along-term interchannel area of fan 3, are domin-

ated by current ripple lamination in both thick-and thin-bedded styles (Fig. 12A). Fines-domin-ated sections between fans include thin-beddedturbidites and concretion-bearing condensed de-posits (Fig. 12B), and distal basin-floor sections

Fan 1 Fan 2

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Fig. 10. Diagram to illustrate the window of observation afforded by the outcrop orientation, compared with theinterpreted architecture of fans 14 at the time of deposition.



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Erosive channel fromnested channel complex atOngeluks Rivier (Fig. 12E).Fan 3 base of slope channelcomplex.

Depositional channel (axis) fromKanaalkop area (Fig. 12E).Mid-fan depositionalchannel.

Amalgamated sheet. Thick-beddedFan 3 mid- to outer-fan.

Erosive, multiple-event channelwith a complex fill.Fan 3 mid-fan position.

A: LOG 17 B: LOG 13 C: LOG 11 D: LOG 18 E: LOG 4 F: LOG 4

Transitional depositionalarchitecture.Note abundant scour and fill.Fan 2 mid- to outer-fan. Thisdepositional style occurs atthe transition between a channeland a sheet environment.

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Amalgamated sheet. Thick-beddedFan 4 mid-fan area.

CLST SILT VFS FSCLST SILT VFS FS

CLST SILT VFS FSCLST SILT VFS FS

CLST SILT FSCLST SILT VFS FSCLST SILT VFS FS CLST SILT VFS FSCLST SILT VFS FSCLST SILT VFS FS

Fig. 11. Type sections along an approximately dip-parallel transect of the fan model. For key, see Fig. 2C. (A) Erosive channel within nested channel complex,upper fan area, fan 3. This section is characterized by abundant rip-up clasts, erosive bedding contacts and thick- to massive-bedded turbidites. (B) Charac-teristic bedding in mid-fan section of a depositional channel axis (4861 m) and alternating thick and thin, high-suspension fall-out climbing ripple-dominatedturbidites of the interchannel succession (6170 m). (C) Example of bedding style from erosive, multistorey channel in an axial position. The lower section isdominated by amalgamated thick- to massive-bedded turbidites, whereas the upper, more depositional section (above 59 m) is dominated by current ripplelamination. (D) In areas characterized by a transitional architectural style, bedding is thick, with localized scour-and-fill structures (e.g. at 5 m) but is oftenmassive. Rip-up clasts occur but are not frequent. (E) Sheets in the mid- to outer fan area of fan 3 show some ripple lamination and parallel lamination, but aregenerally massive. (F) Sheets dominated by thick- to massive-bedded turbidites in the mid-fan environment of fan 4 with noticeable loading, amalgamation andscour.

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CLST SILT VFS FS

Location of sectionsfrom Figs 11 and 12

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Section betweenFans 2 and 3 (Klein RietFontain area; fig. 12E).

Basinward of sand pinchoutin Fan 2

Basin floor sediments:Hemipelagic shales and thin-beddedturbidites and condensed sectionsrepresented by concretionary horizons

Basin floor setting. Dilute silt turbiditedeposition that characterises fanstyle down-dip of the sand pinchout.This is located in the down-dip area ofFan 2 in the ZMF area on the map(Fig. 12E).

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Layered sheets.Thick-bedded style (57 m to 66 m on log) andthin-bedded style (71 m to 76 m on log).Overbank setting in Fan 3 (Klein Riet Fonteinarea; Fig. 12E).

Note the predominance of ripple bedding inthis area

CLST SILT VFS FS CLST SILT VFS FS CLST SILT VFS FSCLST SILT VFS FS

CLST SILT VFS FS

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Slope depositional setting.Section between Fan 4 and Fan 5 inthe Skoorsteenberg area of themap (Fig.12E).

Kanaalkop

Klein Riet Fontain

Ongeluks Rivier

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Fig. 12. (A) Heterolithic sheets dominated by alternating ripple-laminated sandstones and shales from a mid-fan interchannel environment, fan 3. (B)Dilute turbidites in thickening- and coarsening-upward interfan sandstone cycles, within the shaly intervals between the main fans. (C) Hemipelagic shaleand rare turbidites (195 m) and a well-developed 2-m-thick siltstone unit with starved ripples and parallel lamination (205227 m) that represents theexpression of a fan beyond the downdip sand pinchout. (D) Example of the fine-grained slope succession between fans 4 and 5. (E) Location map oflogged sections. (F) Inferred position of logged sections on a basin-floor fan.

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are dominated by hemipelagic sediments. Mas-sive siltstone beds represent the downdip expres-sions of fans and extend for more than 10 kmbasinwards of the mapped distal fan pinchouts(Fig. 12C). The slope depositional setting is char-acterized by facies association 4 heteroliths, wellexposed between fans 4 and 5 in the Skoorsteen-berg area (Fig. 12D).

SEQUENCE STRATIGRAPHY

Introduction

Application of classical sequence stratigraphicmodels to deep-water clastic systems implies thatall volumetrically important turbidite successionsare basin-floor fans or leveed channel complexes,deposited during or shortly after an episode ofrelative sea-level fall (Vail et al., 1977; Mitchum,1985; Mutti, 1985; Pickering et al., 1989; Weimer,1990; Posamentier et al., 1991; Den Hartog Jageret al., 1993). In addition the validity of the basin-floor fan model and the necessity of a period ofrelative sea-level fall to produce turbidites havebeen questioned by many authors (e.g. Kolla &Perlmutter, 1993; Shanmugam et al., 1995; Bur-gess & Hovius, 1998). The basic model, derivedmainly from seismic sections (Mitchum, 1985),does not address the higher frequency cyclicityobserved in outcrop data sets (Mutti et al., 1994),but recent work has attempted to apply sequencestratigraphic principles at outcrop scale (Gardner& Sonnenfeld, 1996). A fundamental differencebetween deep-water and other depositional envi-ronments is that, at any relative sea-level position,there is always accommodation space in which todeposit and preserve sediment in the deep basin.Locally, variable equilibrium profiles may affectaccommodation space on the slope (Prather et al.,1998; Prather, 2000). However, at times of negativeor minimal accommodation space on the shelf,sediment supply to the deep basin is maximized.For example, Kohl & DSDP Shipboard scientists(1985) showed that, for the Mississippi fan duringlow sea level, the non-decompacted rate of sedi-mentation was 6001100 cm/1000 years butdropped to 213 cm/1000 years during high sealevel. This does not mean that all turbiditesrepresent lowstand systems tracts, because thereare active deep-sea fans around the world today,coincident with a highstand systems tract (e.g. theVar Fan; Piper & Savoye, 1993), and calculationssuggest that many rivers can build deltas to theshelf edge and thus source turbidites during

highstands (Burgess & Hovius, 1998). However,the above studies do indicate that a major controlon the sequential development of deep-waterclastic systems is the volume and type of sedimentsupplied to the system at a given time. Thisallows an objective view of vertical stratigraphicevolution in ancient turbidite deposits, based onvolume of sediment input at a given time.

Condensed sections

The most reliable correlation markers in theTanqua system at all scales are condensed sec-tions, which therefore form the basis of thesequence stratigraphic analysis provided here.Three consistently recognizable associations ofsedimentary facies represent condensed sections.

Type 1 horizons

Type 1 horizons are condensed sections markedby nodular concretionary horizons (Fig. 3H). Theconcretions are commonly iron stained withcone-in-cone structures and vary between 7 cmand 50 cm in diameter. They form either exten-sive layers or more localized lozenge forms in thefine-grained packages of facies association 1between the fans. The variation between concre-tionary nodule horizons and concretionary pansis thought to result from variations in the lengthof time that sedimentation was suppressed (Rai-swell, 1987), with the concretionary pans form-ing during longer periods of hiatus. Type 1horizons represent relatively long-duration breaksin sedimentation that allowed the development ofdistinct diagenetic processes and are associatedwith the 20- to 60 m thick, fine-grained basinalshale packages developed between fans. Theseshaly packages are interpreted as transgressiveand highstand systems tracts of low-frequencysequences (see below). Stratigraphically clusteredtype 1 concretionary horizons are interpreted asthe deep-basin equivalent of the maximum flood-ing surface on the coeval shelf (i.e. the time oflowest net rates of sediment supply via the shelfto the deep basin).

Type 2 horizons

Type 2 horizons comprise 20 cm- to 3 m-thickintrafan packages of facies associations 13.These predominantly shale to silty-shale horizons(Fig. 13) are traceable for >20 km laterally withinthe fans and therefore represent significant peri-ods of condensation across whole fan systems.Type 2 condensed horizons are most common



between amalgamated sheets or above channelfills (Fig. 13). Because of their lateral extent andintrafan setting, type 2 horizons are interpreted ashigher order (shorter duration) condensed sec-tions than type 1 horizons. An abrupt reductionin sand supply over a whole fan system mayresult from an increase in water depth on thecoeval shelf, thereby trapping sand updip. Theseincreases in shelf water depth could be related torelative sea-level rises, but this cannot be provedas the coeval shelf is not preserved.

An alternative interpretation is that these sand-poor zones relate to autocyclic switching ofchannels within the fan. For example, updipchannel switching may cause the development ofintersheet/sheet fringe deposits above an oldersheet, producing a succession with the character-istics of a type 2 horizon. In this interpretation,a less highly ordered vertical and lateral strati-graphic signature might be expected within a fanthan those observed herein. Another possibilitymight be climatically driven reductions in sandsupply, but there is no regional evidence forsignificant climate change in the Ecca Group(Wickens, 1994). The present interpretation there-fore favours type 2 horizons being related torelative sea-level rises on the shelf and represent-ing transgressive and highstand systems tracts tohigh-frequency sequences.

Type 3 horizons

Type 3 horizons are commonly

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Log 11. Fan 3.Upper- to mid-faninterchannel setting. Type 2 and type 3horizons are welldeveloped.Deposition is intitiatedby a gradational contactthat marks the sequenceboundary.The sequence boundaryis gradational.

Log 7. Fan 3.Outer-fan.In this area type 2 andtype 1 horizons aredominant.The sequence boundarydeveloped in this area isoften gradational but isoccasionally erosive at thebase of localisedchannels.

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vertical motif between the type 2 horizons. Inareas lateral to stacked channel systems, the sand-rich packages are up to 6 m thick and compriseaggradational to fining-upward units of rippledvery fine unamalgamated sheet sandstones,whereas in axial channel complexes, they maybe represented only by time-equivalent erosionsurfaces, rip-up clast horizons or amalgamationsurfaces (Fig. 14). In downdip sheet deposits,intrafan sandstone cycles are 1- to 5-m-thick sheetsandstone packages of Bouma sequences withtheir upper CD divisions missing. Mid-fan areascontain cycles that show complex arrangementsof sheet and depositional channel sandstones(Fig. 14).

Interfan sandstone cycles

Interfan sandstone cycles are present in the 30- to60-m-thick fine-grained intervals between fans14 (Fig. 19). These sandstone cycles are domin-ated by facies association 2 and are bounded bytype 1 or type 2 condensed sections (Fig. 19).Each cycle can be traced over the whole exposure(20 km +) and has the same mappable extent asfans 13. These cycles are never >8 m thick(Fig. 19), do not grade laterally into thick fans,and sets of two to six cycles exhibit a range ofstacking patterns. These cycles may representmuch longer durations of deposition than theintrafan sandstone cycles, based on the higherproportion of finer, more slowly deposited sedi-ment. Interfan sandstone cycles are tentativelyinterpreted as parasequences within the low-frequency highstand systems tract that separateeach major fan lowstand systems tract.

Initiation of active fan growth

Field relations

Within the stratigraphic framework based oncondensed zones, it is possible to identify epi-sodes of active fan growth (sensu Pickering et al.,1995) at stratigraphic positions coincident withinterpreted regional-scale changes in the calibre,rate and architectural style of deposition. Partic-ular vertical facies juxtapositions that are

mappable over the whole study area show thefollowing characteristics:

1 A clear surface that separates thin-beddedturbidites from an overlying succession of thick-bedded turbidites (Fig. 14). In different areas of anindividual fan, this change can be marked byeither the base of a channel complex overlyingdistal basin plain or fan fringe deposits or the baseof an amalgamated sheet sandstone overlyingmore distal turbidite deposits (Fig. 14). The sur-face can be erosive if at the base of a channel, orplanar with little appreciable erosion at the baseof a sheet succession. It is important to note thatthe expression of the sequence boundary variesdepending on the position on a depositionalprofile. The exact expression of the sequenceboundary is described and illustrated in Fig. 14,with a general tendency towards a more erosive tosharp sequence boundary in updip areas and agradational to sharp boundary in downdip areas.

2 A grain-size increase across the surfacecoupled with a change to more frequent, higherconcentration turbidity current deposits abovethe boundary.

3 A net increase in sedimentation rate abovethe surface.

4 Surfaces can be traced over extensive distan-ces with the above characteristics.

Interpretation

The increase in sediment volume, sand:shaleratio and change in style from low- to high-concentration turbidite current deposition at anygiven point on the fan is associated with theinitiation of a phase of active fan growth and isinterpreted as an abrupt increase in sedimentsupply to the basin floor. This abrupt increase insediment supply over the whole fan system isconsidered to be related to a decrease in accom-modation space on the coeval shelf and thegeneration of an updip sequence boundary.Episodes of fan growth occur at a high-frequencyintrafan scale marked by abrupt increases in sanddeposition immediately above type 2 condensedsections and also on the lower frequency of thefive complete fans marked by a variable sequenceboundary expression.

With limited data sets, it is difficult to be surethat an abrupt increase in sediment supplydefinitely represents a basinward shift in facies(for example, proximal fan strata abruptlyoverlying distal fan strata). The alternativeinterpretation is a lateral shift in fan lobe depos-ition related to autocyclic processes. The key

Fig. 14. Types 1, 2 and 3 condensed horizons andsequence boundary expression in different fan settings:(A) interchannel setting, fan 3; (B) mid- to outer fansetting, fan 3; (C) outer fan setting in fan 3; (D) mid-fansheet-dominated setting in fan 4. (E) Positions of loggedsections on a basin-floor fan.



Fan 3LOG 12

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KEYMAIN SANDSTONE UNITS:SHEET GEOMETRYCHANNELISED UNITSMAIN CORRELATION ZONESTYPE 1 HORIZONTYPE 2 HORIZONTYPE 3 HORIZONSHALESANDSTONE

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Fig. 15. Fan 3, correlation of type 2 condensed sections, using the top of the fan as a datum.

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KEYMAIN SANDSTONE UNITS: SHEET GEOMETRYCHANNELISED UNITSMAIN CORRELATION ZONESTYPE 1 HORIZONTYPE 2 HORIZONTYPE 3 HORIZONSHALESANDSTONE

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Fan 3

Fig. 16. Fan 3, correlation of type 2 condensed sections, using the base of the fan as a datum. Note the basinward-dipping clinoform geometries of thecondensed sections and the lobate geometry of the lowest sandy unit.

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LOG 2ALOG 1A LOG 4 LOG 5 LOG 6 LOG 14

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Good to excellent correlation confidence Moderate to goodcorrelation confidence

Poor to moderatecorrelation confidence

Fan 4

Fig. 17. Fan 4, correlation of type 2 condensed sections in a depositional strike-oriented mid-fan setting, using the top of the fan as a datum. Note the generalsheet-like nature of this system.

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SCALE

5 m

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Location of logs in Fans 3 and 4Log 1a Log 2a Log 4 Log 5 Log 6 Log 7 Log 14

Log 4 Log 6

Log 7

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Log 13Log 14

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Fan 3

SectionbetweenFans 3 and4 is 14 mthick atthis point

Sectionbetween Fans3 and 4 is 18 mthick atthis point

Sectionbetween Fans3 and 4 is 19 mthick atthis point

Section betweenFans 3 and 4 is 20 mthick at this point

Key: Sheets

Channels

Fines-dominated packagesBasin shale dominated package

Main correlatable sand-rich packagesMain palaeoflow of individualdepositional packages

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Fig. 18. Large-scale compensational stacking of fans 3 and 4.

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BURROWS ANDVESTIGE OF WAVERIPPLEDSANDSTONE

HUMMOCKYBEDDING

TROUGHCROSS-BEDDING

SANDSTONE

MUDSTONE

P L A N A RBEDDING

BEDDINGIN

CORE

GRAPI UNITS0 150

PARASEQUENCEBOUNDARY

FS = FORESHORE; USF = UPPER SHOREFACE; LSF =LOWER SHOREFACE; D.LSF = DISTAL LOWERSHOREFACE; SH = SHELF

Slope inter-fan sandstone cycle

Shallow marine parasequence for comparisonRedrawn from Van Wagoner et al., 1990

PS = SANDSTONE CYCLE BOUNDARY

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PS

PS

PS

PS

PS

PS

PS

PS

PS

PS

BASEFAN

PSA B C

Type 1 horizon (concretionary horizon)

PS

Thickness in metres

Fig. 19. Comparison of interfan sandstone cycles (interpreted as parasequences) in basin-floor (A and B) and slope depositional settings (C). A schematicexample of a shallow marine parasequence is included for comparison (after Van Wagoner et al., 1990).

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observation in the Tanqua fans is that certainsurfaces occur fan-wide and are always associatedwith an increase in sediment supply, but theirspecific expression in terms of facies juxtaposi-tions is quite variable (Fig. 14), owing to autocy-clic processes, type of geomorphological elementabove the surface and geographic position on thefan profile.

SEQUENCE STRATIGRAPHICEVOLUTION OF THE TANQUA FANS

Fan 1

Fan 1 has the smallest outcrop extent, and theexposed area is limited to the distal/lateralpinchout area. Based on the presence of adowndip pinchout and palaeocurrent analysis,fan 1 is interpreted as mid- to outer fan deposits(Figs 9 and 10). Owing to the bedding nature,amalgamation and geometry, an unconfineddepositional setting is interpreted, but no specific

lobe geometry can be proved from the limitedoutcrop control. The fan is up to 20 m thick andcomprises three high-frequency lowstand systemstracts separated by type 2 condensed zones,which represent the corresponding high-fre-quency transgressive and highstand systemstracts. The basal unit 1 is characterized bylocalized channel complexes up to 10 m thickand 1 km wide that are erosive with a heterolithicfill. Units 2 and 3 are mid-fan sheets and shallowdepositional channels. The stacking pattern of thehigh-frequency lowstand systems tracts is prog-radational or basinward stepping (Fig. 20), asdescribed from other fan systems by Nilsen et al.(1994) and Normark et al. (1999).

Fan 2

Fan 2 represents the mid- to outer fan environ-ment (mostly zone 3 and 4 deposition, Fig. 9).The fan contains three high-frequency lowstandsystems tracts separated by type 2 condensed

Fan 5 islandward stepping

Landward stepping

Fan 4 is aggradational toslightly basinward stepping

Aggradational

Fans 1,2 and 3step basinward

Basinward stepping

Large-scale stacking pattern analysis in the study areaThis is caused by the successive stacking of the high-resolution sequencesdefined by type 2 starvation zones (each 10-20 m+ thick).This may be a mechanism to accurately subdivide fan systems on seismic data.Tieing condensed sections, defined by borehole biostratigraphic data, to seismicreflectors could enhance sub-surface fan delineation and interpretation.

Fig. 20. Observed styles of large-scale stacking patterns in the KarooBasin fans. The style observed iscaused by the stacking of a numberof high-resolution sequences inbasinward, landward or aggrada-tional patterns.



sections and has a maximum thickness of 42 m.The lowest unit comprises heterolithic channelfills and thin, elongate sand sheets that pinch outbasinwards into siltstones. The middle and upperunits of fan 2 comprise sheets, transitional andchannelized architectural elements (Rozman,2000). The distal pinchout of the upper sectionof fan 2 exhibits a gradational facies change in abasinward direction and a channelized style, asdescribed for fan 1 above. The distal sandy areasof the fan show localized channel fills up to 10 mthick and 12 km wide. Although sandstonedeposition has a definite basinward limit, a 2-m-thick siltstone package (Fig. 3G) with rare centi-metre-thick starved current ripple laminationextends for 15 km further basinwards from thesand pinchout that is a characteristic depositionalstyle of zone 4 deposition.

The base of fan 2 is erosional in the locality ofchannels and sharp elsewhere. Regional mappingshows that the high-frequency lowstand systemstracts step successively further basinwards, defi-ning an overall progradational stacking patternsimilar to that of fan 1 (Fig. 20).

Fan 3

Fan 3 shows the greatest variety of facies andarchitectural elements (Fig. 9) and affords a fullview of the updip to downdip stratigraphic evolu-tion. The principal palaeocurrent orientations areto the north/north-east, and sections are orientedin a rough north to south transect. Thickness trendsare variable but, overall, the fan thins to the northfrom a maximum thickness of 55 m. The base of fan3 is always sharp but variable in facies. In places,thick-bedded sheet sands overlie thin-bedded veryfine sandstones and siltstones, whereas stackedchannel fills overlie shales in proximal/axial areas.The basal contact is sharp where sheet sands occurwith little evidence of large-scale erosion. In thedowndip parts of fan 3, where thin-bedded turbid-ites are the dominant facies, the initiation of activefan growth occurs above a gradational sequenceboundary. Fan 3 represents a low-frequency low-stand systems tract, as interpreted by Wickens(1994).

The main architectural elements in fan 3 includea nested channel complex (in the most proximalsouthern area), thin amalgamated sheets separatedby heterolithic sheets and isolated large deposi-tional and erosional channel complexes. Theamalgamated sheets of thick-bedded and massiveturbidites are interpreted as lobes (sensu Mutti &Normark, 1987) on account of their laterally

extensive, parallel-sided, stacked character. Thefines and unamalgamated sheets represent inter-channel/overbank complexes because of the lackof clear channelization, the ubiquity of currentrippled strata and the dominance of thin beds insmall-scale thinning- and/or fining-upward bed-sets. Thin, base-absent turbidite sheet sandstones,representing local fan abandonment, occur inplaces.

Fan 3 can be divided into four separate high-frequency lowstand systems tracts, separated byregional type 2 condensed horizons (Fig. 16). In asimilar manner to the underlying fans, the high-frequency sequences step successively furtherbasinwards, which is interpreted as a prograda-tional stacking pattern (Fig. 20).

Fan 4

Fan 4 exhibits less internal complexity than fan 3.Palaeocurrents show a 90 swing to a generaleastward trend, providing both dip and strikesections. Fan 4 thins from 65 m in the Skoors-teenberg area to around 35 m in the south andcomprises laterally extensive, stacked amalgama-ted sheets, channels and transitional architecturalstyles (Fig. 9). The sheets comprise medium- tothick-bedded (and massive) turbidites with par-allel tops and bases. Channels occur only in theupper part of fan 4 and are mainly large, singlestorey and erosionaldepositional in type. How-ever, a channel complex is exposed in the southof the study area (Fig. 8B). Transitional channelto unconfined depositional settings are inter-preted where a series of shallow erosive scoursat the base of a sheet complex pass basinwardsinto amalgamated sheets with planar, parallelbases. The base of fan 4 is abrupt and more clearlydefined than the base of fan 3, because of thecommon lack of thickening-upward packagesbelow the basal surface. There is no large-scalebasal erosion, but shale rip-up clasts, woodfragments and organic debris are common. Thissurface is interpreted similarly to the bases of theolder fans as correlative to an updip, low-frequency sequence boundary.

Fan 4 can be divided into five high-frequencylowstand systems tracts, bounded by type 2condensed zones up to 2 m thick (Fig. 17). Thelower four sequences are generally not associatedwith channel deposits, although a channel tosheet transition is seen in places, whereas theuppermost sequence contains the best developedchannel fills. The stacking pattern of the high-frequency sequences is aggradational to slightly



progradational (Fig. 20), in contrast to the prog-radational stacking patterns within fans 13.

Fan 5

Fan 5 has only been examined in the northern partof the study area because of sporadic exposureelsewhere. Unlike the other fans, it shows cleartruncation of underlying facies association 4 slopedeposits with more than 6 m of erosion over lessthan 500 m. This truncation surface is interpretedas a slump scar, which provided local healed slopeaccommodation space (Prather, 2000) that waslater filled. This basal surface may be a sequenceboundary but, as slumping on the slope can occurduring any position of relative sea level, thisinterpretation is tentative. However, the develop-ment of a 50 m thick sandy package above the basalsurface represents active fan growth, which is morelikely when sediment supply to the slope isincreased during relative sea-level fall.

Above the basal erosion surface, fan 5 is com-posed of channels that have amalgamated to form alarge-scale sheet geometry. There is a clear verticalpartitioning of channel fills through the fan. Thelower part comprises large channels with up to2 m of basal erosion that are mainly filled bymassive to structured sandstone with minoramounts of thin- to medium-bedded sandstone.Large-scale internal scours and smaller scour-and-fill bedforms are common in the lower part of thefan. The upper part of the fan is dominated bylaterally offset-stacked depositional channels withheterolithic fills. This fan-wide change in archi-tecture is interpreted as the deep-water equivalentof a backfilling depositional system, evidenced byan upsection decrease in the volume of theturbidity currents. The backstepping stackingpattern (Fig. 20) may represent the first significantincrease in the rate of relative sea-level rise duringlate lowstand, with sand starting to be trapped inshelf-edge deltas and incised valleys updip. Thefan is overlain by prodelta shales and heterolithsand is interpreted as a slope fan that filled anerosive slump-defined intraslope mini-basin.

Interfan fines and interfan sandstone cycles

The fine-grained intervals below fan 1 andbetween fans 1 and 2 are dominated by basinplain claystones of facies association 1 withabundant, well-developed concretionary hori-zons. Minor, thin (

High-frequency sequences

As demonstrated above, the five fans can besubdivided on the basis of intrafan type 2condensed horizons. Each intrafan sand-richpackage and overlying Type 2 condensed horizonis interpreted as a high-frequency sequence, andthese sequences show progradational, aggrada-tional and retrogradational stacking patterns(Fig. 20). According to the concept of sequencehierarchy (Mitchum & Van Wagoner, 1991), eachfan is interpreted as a sequence set, built of thehigh-frequency sequences.

CONTROLS ON STRATIGRAPHICARCHITECTURE

Chronostratigraphy

The literature is replete with examples of botheustatic and tectonic driving mechanisms forsequence development, often lacking directunequivocal evidence for the exact driving mech-

anism or, more usually, the combination ofmechanisms (for a review, see Miall, 1997). Amore objective approach is to compare the timingand duration of stratigraphic features with knownrates of potential driving mechanisms. This isonly possible within the available time frame-work, which is limited in the case of the Tanquabasin. Poor absolute age dating has led severalworkers to assign dates to the turbidites based onassumed linkages between thrust belt tectonicsand certain stratigraphic intervals of the forelandbasin (Wickens, 1994; Scott et al., 2000). Usingthis approach, ashes from the lower Ecca Coll-ingham Formation in the Laingsburg subbasin,interpreted by Halbich et al. (1983) to first dateuplift/shortening in the Cape Fold Belt (278 Ma),and Vissers (1990) estimation from regionalcorrelations suggests that the entire Ecca Grouprepresents 35 My (Wickens, 1994). This shortduration precludes most tectonic driving mech-anisms (Miall, 1997). Bouma & Wickens (1991)also postulated a 100 000 years cyclicity for thefans, and Goldhammer et al. (2000) followed this

Fig. 21. Schematic summary of architectural elements, geometries and stratigraphic positions of the Tanqua Karoofan systems.



argument. None of the above authors delineatedthe intrafan high-frequency sequences.

The most reliable estimate of Ecca Groupduration is based on a recent radiometric date of270 1 Ma for the Collingham Formation (Tur-ner, 1999) and a reptilian assemblage date of255 Ma for the overlying lower Beaufort Groupfluvial deposits (Rubidge, 1991), thus bracketingthe Tanqua fans within a 15 My period. On thisbasis, the five major fans may each represent thelowstand systems tracts to third-order relativesea-level cycles, which have durations of23 My (Haq et al., 1987). The intrafan high-frequency sequences would then representfourth-order (05 My) sequences.

Sequence driving mechanisms

The highly diachronous nature of the Gondwanaglaciation means that glacial/interglacial activityin this known icehouse climatic system remainedoperative during Ecca Group times elsewhere onthe supercontinent (Visser, 1990). Although intra-Dwyka chronostratigraphy is poor, loose analogywith the late Cenozoic glacial cycles providesa plausible Milankovitch cyclicity, producingeustatic sea-level changes on a 10 000100 000years frequency (Abreu & Anderson, 1998). Elderet al. (1994) correlated 100 000 years durationinterpreted Milankovitch cycles across the Creta-ceous Western Interior retro-arc foreland basin,USA, from clastic cycles in the west to carbonatecycles in the east, over a distance of 1500 km. Thiscorrelation effectively precluded thrust belt tec-tonics as the origin of the cycles, as the easternsection of the basin was not directly affected bythe local subsidence variations caused by thrustmovements.

Changes in thrust propagation rates and thedevelopment of local imbricate structures withinthrust belts bounding foreland basins have beensuggested as mechanisms for controlling strati-graphic cyclicity in the basin fill, particularlywhere thrust fault dynamics are complex, withthe development of imbricate structures wheremovement on a hierarchy of faults may occur atdifferent rates (Boyer, 1992). Many authors haveinterpreted a causal link between the shorteninghistory of the thrust belt and cyclicity in the basinfill. The main problem is the paucity of data tolink rates of fault movement quantitatively withthe estimated duration of high-frequency cyclesin the Karoo basin and limited understanding ofthe exact timing of subaerial emergence of thethrustbelt (Turner, 1999). Without better age

dating, it can only be speculated as to the exactduration and hence driving mechanism of theTanqua cycles, but the presence of coeval glacialcycles provides a viable eustatic mechanism forat least the high-frequency sequences. Goldham-mer et al. (2000) speculated that fans 15 com-prise a highstand sequence set controlled byglacio-eustatic sea-level changes.

IMPLICATIONS FOR SUBSURFACEDATA INTERPRETATION

Interpretation of cores and well logs

A basin-floor fan model (Fig. 9), built from themost regionally exposed fan (fan 3) and comple-mented with data from fans 1, 2, 4 and 5,illustrates the sedimentary style and verticalgeometric style at different positions in equival-ent subsurface data sets based on interpretedposition on a basin to slope profile (Fig. 21). A keyfinding is the difficulty of distinguishing reliablybetween channel and amalgamated sheet sand-stones, because vertical bedding trends in a singlelocality can be misleading in terms of lateralarchitectural style (see discussion in Cheng &Hiscott, 1999). For example, many successionsinterpreted from subsurface data as stacked sheets(based on thinning-up trends in a vertical welllog) may be formed of stacked channels with acomplex vertical and lateral heterogeneity.

The Tanqua fans allow characterization of onespecific style of distal fan pinchout, which isgradual rather than abrupt and characterized bylocalized channels and sheets rather than exten-sive sandy sheets as current facies models suggest(Walker, 1992). The Tanqua model predicts thatsignificant volumes of sand lie basinwards oflikely fan pinchouts defined purely from seismicmapping, where the vertical resolution of manyseismic surveys would necessitate a sand cut-off at10 m. Distal fan margins in the Tanqua also pro-vide robust stratigraphic trapping mechanisms.

Lithology prediction from stacking patterns

The recognition of condensed sections and, toa lesser extent, sequence boundaries (Fig. 22)allows a refined interpretation of the mechanismof fan growth over time and a partly predictiveunderstanding of the distribution of lithologiesbased on the ability to map out high-frequencydepositional sequences. The high-frequencysequences define fluid flow units for reservoir



simulation and deterministically mappable verti-cal permeability barriers. Net:gross variationswithin the high-frequency sequences are com-plex, and mapping may be facilitated by combi-ning seismic data, well logs and production testdata in subsurface areas and a knowledge of the

statistical distribution of architectural elementsand size based on analogue data.

When the base of a fan complex is used as adatum, progradational stacking patterns of high-frequency intrafan sequences define subtle clino-form geometries, which indicate that a layer-cake

Initial sea-level Lowest sea-level

Highest sea-level

ZONE 1 ZONE 2 metres

Zone 2:

A decrease of turbidite deposition.Dominantly thin-bedded turbidites, siltstones andclaystones.Stacking patterns controlled by high-frequencysea-level changes and autocyclicity

Zone 1:

Flooding surfaces develop on theshelf that relate to zones of sedimentstarvation in deeper water

Decreased probability of sedimentsfed to canyons during rsl highstand

C

Sea level curve based onKolla & Perlmutter (1993)

Initial sea-level

Lowest sea-level

Highest sea-level

ZONE 1 ZONE 2HST HST

Low &E. Rise

Zone 2:

An increase of turbidite deposition.Dominantly thin-bedded turbiditeswith less frequent thick-bedded turbidites.Stacking patterns controlled by high-frequency sea-level changes and autocyclicity

Area of well-developed key

surfaces ofsequence

stratigraphicsignificance Zone of deep-water fan deposition

Zone of slope mini-basin andchannel transport systems

Zone 1:

During initial sea-level fallturbidites may developfrom shelf bypass

Initial sea-level Lowest sea-level

Highest sea-level

ZONE 1 ZONE 2

10s m

Zone 2:

At this time an increased rate of supply andcalibre of sediment causes a change in depositionalstyle.Thick-bedded turbidites begin to dominatewith less frequent thin-bedded turbidites.Stacking patterns are controlled by high-frequencysea-level changes and autocyclicity

Zone 1:

Sub-aerial exposure surface developson the coeval shelf and incisedvalleys may feed directly to canyonmouths

B

Increased probability of sedimentsfed to canyons during early lowstand

A

Maximum potential for bypassthrough canyons during lowered sea-level

Falling

LateRise

HST HST

Low &E. Rise

Falling LateRise

HST HST

Low &E. Rise

Falling LateRise

Fig. 22. Schematic model for the generation of a single fan unit (low frequency or high frequency) through asingle relative sea-level cycle. (A) Early stage of relative sea-level fall results in more rapid progradation of deltasacross the shelf and increased probability of sand being supplied to shelf-edge canyons. Falling-stage bypass ofthe shelf is possible. Basin-floor response may be thin-bedded turbidites. (B) Sequence boundary formation,subaerial exposure of shelf and cutting of incised valleys to lowered base-level position. Maximum volume ofsand fed to deep basin, resulting in thick-bedded turbidites of fan growth stage. (C) Rapid rate of relative sea-levelrise leads to flooding of shelf and storage of sand in shoreline and fluvial environments. Reduced sediment supplyto the basin and the formation of a type 1 or type 2 condensed zone (dependent on rate and duration of therelative sea-level rise).



stratigraphy is not present in fan systems, andinternal stratigraphic trapping of fluids may occurat the distal ends of turbidite systems. In strikesections of sheet-dominated fans, such as fan 4,a layer-cake interpretation may be more appro-priate.

CONCLUSIONS

1 The linkage of facies associations to archi-tectural elements and regional mapping ofextremely well-exposed strata leads to a modelfor the sequence stratigraphic development ofbasin floor and slope fans in the Tanqua basin.

2 Cyclicity is well developed at both fan andintrafan scales, and the key stratigraphic markersare a threefold hierarchy of condensed fine-grained deposits.

3 Each of the five fans is interpreted as a low-frequency (speculatively third order) lowstandsystems tract, with the intervening fines repre-senting transgressive and highstand systemstracts. Minor interfan sandstone cycles areinterpreted as highstand systems tract parase-quences.

4 Active fan growth occurred as a series ofhigh-frequency episodes, punctuated by periodsof regional sand starvation. The sandy growthpackages are interpreted as high-frequency (spec-ulatively fourth order) lowstand systems tracts,with intervening fines representing high-fre-quency transgressive and highstand systemstracts. Local internal variability within the growthpackages is a result of autocyclic processes.

5 Sequence boundaries are interpreted at thebases of the five fans and at the bases of theintrafan high-frequency sequences on the basis ofa clear surface that marks a fan-wide abruptincrease in sediment supply. This change can bemarked by either the erosive base of a channelcomplex overlying distal basin plain or fan fringedeposits or the planar base of an amalgamatedsheet sandstone overlying more distal turbiditedeposits. There is usually a grain-size increaseacross the surface, and the surface can be tracedfor up to 30 km. This abrupt increase in sedimentsupply over the whole fan system is related to adecrease in accommodation space on the coevalshelf and the generation of a sequence boundaryupdip.

6 Intrafan, high-frequency sequences stack pro-gradationally in fans 13, aggradationally in fan 4and aggradationally to retrogradationally in fan 5.High-frequency sequences may define the deter-

ministically mappable permeability structure ofsimilar turbidite hydrocarbon reservoirs. When thebase of the fan defines a datum, progradationalstacked high-frequency sequences define a subtleclinoform geometry, which may lead to internalstratigraphic trapping of fluids at the distal endsof turbidite systems.

7 Sandstone deposition has a clear basinwardlimit, but local depositional channel fills up to10 m thick and 12 km wide are locally present atthis point. Siltstone extensions to the fans arepresent for up to 15 km further onto the basinplain.

ACKNOWLEDGEMENTS

This paper derives from a two-year researchproject completed in 1997, supported by ShellUK, who are gratefully acknowledged for finan-cial support and permission to publish. Our ideason the Tanqua have been refined during discus-sions with colleagues in the STRAT Group,Statoil, and industry-based field course partici-pants over the last 4 years. Fieldwork was under-taken with the assistance of Pete Sixsmith.Reviewer Brad Prather and editor Jim Best arethanked for their expert help in the shaping of thefinal paper.

REFERENCES

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Basu, D. and Bouma, A.H. (2000) Thin-bedded turbidites ofthe Tanqua Karoo: physical depositional characteristics.In: Fine-Grained Turbidite Systems (Eds A.H. Bouma andJ. Stone), AAPG Mem., 72, 263278.

Bouma, A.H. (1962) Sedimentology of Some Flysch Deposits:a Graphic Approach to Facies Interpretation. Elsevier,Amsterdam, 168 pp.

Bouma, A.H. (1997) Comparison of fine-grained, mud-rich andcoarse-grained, sand-rich submarine fans for exploration-

development purposes. Trans. Gulf Coast Assoc. Geol. Soc.,47, 5056.

Bouma, A.H. and Wickens, H.DeV. (1991) Permian passivemargin fan complex, Karoo Basin, South Africa: possiblemodel to Gulf of Mexico. Trans. Gulf Coast Assoc. Geol.Soc., 41, 3042.

Bouma, A.H. and Wickens, H.DeV. (1994) Tanqua Karoo,Ancient analogue for fine grained submarine fans. In: Sub-marine Fans and Turbidite Systems: Sequence Stratigraphy,Reservoir Architecture and Production Characteristics (EdsP. Weimer, A.H. Bouma and B.F. Perkins), Proc. Gulf CoastSection SEPM 15th Annu. Res. Conf., 2334.

Bouma, A.H., Wickens, H.DeV. and Coleman, J.R. (1995)Architectural characteristics of fine-grained submarine fans:



a model applicable to the Gulf of Mexico. Trans. Gulf CoastAssoc. Geol. Soc., 45, 7175.

Boyer, S.E. (1992) Geometric evidence for synchronousthrusting in the southern Alberta and northwest Montanathrust belts. In: Thrust Tectonics (Ed. K.R. McClay),pp. 277390. Chapman & Hall, New York.

Burgess, P.M. and Hovius, N. (1998) Rates of delta prograda-tion during highstands: Consequences for shelf bypass anddeep water deposition. J. Geol. Soc. London, 155, 217222.

Chapin, M.A., Davies, P., Gibson, J.L. and Pettingill, H.S.(1994) Reservoir architecture of turbidite sheet sandstonesin laterally extensive outcrops, Ross formation, Western

Ireland. In: Submarine Fans and Turbidite Systems: Sequ-ence Stratigraphy, Reservoir Architecture and ProductionCharacteristics (Eds P. Weimer, A.H. Bouma and B.F. Per-kins), Proc. Gulf Coast Section SEPM 15th Annu. Res. Conf.,5368.

Cheng, C. and Hiscott, R.N. (1999) Statistical analysis ofturbidite cycles in submarine fan successions: tests forshort-term persistence. J. Sed. Res., 69, 486504.

De Beer, C.H. (1990) The relative ages of the western andsouthern branches of the Cape Fold Belt. S. Afr. J. Geol., 93,583591.

De Wit, M.J. and Ransome, I.G.D. (1992) Regional InversionTectonics of the Cape Fold Belt, Karoo and CretaceousBasins of Southern Africa, pp. 1521. A.A. Balkema,Rott

johnson et al 2001 ecca basin floor to slope tudbid (4)

Documents

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