u, P

, P

; ac3 N

rouona, anots,

ostratigraphy. In spite of deciencies in the available database, it is possible broadly to reconstruct the probable basin history; a riftedcontinental margin seems a likely setting for the Cape Supergroup, Natal Group and Msikaba deposits.

whereas the Msikaba Formation and the Natal Groupoccur as smaller entities in the eastern part of the country,

basin may have extended into parts of what is nowArgentina, where there are similar outcrops of folded EarlyPalaeozoic metasediments (du Toit, 1927; Cobbold et al.,

cession may be as little as 3300 m although it appears tothicken considerably in the eastern parts of the fold belt

in terms of world-wide sea-level curves and the body- andtrace-fossil content of the Cape strata) is generally believedto range from earliest Ordovician to Mid-Carboniferous(Broquet, 1992). Deformation of the Cape strata, the pre-Cape basement and some of the Karoo cover rocks isthought to have taken place in four major compressional

* Corresponding author.E-mail address: russell.shone@nmmu.ac.za (R.W. Shone).

Journal of African Earth Scienceand are relatively undeformed (Fig. 1). The latter two enti-ties are correlated with the main outcrop area of the CapeSupergroup in the south and western areas, based on lith-ological similarities and age correspondence (Hobday andMatthew, 1974; Marshall and von Brunn, 1999). Metasedi-ments of the Cape Supergroup, the Natal Group and theMsikaba Formation were deposited in what appears tohave been a single sedimentary basin, the Cape Basin. This

(Rust, 1973). The prevalence of thrusting and the presenceof thick, thrust-stacked successions with few marker beds(Booth and Shone, 1998, 1999) make it dicult if notimpossible to measure the original thicknesses of the indi-vidual stratigraphic sub-units which make up the CapeSupergroup. The age of the Cape succession (as inferredfrom its stratigraphic relationships with underlying andolder rocks, interpretations of Cape sedimentary sequences 2005 Published by Elsevier Ltd.

Keywords: Palaeozoic Cape Basin; Cape Supergroup; Depositional sedimentary environments; Thrust-stacking; Stratigraphic implications

1. Introduction

Early Palaeozoic metasedimentary rocks of the CapeSupergroup, Natal Group and Msikaba Formation cropout along the southwestern, southern and southeasternmargins of South Africa. Most of the outcrop lies in a lat-erally continuous fold-and-thrust belt, the Cape Fold Belt

1992). Strata of the Cape Supergroup (Table MountainGroup, Bokkeveld Group and Witteberg Group) consistof clastic metasedimentary rocks, predominantly quartzitesand phyllites (though often referred to as sandstones andshales) which, for the most part, have undergone no morethan lower greenschist grade metamorphism (de Swardtand Rowsell, 1974). The thickness of the whole Cape suc-The Cape Basin, So

R.W. Shone *

Geology Department, University of Port Elizabeth

Received 1 June 2004Available online

Abstract

Sedimentary rocks of the Palaeozoic Cape Supergroup, Natal Gmargin in a variety of terrestrial and shallow marine-shelf depositiiferous. Tectonism, which took place between 278 Ma and 230 MaFold Belt. Rocks of the Natal Group and Msikaba Formation weresuccessions, together with evidence of thrust-eliminated pelitic uni1464-343X/$ - see front matter 2005 Published by Elsevier Ltd.doi:10.1016/j.jafrearsci.2005.07.013th Africa: A review

.W.K. Booth

.O. Box 1600, Port Elizabeth 6000, South Africa

cepted 18 July 2005ovember 2005

p and Msikaba Formation were deposited on a passive continentall environments, from the early Ordovician until the mid-Carbon-ected only the Cape Supergroup rocks and resulted in the Capet tectonized. In the Cape Fold Belt, the presence of thrust-stackedcast doubt on the reliability of some aspects of the accepted lith-

www.elsevier.com/locate/jafrearsci

s 43 (2005) 196210

ReUni

R.W. Shone, P.W.K. Booth / Journal of Afrepisodes during the Permian and Triassic, between 278 Maand 230 Ma (Halbich et al., 1983). This orogeny resulted innorth-verging thrusts and folds in the southern branch ofthe Cape Fold Belt. In the western branch of the fold belt,open folds are indicative of less severe deformation. Thetwo fold branches meet in a syntaxial zone characterizedby a pattern of interference structures (de Beer, 1992). Dur-ing the subsequent breakup of Gondwana, Cape Super-group strata were subject to tensional stresses resulting incomplex horst, graben and half-graben structures. Theseare readily recognized wherever younger (Mesozoic) sedi-ments are preserved in fault-bounded basins (e.g.Oudtshoorn, Gamtoos, and Algoa Basins), but they are

Fig. 1. Map showing the outcrop of Cape Supergroup rocks in theO = Oudtshoorn, P = Piketberg, PA = Port Alfred, S = Steytlerville, U =certainly prevalent elsewhere throughout the fold belt.

2. Lithostratigraphic subdivisions of the Cape Supergroup

Lithostratigraphic subdivision of the Cape Supergroupinto three distinctive groups (from oldest to youngest, theTable Mountain, Bokkeveld and Witteberg Groups; SACS,1980) rests in part, on the dierent rock types and readymappability of the three units (Fig. 2). In some areas,within each group, the presence of numerous closely spacedthrusts in parts of the southern branch of the fold belt, giverise to tectonically thickened sequences. In addition, sincepelitic units are commonly smeared out along thrustplanes, and many fold limbs consist of previously thrust-stacked units, it appears that the established lithostrati-graphic succession is, at least in parts of the fold belt,an artefact of the tectonic history.

2.1. The Table Mountain Group

The Table Mountain Group can be subdivided into thefollowing, from oldest to youngest.2.1.1. The Piekenierskloof FormationRust and Theron (1964) described a reddish conglomer-

ate from the Vanrhynsdorp area of the Western Capewhich they considered to be the very base of the Cape suc-cession. The conglomerate contains angular to sub-rounded boulders, cobbles and pebbles of dolomite,quartzite, vein quartz, jasper, shale and pink gneiss. Theconglomerate varies in thickness from about 1 m atGiftberg, about 15 km south of Vanrhynsdorp to morethan 60 m south of Groot Kobe some 29 km east ofVanrhynsdorp (Rust and Theron, 1964). Later Rust(1967) described similar basal conglomerates from thePiketberg area, which he called the Piekenier Formation.

public of South Africa. C = Ceres, H = Hibberdene, K = Kareedouw,ondale, Ul = Ulundi, VRD = Vanrhynsdorp.

ican Earth Sciences 43 (2005) 196210 197He identied two distinctly dierent members, the RestMember and the De Hoek Member which he consideredto represent proximal/distal facies of the PiekenierskloofFormation. The Rest Member is a thick-bedded, pro-fusely cross-bedded conglomerate consisting predomi-nantly of vein quartz pebbles averaging about 6 cm indiameter. Palaeoow as determined from the cross-beddingwas towards the southeast (Rust, 1967, Fig. 84). The DeHoek Member, which crops out south of Piketberg, isdescribed as a gritty, thick-bedded orthoquartzose (quartzarenite) sandstone. The thickness of the Piekenier is evi-dently greatly variable. Rust (1967) suggests a thickness ofup to 1000 m.

The contact between the Piekenierskloof Formation andthe underlying pre-Cape rocks (including the KlipheuwelFormation) is marked by an angular unconformity,although in some areas the contact is essentially that of adisconformity (Rust, 1967).

According to Rust (1967, 1969) the PiekenierskloofFormation was deposited in a relatively narrow north-west-southeast trending embayment, probably open tothe sea at its southeastern extremity. This might suggestthat the Piekenierskloof Formation was always rather

nAfrlimited in lateral extent. However, a conglomerate at Sardi-nia Bay, near Port Elizabeth, occupies a similar strati-graphic position to the Piekenierskloof Formation at thebase of the Table Mountain Group. Although clearly notconnected to the outcrops of Piekenierskloof Formationin the Western Cape, the Sardinia Bay conglomerate isprobably representative of local early Cape sedimentationon a rugged pre-Cape landscape. The Sardinia Bay con-glomerate does not yet have formation status.

2.1.2. The Graafwater and Sardinia Bay formations

These two formations are spatially separated by some700 km, occurring at the western and easternmost ends of

Fig. 2. Stratigraphic colum

198 R.W. Shone, P.W.K. Booth / Journal ofthe main fold belt. They are included under one headingbecause both overlie basal conglomerates, and stratigraphi-cally lie at or near the base of the Cape Supergroup. TheGraafwater Formation outcrops are conned to a narrownorthwest-southeast trending area of the westernmost Cape(Rust, 1977), whereas the Sardina Bay Formation is exposedonly in a narrow coastal outcrop at Sardinia Bay near PortElizabeth (Shone, 1983). The 70 m thick Graafwater Forma-tion, which rests unconformably on pre-Cape basement,consists of a basal conglomeratic unit (the Bellvue Conglom-erate Member; Rust, 1977) which contains rather angularcobbles and pebbles of vein quartz, quartzite and chert(Rust, 1977). The successively overlying units consist ofthinly-interbedded reddish- to maroon-coloured quartzitesand siltstones (Loop Member), coarse, pebbly white-col-oured quartzites (Tierhoek Sandstone Member) and purplequartzites and interbedded siltstones with abundant tracefossils, especially Skolithos traces (FarooMember). An iden-tiable erosion surface separates the Graafwater Formationfrom the overlying Peninsula Formation (Rust, 1967). Ero-sion surfaces are common throughout the Graafwater suc-cession, typically demarcated by deformed clay pelletconglomerates at the bases of thin sandstone/quartzite units.The Graafwater quartzites are identied as ortho-quartzites by Rust (1977) and quartz arenites byTankard and Hobday (1977). Sedimentary structures iden-tied by Rust (1977) and Tankard and Hobday (1977)include megaripple trough- and planar-crossbedding withset heights of up to a metre. Herringbone patterns formedby dierent foreset orientations are common, and reactiva-tion surfaces have been identied by Tankard and Hobday(1977). Flaser-, wavy- and lenticular-bedding are predomi-nant sedimentary structures in the thinly-interbeddedquartzites, siltstones and mudrocks. Wave- and current-ripple structures are common, including at-topped, dou-ble-crested types and ladderback ripple patterns. Some

for the Cape Supergroup.

ican Earth Sciences 43 (2005) 196210red mudrock layers are characterized by sand-lled cracksassociated with desiccation polygons up to 0.3 m in diam-eter. Other post-depositional sedimentary structuresinclude water-escape structures. Trace fossils are relativelycommon, including the genera Skolithos, Petalichnus andArthrophycus. No body fossils have as yet been found inthe Graafwater Formation.

Palaeocurrent analyses by Rust (1977) show a generalbimodal, but not quite bipolar palaeoow pattern withweak vector means towards the north-northeast andsouth-southwest (Rust, 1977, p. 129), whereas the datapresented by Tankard and Hobday (1977, p. 142) forlarge-scale crossbeds are indicative of local variations inpalaeoow with predominantly unimodal westerly tosouthwesterly ow directions.

Tankard and Hobday (1977) identied a ning-upwardfacies sequence as typical of the Graafwater Formation.This sequence is characterized by a basal quartz-arenite/quartzite with large-scale crossbeds. These rocks are over-lain successively by ner-grained quartzite with ripplestructures, thinly-interbedded mudstone and quartzite(the former with sand-lled cracks dening desiccationpolygons) and red- to maroon-coloured mudrock.

AfrAt the eastern end of the fold belt the 180 m thick Sardi-nia Bay Formation crops out in a narrow strip along thecoast just west of Port Elizabeth. It occurs as a sliver of rel-atively undisturbed strata sandwiched between severelytectonized Table Mountain Group quartzites. It appearsto be close to the base of the Cape succession in the areaas it seems to lie immediately above or close to the tecton-ized conglomerate unit described above (Section 2.1.1).Weathered granite outcrops at the base of this conglomer-ate could either be part of the pre-Cape basement proper orthrust-in fragments of this basement basement. The Sardi-nia Bay Formation itself consists of relatively thin-beddedquartzites and phyllites (formerly siltstones and silt-streaked mudrocks). The quartzites are mostly quartz are-nites and sub-lithic arenites, but some of them containnumerous large subhedral detrital grains of microcline feld-spar (Shone, 1983). Two thin, matrix-supported conglom-erate horizons containing angular vein quartz pebblesoccur near the base of the succession. An 18 m thick turbi-dite containing large rip-up clasts occurs at the top of theSardinia Bay Formation (Shone, 1987). Channelled erosionsurfaces occur in the lower and upper parts of the SardiniaBay Formation, but in general, erosion surfaces are rare.What appear to be burrowed hardgrounds may testify toperiods of non-deposition.

Sedimentary structures found in the Sardinia BayFormation include large-scale trough and planar crossbed-ding (set heights up to a metre) in herringbone cosets,abundant wave- and current-ripple structures and associ-ated aser, wavy and lenticular bedding. The remains ofa large sandwave structure complete with lee-side low-angle erosion surfaces and complex intraset cross-laminaehave also been identied (Shone, 1983). Other syn-deposi-tional sedimentary structures found include low-angle andnear-horizontal lamination, hummocky stratication andgraded bedding (in the turbidite unit). Imbricated pebbleswith their long axes oriented sub-parallel to the inferredow direction occur at the base of the turbidite unit(Shone, 1987). Post-depositional sedimentary structuresinclude water-escape structures, load-casts and ame struc-tures (Shone, 1983).

The Sardinia Bay Formation is notable for its trace fos-sil assemblage (Shone, 1991). Genera identied includeOphiomorpha, Thalassinoides, Diplocraterion, Skolithos,(?)Chondrites, (?)Planolites, (?)Fascifodina and possibleCruziana (Shone, op. cit.). Enigmatic body fossils includewhat appear to be abraded fragments of stromatoporoidorigin (Shone, 1983).

Palaeocurrent analysis of restored crossbed and troughaxis data collected from individual units in the SardiniaBay Formation shows a bimodal, nearly-bipolar distribu-tion of the palaeoow data. Flow was characterized byhigh vector strengths and was mainly to the northwestand east-southeast (Shone, 1983, pp. 273313). Theinferred ow direction at the base of the turbidite unit at

R.W. Shone, P.W.K. Booth / Journal ofthe top of the Sardinia Bay Formation is to the southeast(Shone, 1983, 1987).Sequence analysis of the facies identied in the SardiniaBay Formation suggests that a silt-streaked mud was thebase state in the immediate Sardinia Bay depository, withepisodic introduction of sands (Shone, 1983, pp. 332341).

Although the Sardinia Bay Formation and theGraafwater Formation are spatially separated by some700 km, they are tentatively correlated as time-equivalentdeposits, based on the observation that both occur nearthe base of the Cape Supergroup.

2.1.3. The Peninsula Formation

The Peninsula Formation (SACS, 1980) crops out inboth the western and eastern parts of the Cape Fold Belt.In the eastern Cape Fold Belt the Peninsula consists oflittle more than a succession of stacked thrust-boundedquartzite packages. The closely-spaced thrusts are com-monly, but not always, sub-parallel to bedding surfaces,and many identiable mudrock/phyllite units have evi-dently been smeared out and almost obliterated duringthrusting. This indicates that even if individual bedsare preserved, the exposed sequence as a whole is an arte-fact of the tectonism and cannot be subjected to meaning-ful lithostratigraphic analysis.

In the western Cape exposures, however, where thrust-ing appears to be less pervasive, available lithostrati-graphic and sedimentologic data on the PeninsulaFormation may be more reliable. Here the PeninsulaFormation is reportedly only 550 m thick (Fuller andBroquet, 1990), considerably less than the 3500 m mootedas a maximum thickness (see Visser, 1974; Johnson, 1991).Exposures of undisturbed Peninsula Formation in thePlatteklip Gorge, on the east face of Table Mountain,have been described by Fuller and Broquet (1990). Thesestrata consist of well-bedded, pebbly, supermaturequartzites with channel cut-and-ll structures particularlyin the upper half of the succession (Broquet, 1990). Someof the preserved channel cuts are as much as 40 m deepand several kilometres in lateral extent (Hobday andTankard, 1978). Mudrock is reportedly very rare (lessthan 2% of the Peninsula succession). A thin but later-ally-persistent clast-supported conglomerate occurs in themiddle of the sequence. Some pebbles found in a lateralcorrelate of this conglomerate are faceted dreikanter,suggesting a period of subaerial deation (Macdonald,1989; Fuller and Broquet, 1990). Sohnge (1984) describeda diamictite of probable glacial origin from the PeninsulaFormation near Cape Hangklip. Other sedimentary struc-tures found in the Peninsula Formation include large-scaletrough and planar crossbedding (set heights up to 2 m),low-angle cross-lamination and horizontal lamination. Atleast one sandwave complex has been identied (Hobdayand Tankard, 1978).

Trace fossils identied in the Peninsula Formationinclude Diplichnites (the arthropod trackways describedby Rust (1967) and Anderson (1975)), Cruziana (Potgieter

ican Earth Sciences 43 (2005) 196210 199and Oelofsen, 1983), and a variety of other traces usuallyattributed to arthropods (Rusophycus, Isopodichnus;

AfrBroquet, 1990). Broquet (op. cit.) also identied a numberof other trace fossils (Planolites, Skolithos) and trace fossilassemblages.

Palaeocurrent analyses reveal a variety of localized owdirections (see Hobday and Tankard, 1978, p. 1742). Bimo-dal, bipolar ow patterns characterize a few localities, but,in general, the inferred ow patterns are unimodal withvector means towards the south.

Broquet (1990) and Fuller and Broquet (1990) haveidentied a number of ning-upwards sequences in thelower half of the Peninsula Formation. Individual ning-upwards cycles become thinner-bedded towards their tops.The tops of these ning- and thinning-upwards cycles arefurther marked by the increasing prevalence of trace fossils(Broquet, op. cit.).

2.1.4. The Pakhuis Formation

The Pakhuis Formation (Rust, 1967) is relatively thin(as little as 40 m, SACS, 1980), but up to 190 m thick(Rust, 1981). It can be subdivided into a lower portion,the Sneeukop tillite, and an upper unit, the Kobe tillite,separated by a thin quartzite, the Oskop sandstone (Rust,1967).

The Sneeukop tillite is a relatively structureless quartzitewhich contains faceted and striated erratics. The Oskopsandstone, which unconformably overlies the Sneeubergtillite, contains current ripple structures and its upper sur-face is deeply grooved and furrowed and/or folded; fea-tures which Visser (1962), Rust (1967) and Blignault(1970) attributed to soft sediment deformation caused bythe movement of an overlying ice sheet. The Kobe tillitecontains large angular erratic fragments (up to 50 cm indiameter) of probable Neoproterozoic Nama Group origin(Rust, 1981). Ice movement, as inferred from tillite fabricsand soft sediment deformation features, appears to havebeen variable, but with a strong southerly component(Rust, 1981).

2.1.5. The Cedarberg formation shale

This 150 m thick unit grades almost imperceptibly fromthe underlying Pakhuis Formation via a succession of dia-mictites and varved mudrocks (Rust, 1973). The basal por-tion (Soom Member) consists of a black thinly-laminatedshale which is overlain by a succession of lighter-colouredmudrocks, siltstones and sandstones (Disa Member). TheDisa Member contains a fossil brachiopod assemblage,including the genera ?Plectoglossa, Trematis, Orbiculoidea,Marklandella, Eostropheodonta, and Plectothyrella (Cockset al., 1970).

The Cedarberg Formation shale is a persistent unit thatoutcrops continuously from the western part of the foldbelt to near Port Elizabeth in the east, where it dies outsome 100 km west of the city. Remnants of this formationare preserved in fold noses, but not on limbs of large folds.These characteristics suggest that it represents a structur-

200 R.W. Shone, P.W.K. Booth / Journal ofally favourable detachment zone in the eastern part ofthe fold belt.2.1.6. The Nardouw Formation sandstone

The Nardouw Formation sandstone is (according toSACS, 1980) a 500 m thick succession of quartzites witha few thin conglomerate stringers, some of which are char-acterized by small, angular vein quartz pebbles; others con-tain at shale clasts (Rust, 1967; Thamm, 1984). Winter(1984) has described an unconformity at the base of theNardouw Formation, but this is not referred to by otherresearchers. The Nardouw exhibits abundant large-scalecrossbedding (Rust, 1967). Trace fossils, particularly Skoli-thos are common (Rust, op. cit.) and fossil brachiopods areknown to occur at the top of the formation (Theron, 1970).Palaeocurrent directions are predominantly towards thesouth (Rust, 1967). However, the overall vector strengthfor the accumulated crossbed data from a variety of dier-ent locations is low (about 0.6).

2.2. Bokkeveld Group

The Bokkeveld Group, thought to be as much as 3000 mthick (Theron, 1970, 1972; SACS, 1980) consists of a suc-cession of black (when freshly exposed) mudrocks, darkgrey to olive-coloured siltstones and grey to olive-greyne-grained sandstones. Sandstones containing mudclastfragments are common. In parts of the southern Capethe mudrocks contain disseminated cubes of diageneticpyrite which rapidly alter to limonite in the weatheringzone. The sandstones typically have a dirty appearanceassociated with lithic arenites (Tankard and Barwis,1982). However, according to Johnson (1991), most ofthe Bokkeveld sandstones contain small amounts ofdetrital feldspar and few rock fragments.

The most common sedimentary structures found in theBokkeveld Group sandstones are wave-ripple structuresand hummocky stratication (Tankard and Barwis, 1982;Rust and Shone pers. comm.). Both wave and current rippleforms tend to be associated with mudasers, or occur aspartly isolated lenses in wavy and lenticular bedded mud-rock. Climbing ripple cross-lamination, graded beddingand evidence of occasional emergence (sand-lled mud-cracks, runzel marks) have been described by Tankardand Barwis (1982). Some sandstones contain slump struc-tures in the form of ow rolls.

The Bokkeveld strata contain numerous trace fossilsincluding the genera Skolithos, Zoophycos and ?Planolites.Body fossils are common, including an abundance of cri-noid, brachiopod, gastropod, bivalve and pteropodremains. Some cephalopod, sponge and coral fragmentshave been identied, as well as a few plant fragments(Reed, 1925; du Toit, 1954). Fossils are not, however,equally abundant throughout the Bokkeveld succession.The black pyritic shales, for example, contain very fewbody fossils and sometimes only a few enigmatic tracefossils.

Palaeoow, as inferred for the Bokkeveld, is generally

ican Earth Sciences 43 (2005) 196210towards the south and southwest (Rust, 1973). Localizedvariations in ow and episodic current reversals are

Afrinferred from unspecied data by Tankard and Barwis(1982).

Both ning- and coarsening-upwards sequences havebeen identied by Tankard and Barwis (1982). They con-sider the Bokkeveld succession as a whole to consist of vemajor superimposed coarsening-upwards sequences, eachgrading from mudrock at the base upward through gray-wacke and lithic arenite sandstones to coarser, morequartz-arenitic sandstone.

2.3. Witteberg Group

The Witteberg Group strata have been described indetail by only a handful of researchers, amongst themLoock (1967), Hiller and Dunlevy (1978), Theron andLoock (1988), Hiller and Taylor (1992), Booth et al.(1999) and Cotter (2000).

The Witteberg succession is thought to attain a thick-ness of over 2600 m in the Eastern Cape (SACS, 1980).The lithotypes include mudrock, siltstone and sandstone.According to Johnson (1991) most Witteberg sandstonesare quartz arenites or subfeldspathic arenites. For the mostpart the strata consist of thinly interbedded sandstones andshales. Some cut-and-ll erosion surfaces are evident, butin general individual beds show appreciable lateral continu-ity, except where truncated by thrusting. The thin lenticu-lar beds illustrated by Cotter (2000, p. 5), are almostcertainly thrust wedges.

Sedimentary structures in the Witteberg Group stratainclude trough and planar crossbedding, wave ripple struc-tures, hummocky stratication, and near-horizontal lami-nation. Mudrocks usually contain silt streaks. Wavy,aser and lenticular bedding characterize many of thethinly-intercalated mudrock/sandstone units. Palaeoowdirections determined for the Witteberg are mainly towardsthe south, southeast and southwest (Rust, 1973, Fig. 12).

Trace fossils found in the Witteberg include Skolithosand Zoophycos. Body fossils found in the Witteberg stratainclude the remains of lingulid brachiopods, palaeoniscidsh (Jubb, 1965) and lycopod and psilophyte plant frag-ments. In the eastern part of the fold belt, near Grahams-town, Hiller and Taylor (1992) describe carbonaceousblack shale units containing fossilized sh and plant mate-rial, in association with thinly-bedded quartzites and shales.

Cotter (2000) recognized a typical coarsening-upwardsfacies sequence in the lower part of the Witteberg Groupwhich is repeated at least 40 times in the stratigraphic inter-val he investigated. The sequence, from the bottomupwards, consists of a silt-streaked mudrock grading intomudrock containing isolated wave-ripple lenticles, throughwave-rippled aser-bedded sandstone, and into hummockycross-stratied sandstone.

Structurally, the Witteberg Group is thrusted andfolded, as are the underlying Bokkeveld and Table Moun-tain Groups, particularly on the southern side of structural

R.W. Shone, P.W.K. Booth / Journal ofbuttresses composed of infolded, less competent, massivetillites of the overlying Karoo Supergroup.3. Lithostratigraphy, Natal Group and Msikaba Formation

The Natal Group rocks and those of the MsikabaFormation crop out in a fairly narrow strip along or closeto the eastern coast, in Pondoland and Kwazulu-Natal(Fig. 1).

3.1. Natal Group

Flat-lying Natal Group rocks occur in a coast-parallelstrip from Hibberdene in southern Kwazulu-Natal toUlundi in the north, stretching as far inland as Pietermaritz-burg. The Natal Group attains a maximum thickness of530 m; (SACS, 1980) and can be subdivided into a numberof readily identiable lithostratigraphic entities. Despite thisthere are dierences in the subdivisions recognized by dier-ent workers (SACS, 1980; Marshall and von Brunn, 1999,2000). TheUlundi conglomerate at the very base of theNatalGroup unconformably overlies basement rocks of the Neo-proterozoic Natal Metamorphic Belt (Liu and Cooper,1998). This basal unit is a coarse clast-supported conglomer-ate composed largely of quartzite pebbles with a sparsemuddy-sandy matrix (Marshall and von Brunn, 1999). TheUlundi conglomerate is overlain successively by arkosicsandstones and shales (Eshowe Member of Marshall andvon Brunn, 1999), a well-silicied quartz arenite unit (theKranskloof Member), more arkosic sandstones (SitunduMember) and a second silicied quartz arenite (the Dassen-hoek Member) (Fig. 2). These units make up the DurbanFormation (lower half of the Natal Group, Marshall andvon Brunn, op. cit.). The Durban Formation is overlain bythe Marianhill Formation (Marshall and von Brunn, op.cit.) which has at its base the Tulini Member (a small-pebbleconglomerate). This conglomerate underlies a succession ofarkosic sandstones and shales (Newspaper Member) whichis in turn overlain in some areas by a matrix-supportedpolymict conglomerate, the Westville Member.

Sedimentary structures recognized in the quartz-arenitesandstones of the Kranskloof and Dassenhoek Membersof the Durban Formation include trough and planar cross-bedding, current- and wave-ripple structures and associ-ated aser, wavy and lenticular bedding. Herringbonecrossbedding is common and cyclic variations in the thick-ness of foreset laminae have been recognized as well asreactivation surfaces (see Liu and Cooper, 1998). Post-depositional sedimentary structures identied includemudcracks.

Palaeocurrent analysis of the Natal Group as a wholeshows a southerly ow trend throughout the outcrop area(Marshall and von Brunn, 1999, their Fig. 8, p. 22). For theKranskloof and Dassenhoek Members of the Durban For-mation, Liu and Cooper (1998) measured bimodal roughlybipolar ow directions, the more dominant direction to theeast.

A typical vertical facies sequence characterizes the

ican Earth Sciences 43 (2005) 196210 201quartz arenite sandstones (Kranskloof and DassenhoekMembers). Thinly-interbedded wavy and lenticular bedded

Afrsandstone and mudrock at the base of each cycle give wayupwards to herringbone crossbedded sandstone and thenceto sandstone with tabular (planar) crossbedding (Liu andCooper, op. cit.).

3.2. Msikaba Formation

The Msikaba Formation, which crops out north of PortSt Johns in the southeastern part of South Africa (Fig. 2),overlies Natal Group rocks along the coast at WoodGrange and Rock of Gibraltar near Hibberdene (Marshalland von Brunn, 2000). The Msikaba Formation is there-fore clearly younger than the Natal Group.

The Msikaba Formation consists of a succession of palegrey quartz arenite sandstones almost 700 m thick (du Toit,1954). Hobday and Matthew (1974) identied three distinc-tive facies. These include a sheet sandstone facies charac-terized by relatively thin, but laterally-persistent, bedscontaining the trace fossil Scolicia; a lenticular troughcrossbedded sandstone facies with Planolites traces andinferred palaeoow towards the southwest; and a chan-nel-sandstone facies marked by the presence of quitesteep-proled channels lled by complexly crossbeddedsandstone. Lycopsid plant fragments from the MsikabaFormation were identied by Lock (1973).

4. Age of the Cape Supergroup, Natal Group and

Msikaba Formation

Circumstantial evidence, that is, evidence on which rea-sonable deductions may be based, is by and large the prin-cipal source of information on the age of these successions.There are very few direct radiometric dates available, nota-bly the early Ordovician (490 Ma) date obtained for theNatal Group (Thomas et al., 1992a,b), as well as datesfor pre-Cape basement rocks, including the Darlingbatholith (502 Ma, Schoch, 1975) the Kango Group rocks(519 Ma, Barnett et al., 1997), and volcanic rocks associ-ated with the Cape Granite Suite (515 Ma, Scheepers andPoujol, 2002). Most of the other dates are inferred fromindex fossils, a procedure fraught with pitfalls, aspointed out by Cooper (1984).

These caveats notwithstanding, it is generally acceptedthat the Table Mountain Group rocks range from mid-Cambrian to late Silurian (Broquet, 1992, his Table 1).The Bokkeveld Group rocks are regarded as mid Devonianand the Witteberg Group as late Devonian to mid Carbon-iferous in age (Broquet, op. cit.).

The Natal Group rocks are Ordovician and thereforeprobably coeval with part of the Table Mountain Group.The Msikaba Formation, on the other hand, contains plantfossils which suggest a much younger (Devonian) age forthis unit (Lock, 1973). Neither Marshall and von Brunn(1999) nor Liu and Cooper (1998) regard the MsikabaFormation as part of the Natal Group. Furthermore Liu

202 R.W. Shone, P.W.K. Booth / Journal ofand Cooper (1999), following Anderson and Anderson(1985) point out that the Msikaba Formation could beeven younger than the Witteberg Group. If the latter werethe case then a hiatus between the Witteberg Group andthe Msikaba Formation would have been very shortbecause Dwyka tillite unconformably overlies both groupsof rocks, the ice sheets having stripped o >1000 m ofMsikaba overburden in Pondoland, whilst simultaneouslybringing about soft sediment deformation in Wittebergsandstones along the main Cape Fold Belt.

5. Depositional sedimentary environments

Depositional sedimentary environments are most reli-ably inferred from a variety of sedimentary parameters(geometry, lithology, sedimentary structures, fossils andtrace fossils, palaeocurrent patterns, successions and cycles;Selley, 1978). Given that the known data bases for many ofthe above mentioned lithostratigraphic units are of variablequality, and in some cases relatively incomplete, some ofthe inferred depositional sedimentary environments arespeculative and a few may be controversial.

5.1. Cape Supergroup

The Piekenierskloof Formation at the base of the Capesuccession is clearly the result of coarse clastic terrestrialsedimentation on a rugged, freshly eroded pre-Cape land-scape. That these coarse sediments are found only in twolocalities (in the western Cape in a narrow southeast trend-ing zone near Vanrhynsdorp, and in a coastal exposure ofvery limited extent at Sardinia Bay, near Port Elizabeth)may be misleading. We may not yet have seen the fullextent of the basalmost Cape Supergroup, perhaps becausethe exposed pre-Cape/Cape contact is commonly a tectonicone (cf. Booth and Shone, 1992a). Nevertheless, Rust(1967, 1973) envisaged the Piekenier basin as a smalltrough-shaped one and the coarse conglomerates lling itas scree breccias, uvial conglomerates and even tillites(Rust and Theron, 1964). A braided uvial setting isinvoked by Vos and Tankard (1981), Tankard et al.(1982) and Thamm (1989).

The Graafwater Formation sediments are generallyconsidered to have originated in a shallow marine tidal atsetting characterized by intermittent exposure (Rust, 1977).Tankard and Hobday (1977) opted for a seawards-pro-grading, ebb-dominated, mesotidal, back-barrier settingfor these strata which they envisaged as distal to thePiekenier facies (op. cit., their Fig. 21).

The Sardinia Bay Formation in the Eastern Cape isthought to be a shallow marine shelf deposit which showsevidence of strong tidal activity (Shone, 1983). A turbiditeunit at the top of the Sardinia Bay Formation suggestseither a major storm surge (Shone, 1987) or a seismic event.

Some controversy exists over the depositional sedimen-tary environment inferred for the Peninsula Formation.Rust (1973) weighed the possibilities of marine surf zone

ican Earth Sciences 43 (2005) 196210and aeolian sedimentary environments before opting fora general marine setting. Hobday and Tankard (1978) were

Afrmore specic in their interpretation of the data, favouring ashallow marine shelf/barrier island depositional model.The marine origin of the Peninsula Formation has never-theless been questioned by Fuller (1985) and Turner(1987). The latter considered progradation of uvial braid-plain deposits over the older Graafwater deposits as amodel for the deposition of the Peninsula Formation.

The Pakhuis Formation is certainly glacial in origin(Visser, 1962; Rust, 1967, 1981). The ice sheet deemed tobe responsible for these deposits was probably grounded,at least at its northern extremity with inferred movementof the glacier generally southwards (Rust, 1973).

The Cedarberg Formation shale which overlies thePakhuis Formation is evidently a distal marine/meltwaterfacies of the Pakhuis Formation (Rust, 1973).

The relatively thick succession of quartzites which makeup the Nardouw Formation have been ascribed to deposi-tion in a marine setting by Rust (1973). Thamm (1987) sug-gested an alternative uvial setting on the basis of clastshapes, but his analysis was subsequently questioned byReddering and Illenberger (1988).

The Bokkeveld Group strata have been identied asdelta and shallow marine deposits by Theron (1970), Rust(1973) and Tankard and Barwis (1982). The delta front andinterdistributary bay deposits seem to be conned mostlyto the more northerly outcrops of Bokkeveld, with prodeltaand shallow marine muds in the more southerly areas ofBokkeveld outcrop (Rust and Shone, pers. comm.).Tankard and Barwis (1982) suggested an array of deltafront sedimentary environments for the more northerlydeposits, including tidal channel sand bodies, tidal at,inter-distributary bay and distributary mouth bar deposits.They found evidence of ve coarsening-upwards sequencesin the Bokkeveld which they interpreted as indicating rapidsubsidence, punctuated by periods of relative stability anddelta progradation. The dark pyritic shales of the moresoutherly Bokkeveld outcrops contain few sandy unitsand commonly display a restricted faunal content. Thesestrata, even though characterised by wave ripple structuresand hummocky stratication (and therefore deposited justabove or below ordinary wave base) seem to have beenaccumulated as pro-delta and shallow marine shelf depositsin what might have been an anoxic epeiric marine setting.Rust (1973) alluded to signicant changes in the rates ofsedimentation and subsidence for the second half of thelife of the Bokkeveld depository, which are reected inregional facies dierences in the upper part of theBokkeveld succession.

The Witteberg Group is widely believed to be the resultof sedimentation in a shallow marine environment (Loock,1967; Rust, 1973; Cotter, 2000) although distal delta set-tings and freshwater lagoonal depositional environmentshave been suggested by some researchers for specic unitsin the Witteberg succession (Gardiner, 1969; Johnson,1976, 1991). The abundance of hummocky stratied sand-

R.W. Shone, P.W.K. Booth / Journal ofstone and wave-rippled, aser-bedded siltstone and mud-rock suggests that much of the Witteberg was depositedclose to ordinary wave base in a shallow marine deposi-tional milieu. Loock (in Rust, 1973) evidently felt thatthe Witteberg succession reects an unspecied numberof repeated transgressions and regressions. Cooper (1986)and Hiller and Taylor (1992) reported evidence of shorelineuctuations from Witteberg Group strata. Likewise,Cotters (2000) investigation of cyclic facies repetition inthe lower Witteberg led him to suggest repeated shallowingwhich he related to global sea level changes.

5.2. Natal Group

The Natal Group sediments are almost all inferred to beterrestrial/uvial in origin (Marshall and von Brunn, 1999),except for the two clean quartzite units (Kranskloof andDassenhoek Members of the Durban Formation) whichLiu and Cooper (1998, 1999) ascribe to deposition in ashallow, tide-dominated marine setting, an interpretationchallenged by Marshall (1999).

5.3. Msikaba Formation

The clean quartz arenites of the Msikaba Formation(Cape Supergroup in the southeastern part of SouthAfrica referred to as the Transkei) were identied as shal-low marine and estuarine deposits (Hobday and Matthew,1974). They recognized the sheet sandstone facies as shal-low marine shelf deposits. The lenticular sandstone unitswere compared to modern subtidal deposits and the chan-nel-sandstone units ascribed to deposition in a sub-tidalestuarine setting.

6. Deformation of the Cape Supergroup

Thrusting and folding is restricted to the rocks of theCape Supergroup (Natal Group and Msikaba Formationrocks show no signs of either thrusting or folding). Accord-ing to Halbich et al. (1983), deformation of the Cape FoldBelt began at about 278 Ma (270 million years after thedeposition of the rst Cape Supergroup sediments) andended during the mid Triassic (230 Ma) during the deposi-tion of the Molteno Formation of the Karoo Supergroup.Halbich et al. (op. cit.) identied four tectonic pulses, at278 Ma, 258 Ma, 248 Ma and 230 Ma. The eects of theseepisodes of intense crustal shortening include thrusting andfolding of Cape Supergroup and lowermost Karoo Super-group rocks. The deformation is most intense in the east-west trending southern branch of the Cape Fold Belt,whereas the north-south trending western branch of thefold belt is less severely deformed (Fig. 1). A syntaxismarks the junction of the western and southern branchesof the fold belt. This is an area of coeval interference fold-ing (de Beer, 1992), these structural patterns possiblybrought about by the rotation and interaction of micro-plates during the Cape orogeny (de Wit and Ransome,

ican Earth Sciences 43 (2005) 196210 2031992). Inversion (repeated episodes of compression andextension exploiting pre-existing zones of weakness) is a

characteristic feature of the Cape Fold Belt (de Wit andRansome, 1992).

Thrust faulting is more prevalent in the southern branchof the fold belt, and especially where there is pronouncedarcuation of the fold belt, there seem to be thrusts whichare characterised by relatively large displacements (e.g.Baviaanskloof thrust, 15 km displacement, Booth et al.,2004). Thrusting in the southern branch of the Cape FoldBelt is more often characterized by sets of closely-spacedthrusts and stacked thrust sheets in which individual dis-placements are dicult to ascertain (Fig. 3). Most of thethrust planes dip southwards, with a northward directionof thrust propagation as inferred by slickenbre orienta-tions (Fig. 4). However, there are numerous backthrustsand associated pop-ups, triangles and wedges. Thrust-stacked quartzite packages, hundreds of metres thick,occur in areas of Table Mountain Group outcrop (e.g.Cape Recife, Booth and Shone, 1992b; Kareedouw andUniondale thrust sheets, Booth and Shone, 1998, 1999).Individual thrusts are often parallel to bedding (beddingthrusts), but this is not always the case, and the thrusting

204 R.W. Shone, P.W.K. Booth / Journal of Afralso exploits suitably oriented cleavage planes, particularlyin areas of Bokkeveld outcrop where there is a pervasive,south-dipping cleavage (Shone and Booth, 1993). A com-mon feature of the thrusting is the frequent eclipse of shaleunits, formerly interbedded with the quartzites but thensmeared out along thrust surfaces (Booth and Shone,1992a, 1999). Thrusting can be seen to have developed inboth piggy-back and break-back fashion.

Folding on various scales and in an array of dierentfold styles is the most immediately obvious form of defor-mation in the Cape Fold Belt (Halbich, 1977, 1983, 1992).Gentle, fairly open folds characterize the western branch of

Fig. 3. Closely spaced thrusts in thrust-stacked quartzites of the TableMountain Group, Uniondale. Note the development of S-C cleavages

which demonstrate the direction of thrust propagation, in this case fromsouth to north.the fold belt, whereas tighter north-verging folds occur inthe southernmost parts of the southern branch, where thedeformation becomes markedly less intense and the foldsmore open northwards. Recumbent overfolds large enoughto be identied as nappes occur at Port Elizabeth (Boothand Shone, 1992b), but in general the folds are moreupright with a well-developed axial planar cleavage (Halb-ich, 1983).

The relationship between thrusts and folds is a complexone. Thrusting appears to have preceded folding in manyareas; In places fold limbs occur as thrust-stacked units(de Wit et al., 1998). Subsequent folding was clearly fol-lowed by more thrusting and backthrusting with suitablyoriented fold limbs being exploited as zones of weaknessalong which renewed thrusting took place (see Booth andShone, 1999, for examples of this). Within the WittebergGroup, the development of folds and thrusts is closelyrelated (Booth, 1996). In some areas movement on later-generation thrusts has re-oriented earlier fold structures(Booth, 1998; Booth et al., 2004).

Normal faults occur throughout the Cape Fold Belt.Most of these normal faults are east-west striking withdownthrown blocks to the south, but complex horst andgraben structures with cross-cutting transfer faults are alsocommon in the Cape Supergroup and pre-Cape rocks(Shone, 1976; Shone et al., 1990). Displacements of thou-sands of metres can be determined for some of these faults(Worcester, Gamtoos, Coega and Commando Kraalfaults). At least some of these normal faults co-incide withearlier-formed planes of crustal weakness, but most can belinked more immediately to the breakup of Gondwana.

Many of the east-west striking normal faults have asmall strike-slip component (e.g. Lauries Bay fault, Boothand Shone, 1992a), but there are also a number of north-south striking strike-slip faults in the eastern part of theCape Fold Belt (Booth, 1996) which were probably formedas a result of regional extension during the breakup ofGondwana.

The most important eect of this tectonic deformation(thrusting and folding, as well as the subsequent normaland strike-slip faulting), is the packaging of slivers of CapeSupergroup rocks in such a way that, in places, individualslivers and packages of slivers cannot be readily correlatedwith those nearby. Thrust stacking of quartzites and theeclipse of pelitic units add to the problems of correlationbetween outcrops and stratigraphic analysis withinGroups. Despite this, the occurrence of mappable quartzitepackages has misled many researchers into believing thatthe exposures in certain areas are at the very least, closeapproximations of the original stratigraphic succession.This is not to say that all outcrops are similarly tectonized:the Sardinia Bay Formation, for example, is a 180 m thickintact remnant of part of the Table Mountain Group,sandwiched between strongly tectonized quartzitic units(Shone, 1983). There is every reason to believe that the

ican Earth Sciences 43 (2005) 196210Sardinia Bay Formation exposed near Port Elizabeth isan intact fragment of the Cape succession (it contains many

apelineequ% a

Afrmudrock units and shows no signs of thrusting), but thesame cannot be said of the strata above and below it.

7. The Cape Basin

Fig. 4. Stereograms of structural elements in the eastern sector of the Cforethrusts of a, (c) forethrusts in the Witteberg Group, Steytlerville, (d)Groups, Port Alfred, (f) lineations in forethrusts of e. Data plotted onintervals: (a) = 3.5% per 1% area, (c) = 3.4% per 1% area (e) = 2.8% per 1et al., 1999; Booth et al., 2004.

R.W. Shone, P.W.K. Booth / Journal ofSedimentary basin analysis is no longer simply a ques-tion of lithostratigraphic and biostratigraphic analysis ofa perceived layer-cake succession (Miall, 1990). Modernstratigraphic investigations generally demand a moreexacting interrogation of sedimentary successions throughcareful dating of the rocks, examination of the verticalfacies sequences and the inferred depositional sedimentaryenvironments, the application of the principles of sequencestratigraphy assisted whenever possible by seismic strati-graphic data, and a broad understanding of plate-tectonicsettings and associated basin types. In the case of the CapeSupergroup-Natal Group deposits, we must ask ourselveswhether the time has come for a detailed re-analysis tounravel the complications resulting from tectonic stackingand ll in the data gaps such as the absence of seismic orother geophysical data which might be used to elucidateaspects of the basin architecture. The present dominantlylithostratigraphic-based interpretations may be foundwanting, a point made previously by Fuller and Broquet(1990).

7.1. Previous attempts at basin analysis

Rust (1973) was one of the rst researchers to attempt acomprehensive analysis of the Cape Basin. He consideredthe Cape sedimentation to have been uninterrupted fromthe beginning of the Ordovician until the beginning ofthe Carboniferous (i.e. he did not identify any majorunconformities). Rust identied fault-bounded subsidenceas an early inuence in the history of the Cape Basin(e.g. deposition of the terrestrial pre-Cape KlipheuwelFormation). Subsequent regional downwarping to form

Fold Belt. (a) forethrusts in the TMG at Uniondale, (b) lineations inations in forethrusts of c, (e) forethrusts in the Bokkeveld and Wittebergal area nets (a), (c) and (e), and on Wul nets (b), (d) and (f). Contourrea. Data from Booth, 1996, 1998; Booth and Shone, 1998, 1999; Booth

ican Earth Sciences 43 (2005) 196210 205an elongate basin during Peninsula time was inferred fromisopach data (Rust, 1973). He regarded the basin as tecton-ically stable until the sedimentation of the Bokkeveldwhich he considered to be marked by repeated transgres-sions and regressions associated with accelerations anddecelerations in the rate of basin subsidence. Two maindepocentres in a once-again tectonically stable downwarpwere proposed to explain the accumulation of the Witte-berg sediments.

Winter (1984) and Johnson (1991) regarded the CapeSupergroup sediments as passive continental margin depos-its. Johnson (op. cit.) invoked a wedge-shaped (in cross-section) craton-derived clastic accumulation along aneast-west trending passive margin as a likely scenario. Hefurther suggested downward exure of the continental mar-gin in response to sediment loading to explain the consid-erable apparent thickness (8000 m) of the Cape succession.

Tankard and Hobday (1979) and Tankard et al. (1982)favoured an aulacogen as the principal mechanism ofdownwarp which demarcated the early Cape/Natal basin.They attempted to relate the sedimentation to known glo-bal sea level data by suggesting that the world-wide rise insea level during the Ordovician was balanced in the CapeBasin, by the ready availability of terrestrial sediment.They suggested that the Winterhoek glaciation (as repre-sented by the Pakhuis tillite) did not involve appreciablelowering of sea level, but nevertheless considered the

Afrpostglacial Cedarberg Formation shales to be indicative ofmarine transgression. Regression was invoked to explainthe deposition of the Nardouw Formation. The stackingof deltaic Bokkeveld deposits was considered to be theresult of delta progradation and repeated basin margindownwarping in an epeiric marine setting. The Wittebergsedimentation was similarly thought to be the result ofalternating regressions and transgressions (Tankard et al.,op. cit.).

Winter (1984) identied two major discordances in theCape Supersequence. The rst discordance perceived,between the Peninsula and Nardouw Formations, he corre-lated with the global drop in sea level proposed by Vailet al. (1977) for the end of the Ordovician. The second dis-cordant break identied by Winter is represented by theNardouw/Bokkeveld interface which corresponds with aglobal sea level rise which is presumed to have commencedat the start of the Devonian (Vail et al., op. cit.). Winter(1984, 1989) did not regard the Cape Supergroup andNatal Group deposits as a failed rift accumulation, prefer-ring a miogeoclinal downwarp as a probable tectonicsetting.

Cooper (1986) correlated facies shifts in the Bokkeveldand Witteberg Groups with global sea level curves estab-lished for Europe and North America. He noted theprobable deepening associated with the Bokkeveld sedi-mentation and commented on the anoxic nature of theBokkeveld sea oor. Metre-scale coarsening-upwardsequences in the lower Witteberg Group have been linkedto global sea level changes by Cotter (2000), who suggestedthat the regional architecture of the lower Witteberg Groupcan be explained by an hierarchical series of global sea leveluctuations.

Thomas et al. (1992b) used aeromagnetic data and theoccurrence of Beattie-set anomalies to suggest that twodierent types of Mid- to Late-Proterozoic crust (inferredfrom the magnetic data) controlled sedimentation and thelater deformation (or lack of deformation) of sedimentsin the Cape Basin.

Catuneanu et al. (1998) proposed that orogenic cycles ofloading and unloading (related to the Cape Orogeny)led tothe deposition of overlying sediments of the Karoo Super-group in a retro-arc foreland basin, immediately north ofthe Cape Basin.

All of these attempts at basin analysis draw on a data-base which at its best is incomplete. Little is known aboutthe seismic architecture of the Cape Fold Belt, and few ifany acceptable sequence boundaries have been identied.This does not mean that the attempts to understand theCape Basin are not innovative. They are simply based onvery limited data. Seen in the light of the evidence for wide-spread and pervasive thrusting, thrust-stacking and theaccompanying eclipse of pelitic units which characterizesthe Cape Fold Belt, some of the data used to infer the basinhistory may be controversial and requires a revisit. Appar-

206 R.W. Shone, P.W.K. Booth / Journal ofent vertical facies changes, coarsening- or ning-upwardsand thinning-upwards successions are, in places, likelyartefacts of tectonic stacking and probably do not accu-rately reect the sedimentary history of the basin. Isopachmaps (the basis for much of Rusts pioneering analysis ofthe Cape Basin) could measure the degree of thrust stack-ing rather than the position of a depocentre. On the out-crop scale, there are many potential problems: thephotograph of what are described as lenticular beddedsandstones in Cotters paper on the Witteberg (Cotter,2000, his Fig. 6, p. 5) could very well be nothing more thana series of thrust-stacked wedges.

7.2. A revised model and basin history for the Cape

Supergroup, Natal Group and Msikaba Formation

Given the doubts expressed about the reliability of dataand the problems which arise in using suspect data to infera basin history one might reasonably conclude that anynew model must be as awed as its predecessors. However,simply recognizing the inherent pitfalls may be insucientin the reconstruction of the Cape Basin and its evolution.We therefore propose the following:

1. Broad lithostratigraphic division of the Cape Super-group into the Table Mountain Group, BokkeveldGroup, and Witteberg Group is substantially correctalthough published estimates of depositional thick-nesses are likely to be incorrect. The subdivision ofthe Table Mountain Group into smaller lithostrati-graphic entities (Piekenierskloof Formation,Graafwater Formation etc.) is probably also reliable,at least in the Western Cape, where the succession hasbeen subjected to fairly intensive scrutiny over theyears. Our misgivings about thicknesses, and specula-tions concerning the apparent volume of quartz are-nites (Tankard et al., 1982: the quartz areniteproblem) remain. Thrusting is likely to have beenresponsible for the thinning and even removal ofmany thin interbedded shales thus creating the per-ception of a vast pile of quartz arenite sandstone.Quantication of the degree of thrust displacementhas not to date been adequately addressed, stronglysuggesting that a structural review of the Cape FoldBelt is necessary in order to attempt to resolve thisproblem.

2. Details of the Bokkeveld and Witteberg lithostratig-raphy as listed in SACS (1980) are suspect, especiallyin the Southern and Eastern Cape where closely-spaced thrusts are especially common. Facies analysisof exposed sections in these areas is problematical.

3. Without appropriate seismic data, both the broadand detailed architecture of the Cape successionremains enigmatic. This makes it dicult to identifysequence boundaries. In spite of this, the TableMountain Group/Bokkeveld transition can be identi-ed as a marine ooding surface (Cooper, 1986).

ican Earth Sciences 43 (2005) 1962104. Sediments of the Cape Supergroup (with the possibleexception of the Pakhuis Formation), Natal Group

region of the present southern and southeastern partof the southern arm of the Cape Fold Belt (Fig. 5).According to this hypothesis, the Natal Group andMsikaba Formation would have remained unaectedby the deformation. The far-eld tectonic eects ofsuch a collision remain enigmatic, but could havehad an inuence on distant crustal structures whichmay, in turn, have played a signicant role in the evo-lution of, for example, the Congo Basin (Giresse, thisissue).

8. Conclusions

There is clearly much to be done before the sedimentarybasin history and subsequent deformation of parts of theCape Supergroup, Natal Group and Msikaba Formationcan be more completely understood. There are doubtsabout the nature and origin of some of the lithostrati-graphic units which have not been resolved, and someaspects of the lithostratigraphy which could be artefactsof the tectonism need to be evaluated more thoroughly.Without a series of seismic proles across the Cape FoldBelt, its macro-architecture and the contained sequenceboundaries remain largely unknown. The basin modelwhich we have proposed is, accordingly, a conservative

Afrand Msikaba Formation are typical uvial, clasticshoreline and clastic marine continental shelf depos-its. The Bokkeveld Group sediments appear to bedelta and shallow epeiric marine deposits which showsome evidence of accumulation in anoxic sinks.

5. The basin model which best ts the available data isthe divergent margin basin model as described byMiall (1990). Passive rifting is preferred as the mech-anism responsible for divergence in the Cape Super-group (absence of volcanics), but active rifting mayhave marked the separation in northern Natal wherecontemporaneous volcanism has been reported(Marshall and von Brunn, 1999).

6. Early rifting, probably in the form of listric faultsalong pre-existing lines of crustal weakness, wasaccompanied by the deposition of coarse clastic sedi-ments in fault-bounded graben and half-grabenbasins (e.g. Piekenierskloof Formation in the CapeSupergroup, Ulundi conglomerate in the NatalGroup).

7. Subsidence along the new divergent margin resultedin the deposition of marine sediments over earliernon-marine graben-ll deposits (e.g. Graafwaterand Peninsula Formations in the Cape Supergroup,Kranskloof member in the Natal Group).

8. Subsequent regressions or transgressions (as inferred,for example, by the deposition of the Pakhuis tillite,Cape Supergroup) are probably not simply the resultof global sea level changes as suggested by Winter(1984, 1989), Cooper (1986) and Cotter (2000), eventhough the evidence may appear to t inferred globalsea level records. Tectonically-induced changes of rel-ative sea level are likely in a divergent margin basinsetting. The inferred global sea level rise during theDevonian (Vail et al., 1977) does, however, matchthe evidence for epeiric marine sedimentation of theBokkeveld rather well (Winter, 1984; Cooper, 1986).

9. Lithication of the Cape Supergroup sediments tookplace during deep (up to 8 km) burial under condi-tions of load diagenesis (de Swardt and Rowsell,1974). This happened well before the Cape foldingevents which did not have notable dynamo-metamor-phic eects on the strata (Drong, 1973).

10. Thrusting and folding of the Cape Supergroup beganat about 278 Ma, resulting in the inversion of theCape Basin and the development of a yoked Karooforeland basin to the north of the thrust and fold belt(de Wit, 1992). A number of plate tectonic modelshave been proposed to explain the folding and thrust-ing (Lock, 1980; Johnson, 1991; de Wit and Ran-some, 1992; Halbich, 1992). Problems regarding thedierent orientations and intensities of folding inthe western and southern arms of the Cape Fold Beltand the area of the syntaxis have not yet been satis-factorily resolved. The undeformed, but more or less

R.W. Shone, P.W.K. Booth / Journal ofcoeval Natal Group and Msikaba Formation, posean even bigger problem: How is it that these lattersediments escaped the Cape deformation? A possiblesolution has been put forward by Booth et al. (2004)who propose the collision of a microcontinent in the

Fig. 5. Plate tectonic model for the Cape Fold Belt (modied after de Witand Ransome, 1992). Note the position of the proposed microplate southof the southern African coastline presumed to have collided withthe African continent (Booth and Shone, 2002). The presence of themicroplate could account for the greater degree of deformation in theeastern compared to the western part of the fold belt, and lack ofdeformation in the Natal Group, and Msikaba Formation.

ican Earth Sciences 43 (2005) 196210 207one. A more speculative interpretation would be dicultto justify in the light of the many remaining uncertainties.

AfrThe microcontinent collision hypothesis originally put for-ward by Booth et al. (2004), and re-advanced in our basinmodel (point 10., above) to explain how the Natal Groupand Msikaba Formation escaped the fairly intense defor-mation which characterizes the southern branch of theCape Fold Belt may not stand up to rigorous criticism.For the moment, however, it provides an appealing solu-tion to a vexing problem.

Acknowledgements

We have done eld work in the Cape Fold Belt and havetherefore been able to contribute some of the data formingthe basis for this paper. Much of the data was, however,collected by others, as listed in the references. For theirassistance with this review we would like to thank IzakRust, Maarten de Wit, Stephen Johnston, John Craddock,Gerry Webers, Nick Mortimer and Coenie de Beer for their(occasionally acerbic) comments, ideas and encourage-ment. Callum Anderson digitized the location map, andthe University of Port Elizabeth provided us with researchfunding. We would also like to thank Pat Erikssen andMarc Goedhart for their constructive criticism whichhelped us improve on the original manuscript.

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210 R.W. Shone, P.W.K. Booth / Journal of African Earth Sciences 43 (2005) 196210

The Cape Basin, South Africa: A reviewIntroductionLithostratigraphic subdivisions of the Cape SupergroupThe Table Mountain GroupThe Piekenierskloof FormationThe Graafwater and Sardinia Bay formationsThe Peninsula FormationThe Pakhuis FormationThe Cedarberg formation shaleThe Nardouw Formation sandstone

Bokkeveld GroupWitteberg Group

Lithostratigraphy, Natal Group and Msikaba FormationNatal GroupMsikaba Formation

Age of the Cape Supergroup, Natal Group andMsikaba FormationDepositional sedimentary environmentsCape SupergroupNatal GroupMsikaba Formation

Deformation of the Cape SupergroupThe Cape BasinPrevious attempts at basin analysisA revised model and basin history for the Cape Supergroup, Natal Group and Msikaba Formation

ConclusionsAcknowledgementsReferences