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S.AfrJ.Geol.,1992,95(5/6),181-193 181 Deposition of the Early to Late Permian Whitehill Formation during a sea-level highstand in a juvenile foreland basin J.N.J. Visser Department of Geology, University of the Orange Free State, P.O. Box 339, Bloemfontein 9300, Republic of South Africa Accepted 11 November 1992 The black, laminated, carbonaceous shales of the Whitehill Formation were deposited in a very young, underfilled foreland basin under anoxic bottom conditions. A sea-level highstand, basin tectonics, and climate were the controlling factors - interplay of which resulted in bounding conditions for organic-rich mud deposition during a specific time slot in the history of the basin. Coal-forming environments along the steep palaeo-eastern basin margin were the source of mud and organic matter transported as fresh-water plumes in an offshore direction during episodic flooding and erosion of the organic-rich deposits. Air-borne volcanic ash deposited together with the muds as well as in discrete layers was derived from a tectonic arc in the palaeo-west. The high concentration of organic matter in the water body and the restricted oceanic circulation in the morphologically complex basin created anoxia in the water column. Preservation of organic matter in the absence of benthonic fauna was high. Less anoxic conditions prevailed in the shallow marginal regions where deposition of siltstone and carbonate rocks interbedded with the black shales took place. Continuous inflow of fresh-water plumes in the restricted basin progressively caused brackish conditions suitable for the proliferation of aquatic fauna. Die swart, gelamineerde, koolstofryke skalies van die Whitehill-formasie is onder anoksiese bodemtoestande in 'n baie jong, ondergevulde voorlandkom afgeset. 'n Hoe seevlak, komtektoniek, en klimaat was die kontrolerende faktore waarvan tussenwerking tot begrensingstoestande vir die afsetting van organiesryke modder gedurende 'n spesifieke tydvak in die geskiedenis van die kom, aanleiding gegee het. Omgewings van steenkoolvorming langs die steil oeroostelike grens van die kom was die bron van modder en organiese materiaal wat as varswaterpluime in 'n wegstrandse rigting gedurende episodiese oorstroming en erosie van die organiesryke afsettings, gevoer is. Lugstroomvervoerde vulkaniese as afkomstig van 'n tektoniese boog in die oerweste is tesame met modder asook in onderskeie lagies afgeset. Die hoe konsentrasie van organiese materiaal in die watermassa en die beperkte oseaniese sirkulasie in die morfologies-komplekse kom, het anoksia in die waterkolum tot gevolg gehad. Preservering van organiese materiaal was hoog in die afwesigheid van bentoniese fauna. Minder anoksiese toestande het in die vlak randsones waar afsetting van sliksteen en karbonaatgesteentes tussengelaagd met die swart skalies plaasgevind het, geheers. Aanhoudende invloei van varswaterpluime in die beperkte kom het progressief braktoestande, geskik vir die opbloei van akwatiese fauna, veroorsaak. Introduction The Whitehill Formation, formerly known as the 'White Band', is a very conspicuous stratigraphic unit near the base of the Ecca Group in the western half of the Karoo Basin (Figure 1). This white-weathering unit is persistent throughout the area which makes it an excellent marker horizon and, together with its characteristic fossil assemblage, a time-stratigraphic marker. It was furthermore considered as a potential hydrocarbon source rock on account of its organic content. The Whitehill Formation is probably the one formation in the Karoo Supergroup 'we know little about despite a large number of publications on its palaeontology, sedimentology, and economic potential (e.g. McLachlan & Anderson, 1977a; Anderson, 1979; Oelofsen, 1981; 1987; Cole & McLachlan, 1991). In most of these publications reference is made to the origin of the formation. All authors reached a consensus that the formation was deposited as black, organic, highly sulphuretted muds in a large sea and which were preserved as black shales in the Parana Basin of South America and the Karoo Basin of southern Africa. This is, however, a fairly simplistic view which leaves several questions unanswered. 1. Why is the Whitehill Formation such a distinctive unit in an overall mudrock succession? 2. Why does the organic content of the formation increase with respect to the under- and overlying strata? 3. Why is there an abrupt change in style of mud sedimentation with respect to the under- and overlying strata? The objective of this paper is to formulate answers to these questions and, in the process, define a more acceptable Figure 1 The Whitehill Formation in outcrop north of the Orange River on the farm Aussenkjer. Note the presence of blackweathering shale interbedded in white-weathering shale above the lower dolerite and a white streak below the base of the formation. The top of the Whitehill Formation was taken at the whiteweathering shale above the black horizon (Aussenkjer section in Figure 4). Do = dolerite. S.-Afr.Tydskr.Geol.,1992,95(5/6) 182 \'\, II NAMIBIA BOTSWf/~,A 'I :,; \ \ I ,,/ "---~SOUTH /'-? ,) k.---." AFRICA .,: <: , Main Karoo .. Basin \ : Douglas , Auss enkJe~...........: , '-----..,....-..--,,--,----t ..:' Kenh~';dt '" .. : Lateral cut-off , ':'" . . for Whitehill' Deep-water ' . :~~esfon:~~n '--', Formation~ facies of .. -I lo,. Shallow Whitehill . De Bos i Prince Albert \ facies of Formation ' Whitehill Ceres .... Mer~eviJIe ----, Willowmore Formation Grahamstown PORT ELIZABETH o 200 400km Figure 2 Locality map showing the major facies distribution in the Whitehill Formation. model for Whitehill deposition. This means that in the analysis aspects of the sedimentology of the underlying Prince Albert and overlying Collingham Formations in the southern Karoo need to be covered as well as the regional reconstruction of southwestern Gondwana during the Early to Late Permian transition. This is necessary to get a perspective of the morphology and tectonic setting of the Whitehill basin. predominantly of dark-grey, carbonaceous, pyrite-bearing, splintery shale, olive-grey micaceous shale, dark mudstone, and subordinate bluish-grey chert nodules and lenses, nodules as well as thin beds of phosphorite and carbonate concretions. Viljoen (1990) recognized widely distributed volcanigenic material in the formation, whereas discrete tuff beds are present in the northwestern Karoo (McLachlan & Jonker, 1990) and near the top of the formation between Laingsburg and Klaarstroom. Lithologies within the formation can be grouped into five lithofacies (Figure 3). The mudstone-shale facies which contains subordinate claystone breccia, fine-grained wacke, chert, and dolomitic limestones, pinches out in the Prince Albert area, but merges with the greenish-grey shale facies to the north. The chert-shale facies is a very distinctive unit and forms a useful marker in the formation. It consists of reddish weathering chert, containing abundant pyrite, and subordinate dark-coloured shale. Phosphorite and chert bodies in the south and dolomitic limestone in the north are the main constituents of the greenish-grey shale-chert facies. The olive-grey micaceous shale facies containing phosphorite bodies forms lenticular units in the Laingsburg and Grahamstown areas (ef Wright, 1969). At the top of the formation the dark-grey shale-phosphorite facies is developed in the Laingsburg-Prince Albert area and then again to the east of Klaarstroom. It also contains dolomitic limestone and pyrite. Stratigraphy and lithology Prince Albert Formation Whitehill Formation The Prince Albert Formation attains a maximum thickness of just under 200 m in the Laingsburg area and east of Klaarstroom (Figures 2 and 3). The contact with the Whitehill Formation is sharp, except in the north (Loeriesfontein Aussenkjer area) where thin white-weathering beds are present near the top of the formation (Figure 1). The outcrops of the Prince Albert Formation consist The white-weathering Whitehill Formation which is a synchronous marker horizon (Oelofsen, 1987), is present only in the western and central parts of the Karoo Basin (Figure 2). Mapping of the Whitehill Formation shows that it loses its 'character' laterally and can no longer be recognized as such (ef Cole & McLachlan, 1991). Along strike in the Karoo Basin the lateral cut-off mostly CD Mudstone-shale facies: minor claystone breccia, wacke and chert; chert pinches out southward; dolomitic limestone content increases northward @ Greenish-grey shale-chert facies: phosphorite in the south; dolomitic limestone content increases northwards; chert pinches out towards the north MERWEVILLE ® Chert-shale facies: reddish marker; abundant pyrite Phosphorite boundary LAINGSBURG @ Olive-grey micaceous shale facies: phosphorite in the south m @ Dark-grey shale-phosphorite facies: 100 carbonaceous and dolomitic limestone; pyrite; tuffaceous beds Chert/phosphorite boundary MF = Marine fossils o Chert/phosphorite boundary WILLOWMORE @ WEST EAST Figure 3 Fence diagram of facies distribution in the Prince Albert Formation along the southern basin margin. The heavy line represents a distinctive break within the Prince Albert Formation. en > ::i' o ¥~ (1) "Tl~61~ C. ~ ::s fD a.~§:n ~ (1) ..... 0Cl a:ci Jg (1) II en II II >-j ..... n ~ ~ ~O., '"1 "Tl::rN ~ 3 3g ~ ~~ '"1 q:; CD t=. AUSSENKJER GENERAL KAROO STRATIGRAPHY ~ o a '"1 rz.. ;" (S. 0 I-: o' g:;?Ro~ ~ g. en (") 0 (1) ::s ~s ____ ~ o.."Tl g II ~< ..... (1) '"1 0Cl en ~g:....., ~ § So (1) ,--, ~~~ (") ::r :>_::r II ~~. 61~ 3 en 0 ~ ~ ~N[ ::1. II - c. "Tl ~ ~ 61 o 0 (1) ~ '"1 P :> '"1 3 t:C ~ 3 a g. P II '"1 0: ;? - 0 t;r> ~ _ ~. 0 ? t:C l ' o"Tl (1) en ~ '"1 ::s (1) ~ 3 (1) ,=",>"Tl~ -"Tl ~ ::> rz.. I:t; 0 II g. ~~. a~P~~ a s:::Jg..... ~VJN ~ ':=' en II II '="'> g"~~~ _ (1) '"1 ~OCl2:~So ~ =:: 61 0 0 I'zl ...> ~ • ~ g tI < - 0 ::to ~....., ~g~s• • (1) -----_. I -- \~.-- //; \ \ Notocaria tapscotti Palaeoniscoid fish '\. " ' o \. rz.. \. \. \. "\. \ I%: ", 10 0 \ \ ,, , ~ Dark-grey splintery shale ~ ~ Laminated black carbonaceous shale B:::..: -::1 Grey silty shale (siliceous in south) r';.::':.-:':' :':<1 \ \\F".:.:':.":':'.':.:': Fin~ to mediumgrained sandstone 2 ~ Coal PRINCE ALBERT 2 ~ KLAARSTROOM . ~ Fe-rich carbonates III T \ \ > \ m / LAINGSBURG >t I:t; \ 20 \ A .... I'zl \. \ ::::::: 7. Chert (siliceous mudrock) Tuffaceous T \ \ , \ o .__---- to't~ /.--'-- ;\;e't\ \ ./ /'~~'" \ A ---.__'-- --~\\o~ /'-0-(, ,,- \ ~ ' .-- ./ "'\ Mesosaurus >t ~ __ __ \ 0 ~ I:t; \ I -- -- < \. I \ < \. I Fossil Range Zones I:t; ' ' \ \ "' \ E-! \ \ ::> ' f ~ ~ "? "?. I I o o o 0 ' z o.... \ \ I ... VI 'cc" , " ' L_ __ .. _.. .c .i / ."CC' \ I I 0 I-: 0 3 ::s ::s '"1 / /' // / ' // /: // :, rz.. ~ ~. ::to I I - a -0 E? (") ·: ~ 0 o ..... _. "Tlg. 3 I I I as (1) §. ·, · /// : , · ~ ~ ...-:-.::.:-: .... I I I 0 < ·· I I I I ~ KENHAR DT 2 I 1I e:: / :1" il rz.. l ' ::s ~ (") C'I f--- "Tl P II ·,, . . . ."-,,.--'"":1 f o o o I' >" Jt::L / //~:"-Cc:~ ,, I ~. 61 ~ on~::r '"1 ~ o P 3 a :N:B i.e BOSHOF 1 LOERIESFONTEIN '-" DOUGLAS B. GR. - WELKOM 3 , \ 00 VJ S.-Afr.Tydskr.Geol.,1992,95(5/6) 184 + JOHANNESBURG@ + + DUR,~,7' / ...., / Figure 5 Tentative isopach map of the Whitehill Formation. Orientation of fossils in the formation (arrows) after Oelofsen (1981). comprizes a change in weathering characteristics of the mudrock, whereas a lithofacies change takes place towards the northern basin margin (Figure 4). It is also only along the southern basin margin that the upper and lower contacts are well defined, whereas northwards the formation becomes less distinct (cf Figure 4). There organic-rich shales, which weather black, occur interbedded, below or above the Whitehill Formation (cf Figure 1). This illustrates that the deposition of the Whitehill muds represents a change in bounding conditions in a specific sector of the basin during overall mud deposition. The Whitehill Formation has a maximum thickness of about 80 m in the Loeriesfontein area (Figure 4). Along the Cape Fold Belt, complex folding occurs within the formation with the result that thickness measurements are unreliable. Formation contacts are poorly defined along the northern basin margin. Therefore only a tentative isopach map which shows an apparent northwest - southeast thickness trend with maximum thicknesses in the west, could be constructed (Figure 5). The formation thicknesses show a correlation with the palaeotopography along the northern basin margin in that greater shale thicknesses occur near palaeovalleys. Lithology Lithologically the formation can be subdivided into a 'deepwater' facies characterized by the presence of chert and carbonate concretions, and a 'shallow-water' facies with numerous silty horizons and subordinate carbonate beds, but with chert noticeably absent (Figures 2 and 4). Along the western outcrop belt the shallow-water facies consists of a basal and an upper shale horizon separated by a carbonatebearing zone. Black, carbonaceous, pyrite-bearing shale constitutes between 75 and 100 per cent of the formation. The shale is very thinly laminated and the carbonaceous material consists mostly of amorphous kerogen (Cole & McLachlan, 1991). In the Boshof area the black shales contain siltstones and very fine-grained sandstone beds, laminae, and lenses (Figure 4). These become thicker and more abundant northward. They exhibit wave ripples, horizontal laminations, and minor inverse grading and loadcasting (cf Cole & McLachlan, 1991). Thin, yellowish-weathering rhyodacitic tuffs associated with silty shale are interbedded in the formation in the Loeriesfontein - Kenhardt area, Douglas - Boshof area, and east of Laingsburg along the southern basin margin (Rowsell & De Swardt, 1976; McLachlan & Anderson, 1977a; Viljoen, 1990; McLachlan & Jonker, 1990). The carbonate-bearing zone consists of ferruginous, dolomitic limestone containing organic-rich laminae (McLachlan & Anderson, 1977a), whereas dolomitic concretions are found throughout the formation. Halite imprints were found on bedding planes along the northern basin margin (Van der Westhuizen et al., 1981). Primary gypsum crystals within the carbonate beds in the southwestern Cape were apparently replaced by dolomitic material (McLachlan & Anderson, 1977a) and Cole & McLachlan (1991) reported gypsum lenticles in shale from the Boshof area. Palaeontology The biostratigraphy of the Whitehill Formation suggests synchronous deposition in the Karoo Basin, as the fossil range zones are confined mostly to the upper part of the formation (Figure 4). The remains of a primitive swimming reptile (Mesosaurus tenuidens) (Oelofsen, 1981) and of plants (Glossopteris leaves and fossil wood), sponge spicules, palaeoniscoid fish (Palaeoniscus capensis), and arthropods (No tocaris tapscotti) are common (Figure 6). Rare fossil insect wings were also reported by McLachlan & Anderson (1977b). Worm trails are present in grey silty shale near the middle of the formation at Loeriesfontein. Anderson (1975) reported the presence of limulid trackways at the top of the formation in the Aussenkjer area. Correlation Van Eeden (1973), McLachlan & Anderson (1975), and Anderson (1977) suggested a correlation between the Whitehill Formation and the Middle Ecca coal measures. Such a correlation is also substantiated by borehole sections in the Boshof area where the fine-grained sandstone facies may represent the distal extent of the coal measures (Vryheid Formation) deposited along the northern and northeastern basin margins (cf Cole & McLachlan, 1991) (Figure 4). Mapping of the Dwyka - Ecca transition in the southern Karoo indicates a facies change as the ice retreated towards the palaeo-south and palaeo-east. Therefore facies 1 and 2 and probably also facies 3 of the Prince Albert Formation are the lateral equivalents of the upper diamictites in the eastern and northern parts of the basin (Figure 3 and 7). Van Vuuren (1983) also showed that the coal beds are overlain by transgressive units which enables a correlation with the offshore shale and siltstone facies. The upper part of the Prince Albert Formation is thus correlated with the lower coal-bearing cycle of the Vryheid Formation in the Kroonstad area, whereas the upper two cycles correlate with the Whitehall Formation (Figure 7). The Whitehill Formation is also present in southern and northern Namibia where Horsthemke et al. (1990) correlated the beds lithologically with the Irati Formation in the Parana Basin. Oelofsen (1987) and Oelofsen & Araujo (1987) correlated the succession in the Karoo and Parana Basins ecologically and palaeontologically. S.Afr.J.Geol.,1992,95(5/6) 185 ELAND$VLEILOERIESFONTEIN KROONSTAD AREA AGE z < a: ;:) <.? Z ;:) Basement '" z < ::E -- a: >...J a: < z < ~ • Coal ~ Limestone D Sandstone U (/) z ~ Siltstone ~ Shale < ~ Diamictite G Glauconite Tr Transgression • Highly carbonaceous shale, but not while-weathering, present in Aussenkjer section Figure 7 Generalized sections of the Prince Albert and Whitehill Formations along the western basin margin and of the coal-bearing Vryheid Formation along the northern basin margin (after Stavrakis, 1986), showing correlations and sea-level fluctuations during the Early to Late Permian. PMB = Pietermaritzburg, Wh = Whitehill, Fm = Formation. Modified after Visser (in press). Figure 6 Fossils from the Whitehill Formation from near Loeriesfontein. A. Primitive free-swimming reptile (Mesosaurus tenuidens). B. Arthropod (Notocaris tapscotti). C. Palaeoniscoid fish (Palaeoniscus capensis). Age The age of the Prince Albert and Whitehill Formations is based on palynology, their correlation with the coal-bearing beds of the Vryheid Formation in the Karoo Basin and formations in the Parana Basin of Brazil, the presence of volcaniclastics, and the Eurydesma fossils near the top of the glacigenic Dwyka Group in Namibia. The Eurydesma fossils within the Dwyka Group have a Sakmarian age (Dickins, 1984) which led Visser (1990) to suggest an Artinskian age limit for the deposition of the upper Dwyka beds. A Late Permian age had been assigned to the Irati Formation (Archangelsky & Toigo, 1978). Pollen analyses from the coal-bearing strata which correlate with the Prince Albert and Whitehill Formations, indicate an Artinskian to Kungurian age ~ (Rio Bonita beds in the Parana Basin; Dos Santos et al., 1988) and a Late Sakmarian to Late Artinskian age (Waterberg coalfield; MacRae, 1987). MacRae (1992) assigned a Late Permian age, based on palynology, to the Whitehill Formation in the Douglas - Boshof area. Recent work on the Glossopteris floras of the coal measures also indicates a Late Permian age (Kovacs-Endr6dy, 1991). Tuffaceous beds occur from near the top of the Prince Albert Formation to the Collingham Formation and show the same rhyodacitic composition of tuffaceous beds in similar strata in the Parana, Paganzo, and Sauce Grande Basins of South America. There the rhyodacitic volcanism started at about the Early to Late Permian boundary and continued until the Early Triassic (Coutinho et al., 1988; Breitkreuz et al., 1989; Lopez-Gamundi et al., 1990). Based on these data Visser (1990) suggested a Permian age; possibly Artinskian to Kungurian for the Prince Albert Formation and Late Kungurian to Early Ufimian for the Whitehill Formation. Collingham Formation The formation consists of a rhythmically bedded succession of dark-grey splintery shale, yellowish-weathering tuffaceous beds, dark-grey chert, and subordinate very finegrained sandstone. About 10 m above the base of the Collingham Formation a white-weathering, dark-grey chert (Matjiesfontein chert) forms a prominent marker bed. The tuffaceous beds have a rhyodacitic composition (ef. Viljoen, 1990). S.-Afr.Tydskr.Geol.,1992,95(5/6) 186 Table 1 Chemical data on mudrocks and cherts from the southern Karoo 2 3 4 5 6 80.23 Si02 Ti02 64.86 88.59 58.31 81.05 61.48 0.61 0.20 0.62 0.12 0.61 0.12 A120 3 17.71 5.09 15.95 9.04 18.04 10.12 F~03 5.28 2.91 6.48 0.67 6.70 0.81 MnO 0.10 0.05 0.07 0.03 0.11 0.06 0.09 MgO 1.48 0.59 2.36 0.10 1.86 CaO 0.33 0.05 0.94 1.67 1.01 1.22 Na20 0.97 0.23 1.26 5.11 1.41 5.51 K20 3.10 0.69 3.23 0.07 3.64 0.12 P2 0 S 0.10 0.01 0.16 0.12 0.23 0.05 LOI 5.66 1.41 10.20 1.66 4.85 1.46 100.20 99.82 99.58 99.64 99.88 99.79 159 64 153 92 154 93 0.0055 0.0098 0.0046 0.0033 0.0055 0.0059 1.2 0.8 2.8 0.3 1.1 Total Zr MnO/ Al20 3 Org. C Carbo C 0 0.4 0.6 0.4 <1 <1 32 51 58 67 43 38 0 5 3 20 105 45 *Organic matter 4 2 *Oastic detritus *Biog. silica *Dolomitic limestone tSedimentation rate 1. Prince Albert shale (3 analyses) 2. Prince Albert chert (red marker), Laingsburg 3. Whitehill shale (7 analyses) 4. Whitehill chert, Laingsburg 5. Tierberg/Collingham shale (6 analyses) 6. Collingham chert (3 analyses), Laingsburg - Matjiesfontein *After Tada (1991) tAfter Iijima et al. (1985) Facies interpretation Carbonaceous mudrocks The presence of thinly-bedded shale, mudstone, claystone breccia, silty rhythmite, and fine-grained wacke in the Prince Albert Formation suggests deposition by sediment gravity flow (mostly distal turbidites) and subordinate suspension settling. The muds contain about 2 per cent organic matter (Table 1) indicating accumulation under anoxic conditions as is also shown by the rarity of bioturbation and the presence of pyrite. The anoxic conditions were probably caused by oxygen consumption in the water during the degradation of the descending organic matter through the water column (ef. Demaison & Moore, 1980). The geometry of the mudrock facies along the southern basin margin is explained by the formation of stacked basin floor mud fans - the distribution of which was controlled by the sediment entry points. The thinly laminated Whitehill shales formed by slow suspension settling of mud, whereas the interbedded siltstone and sandstone were deposited by tractional bottom currents in a shallow environment as shown by the crossbedding and ripple-lamination. However, some of the coarser-grained units in the Loeriesfontein and Aussenkjer sections may have formed by rapid suspension fall-out of fine sand and silt from large inflows of sediment-laden water as shown by the presence of slumping and parallel lamination (cf. Domack, 1983). The black muds deposited along the southern outcrop belt contain about 4 per cent organic matter consisting of plant algae, bacterially altered land plant cuticle and pollen, and possible benthic cyanobacterial mats (Cole & McLachlan, 1991). This material accumulated under anoxic conditions as indicated by the presence of pyrite and lamination in the mudrocks emphasizing the absence of bioturbation. The increased preservation of organic matter is attributed, firstly, to a higher input flux of organic matter whereby more organic material was entrained during suspension settling of the sediment with reduced dilution by mud (ef. Pedersen & Calvert, 1990) and, secondly, to increased anoxic conditions in the water column due to a greater demand on oxygen leading to an almost complete absence of scavenging fauna (ef. Demaison & Moore, 1980). Cole & McLachlan (1991) attributed the anoxic bottom conditions to the formation of benthic microbial mats, but up to now no evidence for the presence of such mats has been found and their effectiveness on oxygen drain in the water column over such a large basinal area is doubtful. The presence of trace fossils in the middle and arthropods in the upper part of the formation along the northern basin margin indicates episodic oxygenated bottom conditions. These conditions can be attributed to the shallower environment where oxygen drain in the bottom waters was sporadically reduced. The organic-rich mud formed a blanket deposit on top of the Prince Albert Formation in the southern Karoo. Along the northern basin margin increased formation thicknesses can be attributed to mud and silt accumulations in the vicinity of possible sediment entry points (ef. Figure 5). The average chemical index of alteration for 6 Whitehill shales is 67 (Visser & Young, 1991) which is below that of an average shale (= 72). This lower value can be attributed to the presence of unweathered volcaniclastic material in the black shales (ef. Fritz & Vanko, 1992). Strydom (1950) described worm trails on bedding planes and Viljoen (1990) bioturbation of the Collingham mudrocks. The shales may represent distal mud turbidites which accumulated under oxygenated bottom conditions. Geochemically there is a consistent increase in CaO, Na20, and K20 from the Prince Albert to the Collingham shales (Table 1) suggesting a progressively larger input of volcaniclastic material to the bottom muds. Chert Interpretation of the chert formation is important in that chert occurs interbedded with the deep-water facies of the Prince Albert, Whitehill, and Collingham Formations. The Prince Albert chert has a higher Fe content than that of the S.Afr.J.Geol., 1992,95(5/6) Whitehill and Collingham Formations, whereas all the cherts contain organic material and clastic detritus (el. Table 1). The cherts can be regarded as a binary mixture of biogenic silica and terrigenous detritus. Radiolarian spicules were observed in the cherts (el. Haughton et al., 1953), but most of the skeletal material was dissolved during silica phase transformation. The Ti~ (>0.1 %), A12 0 3, and Zr contents indicate a large input of clastics (el Adachi et al., 1986). The Prince Albert chert has been influenced for a longer period by sea-water as indicated by its F~03 content of about 3 per cent and its low sedimentation rate of about 20 m/Ma (Table 1). It consists of about 67% biogenic silica, 32% clastic detritus and 1% organic matter (Table 1) which, together with the presence of parallel lamination, suggest slow suspension fall-out of largely siliceous skeletal material. The chert formation can be attributed to a high frequency of biogenic silica productivity in the sea (el Tada, 1991). The Whitehill chert consists of about 43% biogenic silica, 51 % clastic detritus, 5% dolomitic limestone, and 1% organic matter. Sedimentation rate was about 105 m/Ma which was about 5 times as high as the Prince Albert chert, but still within the range of biostratigraphically calculated rates (el. Tada, 1991). The higher sedimentation rate may imply emplacement by turbidity currents, but because clastic detritus exceeds the biogenic silica input, high-density suspension fall-out is considered more likely for the origin of the chert. Although a high frequency of biogenic silica productivity may have still occurred in the Whitehill sea, suspension fall-out of skeletal material was partly masked by a higher terrigenous clastic input flux as evidenced by the 24 per cent decrease in biogenic silica content in the chert. The nodular nature of the chert may then in part be attributed to density inversion and diagenesis. The carbonate in the chert is of diagenetic origin. Anoxic bottom conditions during deposition are indicated by the presence of pyrite. The rhythmically bedded Collingham chert alternates with shale and tuffaceous beds. It consists of about 57% clastic detritus, 38% biogenic silica, 4% dolomitic limestone, and 1% organic matter. Theron (1967) ascribed the graded bedding in the Matjiesfontein chert to turbidity current deposition; thereby implying that the organic material had been transported from a shelf to its present position. According to Murray et al. (1992) grading in chert is not necessarily an indication of turbiditic input, whereas Viljoen (1990) suggested that most of the chert beds had a volcaniclastic origin. The sedimentation rate of the cherts was about 45 m/Ma (Table 1) which rather suggests fall-out of predominantly terrigenous material (mostly ash?) together with siliceous skeletal material. The carbonate in the chert is of diagenetic origin. 187 chemistry of the Collingham shales suggests also the presence of volcaniclastic material. The tuffaceous material represents air-borne volcanic ash (Viljoen, 1990). The presence of mostly well-defined tuffaceous beds, as well as disseminated volcaniclastic particles in the host-rocks, suggests different levels of ash concentration in the atmosphere over the Karoo Basin. The deposition of a bed occurred as a sudden episodic event and required an initial introduction of a large volume of volcaniclastic material in an upwind dIrection (el. Moore, 1991). Depletion of the ash source would have resulted in lower levels of ash fall-out and minor contamination of bottom deposits by the ash. The geochemistry of the mudrocks and the clastic detritus content of the cherts suggest an increase in ash contents from the Prince Albert Formation to the Collingham Formation which may imply increased volcanism in the source areas or a change to drier and windier climatic conditions during the Permian. Carbonate bodies. The carbonate nodules, which are mostly confined to the deep-water facies of the Prince Albert and Whitehill Formations, are early diagenetic in origin. Those from the Whitehill Formation are almost black in colour due to their organic material content (Strydom, 1950). They formed below the water-sediment interface during suspension settling of mud and organic matter with high levels of organic preservation. Bottom conditions were anoxic (el. Scotchman, 1991). The absence of carbonates in the Collingham Formation may reflect the change to more oxygenated bottom conditions. Along the northern basin margin, the dolomitic limestones of the Whitehill Formation are associated with silty horizons suggesting a shallower depositional environment (Oelofsen, 1987). The limestone beds contain carbon-rich microlaminations and casts of gypsum crystals, but show a lack of shallow-water sedimentary structures (el McLachlan & Anderson, 1977a). These features suggest deposition below wave-base from bottom waters with greatly increased salinities (brines?), which could be attributed to stratification in the water column in a possible silled basin, or as a result of high aridity or to the influx of brines formed by evaporation in shoreward areas during a sea-level lowstand. Oelofsen (1987) reported the presence of traces made by benthic fauna in the associated silty shales, but normally, the presence of bottom brines would have prevented sessile fauna from entering the depository and establishing themselves (el. Eugster, 1985). However, during deposition of the shallow-water facies episodic oxygenated bottom conditions must have occurred, ruling out stratification as a possible cause for the high salinities. The carbonate horizon correlates with a temporary lowstand (Figure 7) which would have favoured the influx of brines from shoreward regions. Volcaniclastic deposits Thin, yellowish-weathering tuffaceous beds occur towards the top of the Prince Albert Formation, throughout the Whitehill Formation, and increase in number in the Collingham Formation where they constitute between 30 and 40 per cent of the rock volume (Lock & Wilson, 1975). The geo- Phosphorites The phosphorites, which also contain pyrite and up to 4 per cent argillaceous material, are notably absent from the Whitehill and Collingham Formations. The black colour of the rock is due to the presence of carbonaceous material. S. -Afr.Tydskr.Geol., 1992,95(5/6) 188 Deposition of phosphatic material occurred in the muds below the water-sediment interface in a restricted anoxic environment from interstitial waters enriched in phosphorous (ef Cook, 1984). According to Cook (1984), a marine environment with the minimal input of terrigenous sediment and a sea-level rise are prerequisites for the formation of phosphorites. During the early stages of the sea-level rise oxygen-depleted and phosphorus-enriched waters were brought into the basin leading to phosphorite deposition (ef Brasier, 1992). This was followed by deeper anoxic waters which enhanced the deposition of the organic-rich black shales of the Whitehill Formation. The increased concentration of suspended terrigenous material in the water column and the reduced rate of oceanic overturn due to the closing of sea ways also contributed to the absence of phosphorites in the Whitehill shales. The high input of volcaniclastic material into the basin during deposition of the Collingham Formation and the more oxygenated bottom conditions were also unfavourable for phosphorite formation. AGE (Ma) 248 SEDIMENTATION RATE(m/Ma) 0 TECTONIC EVOLUTION 200 100 z .~ : Compressional deformation. w~ I-~ ~ w a. 'tl <CO: ..J 258 .r. .~ .:.: () ~o (1) 0: I (1) .5 .5 ~ ~ w a. >- ..J 0: <C E E c: 'tl (1) (1) a 0 Of w .!l! 286 t Basin tectonics and eustacy The Karoo Basin fonned part of a much larger depository in southwestern Gondwana of which remnants today can be found in South America, Falkland Islands, and Antarctica. It started as a back-arc basin probably during the MidCarboniferous, then became partly segmented and ended as a foreland basin during the Late Pennian and Triassic Substantial change in basin configuration. Start of submerged fold-thrust belt. Rhyodacitic volcanism ~ ~ B u. ~ Z <C : Uplift of fold-thrust belt. • Subsidence of foreland ramp Evidence for marine conditions The remains of a shark, sponge spicules, foraminifera, radiolaria, and acritarchs in the basal shales and the presence of marine invertebrate fossils at higher stratigraphic levels in the Prince Albert Formation are strong evidence for marine conditions (ef McLachlan & Anderson, 1973; Oelofsen, 1986). The phosphorites in the formation, and the Rb/K ratio of the shales suggest marine conditions during deposition of the entire Prince Albert Formation (ef. Cook, 1984; Visser & Young, 1990). The Rb/K ratio of the Whitehill shales suggests marine conditions for at least the lower part of the formation, whereas the Rb/K ratio of the upper Whitehill shales and the palaeontology suggest brackish conditions during deposition (Visser & Young, 1990; Oelofsen, 1981). Glauconitic sandstone overlying coal in the Vryheid Formation indicates a marine transgression (Van Vuuren, 1983). The coal-bearing cycles are correlated with the Whitehill Formation (Figure 7) which implies the presence of marine conditions in an offshore direction. It is concluded that overall marine conditions prevailed in the basin up to at least the end of the Early Permian. Thereafter conditions may have changed to brackish due to changes in basin configuration restricting connections with the ocean. A t PR rn () (1) ~ 0: C) .5 rn tll ~ () en ::;) ~ I ..:.: 0 0: () u. III w tll Decrease in plate movement. Thermal subsidence phase. Passive back-arc basin in extensional setting Z 0 III 0: <C (.) w ,, t \ I<C ..J 315 en ::;) 0 0: IW Qu. ~z 0 ... Windblown volcanic ash ..-----Sasin axis ~. = Whitehill Formation High rate of plate movement. Subduction of Proto-Pacific oceanic crust beneath Gondwana. Partial destruction of shelf III 0: <C (.) Figure 8 Sedimentation rates and tectonic evolution of the Karoo Basin from the Late Carboniferous to Late Permian. A and B represent compressional events in the pre-Karoo strata along the southern basin margin (cf. Halbich et ai., 1983). Quiescent periods shown by dotted lines. Modified after Visser (in press). ::. Fluviodeltaic . :. deposits -::-::;: Deep-water facies + + Highland ..-/ Palaeocurrent direction .....,... -y- Fold-thrust belt ~ ~~I~~~~~m N~ 1000 km Figure 9 Palaeogeographic reconstruction of southwestern Gondwana at about the end of the Early Permian (~ 260 Ma). Distribution of South American basins after Lopez-Gamundi et ai. (1990). Ch = Chaco Basin, Kr = Karoo Basin, Pr = Parana Basin, pz = Paganzo Basin, Sg = Sauce Grande/Colorado Basin, Sr = San Rafael Basin, * = early Late Permian tuff in Sauce Grande Basin (Lopez-Gamundi, pers. comm.). Palaeo-southern limit of Whitehill deposition shown by dotted line. 189 S.AfrJ.Geol.,1992,95(5/6) (Figure 8). Towards the end of the Early Permian the basin configuration consisted of an incipient fold-thrust belt in the palaeo-west, a foredeep (depocentre of the deep-water facies of the Prince Albert and Whitehill Formations), a gently sloping foreland ramp (shallow-water facies of the Whitehill Formation), and a forebulge forming part of a dissected continental highland in the palaeo-east (Figure 9). During the Early Permian the transition from a back-arc to a foreland basin involved the formation of an incipient retro-arc fold-thrust belt, the first evidence of which along the southern margin of the Karoo Basin was a compressional event dated at about 278 Ma in pre-Karoo strata (Hiilbich et ai., 1983; Figure 8). According to Hiilbich et ai. (1983) the main compressional event occurred about 20 Ma later at 258 Ma, but even at that stage sedimentation of the glacial diamictites and post-glacial mudrocks took place uninterruptedly in the region without any evidence of major sediment input from the growing thrust belt. This is supported also by the low sedimentation rates (on average <20 m/Ma - Figure 8) in the basin. Although glacial debris was derived from the palaeo-west, the clast composition suggests that the material was distantly derived. It is concluded that the palaeo-western margin of the Early Permian basin consisted of a broad tectonic belt slowly encroaching into the basin by means of a series of fold-thrust belts (Figures 9 and 10; cf Dalziel, 1986). It may have been comparable to the present North American Cordillera bordering the Pacific margin which has a width of about 1200 km. At the time of the first two compressional events the sea extended at least 200 km beyond the southern margin of the Karoo Basin which implies that the palaeo-eastern margin of the tectonic belt (i.e. the precursor of the Cape Fold Belt) remained submerged until the end of the Early Permian. However, its influence on basin floor morphology was manifested in the formation of the foredeep due to thrust-induced subsidence and oscilladon in the position of the forebulge during periodic crustal deformation ® .. and lithospheric relaxation (cf Flemings & Jordan, 1990). According to these authors rejuvenated thrusting created, by subsidence, more space that can be filled by sediment. Because the thrust belt did not yet feature as a sediment source, rapid subsidence trapped the available sediment during an underfilled stage causing sediment starvation in the basin (c/. Allen et ai., 1986). However, during deposition of the basin floor mud fans of the upper Prince Albert Formation, the growing fold-thrust belt temporarily influenced depositional slope, thereby initiating sediment transport from the palaeo-west. Emergence of the rising fold-thrust belt and its erosion are indicated by the sharp rise in sedimentation rates after deposition of the Whitehill Formation (Figure 8). Sediment was derived from the palaeo-west and deposition occurred as deltas and basin floor turbidite fans. During development of the foreland basin in the interior of southwestern Gondwana, faulted basins formed and acidic volcanism occurred in an extensional regime along the palaeo-western margin of the tectonic belt (cf Breitkreuz, 1991) (Figure 10). The plinian type volcanism was the source of the air-borne ash deposited during the latest Early Permian to Early Triassic in the Karoo Basin. A relative sea-level rise inundating parts of the palaeoeastern highland (Pietermaritzburg transgression) followed on the collapse of the Gondwana Ice Sheet (Figure 7). Dark-coloured marine muds were deposited over the entire basin as part of a transgressive system tract (c/. Haq, 1991). This transgression can be attributed to lithospheric relaxation which followed on the 278 Ma compressional event, resulting in craton-ward migration of the forebulge (c/. Flemings & Jordan, 1990). Additional support for the marine transgression is the deposition of phosphorites in the Prince Albert Formation (cf Follmi et ai., 1992; Brasier, 1992). The distinctive Whitehill Formation was deposited during a relative highstand on which two third-order sea-level falls Proto - Precordillera Late Carboniferous to Permian volcanosedimentary strata .. Permian continental basins ~~ ® Whitehill depository Permian acidic volcanism (Plinian) lJ/, __ Proto Pacific Karoo Basin ~ -- -- -- __ ~17> Transport of windblown - . ash . Foredeep Early Palaeozoic to early Carboniferous strata f + + + + Basement --Exten,'onal ,eglme _ _ / Fold-thrust belt Cape Supergroup . . . . - - Compressional regime 300 km Figure 10 Schematic NW - SE section across southwestern Gondwana at about the end of the Early Permian (± 260 Ma). Northwestern part of the section after Breitkreuz (1991). See Figure 9 for the position of the section. 190 were superimposed (Figure 7). These minor regressions are attributed to basinward migration of the forebulge during the 258 Ma thrusting episode (cf. Flemings & Jordan, 1990). Additional evidence for the overall highstand in the Karoo Basin at the end of the Early Permian is the deposition of the coal-bearing strata in valleys and embayments along the basin margin forming part of a highstand system tract (cf. Haq, 1991). Lithospheric relaxation after the 258 Ma period of crustal deformation resulted in basinward migration of facies derived from the fold-thrust belt. A prograding sequence of distal mud turbidites followed by proximal sand turbidites (Collingham, Vischkuil, and Laingsburg Formations in Figure 4) was deposited during a regression in the southern part of the Karoo Basin. Discussion The study of the characteristics and lithological interpretation of the Whitehill Formation and associated strata raise the following points which require in depth discussion to arrive at a sensible depositional model for the Whitehill Formation. 1. At the time of deposition of the Whitehill Formation the foreland basin was in a juvenile stage. Rapid subsidence during the 258 Ma compressional event resulted in a well-defined foredeep, but part of the fold-thrust belt was still submerged, thereby delaying the input of medium- to coarse-grained clastics (c/. Allen 1986). Mud was therefore the main clastic component fed into the basin. Alternating crustal deformation and lithospheric relax~tion in the foreland basin controlled retrogradation of facies along the palaeo-eastern basin margin creating suitable conditions for the formation of peatlands (Figure 10). Periodic flooding of these peatlands enhanced the supply of suspended terrigenous organic material in the basin. 2. The palaeogeography of the region had an overriding control on sedimentation in that maximum sediment accumulation was adjacent to the rugged topography of the eastern basin margin where major rivers entered the basin via deep valley systems (Figures 5 and 9). In the Orange Free State coalfield which was approximately opposite the palaeo-southern limit of Whitehill sedimentation, deposition of the coal-bearing strata occurred in valleys and small inlets. The coal-bearing strata there form aggradational units with down-valley shale-out occurring over relatively short distances (Stavrakis, 1986). Thus no delta progradation into the basin took place with the result that predominantly clay and organic matter were washed basinwards during shoreline sedimentation. Basin morphology was also rapidly changing during this period and was such that basin floor sinks for the accumulation of organic-rich sediment formed during the Whitehill highstand. 3. The Prince Albert Formation had a bidirectional sediment input from the palaeo-east and the palaeo-west. The isopach map of the Whitehill Formation suggests sediment input from the palaeo-east, thereby supporting the directional data obtained from the fossils (Figure 5). The sediment input volume from the tectonic belt in the S.-Afr.Tydskr.Geol.,I992,95(5/6) palaeo-west, except for volcaniclastics, was probably small. It can thus be concluded that rivers draining the supercontinent in the palaeo-east were the main mud source during Whitehill sedimentation (c/. Figure 9). 4. The concentration of suspended organic matter was exceptionally high in the Whitehill sea and during fallout up to 4 per cent of organic material was preserved in muds of the deep-water facies. Most of the organic matter was transported into the depository by river plumes or washed in during flooding of peatlands during transgressions (c/. Demaison & Moore, 1980; RossignolStrick, 1982; Garzanti, 1991). The subsequent regressions also eroded the coal-forming strata, the extent of which is manifested in the form of up-valley in-seam channelling (Stavrakis, 1986) and the amount of fossil wood reported from the Whitehill Formation. Schlanger & Jenkyns (1976) also reported an abundance of terrestrial plant remains derived from the inundation of vegetated lowlands in deep sea floor muds of the northern Pacific Ocean. The high concentration of terrigenous sediment in the water column may also have contributed to the absence of phosphorites in the Whitehill muds (cf. Cook, 1984). 5. A consistent volume of rhyodacitic volcaniclastic material was supplied to the basin from the palaeo-west either as air-borne ash or as devitrified ash by surface run-off. McLachlan & Jonker (1990) suggested a source for the volcaniclastic material within 100 km from the northwestern margin of the Karoo Basin, but no evidence for Late Permian volcanism has yet been found near the basin. The major centre of volcanic eruption was located about 1000 kIn from the depocentre (Figures 9 and 10), which makes the formation of centimetre-thick tuffaceous layers in the Karoo Basin at the time of a volcanic eruption unlikely (cf. Lowe, 1988). Limarino & Spaletti (1986) and Lopez-Gamundi et al. (1990 and 1992) described Early to Late Permian aeolian deposits containing interbedded acidic volcaniclastics in the central Argentine basins. Recently early Late Permian tuffaceous beds were also reported from the nearby Sauce Grande Basin (Lopez-Gamundi, pers. comm.; Figure 9). The airfall ash was derived from the palaeo-west during cool arid conditions (Lopez-Gamundi et al., 1992). Volcanic ash is chemically stable under dry conditions so that it could have been transported intermittently by regional westerly winds over the supercontinent and blown out for hundreds of kilometres over the Whitehill sea. Windblown ash from the 1991 volcanic eruptions in southern Chile was widely distributed over Patagonia (300 to 400 km from the source) and was consistently blown by westerly winds into the Atlantic Ocean (personal observation). The palaeolatitudinal setting of the region (500 to 700 S after Smith et al., 1981) was most favourable for the presence of a strong westerly wind system. Therefore, the actual distance of the volcanoes from the Karoo Basin becomes less critical. 6. The sharp lower contact of the Whitehill Formation, and thus the change from basin floor mud fan deposition of the upper Prince Albert to suspension settling sedimentation, is attributed to the abrupt termination of mud S.Afr.J.Oeo!., 1992,95(5/6) turbidites entering the depository from the palaeo-west. This change is attributed to the sea-level highstand with landward displacement of shorelines (cf Figure 10). Also, if the upper part of a muddy slope lacked any conspicuous feeder channels through which sediment could be transferred to deeper water, a situation could have developed whereby turbidity flows no longer operated. The diversion or blocking of major drainage systems feeding sediment to the basin may have been the reason for such a change. The late Palaeozoic volcanism in the tectonic belt reached a peak during the latest Early Permian to Early Triassic (245 - 265 Ma) (Kay et ai., 1989; Lopez-Gamundi et ai., 1990; Gonzalez-Bonorino, 1991). Volcanic events occurred on a short time scale and could have severely affected drainage patterns in the region. 7. Bottom conditions were more anoxic during deposition of the Whitehill Formation than during either Prince Albert or Collingham times. Anoxia in a water body is caused by excessive oxygen demand resulting from the degradation of organic matter and a deficient oxygen supply by circulation within the water body (Demaison & Moore, 1980). Increased anoxic conditions resulted in a higher preservation of organic matter primarily due to a lack of benthonic scavenging and absence of bioturbation (Demaison & Moore, 1980). During deposition of the Prince Albert Formation, oceanic circulation probably compensated in part for the oxygen drain caused by the suspension fall-out of organic matter, so that only 2 per cent organic matter was entrained in the muds (cf Table 1). However, during Whitehill deposition vertical mixing and oxygen renewal in the water column were greatly exceeded by the oxygen demand of the high organic matter concentration in the water with the result that extreme anoxic conditions prevailed in the depository. There was a lower input flux of organic matter in the shallow-water environment during regressions and episodic oxygenated conditions thus occurred at the water-sediment interface. During the later Volksrust transgression (Figure 7), large tracts of the highlands were completely inundated thereby reducing the input of organic matter. Bottom circulation in the basin could also have been improved by the presence of turbidity currents with the result that oxygenated bottom conditions prevailed. 8. The presence of marine or brackish conditions in the basin had apparently no effect on sedimentation. The basin initially had marine connections to the palaeo-north and palaeo-west, although the tectonic belt is shown as a continuous landmass on the map (Figure 9). Episodic brackish conditions probably caused by the inflow of large fresh-water plumes into the restricted basin, may have been present during deposition of the upper part of the formation when aquatic fauna (a primitive swimming reptile, fish, and arthropods) were introduced to the basin (Oelofsen, 1987). The vertical salinity gradient stratified the upper part of the sea and may have contributed to overall anoxic bottom conditions (cf Rossignol-Strick, 1982). 9. The sea-level highstand was the major driving force in 191 producing a high organic matter concentration in the basin by flooding the peatlands and indirectly controlled anoxia in the water column (cf Demaison & Moore, 1980). Minor regressions superimposed on the overall sea-level highstand caused a lowering in water depth which led to the deposition of interbedded siltstones and carbonate rocks in the shallow-water facies. The shallow facies was also more susceptible to changes in the anoxic bottom conditions and the concentration of bottom brines in sea-floor lows. To understand the distinctive Whitehill depositional event, the above conclusions must be integrated to give a coherent working model applicable on the scale of the Whitehill and Irati seas. Sedimentation of coarse- and medium-grained clastics and organic material (coal) was confined to valleys and inlets along the steep palaeo-eastern basin margin during the late Early Permian. Deposition of black carbonaceous mud started when the peatlands were flooded during a highstand, thereby increasing the input of terrigenous organic matter in the basin as well as the anoxic conditions in the water body. Mud and terrigenous organic matter were transported into the basin as large river plumes, the dispersion of which was inhibited by restricted oceanic circulation caused by the uneven basin morphology. High preservation of organic matter in the bottom muds occurred in those areas of the basin within the influence of the river plumes. Minor regressions contributed to the flux of organic matter to the basin as well as the deposition of carbonate sediment and silts in the shallow-water facies under oxygenated bottom conditions. Settling of predominantly air-borne ash from volcanism in the palaeo-west episodically masked the sedimentation of mud and organic matter. The seas became progressively more brackish due to the inflow of fresh water and restricted oceanic conditions, and the environment became favourable for the introduction of aquatic fauna to the basin. Black mud deposition was terminated when lithospheric relaxation occurred after the 258 Ma thrusting event whereby basinward migration of facies derived from the fold-thrust belt took place. This caused a progressive return to oxygenated bottom conditions in the basin. The abrupt appearance and disappearance of carbonaceous shales in the stratigraphic record are a function of changing bounding conditions in the depositional environment. The changes in bounding conditions during deposition of the Whitehill muds in the distal areas (deep-water facies) were more limited than closer to the source areas where input flux and external factors in the shallow environment greatly influenced deposition. The formation of the carbonaceous shales within the Lower Ecca mudrock succession results from the concurrence of three related factors. Sedimentation was primarily controlled by basin tectonics which involved sediment supply and a possible silled basin, a sea-level highstand which involved the input of terrigenous organic matter, anoxia in the water column and preservation of organic matter, and climate which involved the proliferation of plants and input of air-borne volcaniclastics. Sea-level was the first-order control on the accumulation of the organic-rich facies (cf Pedersen & Calvert, 1990). The interaction of these controls must be seen in a retro-arc fore- 192 land basin setting when predominantly mud was supplied during the juvenile stage of foreland basin development. The depositional model outlined for the Whitehill Formation can also be applied to the Irati Formation between the Parana and Chaco Basins (Figure 9). The eastern, central, and northern parts of the Irati Formation contain more carbonate rocks deposited in an offshore shallow environment or on carbonate mud flats (Oelofsen, 1987; Horsthemke et al., 1990) than the southwestern facies. The organic carbon content of the shales also increases towards the more proximal and shallower facies and, in the case of the Parana Basin, forms a source of hydrocarbons. In areas away from the entry points of terrigenous organic matter or where prograding deltas enhanced offshore mud and silt deposition (e.g. eastern part of the Karoo Basin) a subtle lateral transition from black, organic-rich mud to dark-grey marine muds occurred. Therefore, the deposition of the distinctive Whitehill- Irati muds was a localized event in part of southwestern Gondwana during a specific time slot in the Permian. Acknowledgements Financial assistance for the study of the Whitehill Formation by the Foundation for Research Development (FRD) and the University of the Orange Free State is gratefully acknowledged. Mr J.C. Loack and Dr W.A. van der Westhuizen are thanked for the fossil samples from the Whitehill Formation and the chemical analyses respectively. Ken Eriksson and Ian McLachlan are thanked for constructive comments on an earlier draft of the paper. References Adachi, M., Yamamoto, K. & Sugisaki, R (1986). Hydrothennal chert and associated siliceous rocks from the northern Pacific: their geological significance as indication of oceanic ridge activity. Sedim. Geol., 47, 125-148. Allen, P.A., Homewood, P. & Williams, G.D. (1986). Foreland basins: an introduction. In: Allen, P.A. & Homewood, P. (Eds.), Foreland Basins. Spec. Publ. Internat. Assoc. Sedim., 8, 3-12. Anderson, A.M. (1975). Limulid trackways in the late Palaeozoic Ecca sediments and their palaeoenvironmental significance. S. 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