An analysis of the Permo-Carboniferous glaciation in the marine Kalahari Basin, Southern Africa

Palaeogeography, Palaeoclimatology, Palaeoecology, 44 (1983): 295--315

295

Elsevier Science Publishers B.V., Amsterdam --Printed in The Netherlands

AN ANALYSIS OF THE PERMO-CARBONIFEROUS GLACIATION IN THE MARINE KALAHARI BASIN, SOUTHERN AFRICA

J. N. J. VISSER

Department of Geology, University of the Orange Free State, Bioemfontein 9301 (Sou th Africa) (Received February 14, 1983; revised version accepted August 16, 1983)

ABSTRACT Visser, J. N. J., 1983. An analysis of the Permo-Carboniferous glaciation in the marine Kalahari Basin, southern Africa. Palaeogeogr., Palaeoclimatol., Palaeoecol., 44: 295-315. The Dwyka Formation, which forms the basal unit of the sedimentary fill in the Kalahari Basin, consists of local reddish basal tillite, an argillaceous sequence of shale, mudstone, siltstone, sandstone, diamictite and carbonate lenses and nodules containing marine fossils, and a well-developed main tillite. In the Hotazel Valley along the southern margin of the basin an upper tillite is also present. The sequence represents two major ice advances separated by a well-defined interglacial and a minor ice advance confined to the area around the ice-spreading centre. Glacial erosion during maximum glaciation in the Late Carboniferous formed a dissected landscape depressed isostatically to below sea level. Ice flow during deglaciation was controlled by the basement topography and upon disintegration of the ice sheet the sea followed the retreating ice inland (Hardap Interglacial). Deposition of lodgement till at the ice grounding line was followed by debris flows, turbidity currents, debris rain and suspension settling leaving sediments typical of an ice retreat sequence. The Tses Glaciation, probably of Early Permian age, followed on the interglacial. A marine ice sheet covered the area and predominantly lodgement tills and bedded and laminated diamictons were deposited during grounding line retreat, underside melting of ice shelves and by icebergs respectively. After rapid collapse of the marine ice sheet, small ice caps remained in the mountainous areas and during a temporary ice advance another till horizon was deposited along the southern margin of the basin. INTRODUCTION T h e P e r m o - C a r b o n i f e r o u s D w y k a F o r m a t i o n f o r m s t h e basal u n i t o f t h e K a r o o S e q u e n c e ( t h i c k n e s s m o r e t h a n 1 0 0 0 m ) in t h e K a l a h a r i Basin o f s o u t h e r n A f r i c a . T h e s o u t h e r n a n d w e s t e r n p a r t s o f t h e basin ( h e n c e f o r t h c a l l e d t h e S o u t h K a l a h a r i Basin) w h i c h m e a s u r e s a b o u t 1 0 0 0 k m a c r o s s , s t r a d d l e p a r t s o f N a m i b i a , B o t s w a n a a n d S o u t h A f r i c a a n d are t h e s u b j e c t o f t h i s i n v e s t i g a t i o n ( F i g . l ) . In this p a p e r t h e W a r m b a d Basin (also k n o w n as t h e K a r a s b u r g o r O r a n g e R i v e r Basin), w h i c h is l o c a t e d t o t h e s o u t h w e s t o f 0031-0182/83/$03.00

© 1983 Elsevier Science Publishers B.V.

296

the Kalahari Basin, is included in the analysis as it forms an integral part of the depository during the Early Permian. It is not intended in this paper to give a detailed description of the glacigene outcrops as these have already been treated in detail by Wagner (1915), Du Toit (1916), Haughton and F r o m m u r z e (1927), Martin (1953, 1981a, b), Frakes and Crowell (1970), Heath (1972) and Schreuder and Genis (1973-74), but to present new sedimentological data and to discuss certain concepts on glacial marine sedimentation in the basin. The relationship of the South Kalahari Basin with the main Karoo Basin (Fig.1 inset) during the glaciation was already dealt with by Du Toit (1921), Martin (1961), Stratten (1968), Martin and Wilczewski (1970) and Frakes and Crowell (1970). These authors postulated the continuity of the t w o basins during glaciation by the Namaland and Transvaal/Botswana ice lobes, despite the fact that the sequences in the basins are dissimilar. One can, however, understand the reasons for their assumption as a b o u t 70% o f the South Kalahari Basin is covered by Tertiary to Recent deposits (Fig.l), tectonic uplift, faulting and erosion had partly destroyed the basin in the west, the importance of facies principles in a glaciomarine sequence was not realized, and the research was done at a stage when the importance of glacial striae in ice sheet reconstruction was overemphasized. Meanwhile a large number of boreholes in search for underground water along the southern margin of the basin (Smit, 1971--72) and mineral deposits in the southern, central and west-central parts of the basin, had been sunk. These boreholes yielded valuable stratigraphic information. It was also found that the pre-Kalahari topography (Cretaceous--Tertiary) to some extent reflects the pre-Dwyka (Carboniferous) topography. Such data supplemented by lithofacies studies in the west and south and models of the Quaternary glaciations make a better reconstruction of the glacial history o f the South Kalahari Basin possible. However, the poor borehole control in the northern and northeastern parts of the study area places severe limitations on stratigraphic correlations and results in these areas must be considered as provisional. LITHOLOGY AND STRATIGRAPHY

Altogether ten lithofacies were recognised in the glacial basin fill, b u t scale limitations and the rapid variation in lithology of the glacigenic sediments necessitate the inclusion of widely different rock-types in some facies.

Lithofacies (1) Breccia facies. Deformed blocks and fragments of Nama sandstone (Eocambrian) with a very small matrix c o n t e n t often occur at the base of the D w y k a Formation in the Karas Mountains. Individual blocks measure up

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to a metre across. Diamictite-filled cracks and injections into the b e d r o c k are present in both the South Kalahari and Warmbad Basins. The breccia represents in situ fracturing of the bedrock by the overriding of probably coldbased ice and the deposits are interpreted as deformation till (Dreimanis, 1976, pp. 19, 37--38; Broster et al., 1979, p. 287). (2) Coarse banded diamictite facies. Reddish brown conglomeratic diamictite, containing irregular lenses and streaks of m u d s t o n e and sandstone, consists of up to 80% of r o u n d e d and subrounded clasts of local origin in an arenaceous matrix (Fig.2A). Minor conglomerate, sandstone and m u d s t o n e are present. The facies is very irregular in thickness and attains its m a x i m u m development in basement lows. It represents a basal till deposited predominantly by lodgement processes. The deformed m u d and sand streaks in the diamicton formed by shearing during deposition by the ice {Edwards, 1978, p. 426). (3) Massive diamictite facies. Highly compacted massive diamictite (Fig.3A) with occasional thin (less than 1 m) interbedded shale and shaly diamictite contains predominantly s u b r o u n d e d distantly derived clasts, some of which are up to 1 m in diameter. The thickness (up to 250 m), absence of bedding or other structures and h o m o g e n e i t y of the facies over a large area indicate subglacial deposition during grounding-line retreat of an ice sheet. Minor oscillations in the position of the grounding line led to the formation of the shaly intervals according to an Antarctic glacial model described by Drewry and Cooper (1981, pp. 119--121). (4) Faintly bedded diamictite facies. The diamictite consists predominantly of mudstone in which a faint bedding can be discerned (mudshale) and which contains subrounded to rounded, distantly derived clasts, hence the name boulder mudstone. Elongated clasts often show vertical long-axis orientation. Granite clasts of up to 30 m across were recorded in the area b e t w e e n Tses and Mariental {Martin, 1953, p. 37; Heath, 1972, p. 12), whereas a conglomerate clast which measures 3 m across, was f o u n d in the Warmbad Basin. Thin deformed sandstone and grit lenses, lag conglomerates and carbonate concretions containing marine fossils (Peruvispiras, crinoids, cephalopods and foraminifera) are present. The diamictite was deposited by debris rain during underside melting of an ice shelf, while reworking by bottom-hugging subglacial streams led to an improved sorting of some units. The d e f o r m e d sandstone and grit lenses suggest slumping and the generation of minor sediment gravity flows. (5) Laminated diamictite facies. Very fine-grained sandstone, siltstone or mudstone with thin debris layers which give the impression of pseudorhythmites (Fig.3B) forms units of up to 80 m thick. Minor carbonate beds are also present. The debris varies from small angular fragments to dropstones

299

Fig.2. A. Reddish conglomeratic basal tiUite from the Hotazel Valley. Note the rounding of the clasts and the development of faint flow banding around clasts towards the top of the photograph. B. A thin debris-flow deposit with mud fragments in upper half (arrows). Fine ice-rafted debris in mudstone below and above the deposit. up to a m e t r e across which d e f o r m e d the b e d d i n g (Fig.4). Well-preserved ripple m a r k s , ripple cross-lamination, graded b e d d i n g and slumping are present. T h e s e d i m e n t a r y structures indicate d e p o s i t i o n by suspension settling, s e d i m e n t gravity flow, debris rain and r e w o r k i n g b y b o t t o m currents. T h e

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Fig.3. A. Massive diamictite of the main tillite unit overlying undisturbed Hardap shale with a sharp contact. B. Laminated diamictite with thin ice-rafted debris layers giving the rock a varve-like appearance. coarse fraction was largely brought into the depository by iceberg rafting. Poorly sorted clots of fine sediment, measuring up to 15 mm across, were f o u n d in the thin debris layers and were interpreted as pieces of frozen debris dropped to the b o t t o m (Ovenshine, 1970, p. 893). Soft-sediment deformation of the beds can be a t t r i b u t e d to sedimentation over an uneven floor, resulting in unstable depositional slopes or large icebergs scraping b o t t o m and thus disturbing the bedding. The facies was deposited either near a shelf margin or in a proximal iceberg zone, depending on the volume and size of the ice-rafted debris present. (6) Bedded heterolithic diamictite facies. This facies definition was applied to a rapid alternation of diamictite, conglomerate, grit, sandstone, siltstone and mudstone. In the Hotazel Valley, interbedded massive diamictite units are up to 10 m thick, but bed thickness, on average, seldomly exceeds 1 m. The subrounded clasts in the diamictite are up to 60 cm in diameter, whereas those in the conglomerate are much smaller. Ice-rafted debris occurs in the

301

Fig.4. Quartzite dropstone in ripple-laminated siltstone. Note the deformation of the bedding. At Klipneus in Warmbad Basin.

interbedded mudstone and siltstone. Graded bedding, ripple cross-lamination, m u d clasts and soft-sediment deformation structures are present (Fig.2B). Shell fragments of Eurydesma were f o u n d in the diamictite beds {Martin and Wilczewski, 1970, p. 226). The thick massive diamictite units probably represent subaqueous morainal banks or lodgement tills formed at the ice-grounding line, whereas the bedded diamictites and conglomerates were deposited by proximal sediment gravity flows. The finer sediments formed by suspension settling of silt and mud, and low-density turbidity currents. (7) Conglomerate--grit facies. The conglomerate grading upwards into grit and sandstone, consists of well-rounded pebbles and cobbles, similar in composition to the clasts in the diamictite, in an arenaceous matrix. Poorly sorted conglomerate with boulders up to 0.75 m in diameter and coarse sandstone form sequences 15 m thick in the Warmbad Basin. Graded and cross-bedding are present. The conglomerate represents reworked glacial deposits and formed, together with m u d flows, as a fluvial valley fill or as a basal conglomerate for thick fluvial sandstone units north of Asab. (8) Fine- to medium-grained sandstone facies. Massive fine-grained sandstone, with thin interbedded siltstone, shale and mudstone, builds sequences of up to 60 m in the Warmbad Basin. Locally the sandstones are less massive,

302

associated with ripple-laminated siltstone, show small-scale cross-bedding and ball and pillow structures, and are arranged in upward-coarsening sequences 10 to 20 m thick. The medium-grained sandstones occur together with conglomerate and grit, consist of angular to subrounded grains of quartz, feldspar and rock fragments, contain fossil wood, and are cross-bedded. The finegrained sandstones formed as distributary m o u t h bars in wave-dominated deltaic sequences. The medium-grained sandstones form part of a fluvioglacial outwash fan. (9) Laminated to thinly bedded siltstone facies. Siltstone with minor finegrained sandstone and shale occurs in the southern part of the Warmbad Basin at the base of the D w y k a Formation and again at stratigraphically higher horizons where it builds sequences of up to 40 m thick. Asymmetrical ripples with sinuous and linguoid crests, interference ripples and flaser bedding are characteristic of the facies. The siltstones at the top of the formation contain small dropstones, b u t these deposits have been grouped with the laminated diamictite facies. The siltstone beds formed in a wave-dominated shallow-water environment with abundant reworking of the fine sediment. (10) Shale facies. Sequences of blackish to dark greenish grey shale and minor mudstone with silty laminae and thin fine-grained interbedded sandstones, often contain carbonate and siliceous concretions bearing radiolaria, goniatites and palaeoniscoid fish remains (Martin and Wilczewski, 1970, p. 226). Ripple lamination is developed in the sandstones and slumped sandstone balls are locally present in the mudstone horizons. Deposition of the shale and mudstone t o o k place by suspension settling of clay, m u d and silt in a marine environment where underflows (distal turbidites) brought also fine sand into the depository.

Changes in stratigraphy and lithofacies The overall stratigraphy and lithofacies changes in the basin can be illustrated by referring to three selected areas.

Mariental--Keetmanshoop--Vreda area. Along a north--south line where outcrops are available, the sequence consists of a basal tillite (lower tillite unit) which transgresses upwards into an argillaceous unit consisting of laminated and bedded heterolithic diamictite and fossiliferous shale (Hardap shale). At Schlip in the north the basal tillite is only developed in basement lows. The presence of the Hardap shale, which attains its maximum thickness o f 76 m near Asab (Heath, 1972, p. 10), depends on lateral facies changes as b o t h towards the north and south the shales are replaced by diamictite (Fig.5). The main tillite, which unconformably overlies the argillaceous rocks, overlaps the lower units towards the south where it either rests directly on basement or is separated from the basement by a thin deformation tfllite. The

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Fig.5. North---south section across the western part of the Kalahari Basin. (See Fig.1 for localities. ) tillite eventually grades upwards into Ecca shale, except in the north, where the Nossob sandstone unconformably overlies the main tillite. This sandstone consists of reworked glacial debris and grades into shale southwards. The Dwyka Formation is covered eastwards by thick Kalahari sand and the Vreda Borehole on the border between Namibia and Botswana ( F i g . l ) i s the only source of information (Martin and Wilczewski, 1970, p. 227; Frakes and Crowell, 1970, p. 2267; Heath, 1972, p. 32). The lower unit consists of 14 m of partly reworked basal tillite which contains numerous clasts similar in composition to the underlying basement. This is overlain by 182 m o f bedded heterolithic diamictite which includes minor black mudstone, sandstone (up to 5 m thick) and laminated diamictite, a zone of silty shale, about 45 m thick, with minor ice-rafted debris, and then the main tillite which is about 160 m thick. The latter contains a high clast percentage and this area was probably located closer to the source than the outcrop belt in the west (Martin and Wilczewski, 1970, p. 227). In the borehole the Nossob sandstone also overlies the main tillite. area. This area forms part of an intricate valley system along the southern margin of the basin, and a schematic section of the valley fill based on borehole data is presented in Fig.6. The basal tillite (lower tillite unit) which attains its maximum thickness of about 100 m in the lower and Khuis--Sishen

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middle reaches of the valley, consists of subrounded to rounded clasts of predominantly local origin in a reddish brown arenaceous to argillaceous matrix. The overlying thick argillaceous sequence consists of sediment gravity flow deposits, massive tillite, laminated diamictite and mudstone. Fish scales were recently f o u n d near Hotazel and fish remains and coprolites occur in calcareous

305

nodules near Areachap (McLachlan and Anderson, 1975, p. 417) in this argillaceous unit. Fluvioglacial deposits in the upper reaches of the valley are difficult to correlate with units down-valley. The main tillite present in the upper and middle reaches of the Hotazel Valley, is not so well developed at Khuis Outlet (Fig.6). It probably correlates with laminated diamictites containing thin carbonate beds and distal turbidites observed in the borehole. The basal contact of the main tillite is sharp without any disturbance of the underlying shale (Fig.3A). Up-valley darkcoloured shale and silty shale separate the main tillite from the upper tillite which probably grades into shale northwards. Warmbad Basin. The basin fill shows remarkable facies changes and whereas a threefold stratigraphic subdivision is possible in the east and centre of the basin, this no longer holds true towards the south and southwest (Fig.7). The floor of the basin is extremely uneven and often small valleys up to 30 m deep (Frakes and Crowell, 1970, p. 2269) are filled with predominantly fluvial deposits consisting of poorly sorted conglomerate, grit, sandstone and siltstone. Deformation tiUite is locally developed. North of Vioolsdrif a softsediment pavement is developed on top of a ripple-laminated siltstone, showing truncated ripple cross-lamination, scribing tools (Fig.8) and small sand flows where the oversaturated deformed sediment became unstable. In the northeastern and central parts of the basin the reddish-brown lower NW

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306

Fig.8. Soft-sediment pavement north of Vioolsdrif in Warmbad Basin. Scribing clast indicates ice flow from right to left. Note flow of sediment partly past the clast.

tillite unit consists of about 90% of proximally derived debris. Large (1.5 m across) lodged quartzite clasts, which are striated and orientated parallel to the general ice-flow direction were found. Southwards laminated diamictite replaces the lodgement tillite so that at Grasdrif in the south only siltstone with minor debris flow deposits occurs in basement lows. The argillaceous unit which could be the equivalent of the Hardap shale, consists of dark grey and greenish mudstone and shale with minor debris flow deposits and laminated diamictites in the lower part. Carbonate beds and concretions containing foraminifera and fish scales are also present (Schreuder and Genis, 1973--74, p. 12). Southwestwards the argillaceous beds are largely replaced by a thick predominantly arenaceous sequence which weathers reddish. A thin debris-flow deposit containing small r o u n d e d clasts (max. 5 cm) is present towards the base of a prominent m u d s t o n e horizon (Fig.7). Arthropod trackways were found in a siltstone horizon (Anderson, 1975, p. 267). The main tillite, which has a maximum thickness of 60 m (Schreuder and Genis, 1973--74, p. 10), overlaps the argillaceous unit towards the north and northeast where a coarse facies is present. Subordinate thin debris-flow deposits are o f t e n associated with the tillite unit which thins to less than 3 m in the south where it consists of ripple-laminated siltstone with small ice-rafted debris. Although an apparent threefold subdivision for the glacial sequence is

307 applicable to the largest part of the South Kalahari Basin, the stratigraphic record becomes more complex when approaching the high-lying areas; this is due to a transition from a predominantly glaciomarine to a terrestrial environment. Basinwards, the sequence often becomes more argillaceous and distinction between the units less obvious. PALAEO-ICE FLOW DIRECTIONS Past reconstructions of palaeo-ice flow relied heavily on directional measurements from striated pavements and roches moutonn~es, despite the fact that such data as palaeo-ice flow indicators are unreliable and must be interpreted with care (Frakes and Crowell, 1970, p. 2280; Embleton and King, 1975, p. 184). This aspect is well illustrated at Khuis where tillite-filled palae~valleys strike perpendicular to the long axes of the roche moutonn~es. Some of the contradictory evidence can be attributed to floating ice, as at the soft-sediment pavement to the north of Vioolsdrif the presence of ripplelaminated siltstone and small sand flows shows that the ice was partly afloat in that area. Furthermore, the age relationship of the striated surfaces is largely unknown as the striations could have been formed by ice either during the onset of the glaciation, m a x i m u m glaciation or deglaciation. Striae directions mostly record the latest ice movement in an area (Flint, 1957, p. 314) and therefore exposures, unless overlain by basal tillite, were tentatively correlated with the ice advance which deposited the main tillite (Tses Glaciation). However, the possibility exists that where intersecting sets o f striae occur, evidence of b o t h an earlier and a later glaciation may be present, although it was c o m m o n l y found that t w o or more ice flow directions on a striated surface are mostly the result of local conditions in the basal ice during a single advance (Embleton and King, 1975, pp. 186--187). In the area between Mariental and Asab, Heath (1972, pp. 18--19) carried o u t a number of fabric analyses on the lower tillite unit and found a fairly consistent north--south clast orientation. The clast composition had n o t been fully exploited in the past and certain clasts in the diamictite such as those of Matsap quartzite, garnet-bearing gneiss and granites with metamorphic over-prints can be traced successfully to the source areas. However, interpretations regarding the ice flow patterns had to be made with care as resedimentation of older tillites probably caused the redistribution of the clasts. Finally, lithofacies changes in the diamictite sequences, as well as the strike and bed slope of palaeo-valleys (Fig.9), were used as indicators of palaeo-ice flow. Palaeo-ice flow during deposition of the lower tillite was largely controlled by the palaeo-topography (Fig.10), whereas flow during deposition of the main tillite was to some extent unconstrained b y the topography as valleys were already partly filled with glacial and interglacial deposits and the ice was afloat over the largest part .of the area. The northward palaeo-ice flow radiating from an area around Upington is in contrast with previous recon-

308

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structions (Du Toit, 1921, pl. XI; Stratten, 1968, pl. 33; Visser et al., 1972, pp. 27--30), but there is no other way to explain the presence of clasts of garnet-bearing gneiss to the east of Aroab and granites with metamorphic imprints occurring together with Matsap quartzite in the Hotazel Valley. Furthermore facies changes in the valley fill (Fig.6) and the shape and bed slope of the valley (Fig.9) substantiate this flow direction. In the Warmbad Basin, ice flow for the lower tillite was mainly from the north and during deposition of the main tiUite largely from the east, whereas the deltaic beds separating the two tiUite units were deposited from the northwest (Fig.10). This shows that the basin was bounded at least on three sides by mountains, which gave it a character somewhat different from the rest of the South Kalahari Basin. A N A L Y S I S OF THE G L A C I A T I O N

The analysis presented is based on the ice flow patterns, the basement topography and the glacial lithofacies; but in the west, where post-Cretaceous erosion had largely removed the basin fill, and in the north and northeast of the study area, data are limited so that reconstructions are unreliable. At least two, and in the Upper reaches of the Hotazel Valley three, major ice advances occurred during the glacial history of the basin (Frakes and Crowell,

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1970, p. 2267). All deposits left by these advances most probably occurred during the final deglaciation stage of the Permo-Carboniferous glaciation.

Southern African Ice Sheet during maximum glaciation The area underlain by the South Kalahari Basin formed part of a large dissected upland with prominent mountain ranges (Cargonian and Windhoek Highlands and Karas Mountains -- F i g . l l ) . The region was drained by a major river system debouching into an e m b a y m e n t in the west. Conditions during onset of the glaciation are almost u n k n o w n except t h a t block faulting could have had an effect on the configuration of the basin (Martin and Wilczewski, 1970, p. 227}. At maximum glaciation a huge unbroken ice mass covered this part of Gondwana, including the main Karoo Basin, and a system of ice streams probably withdrew ice from the interior via the palaeovalley system. Towards the southwest a smaller ice stream probably flowed down the highlands via a basement low, today occupied by the Warmbad Basin. Over the upland the ice was probably cold-based and largely frozen to

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the bedrock except for warm-based conditions where ice was down-drawn into ice streams. These areas were also the zones of maximum erosion. Most of the observed topographical features in the region probably originated by steady-state ice flow during this stage whereby the central area was lowered by erosion as well as depressed by the ice to below sea-level. Very little or no deposition at all of till occurred during this stage.

Deglaciation At the close of the Carboniferous(?), possibly warmer conditions and a rise in sea-level triggered the disintegration of the Southern African Ice Sheet. Erosion by ice streams left a dissected upland characterized by an intricate system of valleys and broad lows ( F i g . l l ) and as the crust was depressed to below sea-level in this area, a marine transgression followed the retreating ice into the interior. As the rate o f disintegration increased calving bays encroached by way of the valley system into the centre of the ice sheet.

311

Isostatic rebound was probably restricted to certain areas in the west, especially around the Warmbad Basin, where uplift caused considerable bedrock erosion. Over the rest of the region the sign of isostatic adjustment was mostly negative, even during the deglaciation stage, and parts of the area remained below sea-leveh Stuiver et ah (1981, p. 430) found that very slow uplift rates during deglaciation are common because isostatic rebound mostly took place during the period of down-draw of the ice mass by ice streams preceding grounding-line retreat and ice-shelf formation, which to some extent would explain the visible absence of uplift and postglacial erosion over large parts of the South Kalahari Basin. Furthermore, overall basin subsidence over Gondwana cancelled the low uplift rates in certain regions and during ice-sheet collapse the grounding line retreated across a largely depressed bed which led to marine conditions during the succeeding interglacial. A basal tillite characterized by locally derived debris was laid down in the valleys and broad lows during ice retreat by lodgement processes at the icegrounding line. Seaward of the grounding line underside melting of the ice tongues and shelves led to the deposition by debris rain of bedded diamictons whereas in the calving bays laminated diamictons formed. This dynamic system resulted in a conspicuous marine ice retreat sequence consisting of massive tills grading both upwards and laterally into waterlain diamictons. A characteristic feature of this sequence is the local occurrence (e.g. near Mariental, in Vreda Borehole and Hotazel Valley)of a subaqueous sediment gravity flow facies. The deposits, which vary from diamictite to graded sandstone and siltstone (Heath, 1972, pp. 6--11), formed by cohesive debris and high-density turbidity flows in the proximal and low-density turbidity flows in the distal environment (Visser, 1983, p. 522}. The gravity flows which were triggered by over-steepening of the depositional slope, impact of ice calving on subaqueous morainal banks (Powell, 1981, p. 132), or ice push of deposits formed at the grounding line, transported coarse debris probably up to 150 km across the basin floor. The formation of the thick subaqueous debris flow fans, together with deposits formed by debris rain and suspension settling, was favoured by (1) a fairly slow but pulsating rate of ice retreat which allowed sufficient time for the build-up of an unstable sediment pile. This situation developed where ice shelves or ice tongues were bounded by valley sides or pinned by ice rises; (2) A high debris content in the basal ice which is normally associated with wet-based conditions; (3) A high melting rate, which required a high ice flow rate (surging ice?), would have resulted in rapid deposition; (4) Shallow to moderate water depths which favoured uncompacted sediment accumulation. The latter aspect is substantiated by the presence of shell fragments in debris flow deposits of Eurydesmas which lived under shallow conditions (Martin and Wilczewski, 1970, p. 226) on morainal banks. In areas where ice shelves or ice tongues disintegrated rapidly, the transition from the basal tillite to the marine shale is almost sharp and only minor laminated diamictons were deposited.

312 The glacial history of the Warmbad Basin is somewhat different from that of the rest of the Kalahari Basin, probably as the basin represents a broad overdeepened glacial trough c u t northwards into the Karas Mountains and thus lay on the opposite side of the ice divide. Disintegration of the ice tongues in the basin must have been very rapid and isostatic r e b o u n d was, for some unknown reason, considerable so that part of the glacial deposits were removed by erosion and the reworked debris deposited in basement lows as fluvial valley fills and as fine-grained blanket sands in a very shallow sea in the southwest. Minor re-advance of the ice, partly afloat in the south, formed the soft-sediment pavements on top of the fine-grained sediments and, where grounded over ice rises, even led to striated 'bedrock surfaces. Thin lodgement tills and only very minor laminated diamictons and debrisflow deposits were left in the north and northeast during final ice retreat.

Hardap Interglacial Although the largest part of the basin was depressed, the valley heads remained above sea-level which curbed the headward migration of the calving bays, and small terrestrial ice caps were left on the higher mountains (e.g. Karas Mountains and parts of the Cargonian Highlands). During this stage, outwash fans formed in the u p p e r reaches of the valleys and ice-rafted debris was deposited where ice tongues entered the fiords. Along the lower reaches of the valleys, trees became established as is indicated by the presence of fossil wood, whereas in the large marine e m b a y m e n t clays were deposited by suspension settling and fine sand and silt by distal turbidites. Free-swimming as well as benthonic organisms flourished in the marine environment, b u t in the upper reaches of the valleys brackish to fresh water conditions, due to the large inflow of meltwater, prevailed and non-marine organisms m a y have occurred. In the Warmbad Basin which was b o u n d e d by mountains, sediment-laden meltwater streams built short-headed wave-dominated deltas from the west and northwest. Minor debris flows occurred on the delta fronts. Over the rest of the basin predominantly marine clays were deposited.

Tses Glaciation During the Early Permian the ice caps again expanded and a marine ice sheet which, at its maximum, behaved as a unified system c o m p o s e d of ice domes, outlet glaciers, ice streams and ice shelves, formed (Fig.12). The ice shelves were confined to e m b a y m e n t s and small basins where they were pinned by islands and ice rises, whereas the ice streams flowed d o w n existing valleys and basement 10ws, causing further erosion of the bedrock as well as of the glacial and interglacial sediments. Areas between ice streams were probably covered by cold-based ice with local thawed patches as is indicated by the occurrence of deformation tillite (e.g. Karas Mountains). High

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mountain peaks probably projected as nunataks through the ice. During steady-state conditions, deposition of bedded and massive tills was primarily confined to the ice shelves (e.g. area around Asab) and the ice-grounding line (e.g. Vreda Borehole and Hotazel Valley), respectively. Oscillation in the position of the grounding line led to the formation of thick massive till sequences. In some areas closer to the ice-spreading centre thick massive tillites are, however, absent (e.g. Khuis Outlet). This could possibly be attributed to overdeepening of valleys so that the ice was never grounded in those particular regions. In the Warmbad Basin the southwestern part remained, even during maximum glaciation, ice free except for occasional icebergs that brought fine debris into the depository. Amelioration o f the climate caused a rapid disintegration of the marine ice sheet. The elevation of the ice domes was greatly lowered by surging ice streams, and the bounded ice shelves were freed and grounding-line retreat was accelerated by a large rise in sea-level. Calving bay migration rapidly disintegrated the u n b o u n d e d ice shelves and as the bed slope of the basin was towards the ice-spreading centre, once grounding-line retreat started, it was

314

an irreversible process (Thomas, 1979, p. 169) and the ice sheet collapsed completely. Only very m i n o r debris-flow deposits and laminated diamictons formed during the disintegration phase, except in some areas (e.g. around Tses) where steady-state conditions were prolonged as indicated by the thicker deposits of laminated diamictite.

Late-Stage mountain ice caps Climatic warming caused snow-lines to rise and only small ice caps remained along the Cargonian Highlands. The rapid collapse of the ice sheet, especially in the west, caused isostatic rebound in the Windhoek Highlands and Karas Mountains which resulted in erosion and reworking of the glacial sediments. Fluvial sand aprons (Nossob sandstone) were deposited to the south and east of these elevated areas, whereas over the rest of the basin the rate of subsidence exceeded t h a t of the isostatic rebound and m u d and prodelta silt were deposited on top of the glacials, even in the upper reaches of the valleys. Minor tills, which could be the equivalent of the upper glacials described by Visser (1982, pp. 88--89) and Visser and Kingsley (1982, pp. 71--79) in the main Karoo Basin, were deposited along the Cargonian Highlands during a t e m p o r a r y expansion of the ice caps. The entire area was subsequently inundated and fine-grained sediments of the Ecca Group were deposited overlying both glacial beds as well as exposed basement, except for some major peaks which occurred as islands in the Ecca sea. ACKNOWLEDGEMENTS

The author wishes to t h a n k the CSIR and the University of the Orange Free State for financial assistance, the management of ISCOR and GENCOR for making borehole core available at Sishen and Hotazel respectively, and Dr. H. J. Blignault of Goldfields who showed me the location of the softsediment pavement north of Vioolsdrif. REFERENCES

Anderson, A.M., 1975. Turbidites and arthropod trackways in the Dwyka glacial deposits (early Permian) of southern Africa. Trans. Geol. Soc. S. Afr., 78: 265--273. Broster, B. E., Dreimanis, A. and White, J. C., 1979. A sequence of glacial deformation, erosion, and deposition at the ice-rock interface during the last glaciation: Cranbrook, British Columbia, Canada. J. Glaciol., 23: 283--295. Dreimanis, A., 1976. Tills: their origin and properties. In: R. F. Legget (Editor), Glacial

Till. Spec. Publ. R. Soc. Can., 12: 11--49. Drewry, D. J. and Cooper, A. P. R., 1981. Processes and models of Antarctic glaciomarine sedimentation. Ann. Glaciol., 2: 117--122. Du Toit, A.L., 1916. Notes on the Karroo System in the southern Kalahari. Trans. Geol. Soc. S. Aft., 19: 1--13. Du Toit, A. L., 1921. The Carboniferous glaciation of South Africa. Trans. Geol. Soc. S. Afr., 24: 188--227.

315 Edwards, M. B., 1978. Glacial environments. In: H. G. Reading (Editor), Sedimentary Environments and Facies. Blackwell, Oxford, pp. 416--438. Embleton, C. and King, C.A.M., 1975. Glacial Morphology. Edward Arnold, London. Flint, R. F., 1957. Glacial and Pleistocene Geology. Wiley, N e w York, NY. Frakes, L. A. and Crowell, J. C., 1970. Late Paleozoic glaciation: II, Africa exclusive of the Karroo Basin. Geol. Soc. Am. Bull., 81: 2261--2286. Haughton, S. H. and Frommurze, H. F., 1927. The Karroo beds of the Warmbad District, South-West Africa. Trans. Geol. Soc. S. Afr., 30: 133--142. Heath, D. C., 1972. Die geologie van die Sisteem Karoo in die gebied Mariental-Asab, Suidwes-Afrika. Mere. Geol. Surv. S. Afr., 61. Martin, H., 1953. Notes on the D w y k a succession and on some pre-Dwyka valleys in South West Africa. Trans. Geol. Soc. S. Aft., 56: 37--43. Martin, H., 1961. The hypothesis of continental drift in the light of recent advances of geological knowledge in Brazil and in South West Africa. Alex. Du Toit Memor. Lect. 7, Annex. Trans. Geol. Soc. S. Aft., 64. Martin, H., 1981a. The Late Palaeozoic D w y k a Group of the South Kalahari Basin in Namibia and Botswana and the subglacial valleys of the Kaokoveld in Namibia. In: M. J. Hambrey and W. B. Harland (Editors), Earth's pre-Pleistocene Glacial Record. Cambridge University Press, Cambridge, pp. 61--66. Martin, H., 1981b. The Late Paleozoic D w y k a Group of the Karasburg Basin, Namibia. In: M. J. Hambrey and W. B. Harland (Editors), Earth's pre-Pleistocene Glacial Record. Cambridge University Press, Cambridge, pp. 67--70. Martin, H. and Wilczewski, N., 1970. Palaeoecology, conditions of deposition and the palaeogeography of the marine D w y k a beds of South West Africa. Proc. Pap. Second Gondwana Syrup., South Africa; 225--232. McLachlan, I.R. and Anderson, A.M., 1975. The age and stratigraphic relationship of the glacial sediments in southern Africa. In: K. S. W. Campbell (Editor), Gondwana Geology. Australian National University Press, Canberra, A.C.T., pp. 415--422. Ovenshine, A. T., 1970. Observations of iceberg rafting in Glacier Bay, Alaska, and the identification of ancient ice-rafted debris. Geol. Soc. Am. Bull., 81: 691--894. Powell, R. D., 1981. A model for sedimentation by tidewater glaciers. Ann. Glaciol., 2: 129--134. Schreuder, C. P. and Genis, G., 1973--74. Die geologie van die Karasburgse Karookom. Ann. Geol. Surv. S. Aft., 10: 7--22. Smit, P. J., 1971--72. The Karoo System in the Kalahari of the Northern Cape Province. Ann. Geol. Surv. S. Aft., 9: 79--81. Stratten, T., 1968. The Dwyka Glaciation and its Relationship to the pre-Karroo Surface. Thesis, Univ. Witwatersrand, Johannesburg (unpublished). Stuiver, M., Denton, G. H., Hughes, T. J. and Fastook, J. L., 1981. History of the marine ice sheet in West Antarctica during the Late Glaciation: A working hypothesis. In: G. H. Denton and T. J. Hughes (Editors), The Last Great Ice Sheets. Wiley, New York, NY, pp. 319--436. Thomas, R. H., 1979. The dynamics o f marine ice sheets. J. Glaciol, 24: 167--177. Visser, J. N. J., 1982. Upper Carboniferous glacial sedimentation in the Karoo Basin near Prieska, South Africa. Palaeogeogr., Palaeoclimatol., Palaeocol., 38: 63--92. Visser, J. N. J., 1983. Submarine debris flow deposits from the Upper Carboniferous Dwyka Tillite F o r m a t i o n in the Kalahari Basin, South Africa. Sedimentology, 3 0 : 5 1 1 . Visser, J. N. J. and Kingsley, C. S., 1982. Upper Carboniferous glacial valley sedimentation in the Karoo Basin, Orange Free State. Trans. Geol. Soc. S. Afr., 85: 71--79. Visser, J. N. J., Botha, B. J. V. and Geringer, G. J., 1972. Gletservloere van Karboonouderdom in Griekwaland-Wes en Gordonia. Tydskr. Natuurwet., 12: 21--31. Wagner, P. A., 1915. The Dwyka Series in South-West Africa. Trans. Geol. Soc. S. Aft., 18: 102--117.