29 Palaeont. afr., 25, 29-38 (1984) PALAEOGEOGRAPHIC IMPLICATIONS OF BRAID BAR DEPOSITION IN THE TRIASSIC MOLTENO FORMATION OF THE EASTERN KAROO BASIN, SOUTH AFRICA by BRIAN R. TURNER Department of Geology, The University, Newcastle upon Tyne, NEl 7RU, U.K. ABSTRACT The Triassic Molteno Formation in the main Karoo Basin, South Africa, forms a northerly thinning intracratonic clastic wedge deposited by sandy braided rivers of South Saskatchewan type. Deposition of the sandy facies was dominated by channel floor mega­ ripples producing trough cross-bedded cosets; transverse bars, represented by solitary, large-scale planar sets are not significant. Departures from this regional pattern of sandstone deposition occur along the north­ ern distal margin of the Molteno basin around Bethlehem in the Orange Free State. Here thickness trends and clast size delineate a deep channel system interpreted as the main braided exit channel from the basin. Because of its depth and constriction by local height differentials the competency and capacity of the flow were able to reproduce features more typical of proximal rather than distal depositional settings. The sandy facies is dominated by fine gravel with lesser amounts of coarse sand. Gravel occurs as longitudinal bars some of which contain low angle foreset stratification whose orientation is consistent with lateral growth and marginal riffle migration. The scale of the bars and simple deposi­ tional form imply that they may have been larger than modern equivalents and the flows deeper. The coarse sand occurs mainly as falling water stage features associated with the gra­ vel bars. Shallow channel-fills, bar edge sand wedges, bar top sheet sands and thicker channel sands have been recognised and compared with similar features in modern and ancient braided stream sediments. When traced to the southeast the deep channel sedi­ ments contain few longitudinal gravel bars and more transverse bars; the vertical sequence from longitudinal to transverse bars at this locality points to the increasing distality of the depositional site through time. CONTENTS Page INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 SEDIMENTOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 7 REFERENCES ......................................................... . 37 INTRODUCTION The Molteno Formation in the main Karoo Basin, South Africa (Fig. 1) forms a northerly thin­ ning intracratonic fluvial clastic wedge covering an area of about 25 000 km2. The age of the forma­ tion is uncertain. It is regarded as Upper Triassic (Carnian) by Anderson and Anderson (1970) and Middle Triassic by Plumstead ( 1969) and Keyser (1973). Regional facies analysis shows the forma­ tion to consist of a number of stacked large-scale fining-upward sequences comprising conglomerate, pebbly sandstones, fine sandstone and siltstone, shale and coal. These sequences are thought to have been deposited by braided streams draining an allu­ vial plain which may have been built on to the dis­ tal slopes of alluvial fan complexes of glacial out­ wash type. Basinward the sediments intertongue MS accepted December 1983 with floodplain-lacustrine sediments of the Elliot Formation (Upper Triassic) and Beaufort Group (Permo-Triassic). Full details of the regional depo­ sitional model are given by Turner (1983). The pebbly sandstones, which dominate the sequence are multilateral and multistoried sheet sandstones from 30-120m thick. Internal sedimen­ tary structures consist predominantly of large-scale trough cross-bedding (average thickness for the en­ tire basin 56 em), with subordinate large-scale soli­ tary planar sets (up to 1,8 m thick) and some flat­ bedding. Palaeocurrent trends based on trough fore­ sets indicate a source area to the south and south­ east of the present eroded edge of the formation. Petrographic studies suggest that most of the sedi­ ment was derived from metaquartzites of the Cape Supergroup to the south and a granitic fault-block terrain probably located off the present southeast 30 l Drakensberg lava Group ~ Clarens Formation Elliot Formation • Molteno Formation D Beaufort Group c:J Ecca Group _ 0 D Dwyka Formation 400 km Fig. 1 Generalised stratigraphic and locality map of the study area The inset map shows the Karoo Basin with study area enclosed. coastline of South Africa (Turner, 1983). Proximal facies equivalents are not preserved because of re­ peated uplifts and peneplanation of the basin mar­ gin from Late Jurassic times onwards (Du Toit, 1954). The pattern of sedimentation shows a stri­ king resemblance to the South Saskatchewan brai­ ded river model of Cant and Walker (1978). Sedi­ ments were deposited by perennial, high energy, low sinuosity braided streams. The relative abun­ dance of trough cosets to solitary planar sets implies that sedimentation was dominated by undirection­ al migration of channel floor megaripples and that channel bars were of little significance. The finer grained sediments were deposited largely from wa­ ning bedload to suspension load sedimentation in response to channel shifting, abandonment and overbank flooding. Shales record deposition from vertical accretion following overbank flooding, with standing bodies of water on the alluvial plain providing the locus for plant growth and in situ accumulation of coal. Well rounded to subangular pebbles, cobbles and boulders composed predominantly of quartzite are present, especially in the middle of the forma­ tion where they form a thin, but laterally persiste­ nt unit at the base of the Indwe Sandstone Member (Turner, 1975). This is the only regionally extensive lithostratigraphic unit in the formation. The pattern of sandstone deposition outlined above remains the same throughout the Molteno basin except along the northern distal margin of the basin (Fig. 1) where the sandstone (Indwe Sand­ stone Member) is locally much thicker and longitu­ dinal bars become less common and sandy trans­ verse bars are important depositional features. The­ se bedforms are generally lacking elsewhere in the basin and represent an important local change in depositional conditions requiring modification of the existing regional depositional model. The pur­ pose of the present paper therefore is to describe and interpret the localised change in sedimentation along this northern margin of the basin within the framework of the established regional model. SEDIMENTOLOGY When traced basin wards the regionally exten­ sive Indwe Sandstone Member of the formation thins out from about 60 m to a few metres except locally along the northern distal margin of the out­ crop in the vicinity of Bethlehem in the Orange Free State, where a pronounced thickening and in­ crease in clast size record the position of a deep channel system (Fig. 2). Regional clast size and th­ ickness trends argue against a local sediment source area ·such as the structural Harrismith dome (Fig. 2), a view substantiated by the fact that the dome was not exposed at this time and apart from some thinning and loss of sequence across the structure it had little influence on sedimentation (Turner, 1975). Limited palaeocurrent data around the dome provides further evidence against the idea of a local intrabasinal sediment source area. Rework­ ing of clasts from underlying coarse lower Beaufort Group sediments is also unlikely because of their more varied lithology, angular nature and much smaller size. In the absence of any recognisable lo­ cal intrabasinal sediment source an alternative ex­ planation must be sought consistent with regional sedimentary trends. One possible explanation is that the channel system and its sediment fill record the convergence of several small channels in to one Contour interval 30m ,1Sm and Jm • uocm Mean maximum clast length of ten largest clasts at each sample locality ORANGE FREE Jo· STATE 0 30 Port St. Johns 40 80 120 Km Fig. 2 Clast size and isopach map of total thickness of the Molteno Formation. Note the local increase in clast size and thickness along the northern margin of the outcrop, contrary to regional trends. The dome-shaped feature delineated by the isopach con tours in the northeast is known as the Harrismith dome (Turner,1975). 31 'E ftl ~ :I I 'i: 0 ir 0 40 80Km Fig. 3 Generalised section showing characteristic features of the Molteno Formation along the north-central margin of the basin. General transport direction from south to north. (A) The lower part of the formation (Indwe Sandstone Member) consists of pebble and granule conglomerate (gravel) overlain by trough cross-bedded very coarse, pebble and granule rich sandstone. The conglomerate is thicker and generally coarser in the north (distal) becoming thinner and slightly finer to the south (proximal) accompanied by a change in the dominant stratification type from longitudinal gravel bars to more sandy transverse bars and subordinate gravel bars confined to a thin basal zone. The remainder of the succession consists of (B) shale, (C) fine-med­ ium sandstone, (D) silty-shale and (E) coarse granule and pebble rich trough cross-bedded sandstone. The sec­ tion line extends from Danielsrust to Oliviershoek Pass and corresponds to sections 32, 41 and 42 in Fig. 10. main braided channel, due perhaps to distal chan­ nel entrenchment or constrictions and height diffe­ rentials composed by local features such as the Ha­ rrismith dome (Fig. 2) - a situation analogous to that of the Knik River, Alaska (Bradley et al., 1972). However, the decrease in overall grain s!ze and th­ ickness to the south, and the accompanymg changes in the dominant stratification pattern and fining­ upward trend (Fig. 3) typical of the ret?~der of the succession favours channel constnctlon and convergence rather than localised short-term chan- nel entrenchment. Palaeocurrent data for this nor­ them margin of the basin provides some additional support for this idea of convergence (Fig. 4) and the development of a main braided exit channel at this locality (Fig. 5). ~Vector mean Fig. 4 Palaeocurrent map of the northern distal margin of the Molteno Basin. Vector means based on 1018 measurements of the axes of trough cross­ bedded sets. At the same time channel constriction im­ proves its competence and transport capacity (Bo­ othroyd and Ashley, 1975) reproducing features that might be expected in more proximal situations. Within this environment gravel and sand were de­ posited contemporaneously. Matrix poor, small pe­ bble and granule gravel is very common and domi­ nates the lower part of the sequence (Fig. 3). It ac­ cumulated as sheet-like longitudinal gravel bars elongate parallel to the transport direction. Indi­ vidual bars are up to 1, 7 m thick, 1 0 m wide and several tens of metres in length. Internally the bars are massive or show low angle foresets which are commonly convex upwards and contain rare sand­ stone clasts (Fig. 6). Foreset dip directions are or­ ientated towards the west and northwest, and de- Distal braided alluvial plain Molteno Beaufort Fig. 5 Generalised depositional model for the Molteno Formation along the north-central distal margin of the basin. 32 viate markedly from the northerly orientated fore­ sets of adjacent trough cross-beds. This difference in foreset dip orientation emphasizes the impor­ tance of lateral bar growth within the channels and invites comparison with the diagonal bars of Smith (1978). Furthermore, foresets tend to predominate on one side of the bar and are absent or poorly de­ veloped on the other, implying growth mainly in one direction. Downstream bar growth also occu­ rred, but the low angle of the foresets and rapid upstream transition into massive or crudely strati­ fied gravel, suggests that they were formed by mar­ ginal riffle migration rather than slip face avalan­ ching (Smith, 1974). The abundance of low angle foreset cross-st­ ratification in sets up to 1, 7 m thick without any intervening erosional events or evidences of multi­ ple deposition suggests that some of the gravel bars are simple depositional bedforms and equivalent to the unit bars of Smith (1974). If this interpretation is correct and the bars are of the longitudinal type then they may have been larger and of greater re­ lief than in modem systems and the flows respon­ sible for their formation deeper (Rust . 1978), par­ ticularly where humid climates prevail and rainfall was high as in the Molteno (Turner, 1983). Such conditions would not only promote deeper flows but also more prolonged flows (low rate of dis­ charge variation), thereby encouraging the forma­ tion of longitudinal bars as primary flood generated bedforms. Some idea of the flow velocity and flow depth can be obtained from the clasts which occur towards the base of the gravel bar sequence, by using Malde's sixth-power law (Malde, 1968) and the Manning equation (Turner, 1975). The average clast size of 16,5 em requires a velocity of 2,0 mfs and a depth of 0.73 m whilst the largest boulder of 42 em requires a velocity of about 3,3 m/s and a depth of about 1,8 m. Theoretically therefore, flow depths were sufficient to be able to generate such bars, assuming little or no loss of bar relief through subsequent erosion. The sands were mostly coarse-grained, imma­ ture feldspathic lithic arenite lenses (Turner, 1975) that were deposited during waning floods, and in many cases they separate individual gravel layers. Comparison with similar features described from modem and ancient braided river sediments (Smith, 1971, 1974; Miall, 1977; McGowen and Groat, 1975; Boothroyd and Ashley, 1975) suggests that the arenites occur: ( 1) within the gravel sequence as channel-fills (Fig. 6); (2) as bar edge and slip face sand wedges (Fig. 7); (3) as bar top sheet san­ ds (Fig. 8); and (4) as thicker channel sands, late- Fig. 6 Thin channel-fill sandstone within gravel bar sequence. Note the presence of low angle stratification dipping into the channel on the right, and small pock marks indicating weathered shale clasts on the left. Low ang~e gravel bar fore sets dipping to the right can be seen beneath the channel-fill sandstone. Some foresets con tam small, discoidal sandstone intraclasts (outlined in black and arrowed in white) with their long axes preferen­ tially aligned down the foreset slope. General transport direction towards the observer. 33 Fig. 7 Bar edge sand wedge with low angle foresets. A thin bar top sand occurs below the wedge (arrowed) which is overlain by a thicker, erosively based sandstone representing part of a sand-filled channel cut into the gravel bar top. General transport direction towards observer. ral to and interbedded with the gravel. The shallow channel-fills are up to 2,5 m wide and 0,5 m deep, and locally contain shale intraclasts. Internally they are massive or contain traces of low angle stra­ tification dipping in to the channel indicative of side-filling (Fig. 6). Trough cross-bedding, as recor­ ded for many modem gravel bar surface channels (Boothroyd and Ashley, 1975) is not developed. The channel-fills record deposition within small channels dissecting the bar surface during falling water stage, under conditions which favoured the rapid dumping of poorly sorted sediment .. and in­ hibited the development and preservation of bed­ forms. Bar crest sands are generally less than 45 em thick, but they may thicken abruptly along the bar flank as shown in Fig. 8. Where individual bars are defined by the bar crest sands, they have a bed re­ lief of between 25 and 170 em. Intraformational sandstone clasts up to 5-6 em in length and 2-3 em thick occur on gravel bar foresets. The clasts have a typical flattened discoidal shape with their long axes aligned down the foreset slope (Fig. 6). The thicker channel sands are up to 2 m thi­ ck and contain trough cross-beds arranged in erosive­ ly-based cosets. Locally they show an abrupt up­ ward decrease in scale from 1 m thick sets to 20-30 em thick sets, and in some cases they may be asso­ ciated with solitary low angle foresets interpreted as transverse bars (Fig. 9). Individual channels with­ in these thicker sands are difficult to detect proba­ bly because of rapid channel shifting, and the un­ consolidated nature of the sediments. Adjacent to the main braided channel, coarse to medium grain­ ed trough cross-bedded sandstone was deposited (Fig. 10). Individual sets are up to 35 em thick, and the sandstones are noticeably lacking in pebble and granule size clasts in comparison with the main braided channel sediments. The environment was essentially one of relatively minor shallow braided streams, where depths of flow and scour were re­ duced, and sand rather than gravel was deposited. However, discrete interconnected channels are not preserved presumably on account of channel shift­ ing, the poorly consolidated nature of the sediments and reworking of any interchannel deposits. When traced some 80 km to the southeast, to Little Switzerland along the edge of the Drakens­ berg escarpment (Fig. 1), the deep channel sedi­ ments change character. Gravel is less common and mainly confined to the lower 1 ,5 m of the succes­ sion where it occurs as longitudinal gravel bars and small granule gravel-filled channels up to about m thick. Clasts of pebble size or larger are not very common and overall tend to be smaller than further north in the palaeoflow direction. Fine gravel-rich layers alternating with gravel-poor layers occur within some sandstones (Fig. 11). The individual layers range from about 3 to 15 em thick, and con­ tain mainly subangular granules with minor amou­ nts of subangular to subrounded small pebbles. The layers do not appear to be erosively-bounded, or form part of discrete small-scale fining-upward seq­ uences In view of this the grain size variation may represent a response to discharge fluctuations and incremental bar growth, with each coarse and fine 34 Fig. 8 Gravel bar crest sand showing abrupt thickening along bar flank. The gravel bar contains poorly defined low angle foresets dipping from the bar crest towards the flank. Other much thinner bar top sands occur at the level of the hammer head. General transport direction towards observer. I . ------------------5M------------------ fig. 9 Stratification sequence in braided channel-fill sandstone. Trough cross-bedding (sinuous cres­ ted dunes) dominates and shows an abrupt up­ ward decrease in scale. A foreset cross-bedded unit of transverse bar origin occurs at the bot­ tom left. Drawn from a photograph. layer representing a depositional couplet recording high and low discharge. Such alternations are par­ ticularly well developed where deposition is domi­ nated by lateral accretion of low slope surfaces such as riffles (Smith, 1974 ). Gravel bars and channel-fills are overlain by solitary sets of large-scale planar cross-bedding in­ terpreted to be coarse sandy transverse bars. These now become important depositional features in this area with individual bars up to 80 em thick and 6 m in length, the foresets are generally straig­ ht, tabular and strongly discordant with the lower bounding surface. Internally the bars show the fol­ lowing features; ( 1) deformed fore sets (Fig. 12); (2) curved convex upper surface tapering off up­ current (Fig. 13); and (3) finer grained small trough cross-bedded bar top sands (Fig. 13), a feature that appears to be lacking from transverse bars elsewhere in the basin possibly due to the rapid rate of dis­ charge fall and disequilibrium between the flow and the bedform. Intervening between the gravel bar and overlying transverse bar shown in Fig. 12 is a thin wedge up to 20 em thick of better sorted, quartz-rich arenite, interpreted as a bar top sand. This could represent reworking of the bar top du­ ring falling water stage, or possibly subaerial expo­ sure of the bar surface which was then subjected to wind action, winnowing out the fines and leav­ ing behind a coarse structureless aeolian sand. Tr­ ansverse bar foreset directions often diverge strong­ ly from the northerly regional palaeocurrent trend based on trough axes. Another feature is the pre­ sence within the gravel sequence of an erosively­ bounded, black, carbonaceous, finely laminated shale lens up to about 1,2 m thick, floored by a 15- 20 em thick matrix supported conglomerate con­ taining large quartzite pebbles and cobbles with an average clast size of 6 em. The erosive floor and conglomerate, and the abrupt lithological change to shale, can best be explained in terms of an aban­ doned braided channel-fill. The presence within the shales of well preserved delicate fossil plant material assigned to the Triassic Dicroidium flora supports this view and emphasises the very quiet local condi­ tions· under which sedimentation occurred, remote from any influence of the active braided channels. The observed vertical relationships between bar types suggests increasing distality of the braided system through time (Smith 1971), an interpreta­ tion consistent with its stratigraphic position be­ neath fluvio-lacustrine sediments (Turner 1983). A generalised interpretative depositional model for the deep channel sediments in the lower part of the Molteno Formation is shown in Fig. 14. ooooo Medium to small - scale trough cross- bedding Large-scale trough cross-bedding Deformed foresets Planar cross-bedding Sandstone lens ooo<>OO Small pebbles () Wood lf. Plants ' Shale clasts @ Max. boulder or cobble size in em 35 --- Fig. 10 Correlation of sections along the northern margin of the Molteno Basin showing lateral variations in thickness and stratification. Fig. 11 Internal structure of longitudinal bar showing systematic variation in grain size, and deposi­ tional couplets of gravel-rich and gravel-poor very coarse sandstone. CONCLUSIONS The Triassic Molteno Formation in the main Karoo Basin, South Africa, forms a northerly thin­ ning intracratonic clastic wedge deposited by coar­ se sandy braided streams. Regional facies analysis and the pattern of sandstone deposition suggest that the streams were probably of the South Saska­ tchewan type and that deposition of the sandy fac­ ies was dominated by curved discontinuously-cres­ ted channel floor megaripples producing trough cross-bedded cosets. Channel-bars of transverse type, represented by large-scale solitary planar sets are not significant within the sandy facies, which fines up in to siltstone, shale and coal deposited mainly from channel sifting, overbank floods and within peat swamps on the alluvial plain during pe­ riods of tectonic quiescence (Turner, 1983). Departures from this regional model occur along the northern distal margin of the Molteno outcrop where the sandy facies shows a pronounced thickening and increase in clast size inconsistent with regional trends and its distal position. In the absence of any local intrabasinal sediment source these marked local differences are interpreted in terms of a deep channel system, thought to repre­ sent the main braided exit channel from the basin. Because of its greater depth and channel construc­ tion, possibly due l:o height differentials imposed by features such as the nearby Harrismith dome, competency and capacity of the flow was consi­ derably increased thereby reproducing more proxi- 36 Fig. 12 Fig. 13 Longitudinal gravel bar with low angle foresets overlain by locally deformed sandy transverse bar. Intervening between the two is a thin lens of winnowed (wind-blown?) bar top sand. High angle simple planar set of transverse bar origin overlain by a separate sedimentation unit of small trough cross-lamination. Note the curvature of the bar surface and its tapering-off towards the bar tail to the right. 37 SUP FACE SAND BAR SURFACE SAND ABANDON D CHANNEL LONGITUDINAL BAR Fig. 14 Block diagram showing generalised interpretive depositional model for the deep channel sediments in the lo­ wer part of the Molteno Formation. Coarse stipple indicates gravel and fine stipple sand. Spatial relationships between major bedforms indicates a dominance of longitudinal gravel bars in the north and transverse bars further south. Characteristic sedimentary structures and sequences produced by the model are illustrated in Figs. 6, 7 and 8. mal characteristics within a distal setting. The sandy facies is generally coarser than elsewhere in the basin and consists of contempo­ raneously deposited granule and small pebble size gravel and coarse sand. The gravel was deposited in the form of longitudinal bars characterised by low angle foreset stratification. F oreset dip directions imply lateral growth within the channel, produced largely as a result of marginal riffle migration rath­ er than slip face avalanching. Some of the bars show features consistent with the simple deposi­ tional bedforms of Smith (1978) but in terms of their size they must have been larger than equiva- lent modem bars and the flows deeper. Comparison with modem and ancient brai­ ded sediments suggests that the sandstones associa­ ted with the gravels were deposited in a variety of ways during falling flood stage as shallow channel­ fill deposits, as gravel bar edge sands, as gravel bar top sheet sands and as thicker internally complex channel sands, lateral to the gravel bars and inter­ bedded with them. When traced some 8 0 km to the southeast the deep channel sediments change cha­ racter. Gravel bars are less common and mainly con­ fined to the lowermost part of the succession. Tra­ nsverse bars become more important bedforms and show a variety of features including deformed fore­ sets, curved, convex-up bar surfaces, tapering off upcurrent towards the bar tail and preserved bar top bedforms. Such bedforms have not been recog­ nised elsewhere in the basin, and the vertical se­ quence of bar types at this locality points to the in­ creasing distality of the depositional site through time. ACKNOWLEDGEMENTS This paper is based on work carried out w}lilst the author was employed as a Research Officer at th.e Be.rnard Price Institute for Palaeontological Research, Umvers1ty of the Witwatersrand, Johannesburg. I am grateful to the Insti­ tute for funding the work, and for the help and encoura.ge­ ment I received from many members of staff. I should hke to thank Elizabeth Walton for typing the manuscript and Christine Jeans for help with some of the diagrams. REFERENCES ANDERSON, H.M. and ANDERSON, J.M. (1970). A preliminary review of the biostratigraphy of the uppermost Permian, Triassic and lowermost Jurassic of Gondwanaland. Palaeont. afr., 13, 1-22. BOOTIIROYD, J.C. and ASHLEY, G.M. (1975). Processes, bar morphology and sedimentary structures on braided outwash fans, northeastern Gulf of Alaska. In: Jopling, A.V. and McDonald, B.C., Eds., Glaciofluvial and Gla­ ciolacustrine sedimentation. Spec. Pubis, soc. Econ. Paleont. Miner., Tulsa., 23, 193-220. BRADLEY, W.C., FAHNESTOCK, R.K. and ROWEKAMP, E.T. (1972). Coarse sediment transport by flood flows on Knik River, Alaska. Bull. geol. Soc. Am., 83, 1261-1284. CANT, D. J. and WALKER, R.G. (1978). Fluvial processes and facies sequences in the sandy braided South Saskatche­ wan River, Canada. Sedimentology, 25, 625-648. 38 DU TOIT, A.L. (1954}. Geology of South Africa. Oliver and Boyd, Edinburgh, 264-332. KEYSER, A.W. (1973}. A new Triassic vertebrate fauna from south west Africa. Palaeontol. afr., 16, 1-15. MALDE, H.E. (1968}. The catastrophic late Pleistocene Bonneville flood in the Snake River Plain, Idaho. U.S. Ceo!. Surv. Prof. Paper 596, 52 pp. McGOWEN, J.H. and GROAT, C.G. (1975}. Van Horn Sandstone, west Texas: An alluvial fan model for mineral ex­ ploration. Bureau of Econ. Geol., Univ. Texas at Austin, Report of Investigations No. 72, 57 pp. PLUMSTEAD, E.P. (1969}. Three thousand million years of plant life in Africa. Ceo!. Soc. S. Afr., Annexure 72, 72 pp. RUST, B.R. (1972). Structure and process in a braided river. Sedimentology, 18, 221-245. ---------- (1978}. A classification of alluvial channel systems. In Miall, A.D., Ed.,Fluvial Sedimentology, Can. Soc. Petrol. Ceo!., Mem. 5, 187-198. SMITH, N.D. (1971}. Transverse bars and braiding in the lower Platte River, Nebraska. Ceo!. Soc. Amer. Bull., 82, 3407-3420. ------- (1974}. Sedimentology and bar formation in the upper Kicking Horse River, a braided outwash stream. ]. Geol., 81, 205-223. ------- (1978}. Some comments on terminology for bars in shallow rivers. In Miall, A.D. Ed., Fluvial Sedimen- tology, Can. Soc. Petrol. Geol., Mem 5, 85-88. TURNER, B.R. (197 5). The stratigraphy and sedimentary history of the Molteno Formation in the main Karoo Basin of South Africa and Lesotho. Ph.D. Thesis (unpubl.), Univ. Witwatersrand, Johannesburg, 314 pp. -------- (1983}. Braidplain deposition of the Upper Triassic Molteno Formation in the main Karoo (Gondwana} Basin, South Africa. Sedimentology, 30, 77-89.