P.G. Eriksson

A review of the stratigraphy and sedimentary environments of the Karoo-aged basins of Southern Africa

Abstract The Karoo Basin of South Africa was one of several contemporaneous intracratonic basins in southwestern Gondwana that became active in the Permo-Carboniferous (280 Ma) and continued to accumulate sediments until the earliest Jurassic, 100 million years later. At their maximum areal extent, during the early Permian, these basins covered some 4.5 million km2. The present outcrop area of Karoo rocks in southern Africa is about 300 000 km2 with a maximum thickness of some 8000 m. The economic importance of these sediments lies in the vast reserves of coal within the Ecca Group rocks of northern and eastern Transvaal and Natal, South Africa. Large reserves of sandstone-hosted uranium and molybdenum have been proven within the Beaufort Group rocks of the southern Karoo trough, although they are not mineable in the present market conditions. Palaeoenvironmental analysis of the major stratigraphic units of the Karoo succession in South Africa demonstrates the changes in depositional style caused by regional and localized tectonism within the basin. These depocentres were influenced by a progressive aridification of climate which was primarily caused by the northward drift of southwestern Gondwana out of a polar climate and accentuated by the meteoric drying effect of the surrounding land masses. Changing palaeoenvironments clearly influenced the rate and direction of vertebrate evolution in southern Gondwana as evidenced by the numerous reptile fossils, including dinosaurs, which are found in the Karoo strata of South Africa, Lesotho, Namibia and Zimbabwe. During the Late Carboniferous the southern part of Gondwana migrated over the South Pole resulting in a major ice sheet over the early Karoo basin and surrounding highlands. Glacial sedimentation in upland valleys and on the lowland shelf resulted in the Dwyka Formation at the base of the Karoo Sequence. After glaciation, an extensive shallow sea covered the gently subsiding shelf, fed by large volumes of meltwater. Marine clays and muds accumulated under cool climatic conditions (Lower Ecca Group) including the distinctive Mesosaurus-bearing carbonaceous shales of the Whitehill Formation. Subduction of the palaeo-Pacific plate reslted in an extensive chain of mountains which deformed and later truncated the southern rim of the main Karoo Basin. Material derived from these “Gondwanide” mountains as well as from the granitic uplands to the north-east, accumulated in large deltas that prograded into the Ecca sea (Upper Ecca Group). The relatively cool and humid climate promoted thick accumulations of peat on the fluvial and delta plains which now constitute the major coal reserves of southern Africa. As the prograding deltas coalesced, fluvio-lacustrine sediments of the Beaufort Group were laid down on broad gently subsiding alluvial plains. The climate by this time (Late Permian) had warmed to become semi-arid with highly seasonal rainfall. Vegetation alongside the meander belts and semi-permanent lakes supported a diverse reptilian fauna dominated by therapsids or “mammal-like reptiles”. Pulses of uplift in the southern source areas combined with possible orographic effects resulted in the progadation of two coarse-grained alluvial fans into the central parts of the basin (Katberg Sandstone Member and Molteno Formation). In the upper Karoo Sequence, progressive aridification and tectonic deformation of the basin through the late Triassic and early Jurassic led to the accumulation, in four separate depositories, of “redbeds” which are interpreted as fluvial and flood-fan, playa and dune complexes (Elliot Formation). This eventually gave way to westerly wind-dominated sedimentation that choked the remaining depositories with fine-grained dune sand. The interdune areas were damp and occasionally flooded and provided a habitat for small dinosaurs and the earliest mammals. During this time (Early Jurassic), basinwide volcanic activity began as a precursor to the break-up of Gondwana in the late Jurassic and continued until the early Cretaceous. This extrusion of extensive flood basalts (Drakensberg Group) onto the Clarens landscape eventually brought Karoo sedimentation to a close.

The 2.7–2.0 Ga volcano-sedimentary record of Africa, India and Australia: evidence for global and local changes in sea level and continental freeboard

Abstract The 2.7–2.0xa0Ga volcano-sedimentary records of the African, Indian and Australian cratons indicate two broadly defined periods of extensive drowning of the emergent continental areas, concomitant with lowered freeboard. Carbonate-banded iron formation (BIF) platforms characterised the first such event, at ca 2.6–2.4xa0Ga (Africa and Australia) to 2.7xa0Ga (India). These earlier globally enhanced sea levels are ascribed to increased mid-ocean ridge activity, possibly related to breakup of a postulated Late Archaean ‘southern’ supercontinent. Alternatively, a transition from global-scale catastrophic mantle overturn events to the onset of plate tectonics may have occurred in the Late Archaean (Nelson, 1998. Earth Planet. Sci. Lett. 158, 109–119). Both explanations of increased mid-ocean ridge activity are compatible with significant Early to Middle Archaean crustal growth (Armstrong, 1981. Phil. Trans. R Soc. London A 301, 443–472), with the emergent high freeboard cratons being subjected to aggressive weathering and erosion. Enhanced continental crustal growth near the Archaean–Proterozoic boundary (McLennan and Taylor, 1982. J. Geol. 90, 347–361), related to the development of significant island arc complexes, would have resulted in common lowered freeboard–enhanced sea level conditions at the passive margins of the ‘southern’ cratons. The diachronous nature of these earlier transgressions in the various cratons may reflect the effect of local tectonic movements and/or the thermal state of the cratons. From ca 2.4–2.2xa0Ga, cratons that make up the present-day continents of India, Africa and Australia had relatively high continental freeboard and lowered sea levels. Glacigenic deposits are preserved on the Kaapvaal (Africa), Singhbhum (India) and Pilbara (Australia) cratons. The second broadly defined drowning event, at ca 2.15xa0Ga, was probably due to post-glacial climatic amelioration. Freeboard was reduced by the combination of eustatic rise and the reestablishment of aggressive weathering as warmer palaeoclimates returned. In India, carbonates were more prominent than the siliciclastic sediments (including prominent black shales) seen in Africa and Australia.

The sedimentary and tectonic setting of the Transvaal Supergroup floor rocks to the Bushveld complex

Abstract The Palaeoproterozoic Transvaal Supergroup floor to the Bushveld complex comprises protobasinal successions overlain by the Black Reef Formation, Chuniespoort Group and the uppermost Pretoria Group. The protobasinal successions comprise predominantly mafic lavas and pyroclastic rocks, immature alluvial-fluvial braidplain deposits and finer-grained basinal rocks. These thick, laterally restricted protobasinal sequences reflect either strike-slip or small extensional basins formed during the impactogenal rifting and southeasterly-directed tectonic escape, which accompanied collision of the Zimbabwe and Kaapvaal cratons during Ventersdorp times. The erosively-based sheet sandstones of the succeeding Black Reef Formation reflect northwand-directed compression in the south of the basin. Thermal subsidence along the Ventersdorp Supergroup and Transvaal protobasinal fault systems led to shallow epeiric marine deposition of the sheet-like Chuniespoort Group carbonate-BIF platform succession. After an estimated 80 Ma hiatus, characterized by uplift and karstic weathering of the Chuniespoort dolomites, slower thermal subsidence is thought to have formed the Pretoria Group basin. Widespread, closed basin alluvial fan, fluvial braidplain and lacustrine sedimentation, as well as laterally extensive, subaerial andesitic volcanism (Rooihoogte to Strubenkop Formations), gave way to a marine transgression, which laid down the tuffaceous mudrocks, relatively mature sandstones and subordinate subaqueous volcanic rocks of the succeeding Daspoort, Silverton and Magaliesberg Formations. Poorly preserved post-Magaliesberg formations in the Upper Pretoria Group point to possible compressive deformation and concomitant rapid deposition of largely feldspathic detritus within smaller closed basins.

An overview of the geology of the Transvaal Sequence and Bushveld Complex, South Africa

The Late Archaean-Early Proterozoic Transvaal Sequence is preserved within the Transvaal, Kanye and Griqualand West basins, with the 2050 Ma Bushveld Complex intrusive into the upper portion of the succession within the Transvaal basin. Both Transvaal and Bushveld rocks are extensively mineralized, the former containing large deposits of iron, manganese, asbestos, andalusite, gold, fluorine, lead, zinc and tin ores, and the latter some of the Worlds major occurrences of PGE, chromium and vanadium ores. Transvaal sedimentation began with thin, predominantly clastic sedimentary rocks (Black Reef-Vryburg Formations) which grade up into a thick package of carbonate rocks and BIF (Chuniespoort-Ghaap-Taupone Groups). These lithologies reflect a carbonate-BIF platform sequence which covered much of the Kaapvaal craton, in reaction to thermal subsidence above Ventersdorp-aged rift-related fault systems. An erosional hiatus was followed by deposition of the clastic sedimentary rocks and volcanics of the Pretoria-Postmasburg-Segwagwa Groups within the three basins, under largely closed-basin conditions. An uppermost predominantly volcanic succession (Rooiberg Group-Loskop Formation) is restricted to the Transvaal basin. A common continental rift setting is thought to have controlled Pretoria Group sedimentation, Rooiberg volcanism and the intrusion of the mafic rocks of the Rustenburg Layered Suite of the Bushveld Complex. The dipping sheets of the Rustenburg magmas cut across the upper Pretoria Group stratigraphy and lifted up the Rooiberg lithologies to form the roof to the complex. Subsequent granitic rocks of the Lebowa and Rashoop Suites of the Bushveld Complex intruded both upper Rustenburg rocks and the Rooiberg felsites.

The transvaal sequence: an overview

Abstract The 15 000 m of relatively unmetamorphosed clastic and chemical sedimentary and volcanic rocks of the 2550-2050 Ma Transvaal Sequence as preserved within the Transvaal and correlated Griqualand West basins of South Africa, and in the Kanye basin of Botswana are described. Immature clastic sedimentary and largely andesitic volcanic rocks of the Wolkberg, Godwan and Buffelsfontein Groups and the Bloempoort and Wachteenbeetje Formations probably represent rift-related sequences of Ventersdorp age. The thin sandstones of the Black Reef Formation, developed at the base of both the Kanye and Transvaal basin successions and correlated with the basal Vryburg siltstones of the Griqualand West Sequence, are considered here to be the basal unit of the Transvaal Sequence. The Black Reef fluvial deposits grade up into the epeiric marine carbonates of the Malmani Subgroup. These stromatolitic dolomites and interdbedded cherts were laid down within a steepened carbonate ramp setting; transgressions from an initial Griqualand West compartment towards the northeast covered both the Kanye and Transvaal basins. Iron formations of the succeeding Penge Formation and Griqualand West correlates are envisaged as relatively shallow water shelf deposits within the carbonate platform model; siliceous breccias of the Kanye basin are interpreted as reflecting subaerial brecciation of exposed silica gels. The Duitschland Formation overlying the Penge iron formations is seen as a final, regressive clastic and chemical sedimentary deposits as the Malmani-Penge sea retreated from the Transvaal basin. The interbedded sandstones and mudstones of the uncomformity-bounded Pretoria Group probably represent a combination of alluvial fan and fluviodeltaic complexes debouching into the largely lacustrine Transvaal and Kanye basins. A strong glacial influence in the lower Pretoria Group is reflected in the correlated Makganyene diamicities of the Griqualand West Sequence. Sedimentation across all three basins was interrupted by the extrusion of the Hekpoort-Ongeluk andesites. Upper Pretoria Group sediments of the Silverton and Magaliesberg Formations probably reflect a marine transgression. These rocks are not present in the Griqualand West basin, and were affected by Bushveld Complex-related thermal doming in the Transvaal basin; post-Magaliesberg sedimentation continued thereafter in separate eastern and western fluviodeltaic-lacustrine sub-basins. The largely volcanic Rooiberg Group (sensu lato) began with catastrophic basin floor collapse and Leeuwpoort Formation fluvial sedimentation in the western sub-basin. The succeeding Smelterskop and Makeckaan Formations reflect a transition from fluvial deposition to volcanism, and are succeeded by the widespread and voluminous, predominantly felsitic lavas of the Dullstroom, Damwal and Selonsrivier Formations. The correlated Loskop, Glentig and Rust de Winter Formations which overlie the felsites conformably, represent the final sedimentary phase of the Transyaal basin.

The sedimentology of the Waterberg Group in the Transvaal, South Africa: an overview

Abstract The Palaeoproterozoic Waterberg Group consists chiefly of a succession of coarse siliciclastic rocks which shows two upward-fining sequences. The main depository evolved as a continental, faultbounded basin in the northern part of the Kaapvaal craton. The Main basin is bounded in the south by the Thabazimbi-Murchison fault zone and in the north by the southern part of the Melinda fault zone. The Swaershoek and lower Sterk River Formations at the base of the sequence are interpreted to have been deposited as fan deltas and were possibly reworked in a littoral palaeo-environment. The Alma and upper Sterk River Formations are interpreted as a series of alluvial fans forming a bajada along the scarp caused by the uplifted block on the southern side of the Murchison fault zone. The Skilpadkop and Setlaole Formations are considered to have been deposited on narrow braidplains. The Makgabeng Formation was deposited during the more stable period that followed and it is considered to represent a large dune field, which may have been coastal in nature towards the south. The upward-coarsening Aasvoelkop Formation was deposited in a shallow through-flow lake, although fluvial deposition was more important towards the top of the formation. The Mogalakwena and Sandriviersberg Formations are interpreted as having been deposited by large braided rivers, forming an extensive braidplain which probably continued to the southwest, through Bostwana into the northern Cape Province, where it may be represented by the Fuller Member of the Volop Group. As sediment input from the north decreased, the sea transgressed over the braidplain and deposited the Cleremont Formation, which is interpreted as a littoral deposit or a tidally influenced shelf deposit. The Vaalwater Formation, which ended the Waterberg episode, formed within a littoral or a shallow siliciclastic sea palaeo-environment.

A review of the sedimentology of the Early Proterozoic Pretoria Group, Transvaal Sequence, South Africa: implications for tectonic setting

Abstract The sedimentary rocks of the Early Proterozoic Pretoria Group form the floor rocks to teh 2050 M.a. Bushveld Complex. An overall alluvial fan-fan-delta - lacustrine palaeoenvironmental model is postulated for the Pretoria Group. This model is compatible with a continental half-graben tectonic setting, with steep footwall scarps on the southern margin and a lower gradient hanging wall developed to the north. The latter provided much of the basin-fill detritus. It is envisaged that the southern boundary fault system migrated southwards by footwall collapse as sedimentation continued. Synsedimentary mechanical rifting, associated with alluvial and deltaic sedimentation (Rooihoogte-Strubenkop Formations) was followed by thermal subsidence, with concomitant transgressive lacustrine deposition (Daspoort-Magaliesberg Formations). The proposed half-graben basin was probably related to the long-lived Thabazimbi-Murchison and Sugarbush-Barberton lineaments, which bound the preserved outcrops of the Pretoria Group.

Thematic Set: Sequence stratigraphy: common ground after three decades of development

Sequence stratigraphy emphasizes changes in stratal stacking patterns in response to varying accommodation and sediment supply through time. Certain surfaces are designated as sequence or systems tract boundaries to facilitate the construction of realistic and meaningful palaeogeographic interpretations, which, in turn, allows for the prediction of facies and lithologies away from control points. Precisely which surfaces are selected as sequence boundaries varies from one sequence stratigraphic approach to another. In practice, the selection is often a function of which surfaces are best expressed, and mapped, within the context of each case study. This high degree of variability in the expression of sequence stratigraphic units and bounding surfaces requires the adoption of a methodology that is sufficiently flexible to accommodate the wide range of possible scenarios in the rock record. We advocate a model-independent methodology that requires the identification of all sequence stratigraphic units and bounding surfaces, which can be delineated on the basis of facies relationships and stratal stacking patterns using the available data. Construction of this framework ensures the success of the method in terms of its objectives to provide a process-based understanding of the stratigraphic architecture and predict the distribution of reservoir, source-rock, and seal facies.

Shear-zone controlled basins in the Blouberg area, Northern Province, South Africa: syn- and post-tectonic sedimentation relating to ca. 2.0 Ga reactivation of the Limpopo Belt

Abstract The extent of the deposition and of the preservation of the Blouberg Formation and Waterberg Group was at least partially controlled by brittle reactivation along the Palala Shear Zone. The Palala Shear Zone in the Blouberg area (Northern Province, South Africa) is characterised by granulite-grade gneiss, and formed by sinistral transpressional collision between the Southern Marginal Zone (Kaapvaal Craton) and the Central Zone of the Limpopo Belt. The Limpopo collision is thought to have occurred either at 2.0 Ga or at 2.7 Ga with reactivation at 2.0 Ga. Deposition of the Blouberg Formation was characterised by syn-sedimentary tectonism, which is reflected by a sudden upward coarsening in sedimentary rocks, and by the presence of a strongly folded and thrusted lower member. Bedding orientations and slickenside lineation orientations suggest that vergence was towards the south, and such a tectonism can be inferred to have produced a highland area to the north, bound on the southern margin by the southern strand of the Melinda Fault. The presence of an inferred northerly upland area is supported by palaeocurrent directions and the preservational extent of the Setlaole and Makgabeng Formations of the Waterberg Group (post-Blouberg Formation). The extent and stratigraphy of the overlying Mogalakwena Formation suggests that these strata onlapped northwards over the denuding highlands. Younger Sibasa basalts of the Soutpansberg Group have been dated at ca. 1.85 Ga. Blouberg and Waterberg strata can therefore be interpreted as syn- and post-tectonic sedimentary rocks, respectively, following a ca. 2.0 reactivation event along the Palala Shear Zone. It is difficult to reconcile the succession of geological events at Blouberg with a ca. 2.0 Ga Limpopo orogeny, and thus sedimentary strata in the study area support a 2.7 Ga date for Limpopo collision, with syn-Blouberg tectonism relating to ca. 2.0 reactivation within the previously assembled Limpopo Belt.

Compressive deformation in the floor rocks to the Bushveld Complex (South Africa): evidence from the Rustenburg Fault Zone

Abstract The north-northwest-south-southeast striking Rustenburg Fault Zone in the western Transvaal Basin, South Africa, has been extensively mapped in order to unravel its tectonic history. In post-Pretoria Group times, but before the intrusion of the Bushveld Complex at ∼2050 Ma, the area surrounding the fault zone was subjected to two compressive deformational events. The shortening direction of the first event was directed northeast-southwest, producing southeast-northwest trending folds, and the shortening direction of the second was directed north-northwest - south-southeast, producing east-northeast - west-southwest trending folds. The second set of folds refolded the first set to form typical transitional Type 1-Type 2 interference folding. This compression ultimately caused reactivation of the Rustenburg Fault, with dextral strike-slip movement displacing the Pretoria Group sediments by up to 10.6 km. The subsequent intrusion of the Bushveld Complex intensely recrystallised, and often ponded against the strata along the fault zone. The fault rocks within the fault zone were also recrystallised, destroying any pre-existing tectonic fabric. Locally, the fault zone may have been assimilated by the Bushveld Complex. After the intrusion of the Bushveld Complex, little movement has occurred along the fault, especially where the fault passes under areas occupied by the Bushveld Complex. It is thought that the crystallisation of the Bushveld Complex has rheologically strengthened the neighbouring strata, preventing them from being refaulted. This model is at variance with previous assumptions, which suggest that continuous regional extension during Pretoria Group sedimentation culminated in the intrusion of the Bushveld Complex.