THE TRANSVAAL SEQUENCE - AN OVERVIEW

The 15 000 m of relatively unmetamorphosed clastic and chemical sedime ntary and volcanic rocks of the 2550-2050 Ma Transvaal Sequence as pre served within the Transvaal and correlated Griqualand West basins of S outh Africa, and in the Kanye basin of Botswana are described. Immatur e clastic sedimentary and largely andesitic volcanic rocks of the Wolk berg, Godwan and Buffelsfontein Groups and the Bloempoort and Wachteen beetje Formations probably represent rift-related sequences of Venters dorp age. The thin sandstones of the Black Reef Formation, developed a t the base of both the Kanye and Transvaal basin successions and corre lated with the basal Vryburg siltstones of the Griqualand West Sequenc e, are considered here to be the basal unit of the Transvaal Sequence. The Black Reef fluvial deposits grade up into the epeiric marine carb onates of the Malmani Subgroup. These stromatolitic dolomites and inte rbedded cherts were laid down within a steepened carbonate ramp settin g; transgressions from an initial Griqualand West compartment towards the northeast covered both the Kanye and Transvaal basins. Iron format ions of the succeeding Penge Formation and Griqualand West correlates are envisaged as relatively shallow water shelf deposits within the ca rbonate platform model; siliceous breccias of the Kanye basin are inte rpreted as reflecting subaerial brecciation of exposed silica gels. Th e Duitschland Formation overlying the Penge iron formations is seen as a final, regressive clastic and chemical sedimentary deposit, as the Malmani-Penge sea retreated from the Transvaal basin. The interbedded sandstones and mudstones of the unconformity-bounded Pretoria Group pr obably represent a combination of alluvial fan and fluviodeltaic compl exes debouching into the largely lacustrine Transvaal and Kanye basins . A strong glacial influence in the lower Pretoria Group is reflected in the correlated Makganyene diamictites of the Griqualand West Sequen ce. Sedimentation across all three basins was interrupted by the extru sion of the Hekpoort-Ongeluk andesites. Upper Pretoria Group sediments of the Silverton and Magaliesberg Formations probably reflect a marin e transgression. These rocks are not present in the Griqualand West ba sin, and were affected by Bushveld Complex-related thermal doming in t he Transvaal basin; post-Magaliesberg sedimentation continued thereaft er in separate eastern and western fluviodeltaic-lacustrine sub-basins . The largely volcanic Rooiberg Group (sensu lato) began with catastro phic basin floor collapse and Leeuwpoort Formation fluvial sedimentati on 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 fels itic 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 th e Transvaal basin.