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.