Changing tectono-sedimentary scenarios relevant to the development of the Late Archaean Witwatersrand Basin

Abstract

Recent studies of the Witwatersrand Basin indicate that contrary to earlier proposals the shape and size of the basin and style of sedimentation were very different early and late in basin history. This reflects a geotectonic evolution which may involve an overall Wilson cycle. Progressive stages prior to, during and after basin development are described in order to map this evolution. Stage 1 saw the structural framework of the Kaapvaal Craton which was to accommodate future basin development. Fault zones representing lineaments and sutures associated with the structural evolution of various greenstone belts of the Kaapvaal Craton were rejuvenated (especially during late stages of Witwatersrand Supergroup deposition) to act as primary synsedimentary fault zones. Stage 2 involved the development of the Dominion Basin which has been interpreted as an extensional basin associated with NNE trending faults, although volcanics show calc-alkaline affinities. Stage 3 saw widespread deposition over the Kaapvaal Craton of a variety of subtidal sediments of the West Rand Group and its correlatives including the Pongola Sequence. This epicontinental style of sedimentation was either associated with a thermal cooling phase related to the Dominion Group extension or with the initiation of a foreland basin. The conformable or disconformable relationship between the West Rand Group and the underlying Dominion Group would point to the former. During stage 4 major changes in sedimentary patterns occurred. Upper Central Rand Group sediments were deposited and draped over compressive basement block-faults, which interacted with the sedimentation causing reverse faults/ monoclines and locally provided sources for coarse gravel. The basin was shrinking considerably in size and fluvial sediments derived from developing pediment areas and uplifted basement interacted with a reduced marine water body. The basin was controlled by NE-SW directed compression which caused a relative tectonic escape of the central part of the Kaapvaal Craton to the SE. The reason for these changes was the initial orogenesis associated with the subduction history preceding and the ultimate A-subduction history during the collision of the Zimbabwe Craton with the Kaapvaal Craton. Stage 5 saw the outpouring of the Klipriviersberg Group flood basalts into the Witwatersrand Basin. Early in this stage the tectonism was similar to that for stage 4, but late in the extrusive history a change in feeder dyke patterns probably heralds the major extensional tectonics developed during stage 6, involving the deposition of the patterns probably heralds the major extensional tectonics developed during stage 6, involving the deposition of the Platberg Group. The latter developed in an extensional rift basin caused by the tectonic inversion of faults from their previous reverse state to a normal movement. This has been interpreted as the result of impactogenal rifting. In the aftermath the collapse of the orogen allowed the deposition of the Pniel sequence and Wolkberg Group in extensional rift basins and the associated thermal cooling led to the development of the Chuniespoort Group chemical sedimentation. At no time did the Vredefort Dome affect this sedimentary history, in fact this was a post-Bushveld event which was followed by the emplacement of the Johannesburg Dome. The Vredefort Dome did, however, have a major structural impact on the Witwatersrand Basin and may have played a major role in its long term preservation up to the present day.