Sedimentology and facies development of an Archaean shelf: carbonate platform transition in the Kaapvaal Craton, as deduced from a deep borehole at Kathu, South Africa

A lithological description and a facies interpretation of Neoarchaean rocks, as penetrated by the 3672 m deep borehole at Kathu, Northern Cape Province, South Africa are presented here. The core penetrated all the formations of the Schmidtsdrif Subgroup and the overlying Campbellrand Subgroup of the. Ghaap Group, Transvaal Supergroup. The core contains the longest continuous section (> 3000 m) through the world's oldest giant carbonate platform and records invaluable information on Archaean development of sedimentary cover sequences. It is proposed that the core may serve as the type section for the bower Transvaal Supergroup rocks in Griqualand West. In the core, the Schmidtsdrif Subgroup, comprising the Vryburg, Boomplaas and Lokammona Formations, is significantly thicker than deduced from surface outcrops. The lithology of these three formations is of mixed siliciclastic and carbonate composition and is thus similar to the outcrop lithology. The depositional conditions are interpreted as having been of shelf to subtidal character and appear to have been generally deeper than those inferred from the surface outcrops. The Campbellrand Subgroup is also of greater thickness in the core than known from outcrop. The Monteville Formation, at the base of the Campbellrand Subgroup, marks shallowing-upward conditions, followed by stromatolitic platform carbonates of the Reivilo, Fairfield, Klipfontein Heuwels, Papkuil, Klippan, Kogelbeen and Gamohaan Formations. Within these platform carbonates, several transgression-regression episodes are recognised. The overall inferred depth of deposition was similar to conditions interpreted from surface outcrops on the Ghaap Plateau, but perhaps with less tidal influence. Only for the Kogelbeen and Gamohaan Formations are somewhat shallower water conditions deduced compared to those previously interpreted from outcrops on the Ghaap Plateau, which is attributed to the Maremane upwarp. A model of a generally westward deepening basin, but with a locally developed palaeohigh at the Maremane Dome, is proposed to explain the inferred depositional patterns observed in the Kathu core and their differences to the surface outcrop facies. (C) 1997 Elsevier Science Limited.