SEDIMENTOLOGY AND FACIES DEVELOPMENT OF AN ARCHEAN 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 Ca pe Province, South Africa are presented here. The core penetrated all the formations of the Schmidtsdrif Subgroup and the overlying Campbell rand Subgroup of the. Ghaap Group, Transvaal Supergroup. The core cont ains the longest continuous section (> 3000 m) through the world's old est giant carbonate platform and records invaluable information on Arc haean development of sedimentary cover sequences. It is proposed that the core may serve as the type section for the bower Transvaal Supergr oup rocks in Griqualand West. In the core, the Schmidtsdrif Subgroup, comprising the Vryburg, Boomplaas and Lokammona Formations, is signifi cantly thicker than deduced from surface outcrops. The lithology of th ese three formations is of mixed siliciclastic and carbonate compositi on and is thus similar to the outcrop lithology. The depositional cond itions are interpreted as having been of shelf to subtidal character a nd appear to have been generally deeper than those inferred from the s urface outcrops. The Campbellrand Subgroup is also of greater thicknes s in the core than known from outcrop. The Monteville Formation, at th e base of the Campbellrand Subgroup, marks shallowing-upward condition s, followed by stromatolitic platform carbonates of the Reivilo, Fairf ield, Klipfontein Heuwels, Papkuil, Klippan, Kogelbeen and Gamohaan Fo rmations. Within these platform carbonates, several transgression-regr ession episodes are recognised. The overall inferred depth of depositi on was similar to conditions interpreted from surface outcrops on the Ghaap Plateau, but perhaps with less tidal influence. Only for the Kog elbeen 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 pa laeohigh at the Maremane Dome, is proposed to explain the inferred dep ositional patterns observed in the Kathu core and their differences to the surface outcrop facies. (C) 1997 Elsevier Science Limited.