Major influences on the evolution of the 2.67–2.1Ga Transvaal basin, Kaapvaal craton

Abstract

The Transvaal is one of three structural basins of the Transvaal Supergroup preserved on the Kaapvaal craton. The evolution of the Neoarchaean–Palaeoproterozoic Transvaal basin is ascribed predominantly to magmatism, palaeoclimate and eustasy, with plate tectonics playing a sporadic role. The supergroup comprises basal ‘protobasinal’ rocks, followed by the Black Reef Formation, Chuniespoort Group and uppermost Pretoria Group. Immature siliciclastic and bimodal volcanic rocks of the protobasinal unit reflect a wide zone of rifting related to the c. 2.7 Ga Ventersdorp (Supergroup) mantle plume. Individual protobasinal successions were laid down in separate fault-bounded basins, controlled at least partially by greenstone belt orientations in the Kaapvaal basement. Post-magmatic, post-rifting thermal subsidence accommodated Black Reef fluvial sheet sandstones and the subsequent thick carbonate-BIF epeiric platform succession of the Chuniespoort Group. Subordinate mechanical subsidence accompanied this long-lived thermal relaxation. Intense weathering due to Neoarchaean atmosphere composition greatly reduced clastic sedimentation and the greenhouse palaeoclimate further encouraged carbonate sedimentation. Globally enhanced sea levels, due to enhanced mid-ocean ridge growth consequent upon either global magmatic events or supercontinent break-up, also played a pivotal role in Chuniespoort epeiric basin evolution. Pretoria Group sedimentation is ascribed to two cycles of rifting and subsequent thermal subsidence. The first cycle appears to reflect plate tectonically induced rifting, with an epeiric sea drowning the rift basin despite reduced sea levels due to the first major global glaciation. The second Pretoria cycle is most likely related to a major continental flood basalt event, with thermal subsidence allowing a second and probably larger epeiric sea to advance onto the northern Kaapvaal craton. Evaluation of the inferred relatively minor importance of plate tectonics in the evolution of the essentially intracratonic Transvaal basin depends to a large degree on the age assigned to the Limpopo orogeny during which the Zimbabwe and Kaapvaal cratons collided; recent evidence for a c. 2.0 Ga collision supports the concept that catastrophic global magmatic events gradually gave way to plate tectonics during Transvaal basin evolution.