Geochemistry and petrology of Witwatersrand and Dwyka diamictites from South Africa: search for an extraterrestrial component

Abstract

Diamictites are poorly sorted sediments characteristically carrying coarse-grained clasts in a fine-grained matrix. They have generally been considered of glaciogenic or glaciomarine origin. Recently, however, it has been suggested that some massive tillite/diamictite layers could represent impact breccias. An earlier petrographic study of rock and mineral clasts from Dwyka Group diamictites revealed no evidence for shock metamorphism, such as planar deformation features. Detailed geochemical studies of diamictite samples from the Archean Witwatersrand Supergroup and the Dwyka Group of the Mesozoic Karoo Supergroup from South Africa are reported. We studied the contents of the siderophile elements in these breccias, as elevated abundances of such elements, especially iridium, could be indicative for an impact origin. By use of γ-γ coincidence spectrometry and other trace element analysis, geochemical tracers of extraterrestrial components were sought. However, no enrichments of indicator elements for extraterrestrial components, compared with ordinary continental crust, were found. Thus, neither geochemical nor petrographic evidence supports an impact origin of the diamictites from the Dwyka Group and the Witwatersrand Supergroup in South Africa.