The Loskop Formation and its relationship to the Bushveld Complex, South Africa

Abstract

The Loskop Formation consists of continental detrital red beds ( sensu lato), 2.06 Ga old, laid down in the Cullinan-Witbank Basin. These beds rest conformably to unconformably on the Rooiberg volcanics and on the Pretoria Group, and have been partly eroded before the deposition of the Waterberg Group. In the western part of the basin, only 3 main relicts of the Loskop Formation have escaped this denudation. They are the object of this study: the area south of Moloto, Renosterkop Hill and south of Balmoral. The investigation focussed on the stratigraphy, the petrography of the clasts, the extrusive and intrusive rocks, and the mineralisations. Synsedimentary tensional tectonics, updoming and epithermal activity are documented. A clast study indicated that the basic rocks of the Bushveld Complex (= Rustenburg Layered Suite), up to the Main Zone and possibly even the basal Upper Zone, were already solid and were rapidly uplifted and partly eroded during the early stages of Loskop sedimentation. The uplift is attributed to diapiric rise of the Bushveld floor before the end of the deposition of the cumulates of the Rustenburg Layered Suite. The main part of the Bushveld Granite (= Nebo Granite) is intrusive into the Loskop Formation, but had cooled down before the end of the last magmatic phase of the Rustenburg Layered Suite. The model proposed recently by some authors, that the Rooiberg Group developed simultaneously with the Rustenburg Layered Suite and that both are due to a plume of short duration, is supported by this investigation. It is suggested that the Loskop Formation was deposited during the waning stage of the plume activity, when the production of new magma became reduced.