Aspects of sedimentological, structural and stratigraphic investigations of the collar strata of the Vredefort structure: contributions towards an understanding of the pre-history and origin of the structure

Abstract

The Vredefort Structure of South Africa is situated on the border of the Orange Free State and Transvaal provinces and occurs roughly near the centre of the oblong Witwatersrand basin, where the strata of the entire 7 000m thick Witwatersrand Supergroup crop out in an arcuate periclinal structure. Although no formal differentiation has previously been made for the Central Rand Group beyond the rank of Subgroup, the uppermost Turffontein Subgroup is tentatively subdivided into five Formations. This paper reports on a sedimentological and preliminary petrographic study of one of these proposed Formations, the Leeufontein Quartzite, and parts of the underlying and overlying Formations. Palaeocurrent distributions in this Formation were determined at 27 localities along the outcrop in the Vredefort Structure, using trough cross-bed axes as palaeocurrent indicators. This revealed bi- and polymodal distributions in the central parts of the arcuate Structure, whereas in the western and eastern regions palaeocurrent distributions are unimodal, with modes towards the east and southeast, respectively. The lithofacies of the Leeufontein Quartzite Formation and those of the uppermost and lowermost portions of the underlying and overlying Formations were studied in three detailed vertical profiles. These revealed that the Leeufontein Quartzite is most probably of shallow marine origin, but that the underlying and overlying successions represent deposition in fluvial environments. The Formation contains several lithofacies, typical of beach and nearshore depositional environments, which may have been deposited no farther offshore than the inner shelf zone. It is proposed that the Leeufontein Quartzite Formation was deposited during successive transgressive and progradational episodes and that the deposits of these contrasting pro￾cesses are partially preserved.The findings of this study have important implications for the configuration of the Wit￾watersrand basin and for its stratigraphy. The probable presence of an ancient shoreline in the central regions of the preserved Witwatersrand basin suggests a basinal location on a continental margin. It follows that marine fades, absent in the northwestern sector of the preserved basin, would most probably occur in the southeastern parts. This may present regional correlation problems to those who are unaware of the facies transition in the Turffontein Subgroup of the Vredefort area.