The Ventersdorp Contact Reef: Final phase of the Witwatersrand Basin, independent formation, or precursor to the Ventersdorp Supergroup?

The Ventersdorp Contact Reef (VCR) is an Archaean conglomeratic gold placer, mined in the Carletonville, West Rand, and Klerksdorp goldfields of the Republic of South Africa. It belongs to a lithostratigraphic formation, the Venterspost Conglomerate Formation (VCF), which is considered to be stratigraphically independent of the underlying Witwatersrand Supergroup sediments and overlying Ventersdorp Supergroup volcanics. In this paper, the present formal stratigraphic placement of the Formation is compared with its alternative stratigraphic assignment to either of the underlying Witwatersrand or overlying Ventersdorp Supergroups. Evidence for a basin-wide, angular unconformity separating the VCF from Witwatersrand rocks, which were regionally lithified prior to deposition of the Formation,indicates that it is stratigraphically divorced from the Witwatersrand Supergroup. While similar angular, internal unconformities are present within the Witwatersrand Supergroup, a significant difference between these and the VCF unconformity is that the internal Witwatersrand unconformities are asserted to have basin ward extents only to the tectonic hinge between continuous deposition and contemporaneous (Witwatersrand) foreland basin destruction. A genetic difference between the VCR and typical Witwatersrand placers is reflected by their differing mineralogical content and metamorphic grade. These differences, by implication, also reflect a substantial time gap separating their deposition. The fact that the conglomerates and sandstones of the VCF are locally interbedded with lava flows that are geochemically similar to the lava of the basal Westonaria Formation of the Ventersdorp Supergroup, indicates lithologic and genetic links to this Supergroup. These relationships are further supported by the presence of tuffaceous sandstone beds within the VCF. The occurrence of deformational load structures at the contact between the formation and the basal lava flows of the overlying Ventersdorp Supergroup indicates an unconsolidated, probably water-saturated state of the VCF sediments at the time of their envelopment by the lava, and implies a conformable relationship with the lower formations of the Supergroup. In conclusion, it is proposed that the VCF be stratigraphically assigned to the Ventersdorp Supergroup.