Stratigraphy and Basin Analysis of the West Rand Group, Witwatersrand Supergroup

Abstract

Abstract The Mesoarchaean West Rand Group displays a layer-cake stratigraphy with lithostratigraphic units correlatable on a basin-wide scale. The ~5 km-thick succession consists of fluvial braidplain and shelf deposits, which range from shallow inner shelf marine orthoquartzites, outer shelf argillites to starved shelf iron-formations. Minor diamictites are of debris flow origin and are possibly related to glacial activity. Three major sequences are present: Sequence I (Hospital Hill Subgroup) is bounded by an angular unconformity at the base of the Orange Grove Formation and a low-angle unconformity at the base of the Promise diamictite. Sequence II (Government Subgroup) extends from the base of the Promise diamictite to a well-defined low-angle unconformity at the base of the Koedoeslaagte Formation. Sequence III (Jeppestown Subgroup) comprises the succession between the Koedoeslaagte Formation and the Maraisburg Formation, up to the low-angle unconformity at the base of the Main Reef. Sequence I was deposited during a period of highstand of sea-level, Sequence II during a period of relative lowstand, and Sequence III during a period of relative highstand coupled with high rates of sediment supply. Isopach, depofacies and palaeocurrent analyses indicate that strata in the western to northwestern parts of the basin were deposited under more proximal sedimentary conditions compared to those in the central or southeastern parts of the basin. There is little relationship between the present outline of the basin and the distribution of depofacies or isopachs of sequences, and it is therefore concluded that the original sedimentary basin was significantly larger in areal extent. Depofacies and thickness distribution, as well as synsedimentary deformation of strata, indicate that the basin was most probably of flexural tectonic origin. These findings strongly support deposition in a wide, shallow, and rather stationary foreland basin, with an axial zone towards the west/northwest and a low amplitude peripheral bulge to the east/southeast. Such shallow foreland basins, with abundant sediment bypassing, are thought to be associated with windward-facing orogenic fronts. High rates of erosion along such fold-thrust belts lead to ineffective loading and advancement of the orogenic front, as well as an oversupply of sediment.