The Witwatersrand Supergroup is divided into two groups:
West Rand Group - which extends over an area of 43 000 sq. km, is up to 5150 m thick and rests on the Dominion Group above an angular unconformity. It is thickest in the Klerksdorp area in the west, and thins to the north-east. Age dating restricts the deposition to between 2985±14 Ma and 2914±8 Ma, the age of volcanics in the upper part of the group. It has been sub-divided into three sub-group, based on varying sandstone/shale ratios and basinwide disconformities, namely the i). Hospital Hill Sub-group, is primarily composed of shallow marine deposits, mainly shales and orthoquartzites, comprising up to 2500 m of conglomerates, sub-greywacke, sub-arkose, ortho-quartzites, siltstones, shales, ferruginous shale and banded ironstone; with no volcanics, approximately 50% quartzites/conglomerates and 50% shales; the ratio of sandstone/shale increases upwards, with the sandstone being predominantly quartz-arenite; the unit contains no economic reefs; ii). Government Sub-group, characterized by more diverse lithologies which formed in a far greater variety of depositional settings, and include diamictites (possibly of glacial origin), interbedded beds of fluvial conglomerates and quartzite, shelf shales and quartzite, and iron formations. The base of the Subgroup is represented by a widely correlated diamictite which lies on a major, regional unconformity; the unit contains three subeconomic auriferous reefs; iii). Jeppestown Sub-group, which commences with a quartzite overlying a major regional unconformity and represents a return to a predominance of shelf deposits. It is up to 1200 m in thickness and comprises interbedded conglomerates, feldspathic quartzites, siltstones, shales, ferruginous shale, calcareous shale and andesitic volcanics; comprising approximately 30% volcanics, 30% quartzites/conglomerates and 40% shales. Two sub-economic reefs have been recognised within the unit.
Central Rand Group - which is up to 2880 m thick in the central part of the basin around the Vredofort structure and unconformably overlies the West Rand Group, covering an area of 9750 sq. km with a basinal extent of around 290 x 150 km. Similar to the West Rand Group, this group is composed of a series of cycles, each comprising fluvially dominated coarse grained siliciclastics overlying an erosional surface, although in contrast to the West Rand Group, the coarser lithologies predominate with subordinate shale or mudstone. Age constraints on the group places deposition between a lower limit of 2902±13 Ma and minimum age of 2840±3 to 2780±3 Ma. Two sub-groups have been defined, namely the:
Johannesburg Sub-group - The base of this unit is the first prominent conglomerate overlying quartzite of the Maraisburg Formation at the top of the Jeppestown Sub-group. This conglomerate is widely recognised and occurs as the North Reef in the Central Rand Gold Field (GF); Rock Tunnel Reef in the West Rand GF; North leader in the Carletonville GF; and the Ada May Reef in the Klerksdorp and Welkom GFs. This conglomerate and the overlying quartzite, which together comprise the Blyvooruitzicht Formation, are both locally truncated by an erosion surface upon which another widespread conglomerate was deposited.
The overlying Main Formation contains the economically important Main Reef, Carbon Leader and Main Reef Leader in the Central and West Rand and at Carletonville, and has been correlated by the prominent, shale-filled erosion channels (Green Bar of Carletonville, Black Bar of the Central Rand), which have been recognized in almost all of the goldfields. Although widespread, this unit was truncated by the overlying sequence in the eastern portions of the Witwatersrand basin.
The overlying Randfontein Formation consists of quartzite and conglomerates, which along the northern margin of the basin contains, at its base, the Nigel Reef of the East Rand, the South Reef of the Central Rand and the Middelvlei Reef in the Carletonville area. Several conglomerates are developed within the succeeding quartzite, which degenerated to pebbly quartzites regionally. This sequence of conglomerates and quartzite was accordingly grouped together in a single formation, termed the Randfontein Formation.
The Randfontein Formation is followed by the sequence of pebbly quartzite and locally developed conglomerates of the Luipaardsvlei Formation, which can be correlated with most of the Krugersdorp Formation of the Carletonville GF, the Stilfontein Formation of the Klerksdorp GF and the Welkom Formation of the Welkom GF, with some small changes in boundaries. This unit is absent on the East Rand, as it has been erosionally truncated prior to deposition of the overlying formation.
An extensive unconformity marks the top of the Luipaardsvlei Formation and the base of the Krugersdorp Formation which contains the Bird Reef and its correlatives Vaal and Basal/Steyn Reefs. This basal conglomerate is overlain by quartzite with locally developed pebbly quartzite or conglomerates, although locally, it fines upwards to siltstone or shale (e.g. Khaki Shale of Welkom). In the eastern sections of the basin, the formation contains the mafic Bird volcanics.
The succeeding Booysens Formation contains the only widespread shale sequence in the Central Rand Group and marks the top of the Johannesburg Subgroup. Both the upper and lower boundaries are generally transitional. The shale coarsens up into the locally prominent orthoquartzite of the Doornkop Member. The marks the top of the Johannesburg Subgroup.
The Turffontein Sub-group - The base of this unit is defined by the widespread unconformity-based conglomerate of the Kimberley Formation which comprises the 'B', Kleinfontein, 'C' and 'LK1' Reefs, and consist primarily of quartzites and conglomerates, deposited in a number of cycles of deposition and erosion. The the Beatrix Reef occurs in conglomerates of one these cycles. Shale-filled erosion channels occur at the base of several of these cycles. The upper boundary of the formation is defined by another widespread unconformity.
The basal conglomerate of the overlying Elsburg Formation contains the Intermediate Reef of the Evander GF, the UE1A of the West Rand GF, the Denny's Reef of the Klerksdorp GF and the VS5 of the Welkom GF. This basal conglomerate is overlain by upward coarsening quartzite throughout the basin. The lower, predominantly quartzitic portion, locally contains economic conglomerates which become increasingly more abundant upwards in the succession. Conglomeratic rocks dominate the upper portion of the sequence, which has been termed the partially equivalent Mondeor Formation. In the Welkom GF, the transition from the Elsburg to the Mondor Formation is characterised by a transition from predominantly oligomictic to polymictic conglomerates.
The top of the Witwatersrand Supergroup is defined by the unconformity which marks the base of the Ventersdorp Contact Reef (Venterspost Formation), where developed, or the base of the volcanic rocks of the Ventersdorp Supergroup.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2013)
