Tectonosedimentary model for the central Rand Goldfield, Witwatersrand basin, South Africa

Abstract

A tectonosedimentary model is established for the Composite Reef at Far East Vertical Shaft, East Rand Proprietary Mine, Central Rand Goldfield. There the Composite Reef comprises the Main Reef and the Main Reef Leader, with the South Reef occurring sporadically in the hanging wall, redefining the Composite Reef stratigraphy for this area. Tectonic controls on sedimentation persisted throughout the deposition of the Composite Reef, influencing the nature and distribution of the conglomerates. Utilising the Composite Reef model, combined with structural and sedimentological modelling of the Central Rand Goldfield, a tectonosedimentary model for the Main Conglomerate Formation is proposed. Pre- and syn-depositional folding associated with regional basin-wide compression resulted in the formation of the Springs Monocline, the West Rand Syncline and associated DRD Anticline. This was overprinted by northwest to southeast oriented folding associated with left-lateral wrenching on the Rietfontein Fault, forming a corrugated palaeosurface, prior to Main Reef deposition that controlled the palaeoflow direction. Brittle deformation initiated during Witwatersrand times in the form of Riedel and Riedel conjugate shears, normal faults, principal shears and P-shears associated with left-lateral wrenching caused northeast/southwest and east/west cross-cutting channel orientations and northeast/southwest oriented erosion channels. The deposition of the Black Bar, associated with a marine transgression, accumulated in topographically lower lying areas, smoothing the palaeotopography prior to Main Reef Leader deposition. This smoothing effect combined with syn-depositional folding resulted in a single Main Reef Leader channel complex associated with the Robinson Deep Syncline, and restricted Main Reef Leader deposition to an area bounded by the Springs Monocline in the east and the DRD Anticline in the west. Brittle deformation continued during Main Reef Leader deposition resulting in cross-cutting channels. The tectonosedimentary model that has been established increases the confidence of modelling the distribution of conglomerates of the Main Conglomemte Formation, thereby facilitating feasibility modelling of the down-clip, un-mined South Central.