Evidence of basin partitioning in the ue7 and upper Elsburg massives of the Turffontein subgroup, Witwatersrand basin (South Africa)

Abstract

The South Deep Gold Mine (SDGM), currently owned by Gold Fields, is located along the eastern edge of the West Rand Goldfield on the northern margin of the Witwatersrand Basin. It mines the Ventersdorp Contact Reef (VCR) as well as the “Upper Elsburg” conglomerates, comprising Elsburg Massives of the Modderfontein Member and the UE7, or Individual Reefs, of the underlying Waterpan Member of the Mondeor Formation (Turffontein Subgoup, Central Rand Group). The Upper Elsburgs are preserved as a rotating onlap stacked unconformity structure and sub-outcrop on a NNE trend against the overlying VCR. The Upper Elsburgs are unusual owing to their massive and thick conglomeratic beds with basal unconformities alternating with overlying quartzites, which contrasts with the traditionally mined thin, unconformity-bound, conglomerate reefs elsewhere in the Witwatersrand Basin. Limited regional palaeo-current data suggests SE to SSE sediment transport. This study investigates whether stratigraphic and sedimentological evidence exists at SDGM that supports the existence of syn-depositional strike-slip tectonics associated with multiple E-W wrench faults transecting the ore resource. A total of 4502 drillholes representing some 18731 stacked reef intercepts were utilised to model corrected bed height, percentage conglomerate, and average and maximum clast size for each of the 16 modelled reef beds. The dataset covers an area of ~10 km² along a sub-outcrop strike length of ~6.5 km. Based on mapped wrench faults, six fault blocks are identified across the study volume. Sedimentologicalstratigraphic data were palinspastically reconstructed to remove post-VCR displacement. Colour-coded contour maps were generated from the palinspastically reconstructed data to graphically present each parameter for each bed for interpretation. Visual interpretation of possible data trends was conducted for comparison with the available regional data. The overwhelming trend for all parameters across all blocks is NNW-SSE except in the northern-most block where trends for all parameters for some of the beds are NW-SE. During early deposition of the Elsburgs “C” Band of the UE7 Zone of the Mondeor Formation all blocks show a NNW-SSE trend, while during later UE7 Zone to early Elsburg Massives deposition, the northernmost block exhibits a predominant NW-SE trend which later reverted to NNW-SSE during …. deposition. Bed height - the largest dataset - presented the best modelled parameter, while maximum clast size, though empirical, presented the poorest results owing to the limited data available. Evidence for repetitive discontinuity in data for successive beds occurring across the wrench faults is interpreted as indicating syn-sedimentary strike-slip displacement on these structures. Each of the wrench faults presents a different syn-sedimentary history. The southern two wrench faults (90L Wrench and Fargo Wrench) appear to have been active throughout most of the deposition of the Upper Elsburgs, while the P1 Wrench started synsedimentary dislocation from early Elsburg Massives deposition onwards. The P2 Wrench appears to have been only sporadically active during deposition of the Elsburgs “C” Band times but became more consistently active throughout deposition of the Elsburg Massives. The rotating onlap stratigraphic pattern at SDGM is interpreted as a single progressive unconformity or syntectonic cumulative wedge system attached to a tilted depositional surface linked to an uplifting structure, resulting v in laterally extensive aprons of alluvial sediment on the upper surface of the orogenic wedge during periods of non-deformation and possibly relate to a wedge-top depozone close to the orogenic front. Evidence for broadly E-W compressive tectonic stresses prevailing during Central Rand Group and VCR times is seen in the Tandeka Thrust located in the footwall of the West Rand Fault to the west of SDGM. The rotating onlap geometry of the Upper Elsburgs on SDGM, requires syn-sedimentary uplift in the hinterland (towards the west) and localised subsidence to the east to accommodate the active sedimentation within an overall compressive tectonic environment. The SDGM wrench fault system is interpreted as a network of right-lateral R’ and X shears within the larger orogenic front fault system.