The stratigraphy and sedimentology of the Black Reef Quartzite Formation, Transvaal Sequence, in the area of Carletonville and West Rand Goldfields

Abstract

In the study area the Black Reef Quartzite Formation lies unconformably at the base of the early Proterozoic Transvaal Sequence of South Africa. The Formation comprises a succession of interbedded siliceous quartzites and shales with erratically-developed basal grits and conglomerates in the area of the Carletonville and West Rand goldfields. Outcrops are restricted to the topographic ridge formed by the Rand Anticline. The pre-Transvaal palaeosurface comprises a variety of rock-types. This heterolithic palaeosurface is a result of extensional tectonism which caused the development of horst, graben and half-gra ben structures. A thickness study revealed that the thickness of the Formation is very inconsistent and that zones of maximum thickness generally correlate with linear depressions in the palaeosurface. This study also showed that during Black Reef times the palaeorelief of the study area was low. The typical lithofacies association of the Formation, as determined by Markov analysis, comprises a basal pebbly quartzite unit overlain by a succession of alternating mudstone and quartzite units. In the study area the Formation mostly has a uni-, bi- and polymodal palaeocurrent distribution. On the Rand Anticline the modes of the unimodal distribution are towards the north, but in the remainder of the study area the modes lie within the second and third azimuth quadrants. For the bi- and polymodal distributions there are no general dominant modes. The orientations of oscillation ripple crests show a general, well-defined east-west trend for most of the study area. 'The pebble size and composition of the conglomerates vary significantly throughout the study area. A petrographic study revealed that the quartzites in the Black Reef succession are generally texturally very mature. It is considered that the basal strata are characteristically bedload deposits, possibly the detritus of a braided stream system. The strata overlying these are characteristic of a tidal flat deposit, signifying a transgression. The sedimentary makeup of the coastal system, in which the upper strata of the Black Reef Quartzite Formation was deposited, probably consisted of estuarine, lagoon bay, tidal inlet, tidal flat and marsh environments. The transgression resulted in a rise in base level causing a decrease in sediment flux, bringing about clear-water conditions for the precipitation of the overlying carbonates of the Malmani Subgroup. The cause of the transgression was probably post-graben thermal subsidence in a three-stage tectonic model. The proposed thermal subsidence basin led to the opening of a linear, shallow epeiric sea that transgressed over the partly-pediplaned palaeosurface.