Sedimentology of the Onverwacht Group (3.4 Billion Years), Transvaal, South Africa, and Its Bearing on the Characteristics and Evolution of the Early Earth

Abstract

Strata of the upper part of the Onverwacht Group (3.4 b.y.), Transvaal, South Africa, are composed mainly of chert representing silicified quartz-poor volcaniclastic detritus. Layers of carbonate, silicified carbonate, volcaniclastic debris, and possible primary silica make up minor parts of most sections. Deposition took place largely in shallow water, but no evidence was found to indicate whether the water was marine, fresh, or of some other composition. Turbidites are locally well developed but there is no compelling evidence for deposition in deep water except in northernmost areas. Some units were lithified, and possibly silicified, at the sediment-water interface whereas others remained soft long after burial. The Onverwacht Group accumulated in an anorogenic basin unaccompanied by major tectonic uplift or subsidence. The lower volcanic part of the sequence was deposited subaqueously and apparently contains little interstratified sediment because subaerial exposures were absent. Subsequent pyroclastic volcanism associated with the construction of subaerial vents provided abundant volcaniclastic detritus for the initiation of significant sediment accumulation. The uppermost unit, the Zwartkoppie Formation, reflects early tectonic instability including local uplift, shallow erosion, and the deposition of eroded intra-formational debris in adjacent shallow-water basins. The suprajacent Fig Tree Group represents an orogenic phase of sedimentation in which major tectonic uplift and exposure of deep-seated plutonic rocks to the south was coupled with subsidence and rapid flysch-like sedimentation in the north. The paucity of shale in the Onverwacht Group probably reflects the relatively limited extent of subaerial exposures and the continued eruption of fresh volcanic material. There is no suggestion of a cold climate, but some evidence that it may have been warm.