Abstract

Various different models have been proposed for the style of fill of the Karoo Basin in South Africa. One of these proposes that the Karoo Basin behaved as a partitioned entity, with reciprocal fills on either side of a hinge line separating proximal and distal facies. Detailed sedimentological, palaeontological, stratigraphic and geophysical data for the Beaufort Group in the central Free State lends credence to this model. In the study area fossils assignable to the Dicynodon, Lystrosaurus and Cynognathus assemblage zones occur in the Balfour, Katberg and Burgersdorp formations respectively. This data shows that certain parts of the Beaufort Group stratigraphic succession that are present in the southern part of the Karoo Basin, are absent in the central Free State, indicating either a depositional hiatus or a period of erosion at the beginning and the end of Lystrosaurus Assemblage Zone, as well as the end of Cynognathus Assemblage Zone times. During these depositional hiatuses or periods of erosion, deposition or preservation of the rock record was restricted to the proximal sector (south of the hinge line). Study of the geophysical data has also allowed for the definition of a basement high north-east of Thaba Nchu, which probably accounts for the provenance of the informally named Musgrave unit (upper Balfour Formation), which also has an anomalous south-westerly palaeocurrent direction.

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