A brief lithostratigraphic review of the Abrahamskraal and Koonap formations of the Beaufort Group, South Africa: Towards a basin-wide stratigraphic scheme for the Middle Permian Karoo

Abstract

The basal strata of the Beaufort Group of the South African Karoo Basin, comprising the western Abrahamskraal and eastern Koonap formations, contain the most time extensive record of Middle Permian fossil tetrapods and hold the key to understanding Middle Permian biodiversity change in the continental realm. To determine faunal stratigraphic ranges a reliable lithostratigraphic framework for Middle Permian Beaufort deposits is essential. Until now this has proved difficult to achieve, largely due to the homogeneity of the fluvial succession coupled with structural complexity as a result of Cape Fold Belt orogenesis. Accordingly, the Abrahamskraal Formation has been only locally subdivided on the basis of sandstone packages but regional stratigraphic subdivision has not yet achieved satisfactorily.Collation of stratigraphic sections from around the Karoo Basin for this study demonstrates the presence of four sandstone packages are present within the Abrahamskraal Formation in the south-western corner of the basin. These sandstone packages are given member status, based upon the nomenclature of Le Roux (1985) with the addition of the newly recognised Grootfontein Member. The Combrinkskraal and Grootfontein Members occur in the lower half of the Abrahamskraal Formation and are laterally persistent along the southern margin of the basin. The Koornplaats Member is more restricted to the south west corner of the basin, where it quite thick, suggesting the narrowing of the highly channelized area. The overlying Moordenaars Member, more extensive towards the north than underlying packages, indicates subsequent northwesterly expansion of the locus of active channelization with time. Although thin sandstone packages in the more easterly positioned and stratigraphically equivalent Koonap Formation, this does not facilitate lithostratigraphic subdivision of this part of the stratigraphic succession and may indicate a different form of fluvial architecture. Our study provides a regional lithostratigraphic scheme for the Abrahamskraal Formation that will facilitate its biostratigraphic subdivision and the investigation of Middle Permian Gondwanan basin evolution.