A litho- and biostratigraphic analysis of the lower Beaufort group in the distal sector of the main Karoo Basin, South Africa- implications for the depositional history of the distal foredeep to back-bulge basin

Abstract

Stratigraphic placement of the diachronous contact between the Ecca and Beaufort groups of the South African Karoo Supergroup has long been contentious. Usage of different criteria during mapping in different parts of the main Karoo Basin has resulted in mismatch of the placement of the Ecca-Beaufort contact between the geographically distant northern and southern areas. Correlation of rock units in the fluvio-lacustrine Beaufort Group has relied largely on biostratigraphy because of few basin-wide lithological markers and the richness of tetrapod fossils. Research on the northern sector of the main Karoo Basin has received much less attention effort than the south, especially with regards to tetrapod biostratigraphy. For this study, geological and palaeontological investigations were undertaken in rocks of the uppermost Ecca and lowermost Beaufort groups along the mapped Ecca-Beaufort contact in the Free State and KwaZulu-Natal provinces. Lithological work demonstrates that the same criteria used to define the Ecca Beaufort contact in the south can be applied to the north, and the presence of the Waterford Formation in the north of the Karoo Basin is recognised for the first time. Petrography shows that similar source regions were active during the deposition of the Waterford and Balfour formations and that these can be distinguished using the distribution of ages derived from detrital zircons. Palaeontology revealed the presence of the middle Permian (c. 262 Ma) Eosimops-Glanosuchus Subzone of the Tapinocephalus Assemblage Zone immediately above the Ecca-Beaufort contact in the southern Free State Province. These strata are immediately overlain by late Permian (c. 255 Ma) rocks of the upper Cistecephalus or lowermost Daptocephalus assemblage zones, which suggests a depositional hiatus, or erosional period, in the mid-to-late Permian lasting up to 6 million years in this part of the basin. Further north in the central and north-eastern Free State and KwaZulu-Natal provinces the late Permian Dicynodon-Theriognathus Subzone of the Daptocephalus AZ is present immediately above the Ecca-Beaufort contact. Litho- and bio-stratigraphic correlation in this study enabled refined correlation of strata between the proximal and distal sectors of the basin and supports the iii reciprocal foreland model for main Karoo Basin development during the mid- to late Permian