New evidence for diachroneity of the Ecca-Beaufort contact (Karoo Supergroup, South Africa)

The rocks of the Karoo Supergroup were deposited in a retro-arc foreland ba sin over an extended period from the late Carboniferous to the Middle Juras sic.(1,2) They are internationally renowned for their wealth of fossils, pa rticularly therapsids, which have enabled biostratigraphic subdivision of t he rocks of the Beaufort Group(3-5) and Elliot and Clarens Formations.(6) A lthough a great deal of lithological amd palaeontological research has been done in the basin, the exact position of the Ecca-Beaufort contact remains a subject of debate. Various authors(3,7,8) have hinted at the diachroneit y of this contact in the main Karoo Basin, but these ideas have lacked dire ct evidence. This paper addresses this issue and reports on the presence of rocks of the Pristerognathus Assemblage(9) (previously uppermost Tapinocep halus Zone(4,10)) immediately overlying the Ecca Group in the southern Free State. This discovery greatly extends the geographic occurrence of the Zon e and has important implications for basin development models.