SCOTT, ERIK D., and ARNOLD H. BOUMA

Local subbasins developed in the southwest comer of the Karoo Basin, South Africa, during the late Permian. The Tanqua and Laingsburg subbasins had near-contemporaneous formation and filling, and contain a number submarine fan systems. The five individual submarine fans in the broad, shallow Tanqua subbasin range from 150 to 450 km² with lateral continuity of individual fans up to 34 km. With a sandstone/shale ratio of 75-90%, the arenaceous fans vary in thickness from 20 to 60 m. Intervening shales range from 20 to 75 m. The four submarine fans in the more typical foredeep style Laingsburg subbasin are thicker and a few hundred km long.

The history of both subbasins was heavily influenced by the events and associated tectonic structures of the Cape Fold Belt. Differential compression on the two branches of the Cape Fold Belt led to a basin floor high that effectively separated the transport to the two subbasins. Paleocurrent data indicate that this divided transport started at the early development of the subbasins. While the subbasin development is associated with the subduction zone on the southwest side of Gondwana, their fill indicate deposition during a tectonic quiescence. Petrographic analysis suggest a single source area located outside the adjacent fold-thrust belt (Cape Fold Belt).

The separation of the subbasins makes it difficult to determine the timing of the deposition of the individual fan systems. No paleontological or palynological data could be obtained. From sedimentological and geochemical data, tentative timing of the deposition of the submarine fan systems can be postulated. Understanding the evolution of the subbasins and the conditions under which the submarine fans were deposited will lead to the determination of the influences on the formation of the fans and their resulting architecture. Those results will be applicable to the Gulf of Mexico and other passive margin basins.

AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana