BOUMA, ARNOLD H., Louisiana State University, Baton Rouge, LA; and H. DeVILLE WICKENS, Soekor, Parow, South Africa

ABSTRACT: Fluvially Dominated Deltaic and Submarine Fan Deposits in Two Adjacent Karoo Subbasins, South Africa.

The Permian Ecca Group in the SW Karoo Basin, South Africa, comprises fluvially dominated, shale-rich deltaic and submarine fan deposits that reflect changes in the original basin configuration. Early evolution of the Cape Fold Belt bordering the Ecca Basin to the south and west, and the development of the syntaxis anticlinoria in the SW bend of the foldbelt, resulted in the formation of the "shallow" Tanqua subbasin to the north and the deeper Laingsburg subbasin to the east of the syntaxis. The Tanqua subbasin may have been less than 500 m deep. Its fill of shales and fine-grained arenaceous turbidites is developed as six submarine fans and overlying deltaic deposits in a setting that mimics a passive margin. The deposits were uplifted and are superbly exposed in a nearly horizontal position that allow lateral continuity studies. The Laingsburg subbasin reflects foredeep conditions, and its submarine fan complex displays depositional characteristics different from those of the Tanqua area. Deep-seated crustal deformation and subsequent uplifting resulted in tilted layers being exposed in eroded folds that can be followed over tens of km along strike, and can best be studied in river-cut outcrops. Significant difference can be seen in the depositional cyclicity of both fan complexes. The syntaxis caused depositional separation and hinders correlation. The overlying deltaic deposits are lithologically similar, but they have an overlapping relationship in the syntaxis area. This points to differences in rate of progradation and deposition, which in turn reflects the te tonic control in the development of these foreland subbasins. These areas provide ideal information to construct models for regions under development.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.