Date of Award

1997

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Arnold H. Bouma

Abstract

The Tanqua and Laingsburg subbasins developed in the southwest corner of the Karoo Basin, South Africa, during the late Permian. The subbasins had near-contemporaneous formation and filling, and each contain a number of submarine fans. Five submarine fan systems in the broad, shallow Tanqua have a lateral continuity of up to 34km. With a sandstone/shale ratio of 75-90%, the arenaceous fans vary in thickness from 20 to 60m. Intervening shales range from 20 to 75m. Four fan systems in the more typical foredeep style Laingsburg subbasin are thicker and a few hundred kilometers long. The history of both subbasins was influenced by events and associated tectonic structures of the Cape Fold Belt. Differential compression on the Western and Southern branches of the Cape Fold Belt led to a basin floor high that separated and directed sediment transport to the subbasins. While the subbasin development is associated with the subduction zone in southwest Gondwana, the fill of the subbasins indicates deposition during tectonic quiescence. Petrographic and microprobe analyses suggest a high-grade metamorphic source area for the sandstones, composed predominately of metapelites and metapsammites, that was located somewhere to the west/southwest. The associated fold-thrust belt (Cape Fold Belt) did not have any expression above the ocean surface and was not the source of the submarine fan sediments. The subbasins formed and filled during the earliest tectonic events of the Cape Fold Belt. The 278 Ma event brought about uplift of the orogenic belt, an increase in volcanic activity in a magmatic arc associated with the subduction zone off the southern edge of Gondwana, initial subsidence of the Tanqua and Laingsburg subbasins, and the emergence of a basin floor high that separated the subbasins. Subsequent erosion and transport of sediment from the uplifted orogenic belt fed the building submarine fan systems in the Tanqua and Laingsburg subbasins. The 258 Ma tectonic event lifted the Cape Fold Belt sufficiently to halt transport of sediment to the subbasins. The event also caused further uplift of the orogenic belt and led to progradation from the same directions of delta systems that filled the subbasins.

ISBN

9780591591606

Pages

235

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