The Cape Trough: An Aborted Rift

Abstract

The thermotectonic events at the end of the Precambrian that brought Malmesbury geosynclinal sedimentation to a close initiated a new style of sedimentation characteristic of the 8000-m-thick Lower Paleozoic Cape Supergroup. Deformation and low-grade metamorphism of Malmesbury formations were accompanied by emplacement of granite plutons between 600 and 500 Ma. Posttectonic rise of the Pan African orogens favored accumulation of molasse-type deposits under shallow-water and subaerial conditions. Both the Franschhoek Formation (Malmesbury Group) and the post-Malmesbury Klipheuwel Formation of the southern Cape are complex intertonguing mudstones, arkosic sandstones, and conglomerates of continental origin; the Franschhoek Formation is distinguished by more intense shearing and folding (Haughton, 1969, p. 304). Germs (1972a) equates the Klipheuwel Formation with the terminal Nama cycle. By Cambrian time the paleogeography of the southwestern Cape had evolved to a braided alluvial coastal plain bordering a shallow silici-clastic shelf (Vos and Tankard, 1981). Marine limestones accumulated in the most distal parts of the basin now represented by the Ellsworth and Transantarctic regions of Antarctica (Stump, 1976; Shergold et al., 1976).