AN ANALYSIS OF GEOMETRIC AND SEDIMENTOLOGIC CHARACTERISTICS OF A MIDDLE PERMIAN FLUVIAL SYSTEM, KAROO BASIN, SOUTH AFRICA

The Middle Permian Abramskraal Formation is the basal stratigraphic unit in the Beaufort Group and represents fully terrestrial deposition in the Karoo Basin, South Africa. Previous studies on these fluvial systems interpreted a sedimentary regime that consisted of either low sinuosity or high sinuosity river systems, with floodplain deposition occurring under a seasonal to arid climate. This present study focuses on an exposure of fluvial sediments along the R354 (S32 31.802’, E020 37.854’), South of Sutherland, Northern Cape Province, to characterize the unit’s stratigraphy, channel-body geometry, and fluvial architecture.

The outcrop houses a channel complex that consists of three lithofacies: a dark greenish-gray coarse siltstone; a light olive-gray, very fine-grained sandstone characterized by trough cross-bedding; and a conglomerate channel lag with pedogenic nodules. The sandstone bodies are arranged in barforms that attain thicknesses up to 2.1 m, with several barforms stacked within a single channel. Barforms exhibit trough and low-angle cross-bedding, and ripples are common. Individual channels are up to 7.5 m thick and the system can be characterized as both multi-lateral and multi-storied.

The fluvial character of this part of the Abramskraal Formation differs from river systems at the contact between the Waterford and Beaufort formations, reflecting a different fluvial style following full deglaciation of Gondwana. The river complex in the present study is interpreted as a high sinuosity, meandering system. But unlike previous interpretations, the present study finds no evidence to support an interpretation that sedimentation occurred on an arid floodplain. Rather, evidence exists to interpret this river as having formed in a seasonally wet paleoenvironment.