Abstract

Two fine-grained turbidite basins of Permian age outcrop in the southwestern area of South Africa: the Tanqua Karoo and Laingsburg basins. The outcrops of the Tanqua Karoo basin cover approximately 650 km² (250 mi²) and include five different fan complexes. Fans 1 and 2 only show outer fans; the updip parts have been eroded away. Fan 3 is a nearly complete fan with toe-of-slope to distal-fan deposits. Only outer-fan deposits are present in Fan 4, whereas middle- to outer-fan deposits are present in Fan 5. This Karoo fan complex is unique in that it has no dip in the north–south direction (approximate updip to downdip direction) and only a 1–3° tilt toward the east. Excellent outcrops, and the lack of dip, make it possible to establish detailed correlations across the basin and to study the changes in deposition from the base-of-slope to where the turbidite deposits pinch out. Several types of individual and nested channel deposits, as well as sheet sands, are present and reveal overall gradational patterns of deposition. The five fans range in thickness from 22 to 66 m (72–217 ft) for the individual sand-rich fans and 22 to 76 m (72–249 ft) for the intervening silt-rich shales.

The Laingsburg basin was a typical foreland (elongate) basin. The basin’s present length is nearly 200 km (124 mi) and its width is 35–47 km (22–29 mi). Subsequent tectonic activity led to the deformation of the strata; the beds are approximately vertical. Parts of four turbidite fans (termed Fans A–D) are present in the Laingsburg Basin. Their thicknesses (sands and shales) range from 220–270 m (722–886 ft). Fans A and B are thicker and more sand-rich than C and D. Channel, sheet, and overbank/levee architectural elements are present. The deposits include the Vischkuil and Laingsburg Formations.

The sedimentary fill of both basins came from the paleo-Patagonia region approximately 200–500 km (124–310 mi) away. The basins and individual fans were not deposited simultaneously; the anticlinoria of the Cape Fold Belt operated like a tectonically operated toggle switch to direct sediment into the different basins. As the Laingsburg basin was filling with Vischkuil sandstones, the Tanqua Karoo basin was receiving silty shales. Following deep-water deposition, shoaling of the basins occurred with the deposition of prodelta shales and delta-front sands.