CHARACTERISTICS OF EARLY TRIASSIC FLUVIAL SYSTEMS IN THE KAROO BASIN - CARLTON HEIGHTS

The Katberg Formation, Upper Beaufort Group, of the Karoo Basin, South Africa, represents bedload dominated fluvial deposition in Gondwana during the Early Triassic. Vertebrate fossils in the Katberg Fm. are used to interpret terrestrial ecosystem recovery following the P/Tr extinction event in the lower Palingkloof Mbr., the uppermost unit of the Balfour Formation which underlies the Katberg. Near the town of Senekal, in the northern, distal exposures of the Beaufort, there is a stratigraphic hiataus where the entire Palingkloof Member and lower Katberg Formation are absent. Elsewhere, the Katberg is believed to record continuous sedimentation across the Palingkloof-Katberg contact. The transition into and the basal deposits of the Katberg Fm. were studied in Carlton Heights to document these Early Triassic fluvial systems.

The basal Katberg is relatively well exposed over a large vertical and lateral section at Carlton Heights. The best exposure occurs in an N9 roadcut where the detailed sedimentology were recorded. These results then were extrapolated to a vertical section of ~100 m across an area of ~ 0.5 km. Two different lithofacies associations were recognized within the Katberg Formation of the study area.

Fluvial system 1 is a sequence of medium bedded, planar or ripple laminated, very fine sandstone interbedded with planar laminated siltstone. The erosional bases of this sequence are flat with incisions into underlying bioturbated siltstone reaching depths of < 0.5 m. System 2 consists of thick bedded, cross laminated, fine to very fine, poorly sorted sandstone. Erosional basal contacts are incised to depths > 0.5 m, but may be up to 2.5 m. Sandstone geometries show barforms that are overlain and underlain by mud-pebble conglomerate. Barforms fine upwards into cross- and ripple-laminated, very fine sandstone. Vertically separated sandstones of this sequence amalgamate laterally, resulting in erosion of much or all of preceding system 1 deposits in some units. Pedogenic carbonate-nodule conglomerate, previously used to define the Katberg base, occur intermittently in both systems. Their presence at the Katberg-Palingkloof contact indicates a stratigraphic hiatus in this and other areas.