A palaeoenvironmental analysis of the Molteno formation in the Natal Drakensberg

Abstract

The Upper Triassic Molteno Formation of the Natal Drakensberg forms part of the upper Karoo Sequence. It rests with an erosional base on the Beaufort Group and has a gradational upper contact with the Elliot Formation. Four lithofacies are defined in the study area. Basal coarse-grained, trough cross-bedded sandstones, interpreted as in-channel dune and bedload deposits, are commonly overlain by fine- to medium-grained, planar cross-stratified sandstones. The latter probably reflect the migration of bar bedforms within braided fluvial systems. These sediments normally pass up into fine- to medium-grained sandstones with trough and planar cross-beds, laid down by sand flat processes which reworked emergent bars. The succeeding argillites of the fourth facies are postulated to be overbank or inter-channel floodplain sediments. The sequence is normally terminated by a second development of the bar facies. The vertical lithofacies arrangement probably represents deposition by low sinuosity channel systems, which derived detritus from east-north-eastern and south-south-eastern source areas. Although this palaeoenvironment is very similar to the depositional models of I.C. Rust (1962) and Turner (1970 to 1983), bar migration and sand flat processes appear to have been more important in the north-east of the basin.