Palaeoenvironments of the Estcourt formation (Beaufort Group), KwaZulu-Natal.

Abstract

At present the Karoo Basin covers approximately 20 000 km2. It is a large intracratonic basin which, from Carboniferous to Jurassic times, was infilled with a succession of sediments ranging from glacial deposits to those deposited in warm, equable conditions. The Beaufort Group forms part of this succession, and was deposited in a terrestrial, river dominated environment. The dominant lithologies exposed in the Estcourt region in the KwaZulu-Natal Midlands belong to the lower and middle Beaufort divided by the PermoTriassic boundary. The Permo-Triassic palaeoenvironment in this region is reconstructed using sedimentary profiles combined with the study of the fossil remains discovered in the area, including plant, body, and trace fossils. The lower Beaufort sediments in this region belong to the Estcourt Formation, and the Middle Beaufort sediments to the Belmont Formation. The Estcourt Formation is dominated by a succession of alternating sandstones, siltstones and mudstones, which are interpreted as representing sediments deposited in a fluvial-floodplain environment, which can be divided into two sub-environments. The first is dominated by sediments that were deposited by meandering rivers on a semi-arid floodplain, and the second sub-environment is represented by those sediments deposited in lacustrine environments. Both of these subenvironments are closely linked and alternate in the rock record indicating many episodes of transgressive-regressive lacustrine episodes. The Estcourt Formation can be closely correlated with the lower Beaufort sediments mapped in other regions of the Karoo Basin, indicating similar climatic and environmental controls throughout the Karoo Basin of southern Africa. The Estcourt Formation also contains a wide variety of body and trace fossils. The PermoTriassic boundary can be traced along the western border of Estcourt by using the distribution pattern of the two mammal-like reptiles Dicynodon and Lystrosaurus. There is evidence of an overlap in the distribution between these to mammal-like reptiles, which together with palaeoflora evidence, indicates that Lystrosaurus evolved during the Late Permian and not Early Triassic as previously thought. The first Triassic sediments are represented in the Estcourt region by a series of maroon shales which can be correlated with the Palingkloof Member.