Palaeoenvironmental and palaeoclimatic reconstruction of the Witbank coal deposits (Karoo Basin South Africa)

Abstract

Coal deposits serve as important archives of palaeoclimate change. Palynomorph assemblages reflect the composition of the parent plant communities which are in turn controlled by the local environment and regional climate. Palynofacies can also be used to further interpret the depositional environment and transport mechanisms. In Main Karoo Basin, the Permian Ecca Group hosts economically important coal deposits within the Witbank and Highveld Coalfields. The most significant deposits are located in the Artinskian/Kungurian-aged No. 2 Coal Seam. The post-glacial fluvio-deltaic deposits represent an important shift in climate in the early Permian. Palynological samples from boreholes of the Witbank Coalfield were analyzed to reconstruct the palaeoenvironment and palaeoclimate signature recorded in palynomorph assemblages and palynofacies patterns of the No. 2 Coal Seam. For comparison additional samples from boreholes of the adjacent Highveld Coalfield were analyzed with respect to distinguish between local and regional signatures. The No. 2 Coal Seam is divided into an Upper Coal Seam and a Lower Coal Seam by a sandstone/siltstone marker bed but this does not extend laterally across the full extent of both basins. The palaeoenvironment is generally swamp-dominated in the Lower Coal Seam but fluctuates between swamp-dominated and lake/pond-dominated in the Upper Coal Seam. There are differences on a local scale which are controlled by the climate and topography. The palynofacies and intraseam partings in the Witbank Coalfield indicate a highly proximal depositional environment and a more distal depositional environment in the Highveld Coalfield. The vegetation in the Lower Coal Seam is dominated by conifers in the upland and ferns in the lowland with Cordaites occupying the higher valleys. In the Upper Coal Seam the Carboniferous/Permian vegetation is replaced by a more diverse glossopteris-gangamopteris-dominated flora. The climate signal as indicated by the pollen-producing plants is a shift from a stable cold climate in the Lower Coal Seam to a fluctuating cool-temperate climate in the Upper Coal Seam. This change in the climate is caused by the movement of Gondwana northwards and away from the South Pole during the Permian.