Analysis of late Palaeozoic glacial to postglacial sedimentary successions in South Africa by geochemical proxies – Response to climate evolution and sedimentary environment

Abstract

Records of incisive climate changes during the late Palaeozoic Era are best documented in sedimentary basins of South Africa. Glacial (Upper Carboniferous–Early Permian Dwyka Group and equivalents) and postglacial deposits (Early to Middle Permian Ecca and Beaufort Groups) are known from the Main Karoo Basin in South Africa, from the Kalahari Basin (Namibia and Botswana) and other late Palaeozoic Basins of south Gondwana. Glacial deposits comprise diamictites and tillites followed by postglacial shales with variable organic matter contents and intercalated carbonate horizons. This study covers the final glaciation phase (Upper Pennsylvanian–Asselian/Sakmarian) and the transition into the postglacial stage of the Middle to Late Permian. For sedimentary successions from the Karoo and eastern Kalahari Basin element geochemical proxies (Chemical Index of Alteration, Zr/Ti, Rb/K, V/Cr and organic carbon content) supported by mineralogical data record pronounced variation in sedimentary conditions. These took place in the course of glacier retreat at the Dwyka/Ecca boundary where climate and weathering processes are known to have changed. The proxy signals indicate an increase in sea level and salinity, and a shift in redox conditions from oxygenated to temporary anoxic/euxinic conditions. Due to the synchronicity of these events, distant sampling localities in the Karoo Basin can be correlated. Despite facies differences between the Eastern Kalahari and Karoo Basin, the geochemical proxies point to a comparable climatic evolution during the postglacial phase. For the Early Permian, proxy signals indicate warm–humid conditions in the Karoo and Kalahari Basin. During the Middle Permian increased aridity hampered chemical weathering processes as indicated by decreasing CIA values. Results show that: (i) proxies reflect provenance, synsedimentary redox regime and late diagenetic alteration, (ii) by combination of different independent geochemical proxies primary conditions of sedimentation and climate can be deduced, (iii) the geochemical proxies applied are independent of local environments and (iv) the data reveal supra-regional or even hemispherical changes in climate and sedimentary conditions. Thus geochemical proxies can be used as reliable markers for basin wide correlations. Knowledge about synchronicity on basin scale contributes to an improved understanding of SW Gondwana's interaction with and influence on global climatic evolution.