Reconstruction of sedimentary environment and climate conditions by multi-geochemical investigations of Late Palaeozoic glacial to postglacial sedimentary sequences from SW-Gondwana

Abstract

The most extensive Phanerozoic glaciation and its termination occurred during the Carboniferous-Permian on the southern hemispherical Gondwana supercontinent. This thesis aims to provide detailed geochemical analyses of the lower Karoo Supergroup sediments in southwestern Gondwana with focus on the Karoo Basin in South Africa. Additional samples derive from the Paraná Basin (Brazil), Kalahari Basin (Botswana), and Warmbad Basin (Namibia). <br /> Multiple geochemical investigations (element-geochemical, organic-geochemical and isotope data) were supported by mineralogical data. By this multi proxy approach, information about sedimentary environment, climate conditions, and postsedimentary processes can be obtained. <br /> orbital frequencyElement geochemical proxies (CIA, Zr/Ti, Rb/K, V/Cr) record three confined shifts in climate and paleoenvironment of the Karoo Basin during deposition of the glacial Dwyka Group. These shifts were induced by changes in sea level, weathering rate, provenance, and redox conditions. Organic geochemical investigations (C_org, C/N, lipid biomarkers) indicate the organic matter in the central Karoo Basin was primarily of algal origin. In agreement with element proxies, the varying δ¹³C_org, values mirror shifts in <i>p</i>CO₂, rather than variations of organic-matter type. A covariation trend between carbon isotope signatures of equatorial carbonates and δ¹³C_org, values from the Karoo Basin argues against local forcing factors and instead implies a global climate-control mechanism. The 5-7 m.y. duration of a complete glacial cycle is not in tune with any known orbital frequency. <br /> The onset of the postglacial phase (Ecca Group) is characterized by significant changes in sedimentary environments. Predominantly fully marine conditions prevailed and the formation of anoxic conditions is indicated by the occurrence of C_org, rich sediments containing mineral phases such as apatite and pyrite. Comparable proxy signals are obtained from localities within the Karoo Basin and from the Warmbad Basin in Namibia. Especially the accumulation of phosphatic sediments points to high bioproductivity due to elevated nutrient supply. At the basin borders (Witbank Basin), in terrestrially dominated sedimentary environments, the onset of postglacial climate conditions was recorded by significant changes in the organic matter composition. <br /> The Dwyka/Ecca boundary coincides with a progressional temperature rise. Favourable climate conditions led to the spread of the Glossopteris flora. High chemical weathering rates, assumedly fast growing vegetation and the documented reptile fauna (<i>Mesosaurus</i>), indicate warm-humid climate conditions during the Early to Middle Permian in south Gondwana. During the Middle to Late Permian the continuous retreat of former marine sedimentary environments commences. Turbiditic deposits were replaced by deltaic basin fills, which were in tune covered by fluvial and lacustrine sediments. The retreat of marine conditions in the Karoo Basin was closely related to the rising Cape Fold Belt, whereby the connection to the Panthalassic Ocean was finally terminated. Arid climate conditions during the Late Permian and Lower Triassic are indicated by redbeds and aeolian deposits of the upper Beaufort and Stormberg/Lebung Groups.