Palaeo-environmental and provenance reconstruction of the Lower Permian mudstones (lower Ecca Group) in the main Karoo Basin, South Africa

Abstract

Assessing the composition and origin of the organic matter-rich mudstones, which dominate the early basin fill of the main Karoo Basin of South Africa, are essential for reconstructing the palaeo-environment, palaeo-climate, and potential provenance areas in southern Gondwana during the Early Permian. This study investigates the sedimentary geochemistry and petrography (e.g., major oxides, trace elements, mineral compositions, sedimentary structures, textures) of the lower Ecca Group, which was sampled in newly drilled boreholes. Our results show that the rate of weathering decreased over time as the environment changed from an ice-house to a more hot-house setting (i.e., the Chemical Index Alteration is 72–85 in the Prince Albert Formation and 60–76 in the Collingham Formation). Comparisons of the V/Cr, V(V + Ni), and Ni/Co ratios and Fe–S-TOC ternary plot show oscillations between anoxic and oxic conditions during deposition. Furthermore, there is evidence, albeit somewhat equivocal, for changing palaeo-salinity levels, low sedimentation rates, and high bio-productivity. Trace-element Principal Component Analysis, the Index of Compositional Variability, and Discriminant Function Analysis of the older mudstones (Prince Albert and Whitehill formations) imply different provenance regions with felsic to intermediate signatures predominantly found in the western MKB and mafic signatures in the eastern MKB. The youngest unit, the Collingham Formation, has a more uniform composition across the region, and was mostly sourced from the quartz-arenites and granites most likely associated with the Cape Fold Belt.