Paleoenvironmental trends of Karoo sediments of the Laingsburg area (SW South Africa): Interpretation of geochemical data

ADELMANN, D. and ALBES, D.

The investigated pile of almost exclusively siliciclastic sediments forms the lower part of the Karoo Supergroup that comprises the Dwyka (Permo-Carboniferous), Ecca (Permian) and Lower Beaufort Groups (Late Permian). These sediments were deposited in a retro-arc foreland basin, termed Karoo Basin (Johnson 1991). The geochemistry of approximately 250 pelite samples of 13 vertical sections of the Laingsburg area has been analyzed. Data interpretation has been accomplished by different element ratios and by various methods of multivariate statistics (cluster and correspondence analysis).A rapid change of the redox environment is significant for the Dwyka/Ecca boundary. The glacial Dwyka sediments were presumably deposited under oxic conditions indicated by low V/Cr and Ni/Co ratios as well as the insignificance of a sulfidic-anaerobic element paragenesis. The pelites of the lower part of the Ecca Group (Prince Albert Shales and Whitehill Formations) show opposite features. An affinity with the sulfidic-anaerobic element paragenesis nickel, zinc, cobalt, copper and iron, high V/Cr and Ni/Co ratios and suitable sedimentary structures indicate a sedimentation in an anoxic, possibly euxinic environment. Slow suspension settling under reducing conditions presumably in a deep water environment during Whitehill sedimentation is confirmed by features like thin-lamination and missing bioturbation. Anoxic deposition was terminated during the following Upper Ecca sedimentation. Geochemistry data indicate a progressive return to normal oxygenated bottom conditions during sedimentation. A main uplift phase of the adjacent Cape Fold Belt at Lower/Upper Ecca Group boundary triggered submarine fan and deltaic sedimentation terminating water stratification within the basin. A sulfidic-anaerobic element paragenesis can not be established for the uppermost part of the Ecca Group (deltaic) and the lower part of Beaufort Group (fluvial). A terrigenous/phyllosilicatic and a phosphate element paragenesis prevail.Additionally, the chemical index of alteration (after Nesbitt & Young 1982) was used to quantify the degree of weathering of fine-grained material in the Dwyka, Ecca and Lower Beaufort Groups. The chemical index of alteration (CIA) for the glacigene Dwyka Group shows a restricted and low range of CIA values while CIA values of the post glacial Lower Ecca black mudrocks (Prince Albert Shales Formations) are significantly higher. The change from relatively unweathered material to weathered appears to be triggered by a climatic change from cold to relatively warm and wet. Changing values of CIA in the transition from the Prince Albert Shales (CIA ~ 83) to the Whitehill Formations (CIA ~ 73) can be explained by changing climatic conditions and/or provenances. Sedimentological features plead for an important influence of changing source areas. Corresponding to models by other authors (e.g. Visser 1992) a significant modification of the basin configuration during the late and post-glacial phases is presumed. While the detritus of the Prince Albert Shales Formation presumably was delivered from western and eastern directions, the influx of a highland in the north and east dominated during Whitehill sedimentation (Visser 1992). Different types of source rock and not different climatic conditions in both areas may caused the varying degree of weathering. No significant change of CIA value is observed for the transition to the following Upper Ecca and Lower Beaufort Groups originated from the south. They show average CIA values between 70 and 75 indicating a moderate weathering.

References
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Visser, N. J. N. 1992. Deposition of the Early to Late Permian Whitehill Formation during a sea-level highstand in a juvenile foreland basin. South African Journal Geology 95 (5/6), 109-115.