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Early Triassic Aggradational and Degradational Landscapes of the Karoo Basin and Evidence for Climate Oscillation Following the P–Tr Event

¹ Department of Geology, Colby College, Waterville, Maine 04901, U.S.A.; present address: Mackay School of Earth Sciences and Engineering, Mail Stop 168, Reno, Nevada 89557-0047, U.S.A.
² Department of Geology, Colby College, Waterville, Maine 04901, U.S.A.; ragastal{at}colby.edu
³ Council for Geosciences, Private Bag x112, Silverton, Pretoria, 0001 South Africa

The Lower Triassic Katberg Formation (Beaufort Group, Karoo Supergroup) in South Africa is a fine-grained, arenaceous unit deposited under ephemeral bedload-dominated fluvial conditions and previously interpreted as representing a continuous stratigraphic record. Its genesis was linked directly to the end-Paleozoic extinction event and was considered to exhibit the results of a trend towards aridification. The present study documents the lowermost Katberg Formation exposed at Carlton Heights, Northern Cape Province, to test the aridification hypothesis.

Four lithofacies vary in both vertical and lateral relationships, with feldspathic sandstone dominating the stratigraphy. Pisolith-size nodular conglomerate consists of pedogenically derived carbonate nodules originally precipitated within aridisols, none of which are either exposed or reported from the formation. Rather, the only evidence of extreme aridification is found within channel lag, barform, and overbank deposits marking landscape degradation. Landscape aggradation is characterized by two feldspathic sandstone geometries where (1) medium-bedded, planar bedded, or ripple-laminated sheet sandstone is interpreted as ephemeral sheetflood deposits, and (2) thick-bedded, cross-laminated multi-lateral, lenticular sandstone is interpreted as associated with deeper, sand-bed braided (anabranching) systems. Overbank fines consist of polyphase siltstone paleosols in which bioturbated inceptisols were overprinted by gleysols wherein large pedogenic carbonate nodules precipitated under a high regional water table. Evidence exists for periodic drying in these wet regimes, but no evidence exists for extreme aridity. Hence, the lowermost Katberg Formation superficially exhibits only an aggradational sedimentological record, but significant diastems exist as evidenced by the concentration of aridisol precipitates at the base of each degradational cycle. A model for the Early Triassic in this part of the Karoo Basin is presented wherein this record is interpreted as a function of climate oscillations rather than either episodic uplift in the Cape Fold Belt or any ecosystem response to the end-Permian extinction.