ASSESSING PERMO-TRIASSIC TERRESTRIAL PALEOCLIMATE TRENDS OF CALCIC PALEOVERTISOLS WHEN STRATIGRAPHICALLY ABSENT IN THE KAROO BASIN, SOUTH AFRICA

The widely accepted model for the terrestrial response to the end-Permian crisis is a climate shift from seasonally wet to seasonally dry/arid conditions, associated with a turnover from the Daptocephalus (formerly Dicynodon) to Lystrosaurus Assemblage Zones (AZ) in the Balfour Fm, Karoo Supergroup. Elements of the Daptocephalus AZ range into the Palingkloof Mbr, where the vertebrate turnover is reported, and overlain by the Katberg Fm, a more arenaceous interval in which Lystrosaurus dominates. The presence of calcite-cemented pedogenic nodules in lag deposits of fluvial systems in the Katberg Fm is used by other workers as evidence of post-turnover aridification associated with the Lystrosaurus AZ. Their origin is attributed to the formation in calcic paleoVertisols which formed under a seasonally dry climate. Recently, we have demonstrated that pedogenic nodular conglomerates (PNC) are not restricted to sandstone bodies in the Lystrosaurus AZ, but occur in both the upper Daptocephalus AZ, extending to more than 100 m below the turnover level.

Calcite-cemented nodules form in Vertisols over a wide range of climate conditions. When nodular hozions are recovered from in situ paleoVertisols, stable isotope analyses provide a means to evaluate the prevailing climate at the time of their formation. Yet, very few intact paleoVertisol profiles have be identified in the upper Balfour Formation. In contrast, remnants of these paleosols, a product of erosion and landscape degradation in response to changes in fluvial gradient, are encountered more frequently as intraformational conglomerates. Stable-isotope analyses of calcite-cemented nodules in PNCs provide insight into changing climatic trends when no other physical record has been preserved.

δ¹³C and δ¹⁸O values from fifteen intraformational PNC conglomerates, spanning a stratigraphic distance of ~220 m, have been evaluated at Old Lootsberg Pass, Eastern Cape Province. δ¹³C values range from -1.7 ‰ to -13.9‰, and δ¹⁸O values range from -12.7‰ to -22.8‰, with no unidirectional trend in either data set. Rather, both show fluxes in positive and negative excursions, with a trend towards seasonal wet conditions near the assemblage-zone boundary. Hence, these data do not support a trend of increasing ardification as a factor that may be responsible for vertebrate turnover.