TESTING FOR CARBONATE-RICH PALEOSOLS IN THE LOWER LYSTROSAURUS ASSEMBLAGE ZONE, KAROO BASIN, SOUTH AFRICA

The Karoo Basin encompasses central South Africa and it is reported to contain a record of the Permian-Triassic Extinction in the terrestrial realm. Workers have focused on localities in which boundary strata occur, encompassing the Balfour and Katberg formations. Rocks in the upper part of the Balfour Fm., assigned to span the Daptocephalus – Lystrosaurus Assemblage Zone (AZ) contact, are well studied but those of the Katberg Fm, in which the Lystrosaurus AZ occurs, are not. This project focused on poorly documented rocks of the lower Katberg Fm., containing lower to middle horizons of the Lystrosaurus AZ. The field location, Fairydale (Donald 207) farm, accounts for 47% of the vertebrates assigned to the lower Lystrosaurus AZ at the classic Bethulie boundary section. Rock exposures in four laterally correlative erosional gullies, representing ~100 m of section, were measured using standard field methods. Several siltstone intervals contain calcite-cemented nodules that may represent paleosols. This study tests whether these are potential paleosols and, if correct, what environmental conditions they indicate. Analysis of these intervals may provide insights on the current environmental models advanced for the Lystrosaurus AZ.

The basic lithologies identified are: greenish gray, reddish-gray, or mottled siltstone; greenish gray to olive gray wacke; pedogenic nodule conglomerate; and dolerite. Siltstones are stacked in fining upward cycles of coarse greenish-gray to fine reddish-gray, in which calcite-cemented nodules, as well as mudcracks, were observed. Two of these siltstone intervals were trenched and sampled for geochemical analysis: one interval with mudcracks (312-327 cm), the other without (120-150 cm). These are compared geochemically for evidence of paleosol development.

Geochemical data shows no trend in TOC associated with soil development in either interval. However, there is a positive enrichment in Na as well as enhanced mineral maturity down each profile. Other geochemical plots, including salinization, clayeyness, calcification, and TON show no trends. The Chemical Index of Alteration minus potassium (CIA-K) ranges from [70-80] in the upper and [70-85] in the lower interval. These preliminary data suggest an interpretation that neither profile represents soil development.