SANDSTONES, SILTSTONES, AND TRACE FOSSILS IN A SUCCESSION OF LOWERMOST TRIASSIC ROCKS NEAR LOOTSBERG PASS, EASTERN CAPE PROVINCE, SOUTH AFRICA

The Karoo Basin, South Africa, is reported to contain a continuous terrestrial record in which the Permian-Triassic Boundary (PTB) extinction event is recorded. This extinction model suggests that the basin grew increasingly warm and arid leading up to the extinction and ties these climate changes to ecosystem collapse. The extinction event is expressed as: a turnover from the Daptocephalus to Lystrosaurus Assemblage Zones, a change from primarily green to primarily red siltstones, and a shift from meandering to braided river architectures. Our research continues to test this model at a site near Lootsberg Pass, Eastern Cape Province.

We characterized a 204-m section of rock in an erosional gully (donga) on Tweefontein farm, reported to be above the vertebrate-defined PTB. The section is dominated by thick sandstones separated by thinner sandstone-and-siltstone intervals. A characterization of these sandstone bodies is used to differentiate fluvial architecture through time to test whether a shift in architectural elements occurred. Sandstone bodies can be traced for kilometers, allowing for placement of the section in a broader stratigraphic context. Magnetostratigraphic data in correlative sections eastward, towards Lootsberg Pass, reveal a long normal polarity chron with no evidence of a reverse polarity chron.

Thick siltstone intervals often separate sandstone bodies. These are interpreted as paleosols burrowed by Katbergia in the current model. Total organic carbon (TOC) and bulk chemical composition (XRF) data through a potential Katbergia-bearing paleosol interval indicate that this trace fossil may not be restricted to overbank deposits. Preliminary data and calculation of a proxy for base loss reveal little evidence of paleosol formation. Absence of a soil profile calls into question the classification of other Katbergia-bearing mudrocks as paleosols. Yet, non-Katbergia burrowed paleosols do exist in the section.

Large vertebrate burrows are preserved in several stacked paleosol horizons high in the stratigraphy. While vertebrate bones were found associated with burrows, skeletal material was insufficient for burrow-inhabitant identification. However, analysis of these dwelling structures may provide a more complete picture of paleontologically-defined Triassic environments.