LITHOSTRATIGRAPHY AND LATERAL VARIATION IN VERTEBRATE BIOSTRATIGRAPHY NEAR THE PERMIAN-TRIASSIC BOUNDARY AT BETHULIE, SOUTH AFRICA

The Permian-Triassic Boundary (PTB), while marking the most catastrophic extinction event in Earth’s history, is not well documented in the terrestrial record. Among the few known sequences that expose the terrestrial PTB, the Karoo Basin, South Africa, offers a reportedly complete stratigraphic record across the extinction event. Although the region has been the focus of multiple studies for decades, many questions remain regarding the physical conditions surrounding the vertebrate-defined extinction event. Our study, conducted on the Bethel Farm near Bethulie, Free State Province, where the greatest number of vertebrate fossils have been reported, documents a high resolution record of the vertical and lateral stratigraphic relationships into which these vertebrate fossils can be placed. Our research goal is to understand the physical conditions that shaped the late Permian.

The currently accepted vertebrate-defined PTB is thought to coincide with the upper boundary of a homogenous, thick maroon siltstone extending horizontally across the region. Additionally, the latest model proposes a shift from wet, cool conditions in the latest Permian, to a more arid climate with increasingly short and intense wet seasons in the earliest Triassic. In the Karoo Basin, olive grey siltstones are reported to represent conditions under a wet climate, whereas greyish red siltstones are thought to represent a transition to drier conditions.

We demonstrate that several discrepancies exist in the most recently published model for the area. The inferred PTB interval exhibits extensive lateral and horizontal variability around the Bethel Farm, as the reportedly homogenous maroon siltstone laterally grades to olive grey siltstone within a few tens of meters. Additionally, when the records of vertebrate fossils are placed in a robust stratigraphic context, considerable variance is found between specific reported collection sites and actual stratigraphic position relative to the inferred PTB. For example, several uppermost Permian vertebrate taxa appear to have been collected in overlying lower Triassic rocks. Therefore, the vertebrate-defined PTB may not be exclusively correlated with a maroon bed at the Bethel Farm.