STRATIGRAPHIC EVIDENCE FOR EARLY TRIASSIC GLOSSOPTERIS IN THE KAROO BASIN, SOUTH AFRICA

The continental record in the Beaufort Group of the Karoo Basin plays a critical role in understanding the response of terrestrial ecosystems to the End-Permian mass extinction. Current models propose environmental change as the catalyst of a catastrophic plant-die off which resulted in a phased vertebrate faunal extinction. Palaeoenvironmental models for the Beaufort Group have been constructed in the absence of palaeobotanical data because the megafloral record is extremely depauperate for the southern and central parts of the basin.

We previously reported a plant-fossil assemblage from a horizon approximately 70 m below the vertebrate-defined Permian-Triassic boundary at Wapadsberg Pass. Here we report the occurrence of Glossopteris macrofossils, permineralized wood assignable to Agathoxylon africanum, and a palynological spectrum including glossopterids, corystosperms, peltasperms, and conifers, from above the PTB, as defined by vertebrate biostratigraphy, at Old Lootsberg Pass. These fossils occur in a stratigraphic interval that displays significant lateral and vertical variation. To understand this variation, we measured and correlated eleven stratigraphic sections over a lateral distance of ~1.5 km, using bounding surfaces of fluvial complexes as datums.

Exposures are dominated by olive grey, light olive grey, and greyish red siltstone organized into stacked fining up sequences in which thin, lenticular or planar, very fine wacke sandstones are overlain by coarse-to-fine siltstone. Glossopteris occurs in these olive-grey siltstones below, and laterally equivalent to, multi-storied sandstone bodies wherein meter-scale bedforms are well developed and lenticular pedogenic nodular conglomerates are concentrated as lag deposits above erosional boundaries at the base. Our Triassic placement of Glossopteris is based on their relationship relative to published vertebrate biostratigraphic patterns at Old Lootsberg Pass, supplemented with sedimentological data used by others to identify Triassic fluvial systems. If the current biostratigraphic model is valid, our results contradict the clade's reported extinction at the boundary and support previous reports of this genus as both megafloral and palynological elements in the Triassic.