STRATIGRAPHIC CONTEXTUALIZATION OF VOLCANIC ASH DEPOSITED NEAR THE PERMIAN-TRIASSIC BOUNDARY IN THE KAROO BASIN, SOUTH AFRICA

With its wealth of vertebrate fossils and well-exposed stratigraphy, the Karoo Basin serves as a primary location for terrestrial evidence of the Permian-Triassic Mass Extinction (PTME). Few volcanic ash deposits have been identified in upper Permian strata in South Africa. However, a porcellanite-bearing interval recently was discovered at Old Lootsberg Pass (Eastern Cape Province, South Africa) only 72 m below the vertebrate-defined Permian-Triassic boundary. These silicified volcanic ash deposits mark an important geochronometric data point, and their characterization will provide insight into the continental depositional regime prior to the reported PTME.

Porcellanite was examined within a 26 m interval containing five fining-up sequences, each of which begins with cross-bedded wacke and ends in olive-grey (5Y 4/1) siltstone. Fining-up intervals are interpreted to represent cycles of point bar or lateral accretion deposits, characteristic of a meandering river system. The porcellanite beds are concentrated at the top of a fining-up cycle in 1.6 m of laminated and rippled siltstone and claystone. Thin sections of the siltstones exhibit primary structures, including micrometer-scale cross beds, small-scale ripples, mud balls, and mm-scale burrows. Anisotropy of magnetic susceptibility (AMS) data reveals a primary, depositional fabric. A thin section from the uppermost porcellanite bed shows structureless texture containing euhedral zircon grains, which have been analyzed by ID-TIMS dating techniques.

The grain size, lithology, and primary structures present in the high-resolution, 1.6 m porcellanite section are indicative of deposition in a low-velocity fluvial regime, with little evidence for sediment reworking. The olive color that dominates the section also supports the interpretation of a wet environment. Research continues and the results of this project will serve to improve our understanding of the depositional environment of this latest Permian porcellanite.