A multidisciplinary approach to review the vertical and lateral facies relationships of the purported vertebrate-defined terrestrial Permian–Triassic boundary interval at Bethulie, Karoo Basin, South Africa

Abstract

The stratigraphic section at Bethulie, South Africa, is reported to contain the vertebrate-defined Permian–Triassic boundary succession in the terrestrial realm of the Karoo Basin. The model of vertebrate turnover, from the Daptocephalus to Lystrosaurus Assemblage Zones, tightly constrains the boundary sequence to a short stratigraphic interval where siltstone color begins to change from greenish gray to grayish red, the latter color interpreted to be a consequence of aridification. The biological response to this facies change has been termed “the Great Dying,” and the sedimentary rocks that are preserved are ascribed to a playa lake depositional setting. This drying event is believed to be contemporaneous across the basin, although previous studies have shown that the facies appears at multiple horizons at all purported Permian–Triassic boundary sections in the basin.Here, we report results of a multidisciplinary effort to characterize the vertebrate assemblage-zone boundary interval exposed at Bethulie using the lithostratigraphic, petrographic, geochemical, and rock magnetic properties of these rocks. In this stratigraphic succession at its “type” location, the 3-m thick assemblage-zone boundary interval is distinguished by thick beds of greenish-gray, greenish-gray mottled to grayish-red, and grayish-red siltstone, all of which change facies characteristics laterally along strike. Specifically, about 220m to the southeast of the type section, sediments lose all grayish-red coloration, whereas the interval becomes laminated to the northwest. Petrographically, most siltstone is homogenized, with few burrows and small-scale cross-bedded structures with mudchips. There are no gypsiferous or calcareous beds, nor is there evidence of disturbed structures, authigenic breccia, or pseudomorphs associated with dessication. Mean elemental composition of both greenish-gray and grayish-red beds are indistinguishable, geochemically, and both are dominated by illite and chlorite clay species. Mössbauer spectroscopic analyses reveal the presence of a small concentration of fine-grained hematite in the grayish-red siltstone, with its presence mainly found as coatings on clay minerals. Rock magnetic experiments (isothermal remanent magnetization, acquisition and backfield DC demagnetization; magnetic hysteresis; susceptibility vs. temperature) yield data that demonstrate no essential differences between the different colored siltstones. And, both lithologies host mangnetite/maghematite and hematite. Our results do not support the previous interpretation that this inferred Permian–Triassic boundary interval represents the onset of playa lake deposits under conditions of aridification. Rather, our evidence supports the existence of a “wet” landscape at what is considered the Daptocephalus/Lystrosaurus assemblage-zone boundary.