C-ISOTOPE CHEMOSTRATIGRAPHY OF THE ELLIOT FORMATION OF SOUTHERN AFRICA

The Upper Triassic to Lower Jurassic Elliot Formation of the main Karoo Basin in South Africa and Lesotho preserves evidence for a diverse continental ecosystem. Significant syn-sedimentary paleo-environmental change is arrested by the meandering channel, crevasse splays and floodplain facies associated with the lower Elliot Formation and ephemeral fluvio-lacustrine facies associations in the upper Elliot Formation. Based on detrital zircon geochronology and magnetostratigraphy, this change may be tied to global environmental changes initiated in the Central Atlantic Magmatic Province (CAMP), which has been linked to the end-Triassic Mass Extinction (ETE). This study uses C-isotope chemostratigraphy as an additional stratigraphic proxy to identify the end-Triassic Extinction interval as well as the Triassic-Jurassic boundary in the main Karoo Basin.

Organic carbon isotopes (δ¹³C_org) were obtained from cleaned, crushed, and decarbonated paleosol and sandstone samples from the type section in the Elliot Formation. Preliminary results yield two negative carbon isotope excursions (NCIE), of magnitude 6‰ and 4‰ respectively, below the lower part of the upper Elliot Formation. In global records (e.g., Newark and Hartford basins, St. Audrie’s Bay, and Kuhjoch, Austria), the end-Triassic extinction interval is marked by at least one 5 to 8‰ NCIE in the δ¹³C content of organic and inorganic materials from marine and continental (terrestrial and lacustrine) sedimentary rocks. Moreover, the Triassic-Jurassic boundary is marked by a persistent NCIE interval often called the “main NCIE.” Magnetostratigraphic interpretations tentatively suggest that the E23r reversal (Newark APTS), which occurs at the ETE, is present just below the lithostratigraphic boundary of the lower and upper Elliot Formation. This is much higher in the section than what this preliminary C-isotope record suggests. Additional geochronological data and a more robust correlation to the Triassic-Jurassic magnetostratigraphic record are necessary to confirm this C-isotope record. Nonetheless, the observed δ¹³C variation in the Elliot Formation suggests the release of isotopically light carbon into the ocean‐atmosphere system, which, given additional chronologic constraints, may be tied to the CAMP volcanism.