C-ISOTOPE CHEMOSTRATIGRAPHY OF THE ELLIOT FORMATION OF SOUTHERN AFRICA

The Upper Triassic to Lower Jurassic Elliot Formation (EF) of the main Karoo Basin in South Africa and Lesotho preserves the transition from a Late Triassic to a diverse Early Jurassic continental ecosystem. This transition includes the end-Triassic extinction (ETE) event. The ETE is correlated to a major C-cycle perturbation and associated global climate change induced by carbon released from the Central Atlantic Magmatic Province (CAMP). However, the exact stratigraphic location of the ETE interval, Triassic-Jurassic boundary (TJB) and their relationship to fossil occurrences are poorly constrained. This study uses C-isotope chemostratigraphy constrained by detrital zircon geochronology as a stratigraphic proxy to identify the ETE interval and TJB in the Karoo Basin.

C-isotopic (δ¹³C_org) composition of bulk organic carbon and six detrital zircon samples were collected from the lithologic type-section of the EF at Barkly Pass. Average pre-excursion δ¹³C values are –24.0 ‰. A -6‰ negative C-isotope excursion (N-CIE) occurs starting at 154 m above the base of our floating section to a minimum of -30‰ at 159 m. A slight positive CIE between 159 - 163 m within the overall negative trend. A second NCIE occurs from 165.75 m to a minimum of -29‰ at 167 m. C-isotope values increase to pre-excursion values above 168 m. Maximum depositional ages (MDA) were estimated from detrital zircon grains analyzed via CA-ID-TIMS. A maximum depositional age of 205.47 ± 0.08 Ma 76.75 m below the initial N-CIE. An MDA of 201.31 ± 0.10 Ma 9.5 m above the most negative point of the first N-CIE, make it plausible that it is correlated to the ETE CIE dated to 201.564 ± 0.015/0.220 Ma. Our results suggest that at Barkly Pass 1) the lower EF is latest Triassic (Rhaetian) 2) uppermost part of the lower EF is likely earliest Jurassic in age, 3) the upper EF is likely earliest Hettangian to middle Sinemurian and 4) there is evidence for a perturbation to the C-cycle that may be related to CAMP-induced volcanism recorded in the lower EF. This perturbation likely represents the ETE interval suggesting that the ETE is preserved within the upper part of the lower EF. Careful analysis of the occurrence and diversity of vertebrate fauna at this and other outcrops of the EF will refine the expression of the ETE within the continental ecosystem of the Southern Hemisphere.