A NEW LOOK AT THE COTHAM MARBLE (CARBONATE) MICROBIALITES: FACIES ANALYSIS AND RELEVANCE TO THE END-TRIASSIC MASS EXTINCTION

The Upper Triassic beds of the southwestern United Kingdom contain an aerially extensive (~2,000 km²) biohermal carbonate deposit known regionally as the Cotham Marble. Stratigraphically, the Cotham Marble microbialites occur in the upper Cotham Member, at the same level as the initial carbon isotope excursion (CIE) that is coincident with the end-Triassic extinction interval. The bioherms occur as discrete m-scale mounds and are about 20 cm thick. Cross-sections perpendicular to bedding reveal at least five growth phases that alternate between laminated and dendritic mesofabrics. Microscopically, the dendrolites contain pyrite-coated, putative microfossils and have elevated total organic carbon (TOC) content relative to the interstitial fill. The dendrolite fill contains abundant clusters of the green algal “disaster” prasinophyte Tasmanites as well as a sparse shelly fauna. Stable isotope values of the carbonate exhibit ∂13C_carb values of -0.58‰ to -1.86‰ and ∂18O values of -2.15‰ to 0.02‰ (n=25). Stable isotope values of the microbialite organic carbon (∂13C_org) range from -28.9‰ to -26.0‰ (n=14).

Our stable isotope results indicate that 1) ∂13C_carb values are more negative than overlying carbonate (Langport Member) values and 2) ∂13C_org values are within the range of values of the initial CIE in other sections across the southwestern UK. Petrographically, our results suggest the Cotham Marble formed via the calcification of rapidly lithifying microbial mats in an environment supersaturated with respect to calcium carbonate. High saturation state possibly resulted from accelerated weathering conditions fostered by the emplacement of the Central Atlantic Magmatic Province (CAMP). Considered collectively, the striking aerial extent of microbialite facies, their occurrence at the level of the initial CIE (suggested by our stable isotope data), and co-occurrence of abundant clusters of algal prasinophytes, all indicate the Cotham Marble microbialites record some of the unusual environmental conditions associated with the end-Triassic extinction.