INCREASED OXYGENATION AND NORMAL-MARINE CONDITIONS ENHANCED AUTOMICRITE (MICROBIALITE) ACCUMULATION ON TRIASSIC CARBONATE PLATFORM SLOPES

Microbial and skeletal modes of sedimentation are used to interpret paleoenvironmental conditions in carbonate sedimentary environments, in part because they are typically assumed to have an antithetical relationship, but the environmental controls on their occurrence remain poorly constrained. The end-Permian extinction was the most severe of the Phanerozoic, and in its aftermath, Early Triassic oceans were characterized by high temperature, high carbonate saturation state, expanded ocean anoxia, and depleted metazoan ecosystems—factors assumed to enhance microbialite precipitation. In this study, we assessed the influence of Triassic ocean oxygenation on the occurrence of Tubiphytes-associated automicrite (autochthonous micrite with an interpreted microbial origin) on the slope of the Great Bank of Guizhou (GBG). The GBG is an uppermost Permian to Upper Triassic isolated carbonate platform in South China providing an opportunity to assess changes in carbonate sedimentation across a shallow to deep-water gradient. To assess the relationship between oxygenation, benthic ecosystem evolution, and carbonate sedimentation, we integrated petrographic observations and rare earth element (REE) analyses for the cerium (Ce) redox proxy across slope and basin stratigraphic sections. Our results indicate that the Induan (lowermost Triassic) slope was dominated by fossil-poor lime mudstone and an absence of automicrite boundstone during an interval of expanded marine anoxia. In contrast, Olenekian (upper Lower Triassic) strata were characterized by an increase in both skeletal fossils and automicrite. Preliminary REE analyses demonstrate a negative Ce anomaly in Olenekian slope strata, suggesting oxygenated conditions by the time of automicrite first occurrence. The initiation of slope automicrite coincided with global evidence for increased marine oxygenation, reduced ocean temperature, and more biodiverse benthic ecosystems. Our findings suggest that, instead of having an antithetical relationship, microbial and skeletal carbonates can both be favored by well-oxygenated, normal-marine conditions.