TRACKING RECOVERY AND GEOCHEMICAL CORRELATION FROM THE LOWER TRIASSIC DINWOODY FORMATION AT BLACKTAIL CREEK, MONTANA

The recovery of marine communities after the end-Permian extinction is complex, with several studied sections exhibiting regional accelerated or temporary recovery while others remain depauperate. These recovery resets have been attributed to local perturbations due to continued Siberian Traps outgassing, causing temperature spikes, climatic change, or OMZ expansion in deeper settings. The Lower Triassic Dinwoody Formation of Montana, Wyoming, and Idaho records a mixed carbonate siliciclastic ramp in the immediate aftermath of the end-Permian extinction, and is therefore an excellent site for tracking recovery in the Panthalassic Western U.S. Biostratigraphic constraints of the Dinwoody are limited, so geochemical correlation using δ¹³C_carb values is used. Negative excursions at the bottom of the section (-3.6 ‰) and at the top (-2.8 ‰ and -3.2 ‰) help constrain the Dinwoody at Blacktail Creek to lower Griesbachian through lower Dienerian. Bulk samples of fossiliferous beds were collected and analyzed for ecological metrics of recovery, including richness, evenness and biomass. Griesbachian assemblages record temporally transient blooms of the terebratulid Periallus, while the disaster brachiopod Lingularia is prevalent throughout. Upper Griesbachian assemblages record blooms of microgastropods averaging 0.4mm in size. Lower Dienerian assemblages are low in diversity and evenness, with blooms of the disaster bivalve Eumorphotis. Microconchid encrusters are observed throughout, with some assemblages having 10% of shells encrusted. Winnowed Eumorphotis assemblages in Dienerian beds indicate increasing energy and shallowing. Overall, a decrease in diversity and evenness is observed through the section, recording a relapse of recovery coincident with negative δ¹³C_carb excursions indicating pulses of volcanic outgassing. This is, in turn, coincident with an increase in siliciclastic input within the section, indicating changes in runoff. The observed relapse in recovery associated with these negative carbon isotope excursions lends support to the hypothesis that continued Siberian Traps outgassing stalled recovery in some regions, specifically in shallower settings susceptible to temperature spikes and associated climate change.