FORAMINIFERAL ASSEMBLAGES OF THE BASAL TROPIC SHALE (UPPERMOST CENOMANIAN-LOWERMOST TURONIAN) IN RELATION TO THE PLIOSAUR BRACHAUCHENIUS LUCASI, SOUTH-CENTRAL UTAH

The southwestern side of the Cretaceous Western Interior Sea underwent dramatic oceanographic fluctuations during the latest Cenomanian-early Turonian transgression. The basal Tropic Shale near Big Water, Utah records the westward expansion of the seaway and a deepening upwards succession of mud-dominated lithofacies. Here, two nearly complete specimens of the pliosaur Brachauchenius lucasi were described from the basal Turonian (Albright et al., 2007; Journal of Vertebrate Paleontology). The Tropic Shale section studied here was located about 100 miles (175 km) from the western shoreline. It is an area with diverse scientific importance based on a shallow open marine record of Oceanic Anoxic Event 2 (OAE2), and the occurrence of numerous large marine vertebrate fossils. Eight shale samples spanning 7 m of the Tropic Shale collected in the section where the B. lucasi specimens were excavated were processed and analyzed for planktic and benthic foraminifera in order to document the nature of the environment in which these large marine vertebrates lived. Foraminiferal assemblage data indicate cycles of relative transgression and regression, or fluctuating water mass conditions and low oxygenated water across the Cenomanian- Turonian boundary interval, and low diversity assemblages. Planktic:benthic ratios display cyclical fluctuations of up to 30% indicating the dynamic nature of this neritic environment that may have been responding to changing water mass conditions, including temperature, salinity, and/or turbidity. These fluctuations may have been driven by climate cycles, which are well-developed in more distal, correlative facies. There is a general increase in percent planktics through the study interval with a change in dominance from Guemblitria in the lower part to Heterohelix in the upper part. Both were opportunist taxa, but the stratigraphic changes suggest a deepening water column. Benthic foram assemblages are all dominated by the infaunal taxon Neobulimina albertensis, suggesting that conditions on the seafloor were oxygen stressed due to stratification and/or an abundance of organic matter. The pliosaur Brachauchenius lucasi survived OAE2 across the Cenomanian-Turonian boundary and thrived along the western margin of the seaway despite fluctuating oceanographic conditions.