Depositional Environment and Taphonomy of Marine Vertebrate Biofacies in the Lower Cretaceous (Albian) Thermopolis Shale, South-Central Montana

The Lower Cretaceous Thermopolis Shale records deposition in an offshore to deeper marine setting in the Western Interior Cretaceous Seaway as the northern, boreal arm of the seaway transgressed into the Western Interior Basin, joining with the southern arm of the seaway for the first time in Albian time. In an area west of the Pryor Range near Bridger, Montana, an approximate 5 km-long exposure of the Thermopolis Shale contains two distinct biofacies (sandstone and mudrock) that collectively include an unusually high abundance and diversity of marine vertebrate skeletal material not previously reported elsewhere in this unit. Although, previous workers have interpreted development of coarse-grained beds within the Thermopolis as transgressive lags produced by winnowing in a relatively shallow water environment, other potential depositional scenarios include lateral transport of material to or suspension fall-out in a deeper water setting. Coarse-grained biofacies constituents comprise well-rounded, terrigenous clastic (chert) pebbles and marine vertebrate bioclasts of terrestrial, shallow, and deep water forms including plesiosaur, marine crocodile, hybodont shark, saw fish, turtle, ray, and fish skeletal material, teeth, scales, and coprolites primarily within lenticular and thinly bedded, ripple and micro-hummocky cross-laminated fine-grained sandstone units interbedded with mudstone in the lower portion of the Thermopolis. This sandstone biofacies records relatively high depositional energy resulting from either storms or fluvial discharge events producing turbidity, combined, or hyperpycnal flow that transported allochthonous skeletal material to an offshore to deeper water setting. Relatively larger isolated skeletal elements, coprolites, and large chert pebbles occur less frequently within overlying mudrock that is interbedded with ash units. This mudstone biofacies records a low energy, suspension deposition-dominated environment (or condensed section), with skeletal accumulation primarily driven by suspension settling of components from floating carcasses.