ENVIRONMENTAL AND TAPHONOMIC CONTROLS ON THE DIVERSE UPPER CAMPANIAN (UPPER CRETACEOUS) MARINE FAUNA AT KREMMLING, GRAND COUNTY, COLORADO, USA

The Kremmling Cretaceous Ammonite Locality (KCAL) is a Bureau of Land Management Research Natural Area located in the Middle Park Basin of Colorado. This site is well-known for its rich and abundant marine invertebrate macrofauna derived from a concretionary zone capping a ridge north of Kremmling, Grand County, Colorado. The site is protected due to its abundance of large (>60 cm diameter) Placenticeras ammonite macroconchs, which have been extensively collected over the decades by fossil collectors and paleontologists. This activity has left numerous concretions with large, bowl-like external molds of ammonites colloquially referred to by locals as ‘bird baths’. The goal of this project is to examine the diversity and taphonomy of this fauna in an environmental context. The main fossil-bearing concretionary zone at the KCAL occurs in the unnamed member of the Pierre Shale, which correlates with the upper Campanian Baculites compressus–B. cuneatus Western Interior ammonite zones. The fauna represents within-habitat, time averaged assemblages with limited evidence of spatial averaging. The KCAL has close to 100 species of molluscs, crustaceans, bryozoans, corals, and vertebrates. The most abundant fossils at the site are ostreid and inoceramid bivalves, along with aporrhaid gastropods. The diversity of this assemblage is substantially higher as compared to age-equivalent marine faunas in eastern Colorado, South Dakota, and Montana. Based on paleogeographic reconstruction of the Western Interior Seaway, the KCAL was >60 km east of the paleoshoreline. Published paleocurrent directions suggest a southward seaway current, influenced by tectonically-controlled seafloor topography. Faunal data and the environmental preferences of taxa indicate that the KCAL had normal, open-marine salinities with a well-oxygenated water column and seafloor. Trace fossil evidence and the abundance of infaunal species indicate that sea floor oxygenation extended well-below the sediment water interface, indicating a well-mixed water column. Tectonically-driven shoaling and acceleration of seaway currents in the Kremmling region likely promoted the well-oxygenated sea bottom and higher richness.