FAUNAL DIVERSITY AND TAPHONOMY OF THE STEELE SHALE, MESAVERDE GROUP, AND PIERRE SHALE IN SOUTHEASTERN WYOMING

Collections at the University of Wyoming and the United States Geological Survey were analyzed and subsequent field work was performed to determine the relationship between faunal diversity, depositional environments, and the taphonomy of the Middle and Upper Campanian Steele Shale, Mesaverde Group, and Pierre Shale of southeastern Wyoming. Faunal lists from each unit were compiled or updated and ten major taxa were analyzed in this study, which included: Bivalvia, Gastropoda, Cephalopoda, Scaphopoda, Inarticulata, Cheilostomata, Scleractinia, Polychaeta, Crustacea, and Echinoidea. The faunal diversity increases up-section throughout the Middle and Upper Campanian, punctuated by low diversity horizons, possibly caused by fluctuating environmental conditions. The fossils in the marine Rock River Formation of the Mesaverde Group are believed to have been deposited in a storm dominated, delta influenced, shore-face environment, whereas the fossils from the Steele Shale and Pierre Shale represent prodelta to outer-shelf facies typically deposited below storm wave base in the Western Interior Seaway. The fossils in the lower Middle Campanian Steele Shale and Pierre Shale show current influence, indicated by preferred orientation of Baculites shells and infilling of the body chambers with other mollusk tests. Fossil mollusks of the Mesaverde Group display the greatest variation in shell orientation, with random positioning, current orientation, shell beds, and shell pavements being the most prevalent. Fossil mollusks in the Baculites sp. (smooth species) through the B. jenseni Ammonite Zones of the Pierre Shale commonly exhibit random orientation believed to have been caused by current activity and post-burial bioturbation.