ASPECTS OF THE PALEONTOLOGY AND STRATIGRAPHY OF THE LOWER TRIASSIC-LOWER CRETACEOUS STRATA OF THE EASTERN BIGHORN BASIN, WY

A multidisciplinary study of the Mesozoic strata of the eastern Bighorn Basin of Wyoming was completed as part of an NSF-REU project during the summer of 2005. Stratigraphic sections of the Lower Triassic Chugwater through Lower Cretaceous Sykes Mountain Formations were measured and correlated in the Red Canyon Ranch, Red Gulch, Coyote Basin, and Howe Quarry areas near Shell, WY. Samples were collected for plant/pollen, carbon isotope chemostratigraphy, and petrographic and provenance analysis. A detailed stratigraphic section of the Triassic Chugwater Formation was compiled, and samples were collected and studied interpretaton of a playa lake facies and possible paleoclimatic cyclicity. The Middle Jurassic Sundance Formation was found to have been deposited in shallow marine and tidal environments with periodic intervals of subaerial exposure identified by horizons containing dinosaur tracks. Plant assemblages and organic-rich deposits were identified in several continental and marginal marine facies throughout the study interval. In the Red Canyon Ranch, Coyote Basin, and Howe Quarry areas, productive dinosaur fossil-bearing horizons were correlated to determine relative stratigraphic position within the Morrison Formation. The boundary between the Upper Jurassic Morrison and Lower Cretaceous Cloverly Formations was found to be characterized either by the presence of a channelized conglomeratic sandstone (Pryor Conglomerate) or a thick, mature interfluve paleosol. Within the fossil-bearing horizons at Red Canyon Ranch, taphonomic analyses and mapping were completed for two bone beds, dominated by sauropod elements, from different horizons in the Upper Morrison. These bone beds, associated with fluvial and overbank deposits at the locality, were also sampled for microvertebrates. Vertebrate fossils were collected for REE geochemical analysis of differences caused by varying depositional environments. Taphonomic analyses were completed on fossil vertebrate remains preserved in micrite nodules from the Little Sheep Member of the Cloverly Formation.