PALEOENVIRONMENTAL AND DEPOSITIONAL SETTING OF TWO DINOSAUR BONE-BEARING QUARRIES IN THE MORRISON FORMATION (UPPER JURASSIC) OF WYOMING

The Red Canyon Ranch site near Shell, Wyoming, includes two dinosaur bone-bearing quarries (the Upper and Lower Quarries) in the Upper Jurassic Morrison Formation. The bone layers of each quarry are sedimentologically distinct, indicating different depositional conditions. The Lower Quarry's bone-bearing layer is a light-gray, fine-grained, well-cemented sandstone whereas the Upper Quarry's bone-bearing layer is poorly indurated olive-green sandy siltstone. Both quarries contain sauropod bones and carbonized plant remains.

In order to compare the paleoenvironments and depositional histories of each quarry, a stratigraphic section was made through the two quarries and compared with existing fluvial facies models. The section totals 31.65 m and is mostly made up of muddy siltstones and fine-grained trough cross-bedded sandstones. Several fining-upward sequences and the presence of fines indicate that much of the section, including the Upper Quarry, is made of overbank deposits. The fine sandstone in the Lower Quarry may represent a crevasse-splay event. The regional climate of the western United States is considered to have been arid to semi-arid at the time of deposition of the Morrison Formation. Rivers and vegetated wetlands, however, did exist on a local scale, as represented in this study. The occurrence of sauropod bones in these deposits sheds light on the local fluvial environments inhabited or frequented by large herbivorous dinosaurs.