RECONSTRUCTING THE LATE JURASSIC PALEOENVIRONMENT THROUGH CHRONOSTRATIGRAPHIC CORRELATION OF MORRISON FORMATION SUBSURFACE WELL LOGS, OUTCROPS, AND FOSSIL BEARING LAYERS IN THE EASTERN BIGHORN BASIN, WYOMING

Sections of Morrison Formation outcrop were measured at Sheep Mountain, Red Gulch, Hyattville, Coyote Basin, and Red Canyon Ranch near Shell, Wyoming. Correlations of the sections with oil well log data of the eastern Bighorn Basin demonstrate a regional fluvial environment with local lacustrine systems characterized by cross-bedded sandstones, rip-up clasts and pockets of silty mudstone. The “Scorpion” sandstone at Red Canyon Ranch and similar fluvial sandstones across the study area are rich in both vertebrate and plant remains. In the upper Morrison, thick paleosols are locally well developed and indicate long periods of non-deposition. Large stacked channel sandstones of ancient river valleys occur where paleosols are thin or absent. An abundant record of dispersed cuticle is preserved in the highly sinuous, calcite cemented, fluvial deposits of mud and sand of the mid to upper Morrison Formation and occurs at the same horizon as dinosaur remains and in direct association with bone. Preliminary analysis of the cuticle flora indicates the presence of at least 5 species of gymnosperms, which are characterized by thick cuticles, sunken stomata with overarching papillae, and other features found today in evergreen, dry-adapted plants. Identified taxa include conifers belonging to the genus Brachyphyllum, the family Cheirolepidiaceae, and other taxa. The close relationship of vertebrate remains and abundant plant cuticle in the “Scorpion” sandstone and fluvial valleys of the Morrison Formation provide a rare opportunity to reconstruct the paleoenvironment of the eastern Bighorn Basin and to directly associate fossil vertebrates with the vegetation in which they lived.