DEVELOPMENT OF A STRATIGRAPHIC COLUMN AND GEOLOGIC MAP OF A MORRISON FORMATION DINOSAUR SITE, BIGHORN BASIN, WY

We created geologic maps and a composite stratigraphic column through detailed measurements and characterization of the upper Sundance, Morrison, and lower Cloverly Formations exposed near the base of the Bighorn Mountains north of Shell, WY. The composite stratigraphic column documents the relative location of dinosaur bone-beds in the stratigraphic succession and allows for comparison other studies in the surrounding region. The lithostratigraphic boundaries of the Morrison in our sections do not correlate exactly to previous studies, but they are similar in regard to overall thickness and position in the strata. We recognize the top of the Sundance Formation as a set of widespread shell beds in calcareous sandstones, which grades into the overlying silty muds of the lower Morrison. A distinct channel sand, informally termed the “scorpion sandstone”, may also be present in another section from a previous study nearby. This channel sand may be useful in localized research. We provisionally placed the top of the Morrison Formation at a series of gray, silty muds and dark, reddish-brown muds, which grade into a series of alternating beds of yellowish-orange and gray siltstones of the Cloverly Formation. Several candidates for widespread unconformities were identified in the section, but await confirmation by regional correlation. The mapping portion of this project gathered data using field techniques and GIS to map the surface and sub-surface geology of the local area surrounding the site. Geologic maps on a topographic base and cross-sections were developed and then transferred to a digital format using ArcGIS and Adobe Illustrator. The upper Sundance, Morrison, and lower Cloverly Formations were mapped, along with three previously unmapped Quaternary terrace deposits and one unmapped normal fault. These maps will be used for further research on the site and comparison with previously published maps. By recognizing distinct beds of limestone, channel sands, and chert nodules within the Morrison Formation, and distinguishing between overlying Quaternary terrace deposits, our composite stratigraphic column and geologic map create a foundation for further research in the vicinity of the dinosaur bone beds. This research is the result of participation in a REU, funded by the National Science Foundation (EAR-0353589).