The Morrison Formation of the Western United States is famous for the dinosaurs and other fossils that have been excavated from its beds. It was deposited during the Late Jurassic in a semi-arid, savannah-like environment. The Morrison Formation is remarkably extensive, with outcrops across eight states; however attempts to correlate between Wyoming and the Colorado Plateau have proven difficult. The goal of this research is to determine the origin of carbonate rocks from the Morrison Formation beds exposed on the Spring Creek Preserve in southeastern Wyoming, and to assess their potential for lithostratigphic correlations. A wide range of research techniques were employed, ranging from macroscopic observations in the field to geochemical and isotopic analyses. Field relationships and macro- and microtextures of nodular and calcareous units are consistent with carbonate mineral deposition of freshwater lacustrine muds and the development of palustrine limestones, nodules and calcrete as the micritic muds were exposed and desiccated. Strontium isotope data suggest that the nodules developed from alteration of nearby lacustrine carbonates with little exogenous input. The thickness and level of development of the nodular carbonate units below the mid-Morrison unconformity at 42 m and observed in nearby localities suggests an extended period of subaerial exposure and desiccation in the study area during middle Morrison time. If the presence of these palustrine carbonates found throughout Wyoming is the result of a synchronous regional lake-level lowstand, it is plausible that this nodular horizon correlates to the Mid-Morrison paleosol unconformity identified Utah and Colorado. The strontium isotope composition of a belemnite from the underlying Sundance Formation corresponds to a Late Jurassic Oxfordian age (161-157 million years ago) for this unit and places a maximum age for the Morrison Formation in the Spring Creek Reserve study area
