Geochemical and Stratigraphic Characterization of the Morrison Formation and Cedar Mountain Formation Contact in the Island Park Quadrangle, Uintah County, Utah

The Upper Jurassic Morrison Formation and Lower Cretaceous Cedar Mountain Formation are among the richest sources of dinosaur fossils. Despite a 24-million-year gap in time (the Jurassic–Cretaceous unconformity), these formations are difficult to distinguish due to their similar rock types and colors. In Utah's Island Park quadrangle (IPQ), these formations are not separated on existing geological maps. This study focuses on identifying and understanding the formational boundary between them using field observations, stable isotope analysis, clay mineralogy, and chemical techniques such as X-ray diffraction (XRD) and X-ray fluorescence (XRF). Unlike other areas where a conglomerate layer marks the boundary, the IPQ lacks this feature. Instead, distinct yellow-orange and red layers, along with changes in mineralogy and chemical composition, help pinpoint the boundary. For example, the Morrison Formation contains kaolinite, whereas the Cedar Mountain Formation includes calcite, gypsum, and sepiolite. The chemical data also indicate intense weathering at the boundary, corresponding to the observed color changes. The two formations reflect different climates: the Morrison Formation was deposited in a wetter environment, whereas the Cedar Mountain Formation formed under drier and cooler conditions influenced by nearby mountains. These climatic differences are also evident in carbon and oxygen isotopes, which align with regional and global climate trends. This study provides new tools to identify the Morrison–Cedar Mountain boundary in the IPQ and enhances understanding of the climate and environmental shifts during this time. These findings can help clarify similar and widespread unconformities in the geological record.