Across the western United States, the Morrison Formation documents varied paleoenvironments, associated fauna, and their evolution throughout the Upper Jurassic. While the Morrison has been well-explored, there remain gaps in knowledge about the stratigraphy of the formation. In the Bighorn Basin of northwestern Wyoming, a disconformity exists between the uppermost Morrison and the overlying Cloverly, leaving an incomplete section. In addition, limited exposure of the surviving upper Morrison beds has hindered exploration of the upper Morrison in this basin. In this study, three new sections are reported from the upper Morrison in the Indian Pass 7.5 min Quadrangle of the northeastern Bighorn Basin. Compared to other Morrison sections, the upper strata of these new sections contain abnormal concentrations of a type of weathered volcanic ash, bentonite. Bentonite is common throughout the Morrison, where it generally occurs as thin beds measuring less than a meter in stratigraphic thickness. At the new sites, bentonite is much more abundant, with the upper thirty meters or more of each section being dominated by bentonite. Other lithologies, including shales and sandstones, are present, but occur primarily as lenses within the bentonite beds. Such lithologic data, coupled with chemical composition data obtained with a pXRF, indicates that these bentonite-dominated exposures belong to a new member of the Morrison Formation, for which the name Spirit Mountain Bentonite is proposed. The thickness of the bentonite likely reflects infilling of a syndepositional low. High abundances of calcium and magnesium, as well as the presence of gypsum nodules and microbialites, indicate that this low was an evaporative lake and its adjacent mudflats. A decline in the concentration of calcium and magnesium indicates that this lake gradually desiccated over the course of deposition of the Spirit Mountain Bentonite.