Paleoenvironment of the Morrison Formation in the Bighorn Basin of Montana, USA

Ashley L. Adams

doi:10.17918/00000563

The Morrison Formation is one of the most prolific dinosaur-bearing formations in the world, ranging from the Kimmeridgian-Tithonian in age. Stretching from northern New Mexico throughout much of the western United States and into southern Canada, the units of this Formation have been explored, particularly for dinosaur fossils, since the 1800s (Foster, 2003). However, the majority of research has focused primarily on exposures in Colorado, Utah, and Wyoming, where many of the most famous Jurassic- aged dinosaurs were initially found (Foster et al., 2016). The northern areas of the Morrison, however, have not been thoroughly explored until more recently, and have revealed a mix of unique and relatively common dinosaur species of the Jurassic age (Maltese et al., 2018). An exposure of the Morrison Formation located in the Bighorn Basin of Montana, here referred to as the Suuwassea Morrison Formation Site (SMFS), provides an excellent location to explore and compare to other northern sites within the Morrison Formation, as well as the longer-studied southern areas. In addition to more typical Morrison specimens of apatosaur, stegosaur, and allosaur material, a relatively unique specimen of the sauropod Suuwassea emilieae was initially discovered at this site (Harris and Dodson, 2004). The geology of this site, however, has remained largely unstudied, and the question remains if Suuwassea emilieae is unique to the area due to rarity, temporal differences, or environmental differences restricting its range. The primary goal of this research was determine the local paleoenvironment of the SMFS, particularly in comparison to the rest of the Morrison Formation. In order to accomplish this, I first explored the sedimentology and stratigraphy of the area, describing the sedimentology in detail, with special focus on areas with fossils. In addition, I drafted a detailed stratigraphic column, and identified paleosols, fossils, and photographed localities. I then focused further on the paleosols of the area, with detailed field descriptions, x-ray diffraction analysis, and thin section analysis, to determine the types of soils these profiles originally represented. Based on these data, I conclude the Morrison exposure of the SMFS represents an overall wet, seasonal environment, with fluctuating water tables enhanced by a combination of low base level related to the proximity of this region to the Sundance Sea.

Paleoenvironment of the Morrison Formation in the Bighorn Basin of Montana, USA

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Paleoenvironment of the Morrison Formation in the Bighorn Basin of Montana, USA

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