Stratigraphy of Morrison Formation (upper Jurassic) bentonite beds, northeastern Bighorn basin, Wyoming
Dalton, Jack. 2023. Stratigraphy of Morrison Formation (upper Jurassic) bentonite beds, northeastern Bighorn basin, Wyoming. -- In Proceedings: 22nd Annual Undergraduate Research and Creative Activity Forum. Wichita, KS: Wichita State University, p. 13
Beds of weathered volcanic ash (bentonite) are prevalent throughout the Upper Jurassic Morrison Formation of the western United States. However, most reported bentonite beds are thin and tend to be interlayered with other lithologies. Here, a new Morrison site is reported from the northeastern Bighorn Basin of Wyoming where bentonite beds are anomalously-thick and dominant, constituting the majority of the studied forty-seven-meter interval. The purpose of this study is to produce a stratigraphic framework of this new site and interpret both its depositional environment and overall position within the Morrison Formation. The abundance of bentonite indicates that the site lies within the upper Brushy Basin Member of the Morrison, deposited during the Tithonian stage of the Jurassic. However, the prevalence of the bentonite in this section indicates that it represents a localized subunit of the Brushy Basin that is here named the Cody Bentonite. Currently-ongoing research involving x-ray fluorescence (XRF) and lithostratigraphic data has been utilized to subdivide the deposition of the Cody Bentonite into two distinct stages, with subunits of the Cody Bentonite being identified through similar means. Additionally, interpretations of XRF data indicate that the Cody Bentonite was deposited over a prolonged period of time, potentially ranging from thousands to millions of years, corresponding with prolonged volcanism and contemporaneous with a minor transgressionregression of an inland sea.