Paleoenvironments and geochronology of the Eocene Wagon Bed Formation, central Wyoming

Palko, Emma Elizabeth (2023) Paleoenvironments and geochronology of the Eocene Wagon Bed Formation, central Wyoming. Masters thesis, Northern Arizona University.

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Abstract

The subduction of an oceanic plateau and consequent shallow subduction of the Farallon plate beneath North America have long been proposed to explain the contractile deformation that led to the formation of the Rocky Mountains and associated basins across a craton. However, questions remain about the extent of emplacement, timing of slab removal, and its influence on the land surface above. In this study, I investigate the Eocene Wagon Bed Formation in central Wyoming to better understand the relationship between its lacustrine deposits and late-stage Laramide tectonics and magmatism, as well as to provide a comprehensive record of deposition in the region during the Eocene. Eight new 40Ar/39Ar radioisotopic dates for tuff beds and volcaniclastic sandstone reveal the Wagon Bed Formation was likely deposited between 50 and 35 Ma, on far longer timescales than previously recognized, and includes a ~5 Ma unconformity in the middle. Geochronology at Lysite Mountain suggest deposition occurred only from 50 to ~43 Ma. The formation shows significant spatial and temporal variation along the Beaver Rim, with lacustrine paleoenvironments confined to the west of the Conant Creek anticline, and well-drained volcaniclastic-rich alluvial environments to the east. Similar lacustrine lithofacies to the Beaver Rim area also exist in the north at Lysite Mountain. Stable isotope data from carbonates and volcanic glass suggest the paleolakes in both locations were likely closed or semi-closed, consistent with saline lake indicators such as stromatolites, oil shales, and authigenic zeolites. Deposition of the Wagon Bed Formation suggests the collapse of the Granite Mountains began by at least 45 Ma, 30 million years earlier than currently documented. The removal of compressional stresses during rollback caused the Granite Mountains, a rootless uplift, to extend along preexisting thrust faults or create new normal faults. Evidence of drainage disruption, lacustrine sequences, an increase in volcaniclastic material, unconformities, and extension indicate that the Wagon Bed Formation could record the surficial expression of Farallon plate removal on the overriding North American plate.

Item Type:	Thesis (Masters)
Publisher’s Statement:	© Copyright is held by the author. Digital access to this material is made possible by the Cline Library, Northern Arizona University. Further transmission, reproduction or presentation of protected items is prohibited except with permission of the author.
Keywords:	Eocene; Farallon plate; geochronology; lacustrine; paleoenvironment; Wyoming; Wagon bed Formation
Subjects:	Q Science > QE Geology
NAU Depositing Author Academic Status:	Student
Department/Unit:	Graduate College > Theses and Dissertations College of the Environment, Forestry, and Natural Sciences > School of Earth Sciences and Environmental Sustainability
Date Deposited:	18 Jun 2025 23:39
Last Modified:	18 Jun 2025 23:39
URI:	https://openknowledge.nau.edu/id/eprint/6186

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