Master of Science / Geology / Matthew E. Brueseke / The Upper Wind River Basin (UWRB) is located in north-central Wyoming, to the south of the Yellowstone National Park boundary and east of Jackson Hole. Both Lava Mountain and Spring Mountain are Quaternary volcanoes in the UWRB. Lava Mountain is a shield volcano composed of 26 separate lavas capped by a scoria cone. Spring Mountain is located about ~36 km east of Lava Mountain, north of Dubois, WY, where eruptions of basalt cut through Paleocene and Eocene strata. The goal of this study aims to reconstruct the petrogenesis of magmas erupted at both volcanoes using geochemical, petrographic, and isotopic analyses. Important local events in geologic history played a large role in the development of the UWRB. This includes a long history of ancient and Cenozoic subduction, regional extension, and also the migration of the North American plate over the Yellowstone hotspot. The few previous studies on Lava Mountain claim the rocks are mafic in composition, however this was based solely on reconnaissance geological mapping. Geochemical evidence presented in this thesis show Lava Mountain rocks range from basaltic andesite to dacite. Basaltic andesite and dacite are interstratified at the base until approximately 2774 m; the rest of the volcano is andesite. All Lava Mountain samples are largely aphanitic and crystal-poor. Conversely, at Spring Mountain, localized normal faulting controls the location of eruptions of olivine-rich basalt. Petrographic analysis for both Lava Mountain and Spring Mountain display a range of evidence for open system processes, including sieved and/or resorbed pyroxenes, olivines and feldspars, as well as xenocrysts that suggest an influence from crustal assimilation. A petrogenetic model is introduced that discusses how Lava Mountain magma production occurred via fractional crystallization of basalt to dacite, then magma mixing of basaltic andesite and dacite, coupled with small amounts of crustal assimilation, to form the locally erupted andesites. All samples, including Spring Mountain basalts, have ⁸⁷Sr/⁸⁶Sr isotopes of 0.70608 and 0.70751, with ¹⁴³Nd/¹⁴⁴Nd isotopes of 0.51149 and 0.51157 and εNd values of -18 to -22. Pb isotopes plot to the left of the Geochron and directly on to slightly above the Stacey-Kramers curve. Strontium, neodymium, and lead isotope data suggest that Spring Mountain basalts are melts of ancient (e.g., 2.8 Ga Beartooth province) lithospheric mantle. The high ⁸⁷Sr/⁸⁶Sr values and exceptionally low εNd values separate the UWRB rocks from both Yellowstone and Snake River Plain volcanics, and suggest they originated from a different magma source. Finally, thermal evidence suggests melting genesis for UWRB rocks may not be Yellowstone plume related; rather it is more likely linked to Cenozoic extension.

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The Cretaceous Mowry Shale is an organic-rich, siliceous marine shale, and as such is a known source rock in the Western United States. Studies have documented that total organic carbon (TOC) in the Wind River Basin, Wyoming increases to the southeast. These studies cover large areas with limited sample sets. In this study, over 250 samples were collected near Lander, Wyoming to address spatial heterogeneity of TOC within the Mowry Shale at a much finer scale than previously examined. Samples were collected along five vertical sections at three localities, and following correlation of the vertical sections, which was strongly aided by the presence of regional bentonite horizons, samples were collected laterally from the same unit at regular 25-foot intervals. These samples were analyzed using pyrolysis and x-ray diffraction techniques. Average TOC values are fairly consistent within the study area (1.65%, with a range of 2.10% to 1.15%). Average Tmax values for vertical and lateral samples is 433 °C with a standard deviation of 7.25 °C suggesting immature to very early oil window thermal maturity. Kerogen types are determined to be dominantly type III, suggesting a dominance of terrestrial input, becoming slightly more mixed type II/III to the southeast. Redox-sensitive trace metals such as uranium, thorium, vanadium, chromium, cobalt, and molybdenum values all suggest a slightly oxygenated sediment water interface during time of deposition. These pyrolysis and trace metal data suggest that the study area was in a prograding proximal marine/prodeltaic depositional environment during Upper Mowry time with influences from higher energy bottom flows. Lateral homogeneity of strata and the low variability in geochemical character across the study area suggest that the local basin in the study area was not segmented by structural or oceanographic conditions. While efforts were made to collect unaltered outcrop samples (digging back into what appeared to be unfractured, unaltered rock), alteration or weathering of organic material is a concern for source rock evaluation of near-surface outcrops. In order to address this concern, a 5-foot-deep trench was dug back into the outcrop at the target horizon in one locality. Samples were taken at regular three-inch intervals from this trench as it was excavated to determine the effect of weathering on TOC in the study area. Based on pyrolysis results, TOC was affected by weathering only along fracture sets (several samples intersected fractures in the shallow subsurface) and did not appreciably increase from the surface to a depth of five feet. Due to the impermeable nature of shale rock, decreases of TOC due to weathering appear to be limited to the immediate surface of samples and along fracture sets.