PALEOENVIRONMENTS AND CARBON CYCLING IN THE PALEOGENE HOBACK BASIN, WY

The late Paleocene and early Eocene were marked by major carbon cycle perturbations, including the Paleocene-Eocene Thermal Maximum (PETM) and subsequent early Eocene hyperthermals. These perturbations are recorded in sediments as distinctive negative carbon isotope excursions (CIEs). Terrestrial records of these events are valuable for understanding the response of terrestrial ecosystems to warming, and for comparison with marine records to understand the global carbon cycle. The Laramide orogeny resulted in numerous Paleocene-Eocene basins in the Rocky Mountains, including the Hoback Basin which sits between the Hoback and Gros Ventre ranges in Wyoming, separated from the better-known Green River Basin to the south by a low ridgeline. Ages of the Hoback Formation (late Paleocene – early Eocene) and the Pass Peak formation (early Eocene) have been constrained by vertebrate paleontology and palynology. Despite concurrent deposition with the well-studied, PETM-recording Fort Union and Wasatch formations, there has been no quantitative geochemical investigation conducted on Hoback Basin formations until now. New sampling of the synorogenic Hoback Formation spanned 130 meters, with mudstones, lignite seams, Histosols, and lenticular sandstone indicating a low-energy floodplain environment with braided streams and swamps. The post-orogenic upper Pass Peak Formation exposure spanned 55 meters with conglomerate channel deposits, Entisols, and floodplain sequences that reflect a forested alluvial plain proximal to the Wind River Range, preserving fossil wood, charcoal, and a macginitiea leaf impression. Both sections yielded organic material including lignite, plant material, rhizoliths, and gastropods suitable for carbon isotope analysis. This chemostratigraphic record will be compared with published records from other sources, allowing identification of any hyperthermals recorded as CIEs within these Hoback Basin outcrops. Calculated δ­¹³C_atm­ values using preliminary δ­¹³C­­^­_org ^­results show a mean of -5.55‰ ± 1.0‰ for the Hoback Formation and -5.66‰ ± 1.1‰ for the Pass Peak Formation, comparable to literature values for Early Eocene δ­¹³C_atm estimated with benthic foraminifera and consistent with a volcanic CO₂ source maintaining globally warm conditions in the Paleogene.