Late Eocene Terrestrial Paleoclimatie Record From The White River Formation At Flagstaff Rim, Wyoming, USA

Abstract

Flagstaff Rim near Casper, WY preserves the most complete late Eocene section of the White River Group with over 219 m of late Eocene age sediment compared to 35 m preserved at Toadstool Park, NE. While Flagstaff Rim does not span the Eocene-Oligocene transition, it holds the earliest clues in the White River Group of a late Eocene (37-34 Ma) climatic deterioration. In this study 8 paleosols were collected, described and analyzed based on pedogenic features, mineralogy, and geochemistry, above and below dated volcanic ash beds. The lowermost paleosol is composed of smectite- rich red mudstone, with greenish gray drab haloes, and weather into hummocks. The sediments within this part of the section are the lithologic equivalent of the Peanut Peak member of Toadstool Park, NE and reflect a moist humid environment. Overlying these sediments is the lithologic equivalent of the Big Cottonwood Creek member. These sediments are comprised of smectite poor mudstones and yellow/beige sandstones, are indurated with calcium carbonate, and reflect a more arid environment. The transitional zone between the Peanut Peak and Big Cottonwood Creek lithologies corresponds with an increase in volcanism from the Great Basin, impact events, and building of ephemeral glaciers on Antarctica. The impact events and increase in volcanism, while synchronous with the transitional zone between the Peanut Peak and Big Cottonwood Creek lithologies, cannot explain the long term climatic perturbation, which persists within the White River Group. Instead, the climatic deterioration is likely explained by the building of ephemeral Antarctic ice sheets, which was compounded by the increase in volcanism and impacts. Regional variations in ä18O isotopes within the White River Group can likely explain the suggested variations in paleoclimate across the Eocene-Oligocene transition.