VERTICAL CHANGES IN PALEOSOL MORPHOLOGY WITHIN THE WHITE RIVER SEQUENCE AT FLAGSTAFF RIM, WYOMING: IMPLICATIONS FOR PALEOCLIMATIC CHANGE LEADING UP TO THE EOCENE-OLIGOCENE TRANSITION

Flagstaff Rim near Casper, WY preserves the most complete late Eocene section of the White River Sequence, approximately ~214 m compared to the ~35 m of section preserved at Toadstool Geologic Park, in northwest NE. While the Rim does not span the Eocene-Oligocene boundary it may hold clues to climatic perturbations. Some studies suggest that climate change across the Eocene-Oligocene boundary occurred in steps, possibly throughout the late Eocene. In this study 6 paleosol profiles were collected and analyzed above and below dated volcanic ashes at Flagstaff Rim. The lowermost paleosol is composed of red mudstone with clay filled and grayish green reduced root traces. The sediments within this profile are similar to those of the Interior Paleosol Series of the Chamberlin Pass Formation in South Dakota and Nebraska. This paleosol is smectite rich, and formed in a moist, oxidizing environment. Overlying this section is the lithologic equivalent of the Peanut Peak Member of northwest Nebraska. The sediments in this section are more olive yellow/beige in color and weather into hummocks. The paleosols within the Peanut Peak equivalent contain both clay filled and crystalline roots; there is also less smectite in this section than in the underlying Chamberlin Pass equivalent. Overlying the Peanut Peak equivalent is the lithologic equivalent of the Big Cottonwood Creek Member. These paleosols are cemented by calcite and preserve calcite filled root traces. The increase in glass, pumice fragments, calcium carbonate, and a decrease in smectite all suggest that this soil formed either in a drier environment or that there was an increase in sedimentation rates. Changes in the resistance to modern weathering have also been noted in Flagstaff Rim. The exposures at the base of the Rim weather into large hummocks, whereas up section the sediments are more cliff forming. This same change in weathering has been noted at Toadstool Geologic Park, NE and has previously been interpreted as a change in climate, and/or also as an increase in sedimentation rates, suggesting that either may be at work during the late Eocene at Flagstaff Rim, WY.