GEOMORPHOLOGIC CONTROLS ON CLIMATIC INTERPRETATIONS IN A MIXED FLUVIAL-AEOLIAN SYSTEM: A DEEP TIME PERSPECTIVE FROM THE PALEOGENE WHITE RIVER GROUP OF NORTHWEST NEBRASKA

The White River Group of the North American midcontinent preserves a gradual shift from fluvial to aeolian depositional environments across the Eocene-Oligocene greenouse to icehouse climatic transition (EOT). At Toadstool Geologic Park (TGP) in northwest Nebraska, the late Eocene Chadron Formation consists of predominantly fluvial deposits, whereas the early Oligocene Brule Formation contains mixed alluvium and volcaniclastic loess in the Orella Member and predominantly volcaniclastic loess in the overlying Whitney Member. Alluvial paleosols of the Chadron Formation transition from sub-humid to semi-arid environments leading up to the E-O boundary. We analyzed 20 paleosols from 8 profiles that span the fluvial-aeolian transition in the overlying Brule Formation to test whether drying trends continued into the earliest Oligocene. Reconstruction of paleogeomorphology was achieved by combining paleosol analysis at key stratigraphic intervals and using marker ash beds to trace paleotopographic surfaces. In the mosaic landscapes of the Orella Member, upland paleosols formed on strictly loess while lowland paleosols formed on mixed loess and alluvium. Across the Orella-Whitney Member transition, alluvial landscapes interfinger with loessic landscapes, and then give way to strictly aeolian environments in the Whitney Member. Alluvial and loessic paleosols followed separate pedogenic pathways, wherein alluvial paleosols developed via top-down pedogenesis and loessic paleosols were constructed through pedogenic upbuilding. Although macroscopic features differ drastically between fluvial and aeolian paleosols in the Brule Formation, microscopic analysis shows no great change in pedogenic maturity. Geochemical profiles in all paleosols of the Brule Formation are uniform, which is likely a result of continuous loess deposition buffering base loss rather than diagenesis. The results of climofunction calculations from five paleosol B horizons indicate that the MAP of 600-800 mm/y and MAT of 9-10.5 °C were consistent in the early Oligocene. These findings indicate that sedimentation rate and style—not climate—were the primary control on paleosol morphology and maturity in the Brule Formation.