PALEOCLIMATIC CHANGE ACROSS THE TERRESTRIAL EOCENE-OLIGOCENE BOUNDARY OF THE NORTHERN GREAT PLAINS: THE PALEOSOL PERSPECTIVE

Paleosols of the White River Group preserve a detailed record of paleoclimatic change across the terrestrial Eocene-Oligocene transition (EOT) throughout the northern Great Plains, although differences in basin histories across this region have resulted in different degrees of stratigraphic completeness and subsequent pedologic responses. Once placed within a regional stratigraphic framework, the data from paleosols can be compared to other forms of paleoclimatic records, such as stable isotopes, facies models, and paleontology. In general, the degree of stratigraphic completeness decreases from southwest to northeast. The most complete sections are from the Douglas, WY and Crawford, NE areas, whereas the section in Badlands National Park, SD is condensed with pronounced unconformities. Paleosols from northwest Nebraska show a transition from forested conditions within the middle of the late Eocene Chadron Formation to progressively more open conditions toward the top of the unit. Drier conditions are suggested by a change in soil types, root morphology, and progressively shallower pedogenic carbonate accumulations. This trend continues within the overlying early Oligocene Brule Formation, with progressively arid climates manifested as well drained soils with small roots that formed within a volcaniclastic loessite. This same trend is seen in South Dakota, although the EOT is lost due to an unconformity of approximately 1.7 my at the Chadron-Brule contact. Paleosols in the Big Badlands, SD area appear to be responding to a combination of asymmetric basin subsidence, sediment supply, and climate change. This has manifested as well developed, widespread paleosols that reflect periods of basin stability and/or low sediment input, laterally diverse paleosols that formed as a function of their proximity to channel systems, and weak paleosols that formed during periods of rapid eolian aggradation. Paleosols from northeast Colorado, although temporally equivalent to the Chadron Formation, are more similar to drier soils of the Brule Formation of NE and SD. This suggests that climate change was diachronous across the region.