THE INFLUENCE OF DISTRIBUTIVE FLUVIAL SYSTEMS ON INTERPRETATIONS OF PALEOCLIMATE ACROSS THE TERRESTRIAL EOCENE-OLIGOCENE BOUNDARY (EOB) OF THE NORTHERN GREAT PLAINS

Modern Distributive Fluvial Systems (DFS) form in tectonically active areas and display proximal to distal changes in fluvial style, facies and pedology that are mediated by lateral changes in hydrologic budgets. Down-gradient flow of groundwater manifests as spring lines, with deposits below the spring line dominated by relatively wetter soils and depositional environments. As DFS prograde, areas previously dominated by wetter environments and soils are replaced by deposits and soils of drier environments. In the rock record, the progradation of such a DFS would generate up-section changes in nonmarine facies and paleopedology, with lower parts of the section dominated by paleosols influenced by greater amounts of water and lower sedimentation rates overlain by progressively drier soil conditions. At any one locality this upward trend could be interpreted as paleoclimatic change. Analyses of temporally constrained sections across a broad region within a particular depositional system are required to test hypotheses of climate change versus DFS. The Eocene-Oligocene White River Group (WRG) of the northern Great Plains provides a unique opportunity to determine the relative influence of climate vs. DFS across a major period of global climatic change. The WRG is a succession of fluvial, lacustrine and eolian deposits that have long been interpreted as a record of climate change from warm and humid to cooler and drier conditions. The data used to support this interpretation are varied and include paleontology, sedimentology, fluvial geomorphology, and paleopedology. Recent investigations utilizing stable isotopic values of fossil bone carbonate suggest a large temperature drop across the EOB, but negligible change in water stress. Although phytoliths indicative of forests are present into the Oligocene, paleosols exhibit a decrease in development and root size. The disparity between various datasets regarding the rate and magnitude of this climate shift may be due to regional stratigraphic variability, variable preservation of the EOB interval, and insufficient chronologic control within and between sections. A DFS model for the WRG may bridge discrepancies between datasets and account for stratigraphic changes without invoking complex external forces such as climate change.