Evolution of Fluvial Landscapes during the Eocene-Oligocene Transition in Central North America: Reconstructions from Toadstool Geologic Park, Nebraska, U. S. A.

Rivers drive geomorphic change at Earth’s surface by transporting sediment from mountains to sedimentary basins. They are sensitive to changes in water and sediment flux driven by tectonism, climatic perturbation and or volcanism. We characterized changes in fluvial landscapes during a major Cenozoic cooling event, the Eocene-Oligocene Transition (EOT) at 33.9 Ma. The EOT is characterized by a long term ~4-5°C decrease in global mean annual temperature, punctuated by short, intense climatic fluctuations. In the Great Plains region of central North America, the temperature decreased by ~7°C and mean annual precipitation declined by ~50% across the EOT. Preceding the EOT, renewed Cordilleran uplift near northeastern Nevada began at ~39 Ma and extensive volcanism in the Great Basin region occurred at ~36 Ma. We reconstruct characteristics of rivers and floodplains through the Late Eocene-Early Oligocene deposits of White River Group (WRG), exposed at Toadstool Geologic Park in northwestern Nebraska, U.S.A., and evaluate how river landscapes responded to these events. We identified five stages of change in the paleo-rivers and -floodplain strata of the White River Group: (1) Following the Laramide uplift, the rivers of the Late Eocene Chamberlain Pass Formation adjusted from steep gradients (~10-3) to gentler ones (~10-4), and transitioned from shallow, mobile channels to deeper, stable channels and floodplains. (1) The transition from the Chamberlain Pass Formation to the Late Eocene Chadron Formation saw a shift from a relatively coarse-grained fluvial system with mobile channels to an extremely fine-grained, aggradational, floodplain-dominated system influenced by high volcanogenic sediment loads. (3) The transition from the Late Eocene Chadron Formation to the Early Oligocene Orella Member of the Brule Formation is associated with the onset of the Eocene-Oligocene Transition (EOT) and is marked by coarse floodplains and ephemeral river deposits that display evidence of significant, likely seasonality-driven, discharge variability. (4) Early Oligocene strata (33.9 - 31.6 Ma) also show compelling evidence of recurrent episodes of sustained channel incision and fill that we connect to climate fluctuations associated with the early phase of relatively rapid EOT cooling. (5) Early Oligocene strata deposited from 33.4 to 31.6 Ma show no significant erosional surfaces and continue to reflect discharge variability, high volcanogenic sediment loads, and a relatively stable climate system after the end of the initial phase of rapid cooling.