FLUVIAL LANDSCAPES DURING THE EOCENE-OLIGOCENE TRANSITION IN CENTRAL NORTH AMERICA

Rivers play a critical role in shaping the Earth’s landscapes by transporting sediment from mountains to sedimentary basins. However, river systems are highly sensitive to tectonic or climatic perturbation, which can alter hydraulic geometry of channels, mass fluxes, and flood frequencies and duration. The Eocene-Oligocene Transition (EOT) at 33.9 Ma was a major global cooling event in the Cenozoic. It is characterized by a long term ~4-5°C decrease in global mean annual temperature, punctuated by short, intense climatic fluctuations. In central North America, including the Great Plains region, 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 use the sedimentary record to quantify changes in river and floodplain morphodynamics that occurred in the Great Plains region as a result of these dramatic paleoenvironmental changes.

We characterized morphodynamic changes through the Late Eocene-Early Oligocene White River Group, northwestern Nebraska, using sedimentary structures keyed to grain-size distributions and reconstructed channel hydraulic geometries. During the latest Eocene, landscapes transitioned from perennial streams with stable floodplains, coarse-grained sediment loads and minimal discharge variability, to rivers with high silt and clay sediment loads and extensive floodplain deposits. This transition reflects the influence of transported volcanic ash in streams, which increased the fine-grained sediment load. At the onset of the Oligocene, rivers transitioned to ephemeral streams with significant discharge variability and high avulsion frequency. This major shift in river dynamics likely reflects the short, intense climatic fluctuations associated with the early phase of the Eocene-Oligocene transition. In conclusion, the fluvial system records the long term influence of topographic change but is fundamentally reshaped by enhanced volcaniclastic sediment input and short term climatic fluctuations, superimposed on the regional aridification and cooling associated with the EOT.