RIVERS, CLIMATE AND TECTONIC UPLIFT: RAPID RIVER RESPONSE TO EOCENE-OLIGOCENE CLIMATE CHANGE IN CENTRAL NORTH AMERICA

During the Late Eocene, moderate uplift of the southern North American Cordillera triggered aridification of central North America due to the development of a regional rain shadow. Regional drying was reflected in a time-transgressive wave of loess deposition that started at approximately 36 Ma in the central Rockies and expanded eastward to the Great Plains. This expansion occurred across the ~34 Ma Eocene Oligocene Transition (EOT), a global climate change event associated with ~6°C decrease in regional temperature and ~50% decline in mean annual precipitation. In western Nebraska, the onset of loess deposition occurred at ~31.6 Ma. We studied the late Eocene - early Oligocene fluvial strata that predates eolian deposition in this region to ascertain potential impacts of climate change on river landscapes of this time.

From Early Eocene to Oligocene, floodplains transitioned from relatively static with well-developed paleosols, to highly dynamic environments marked by abundant crevasse splays and weakly developed paleosols. Streams transitioned from perennial channels with limited discharge variability, to ephemeral with significant discharge variability. The Eocene streams produced channel sand bodies ~11 m thick (~3 times channel depth), indicating relatively low avulsion frequency. On the other hand, Oligocene channel sand bodies were ~4 m thick (1-1.5 times channel depth), indicating relatively high avulsion frequencies. These changes in fluvial dynamics and stratigraphic architecture occurred within a 1 - 2 Ma timeframe that spans the EOT at ~34 Ma. We therefore conclude that, superimposed upon the longer term trend of regional drying in North America that is associated with Cordilleran uplift, a rapid reorganization of fluvial landscapes pre-empted the eastward migrating wave of eolian sedimentation in central North America. Global cooling and drying during the EOT was the most likely driver of this change.