PLEISTOCENE STRATIGRAPHY AND GEOCHRONOLOGY OF THE COLORADO RIVER AT LEES FERRY, ARIZONA

Debate about the timing and causes of uplift and incision of the Colorado Plateau has been ongoing since the late 1800's when geologists first began to explore the deep canyons and excellent rock exposures of the region. Recent studies from sites along the Colorado River in western Grand Canyon, eastern Grand Canyon, Glen Canyon, and along the tributary San Juan and Fremont Rivers have reported variable bedrock incision rates that indicate differential incision along the Colorado River drainage through the Quaternary. Lees Ferry lies in the heart of this region at an inflection in long-profile gradient and contains a well-exposed suite of Pleistocene deposits and terraces. We are working to establish an accurate chronostratigraphy of the fill terraces at Lees Ferry through cosmogenic exposure (10Be), optically-stimulated luminescence (OSL), stratigraphic, and sedimentologic methods in order to calculate long-term incision rates of the Colorado River and understand terrace formation at this central location.

Seven distinct deposits (Qt1-Qt7) and two additional erosional terrace levels (Qt4y and Qt6y) have been mapped and surveyed with total-station equipment. Cosmogenic samples have been analyzed from desert pavement surfaces on six terraces, including two sets of cosmogenic depth profiles to correct for inheritance. In addition, several OSL samples have been analyzed from stratigraphic profiles in order to constrain the timing of deposition and subsequent floodplain abandonment and calculate long-term incision rates. Excellent initial cosmogenic-profile results appear to suggest that incision rates at Lees Ferry are relatively higher than others calculated in the region. However, the rates are in fact broadly consistent with values reported downstream in Grand Canyon when the calculation is integrated over more than a single data point from the oscillating fluvial history.