COSMOGENIC 3HE AGES OF ALLUVIAL-FAN TERRACES IN THE LOWER COLORADO RIVER

Along the lower Colorado River corridor below Hoover Dam, Quaternary alluvial fans shed from the adjacent Black Mountains near Topock, AZ interfinger with and overlie main-stem Colorado River sands and gravels. These alluvial fans, referred to as piedmont gravels by previous researchers, grade to various terrace levels that correspond with previous elevations of the Colorado River. Using cosmogenic ³He, we have dated seven of these alluvial surfaces with ages ranging from 5 ka to 2.5 Ma. Quaternary geologists have suggested that hillslope response to climate change in the southwestern United States is largely controlled by an increase in drainage density which follows aridification and a decrease in vegetation cover during a glacial-to-interglacial transition. For example, it has been reported that during the Pleistocene-Holocene climate transition in the southwest, summer precipitation increased, runoff decreased, and desert areas with intense monsoonal rains had a major phase of hillslope debris-flow activity followed by fanhead trenching and distal fan progradation. Researchers have published a few cosmogenic isotopes, magnetostratigraphic, and uranium-series dates in California, Arizona, and Nevada that suggest a regional correlation between the deposition of alluvial terraces and transitions from glacial to interglacial climates. Geologists have also hypothesized that the aggradation and downcutting of the Colorado River is controlled primarily by climate and that mainstem and tributaries response to climate changes is recorded in fluvial and alluvial-fan terraces. Absolute dates for the ages of Quaternary deposits on the lower Colorado River are rare. Our preliminary ³He ages constrain the timing of aggradation and incision in the lower Colorado River corridor and can be used to test these hypotheses. Our exposure ages can also be used to develop a regional correlation of coarse-grained sediments deposited during periods of climate change.