TESTING THE SIGNATURE OF CLIMATE CHANGE ON SEDIMENTATION AND GEOMORPHOLOGY IN EASTERN GRAND CANYON TRIBUTARIES

The Pleistocene stratigraphy of eastern Grand Canyon provides insight into desert responses to past climate change. For example, hillslope and fluvial deposits in the Lava Chuar and Comanche tributary drainages record strong cycles of aggradation and incision resulting from Milankovich-scale shifts in climate. Previous work on these deposits reveals a possible 10+ ky depositional lag time of tributary sedimentation behind the mainstem Colorado River. To test and quantify the possibility of time-transgressive sedimentation along tributary drainages, we are recording patterns in process and age by applying detailed sedimentology and geochronology to deposits resting in the heads, trunks, and mouths of the study tributaries.

The optically-stimulated luminescence dating method is being used to date tributary stream aggradation, and facies descriptions taken along key deposits are being used for interpretation of processes. Initial results show the S3 stream deposit (50-34 ky) thickening in upper-middle catchments, where debris flow deposits are present but largely reworked by streamflow processes. The S3 then grades to massive colluvium at the headwaters of catchments, which represents simultaneous accumulation from mass-movements of hillslope materials on toes of escarpments. By contrast, the S2 deposit (11-7 ky) does not thicken up drainages; rather it thins up and issues from gullies cut into the S3. These observations are consistent with similar processes acting today, but they attest to changing sediment storage and transport capacity with variable climate.