CLOGGING THE GRAND WASH AND LOWER COLORADO RIVER WITH SEDIMENT: TRIBUTARY AND MAINSTEM RIVER RESPONSE TO MIS 5-4 CLIMATE CHANGE

Downstream of the Grand Wash Fault, the Lower Colorado River exits the Colorado Plateau and enters the Basin and Range. Although largely obscured by reservoirs today, early geologists noted prominent high terraces fills throughout the Lower Colorado valley. Equally impressive are the thick Pleistocene terrace fills of Grand Wash, the first major tributary to the Colorado downstream of the Grand Wash Cliffs.

Terrace and alluvial sequences from Grand Wash and the Lower Colorado were mapped, described and sampled for optically stimulated luminescence (OSL) dating. Age results from Grand Wash indicate that the T5 terrace (40 m above the modern wash) is middle Pleistocene in age (300-200 ka) and the T4 terrace (25-30 m) is 130-120 ka. The prominent T3 terrace (17-20 m) dates to 100-80 ka, with an OSL age of 60 ka at its surface. The T2 terrace (5-7 m) is 23-11 ka and the T1 terrace (2-4 m) is Holocene in age.

OSL ages from Lower Colorado River indicate that the bulk of the fine-grained alluvium near Laughlin NV (170 km from the Grand Wash Cliffs) was deposited between 70-60 ka. Capping and inset coarse sands and gravels near Laughlin and upstream at Sandy Point (15 km from the Grand Wash Cliffs) date to 57-50 ka. Basal sandy deposits were dated to 95 ka.

Results from this study indicate that the main fill terrace in the Lower Colorado River correlates to the dominant M3 terrace in the Eastern Grand Canyon (70-60 ka) (Anders et al., 2005). Similar minor incision and strath terrace formation at 60-50 ka is seen in both locations. These correlations provide the first long distance long profile of the Colorado River and indicate nearly synchronous aggradation of the whole system in response to MIS 4 climate change and headwater glaciation.

Perhaps more interesting is the record of aggradation in Grand Wash at 100-80 ka (MIS 5c/b). This time period follows last-interglacial conditions and represents initial climate instability leading up to MIS 4 glaciation. While little is know about the regional climate during this interval, it is likely that increased precipitation caused striping of hillslope sediments and aggradation of Grand Wash. Similar aggradation is seen in Eastern Grand Canyon tributaries at this time (DeJong, 2006) and represents a previously unrecognized significant geomorphic event in the region.