ENHANCED HILLSLOPE SEDIMENT SUPPLY TO THE COLORADO RIVER AND ITS TRIBUTARIES DURING MIS 5-3

Fluvial deposits from the Colorado River and its tributaries in eastern Grand Canyon and Grand Wash Trough were mapped and described. Results from the Eastern Grand Canyon indicate mainstem Colorado River aggradation of the M3 terrace at 60-75 ka was followed by incision and fill-cut terrace formation at 40-55 ka. Tributaries show out-of-phase aggradation at 90-100 ka (S4), 30-50 ka (S3), ~20-10 ka (S2) and during the Holocene (S1). Aggradation of the dominant S4 and S3 terraces can be linked to increased hillslope sediment supply, while smaller S2-S1 terraces represent reworking of older sediment.

Terrace deposits from Grand Wash, the first tributary of the Colorado River downstream of the Grand Wash Cliffs, were described and sampled for optically stimulated luminescence (OSL) dating. Age results indicate that the T4 terrace (25-30 m) is 120-130 ka, and the aerially extensive T3 terrace (17-20 m) deposit dates to 80-100 ka. The T2 terrace (5-7 m) is 11-23 ka and the T1 terrace (2-4 m) is Holocene in age.

OSL ages from Lower Colorado River indicate that the fine-grained alluvium near Laughlin NV (170 km downstream from the Grand Wash Cliffs), in part mapped as the Chemehuevi Formation, was deposited between 60-70 ka. Capping and inset coarse sands and gravels near Laughlin and upstream at Sandy Point (15 km from the Grand Wash Cliffs) date to 50-60 ka.

Compilation of these results suggest that the dominant M3 terrace in the Eastern Grand Canyon (60-75 ka) correlates to main fill terrace in the Lower Colorado River (60-70 ka), although there is evidence for aggradation becoming younger downstream. Minor incision and fill-cut terrace formation at 40-60 ka is seen in both locations. However, enhanced sediment supply from Grand Canyon tributaries at 30-50 ka (S3) stayed off ensuing headwater driven incision at this time. These correlations provide the first long profile of the Colorado River in the region and indicate nearly synchronous whole system aggradation in response to MIS 4 headwater glaciation.

Tributary records from the eastern Grand Canyon and Grand Wash indicate significant aggradation during MIS 5 (80-100 ka) and MIS 3 (30-50 ka). These periods of regional aggradation were due to increased stripping of hillslope sediment and represent previously unrecognized climate-driven geomorphic events in the region during MIS 3-5.