EARLY PLIOCENE SEDIMENT PULSE DOWN THE NEWLY INTEGRATED COLORADO RIVER — A RESPONSE TO DOWNCUTTING THAT DESTABILIZED THE MIOCENE COLORADO PLATEAU LANDSCAPE

The early Pliocene Bullhead Alluvium records a sediment pulse in the lower Colorado River downstream from Grand Canyon shortly after the river became fully integrated to the Gulf of California from its huge drainage area in the Colorado Plateau and Rocky Mountains. Abundant sediment loads temporarily aggraded the river’s bed by over 200 m, forming sand-and-gravel braidplains as wide as 50 km in broad valleys interspersed with canyon reaches. Analysis of the longitudinal profile of the built-up bed suggests that the river temporarily extended its delta far out into the Gulf of California after 4.25 Ma and before 3.3 Ma. Sedimentation rates had to out-compete both rapid subsidence of the delta (2 mm/yr) and rapid translation of the delta by plate-boundary faulting on the San Andreas system (43-45 mm/yr). Detrital zircons in these sediments implicate much denudation of Tertiary cover strata on the Colorado Plateau (Kimbrough et al., 2015), and the cosmogenic ¹⁰Be in the sediments implies slow watershed erosion rates (Matmon et al., 2012). We infer the cosmogenic data record provenance from a relict landscape of regolith, colluvium, and their locally stored detritus. Downcutting triggered by river integration evidently destabilized an erodible, relict Colorado Plateau landscape, which supplied debris for river delivery to the rapidly accumulating Bullhead Alluvium and the prograding Colorado River delta.