A MAJOR PLIOCENE FLUVIAL AGGRADATION ALONG THE LOWER COLORADO RIVER AND ITS IMPLICATIONS FOR COLORADO RIVER EVOLUTION AND TECTONICS
Perched Bullhead deposits overlook the modern delta of the Colorado River from as high as 175 m asl and project toward intersection of Pliocene sea levels 300–450 km away. This is about twice the river distance to both the modern river’s mouth and a paleoshoreline correlated to 4.25 Ma (in the Fish Creek-Vallecito basin as restored for San Andreas faulting). The projection suggests the likelihood that aggradation peaked after 4.25 Ma and lengthened the delta seaward despite delta subsidence and foreshortening by steady northwestward tectonic transport on the San Andreas Fault. Incision that followed the end of Bullhead aggradation implies a declining sediment load, shorter river profile and delta, and lowered delta progradation rate that could not outcompete plate-motion foreshortening and subsidence of the delta. Longitudinal profiles of the Bullhead sequence argue against younger regional tilting or uplift affecting the length of the lower Colorado River corridor, but suggest that the upper Lake Mead area and lower Grand Canyon block have uplifted >200 m on the Callville and Wheeler faults and that the Blythe basin locally subsided the basal Bullhead >100 m to below sea level.