A MAJOR PLIOCENE FLUVIAL AGGRADATION ALONG THE LOWER COLORADO RIVER AND ITS IMPLICATIONS FOR COLORADO RIVER EVOLUTION AND TECTONICS

Analysis of the ~200-m thick Pliocene river-laid Bullhead alluvium along the lower Colorado River downstream of Grand Canyon offers insights into (1) the rapid early erosional history of the river’s drainage basin, (2) successive longitudinal profiles of the river with implications for the evolving position of the river’s mouth and delta, and (3) constraints on regional and local deformation. The base of the unit is inset well below deposits of the Hualapai and Bouse Formations that immediately pre-date the Bullhead deposition, and defines a locally deformed approximate pre-aggradation river profile. The Bullhead aggradational sequence temporarily raised the lower Colorado River’s grade by at least 200 m and built up braid plains as wide as 50 km. It represents a pulse lasting roughly from about 4.5 to 3.5 Ma that suggests invigorated incision of the Colorado Plateau. We interpret the top of the aggradational sequence as untilted from Lake Mead downstream >500 km to near the plate boundary because its downstream slope of 0.05–0.06% matches that of the Chemehuevi Formation, a late Pleistocene aggradational sequence.

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.