THE ANCIENT CALFORNIA AND ARIZONA RIVERS AND IMPLICATIONS FOR THE UPLIFT AND EROSION HISTORY OF THE SOUTHWESTERN US

Recently published thermochronological and paleoelevation studies in the Grand Canyon region, combined with sedimentary provenance data in both the coastal and interior portions of the Cordillera S and E of the Sierra Nevada, place important new constraints on the paleohydrological evolution of the SW US. Review and synthesis of these data suggest incision of a large canyon from a plain of low elevation and relief in the Cordilleran foreland, to a canyon of roughly the length and depth of modern Grand Canyon, occurred primarily in Campanian time (80-70 Ma). Incision was accomplished by a main-stem, NE-flowing antecedent river with headwaters on the NE slope of the Cordillera in California, referred to after its source region as the California River. Subsequent topographic collapse of the headwaters region and coeval uplift of the Cordilleran foreland during the Laramide orogeny reversed the river’s course. After reversal, its headwaters lay in the ancient Mojave/Mogollon highlands region of Arizona and eastern California. This system is also referred to after its source region as the Arizona River. From Paleogene until 6 Ma, the interior of the Colorado Plateau was separated from the Arizona River drainage by an asymmetrical divide in the Lees Ferry-Glen Canyon area, with a gently sloping NE flank that drained into large interior basins, fed by in part by recycled California River detritus shed from Laramide uplifts on the plateau. By 20 Ma, a pulse of unroofing had lowered the erosion level of eastern Grand Canyon to close its present level, and the Arizona River drainage below modern Grand Canyon was tectonically disrupted, cutting off the supply of interior detritus to the coast. Increasing moisture in the Rocky Mountains in late Miocene time reinvigorated fluvial-lacustrine aggradation NE of the asymmetrical divide, which was finally overtopped between 6 and 5 Ma. This event reintegrated the former Arizona drainage system through a cascade of spillover events through Basin and Range valleys, for the first time connecting sediment sources in Colorado with the coast.