THE PALEOGENE CALIFORNIA RIVER: EVIDENCE OF MOJAVE-UINTA PALEODRAINAGE FROM U-PB AGES OF DETRITAL ZIRCONS

U-Pb age spectra of detrital zircons in samples from the Paleogene Colton Formation in the Uinta Basin of northeastern Utah and the Upper Cretaceous McCoy Mountains Formation of southwestern Arizona are statistically indistinguishable. This finding refutes previous inferences that arkosic detritus of the Colton was derived from cratonic basement exposed by Laramide tectonism, and instead establishes the Cordilleran magmatic arc (which also provided sediment to the McCoy Mountains Formation) as the primary source. Given the existence of a north-south trending drainage divide in eastern Nevada and the north-northeast direction of Laramide paleoflow throughout Arizona and southern Utah, we infer that a large river system headed in the arc of the Mojave region flowed northeast ~700 km to the Uinta Basin. Named after its source area, this Paleogene California River would have been equal in scale but opposite in direction to the modern Green-Colorado River system, and the timing and causes of the subsequent drainage reversal are important constraints on the tectonic evolution of the Cordillera and the Colorado Plateau.