Further support for a revised age of colorado river integration

New detrital sanidine geochronology from the Bouse Formation in Blythe basin supports recent research that redefined the age of lower Colorado River integration. Sanidine dating along with magnetostratigraphy indicates that the Colorado River arrived at Cottonwood Valley after 5.24 Ma and reached the proto–Gulf of California between 4.80 and 4.63 Ma. Earlier studies had proposed that initial integration to the evolving proto–Gulf of California occurred at 5.3 Ma; this was based largely on magnetostratigraphy tied to ~2 Ma old ashes ~4 km higher in the section than the deposits containing the first Colorado River sand. The discrepancy in ages was ascribed to duplication of section across newly identified faults in the Fish Creek-Vallecito basin. Detrital sanidine dating from near the first arriving Colorado River sand in this basin placed a much tighter constraint on the timing of integration than was previously available. The new detrital sanidine results reported here come from near the base of a thick section of siliciclastic Bouse Formation in the northern Blythe basin, which was the last in a series of basins that the Colorado River filled before spilling into the proto–Gulf of California. New single grain sanidine ages indicate a maximum depositional age of 4.8 ± 0.1 Ma for this section. This age should approximately correspond with the first arrival of Colorado River sediment to that basin. Previous dating of single sanidine grains from the Bouse Formation indicated a similar age; currently 6 grains have been dated at younger than 5.3 Ma with increasing precision. Because the Colorado River had to pass through this basin prior to reaching the proto-Gulf of California, deposition should predate not postdate integration, supporting the new younger age. These new results and previous work are consistent with downward integration of the Colorado River through a series of closed basins in the lower Colorado River corridor, culminating in integration to the proto–Gulf of California between 4.8 and 4.63 Ma. During and after integration, the Colorado River quickly incised through sediment-filled basins and the liberation of sediment from throughout the entire system led to deposition of hundreds of meters of Bullhead Al­luvium starting around 4.6 Ma as the river adjusted to its lower base level.