REDEFINING THE AGE OF THE COLORADO RIVER

Detrital sanidine dating coupled with magnetostratigraphy indicates that the Colorado River was first integrated from the Colorado Plateau to the proto-Gulf of California at least half a million years later than previously argued. In Cottonwood Valley, ⁴⁰Ar/³⁹Ar dating of a 5.37 Ma ash in pre-Colorado River axial-basin deposits plus magnetostratigraphic analyses indicate that the overlying Bouse Formation, which records arrival of the Colorado River, was deposited after the beginning of the Thvera subchron, which started at 5.24 Ma. Detrital sanidine in the Bullhead Alluvium, the first coarse-grained aggradational package of the Colorado River, indicates a maximum depositional age of 4.6 Ma for that unit in the same area. At Split Mountain Gorge, new detrital sanidine dating coupled with previously published magnetostratigraphy and detrital zircon dating of Imperial Group sediments indicate that the first Colorado River sediment arrived at the proto-Gulf of California between 4.8 and 4.63 Ma (during the C3n.2r subchron), not at 5.3 Ma as has been previously proposed. The new geochronology supports models for rapid downward integration of this continental-scale river system extending its reach from Cottonwood Valley after 5.24 Ma to the opening Gulf of California between 4.8 and 4.6 Ma. This is consistent with the previously dated 5.0-4.9 Ma Lawlor tuff interbedded in the Bouse Formation at the highest levels in the Blythe basin, which records the last filling of that basin prior to integration of the river system to the proto-Gulf of California. Additionally, the data suggest there was little or no hiatus between integration of the Colorado River, incision into the siliciclastic Bouse Formation, and initial deposition of the Bullhead Alluvium, which seems to be a response to rapid profile changes caused by integration.