NEW CONSTRAINTS ON THE TIMING OF COLORADO RIVER INTEGRATION FROM THE DATING OF AXIAL-BASIN DEPOSITS IN COTTONWOOD VALLEY, AZ

Controversies about the timing and processes of Colorado River integration are testable with improved geochronology on the Bouse Formation, which records the arrival of Colorado River sediment to the lower Colorado River corridor (LOCO), downstream from Lake Mead. Previous work indicates that the highest Bouse outcrops in the southernmost LOCO area are 4.87 ± 0.02 Ma based on Ar/Ar dating (all Ar/Ar ages recalculated to a common decay constant and monitor age) and magnetostratigraphy suggests an age of 5.33 Ma for the arrival of Colorado River sediment to the proto-Gulf of California.

Upstream, in the northern LOCO area (Cottonwood Valley, AZ), pre-integration axial-basin deposits (Lost Cabin beds) constrain the maximum depositional age of the overlying Bouse Fm. Previous Ar/Ar dating of a 5.66 ± 0.06 Ma ash in the Lost Cabin beds indicates the Bouse Fm. there is younger. A stratigraphically higher ash newly dated by Ar/Ar at 5.35 ± 0.07 Ma places a younger constraint on the base of the Bouse Fm. The thickness between the ashes allows for estimates of end member sedimentation rates, between 20 and 300 m/Ma, assuming a wide range of basin geometries. Extrapolation of those rates to the top of the axial-basin deposits, >15 m higher, (and the base of the Bouse Fm.) and error propagation suggests that the Colorado River arrived to the northern LOCO area between 5.33 and 4.53 Ma. The presence of multiple soil horizons above the newly dated ash suggests slow depositional rates, which correspond to younger estimates for the age of the Bouse Fm.

Pre-integration basin geometry, defined by Miocene extension, requires that the river arrived in the northern LOCO area before reaching the proto-Gulf of California. Thus, the river can’t first arrive to Gulf at 5.33 Ma and then to the upstream northern LOCO area afterwards. This implies near instantaneous integration at ca. 5.3 or that some or all of the geochronology or its interpretation is flawed. The Ar/Ar data could be consistent with river integration throughout the LOCO area and to the Gulf at ca. 5.0 - 4.9 Ma if the 5.33 Ma magnetostratigraphic correlation is flawed. Ongoing work focuses on additional dating of deposits and magnetostratigraphy to further test the chronologies and refine estimates of the timing of Colorado River integration throughout the lower Colorado River corridor.