GEOCHRONOLOGIC STUDY OF PRE-COLORADO-RIVER DEPOSITS IN COTTONWOOD VALLEY, AZ: IMPLICATIONS FOR THE TIMING OF RIVER INTEGRATION

New age constraints allow for a refined assessment of the timing of Colorado River integration through a series of Miocene-aged extensional basins downstream from Grand Canyon. Here, we summarize existing constraints from upstream to downstream, present new Ar/Ar and paleomagnetic analyses, and describe their implications for Colorado River integration.

In eastern Lake Mead, dated pre- and syn-Colorado River deposits record river arrival between 6 and 4.5 Ma. In western Lake Mead and Black Canyon, dated basalt flows graded to well above river level support significant river incision after 4.9 Ma. In Parker Valley, midway through the river corridor, a paleomagnetic reversal in the Bouse Formation, which records integration, suggests river arrival there before 5 Ma. Further south in Palo Verde Valley, the 4.9 Ma Lawlor tuff is intercalated with Bouse Formation at the highest elevation outcrops in the valley. First arrival of Colorado River sand to the proto-Gulf of California is thought have occurred at 5.3 Ma based on magnetostratigraphy, but detrital zircon dating suggests a younger maximum depositional age. A recent model used these existing constraints and field observations to suggest initial sediment arrival to the proto-Gulf of California followed by upstream sediment trapping and marine reflooding of the basin in the Palo Verde area.

New Ar/Ar dating of a reworked ash in pre-Colorado River deposits in Cottonwood Valley, between Black Canyon and Parker Valley, and paleomagnetic analysis of the ash and overlying sediments indicates an eruption age of 5.35 Ma and deposition during the Thvera subchron. Multiple soil horizons between the ash and the base of the Bouse Formation require that the Colorado River arrived in this area well after 5.24 Ma, the beginning of the Thvera. Preliminary magnetostratigraphy on low elevation basal Bouse deposits in Palo Verde Valley indicate that Colorado-River-derived sediment there is mostly to entirely reversed polarity, consistent with deposition after the end of the Thvera, at 5 Ma. At least one reversal is present lower in the section, where both marine and lacustrine deposition have been argued. Together the data suggests more recent integration of the Colorado River to the proto-Gulf of California and that reflooding of the Palo Verde Valley might not be necessary.