DECIPHERING LOWER COLORADO RIVER INTEGRATION PROCESSES THROUGH GEOCHRONOLOGIC STUDIES OF THE BOUSE FORMATION: PRELIMINARY RESULTS AND FUTURE DIRECTIONS

Long-standing debates have focused on the depositional setting and age of the Bouse Formation with implications for lower Colorado River integration and timing of regional uplift. Competing models include: (1) entirely lacustrine deposition as the Colorado River spilled through a series of internally drained basins; or (2) downward-directed lacustrine integration into a pre-existing marine embayment in the southern Blythe basin. The marine hypothesis requires older and longer durations for Bouse Fm deposition.

We use multiple geochronometers to test these models. Previous work suggests a maximum age for the Bouse Fm in Cottonwood and Mohave valleys of 5.6 Ma, while the upper Bouse Fm in the Blythe basin is 4.8 Ma. New U/Th work on Bouse tufa indicates open system behavior, likely due to U gain during secondary carbonate precipitation, which could also affect other geochemical systems. U/Pb work on Bouse marl indicates a lack of heterogeneity, so a robust isochron is unattainable. Analyzed Bouse silica and Bullhead Alluvium petrified wood are post-depositional and may be as young as 2 Ma. New Ar-Ar ages on basalts and sanidine-bearing ash in units underlying the Bouse Fm yield ages of 9 – 10 Ma; similarly, the youngest detrital sanidine and zircon dates from Bouse sand are between 7 and 17 Ma. This is too old to inform the depositional age of the Bouse Fm, but the detrital ages provide important provenance information. New detrital zircon analyses on the stratigraphically lowest Colorado River sand in the Lake Mead area and in Cottonwood valley, compared to younger Colorado River deposits, suggest that Paleozoic zircon content increases with decreasing age. Detrital sanidine from some samples in both the Mohave and Blythe valleys show a majority of 17.2 Ma grains, which suggests downstream mixing or multiple sources throughout the lower Colorado River corridor.

Ongoing work focuses on Ar-Ar dating of sanidine and/or U-Pb dating of zircon in ashes from 3 locations: the proposed marine portion of the Hart Mine Wash section; in between the Bouse basal marl and the upper bioclastic unit of the Bouse Fm; and stratigraphically above the 5.6 Ma ash in the Lost Cabin beds. We are also in the process of analyzing a large suite of detrital sanidine samples and doing detailed magnetostratigraphic analysis on key Bouse Fm sections.