CIRCA 4.8 MA AGE FOR INCEPTION OF THE MODERN COLORADO RIVER

The lower Pliocene Bouse Formation consists of basal travertine, marl, and overlying siltstone deposited in the lower Colorado River valley of western Arizona, southeastern California, and southern Nevada. The interpretation that the Bouse Formation was deposited in a chain of lakes following first arrival of Colorado River water to closed basins inherited from earlier Basin and Range extension is supported by (1) Sr isotopic similarity of Bouse carbonates and shells to Colorado River water, (2) flood deposits, derived from northern sources, directly beneath Bouse strata, and (3) maximum elevations of Bouse exposures that define individual lakes at maximum water level. Maximum Bouse Formation elevations of ~330m in Blythe basin, which is the southernmost of the Bouse basins, indicate that Bouse lake water would have spilled westward into Bristol basin. Geochemical correlation of a tephra layer interbedded with marl at two locations, one interbedded with Bouse marl at Buzzards Peak in Blythe basin (306m elev.) and the other at the only known exposure of Bouse-like strata in Bristol basin (292m elev.), indicates that these basins were flooded at the same time. Comparison of six major elements (Mg, Al, Si, Ca, Ti, Fe) in glass from these two tephra layers, and comparison with ~5600 samples in the USGS tephra database, indicate that these two tephra layers are more similar to each other (correlation coefficient = 0.9839) than to any of the other samples in the database. Statistical analysis also indicates correlation with the Lawlor tuff originating from the Sonoma volcanic field in the northern San Francisco Bay region. Plagioclase from the Lawlor tuff yielded a ⁴⁰Ar/³⁹Ar incremental-release isochron age of 4.834 ± 0.022 (2σ) Ma (Sarna-Wojcicki et al., 2011, Geosphere). Southward spillover of lake Blythe marks the final event in integration of the Colorado River drainage basin with the Pacific Ocean, and resulted in a major eastward shift in the continental divide. Inception of a through-flowing Colorado River initiated a hydrologic corridor for migration and dispersal of aquatic species and a hydrologic barrier that potentially segregated terrestrial species. Accurate dating of this event provides an opportunity to calibrate rates of molecular evolution for affected lineages.