Zircon age and oxygen isotopic correlations between Bouse Formation tephra and the Lawlor Tuff

The Bouse Formation in the lower Colorado River trough holds an important record of the onset of the modern drainage patterns in the southwestern United States. It comprises calcareous and clastic infi ll deposited during fl ooding of several basins, including the Bristol and Blythe subbasins of Lake Bouse. An intercalated ash bed, which is key to constraining its depositional age, is exposed in two locations, Buzzards Peak and Amboy. Comparative zircon tephrochronology by secondary ion microprobe analysis of U-Pb zircon crystallization ages, U-Th trace element abundances, and oxygen isotopic composition confi rm a correlation between the Bouse Formation tephra and the 4.834 ± 0.011 Ma Lawlor Tuff (40Ar/39Ar eruption age). Zircon in a coeval tephra associated with the Heise volcanic complex in the Snake River Plain has distinctly lower (by ~4.8‰, δ18O VSMOW [Vienna standard mean ocean water]) oxygen isotopic compositions than zircon from Bouse tephra, and can be ruled out as a source. The ca. 4.834 Ma depositional age for the Bouse Formation tephra in fi ne-grained sedimentary beds of the fl ooded Bristol and Blythe subbasins requires widespread Colorado River inundation in the lower Bouse basins at that time.