BOUSE FORMATION: THE PERSPECTIVE FROM THE BRISTOL BASIN, CALIFORNIA

Limestone beds in the Bristol basin, California, indicate that an arm of the Bouse water body extended across a wide part of the southern Mojave Desert. The Bristol basin outcrops contain the Lawlor Tuff (4.834 ± 0.011 Ma), which was also found in the Bouse Formation 180 km to the southeast. We confirmed the previously published glass chemistry correlation (Sarna-Wojcicki et al., 2011) by dating zircon and comparing age spectra, U-Th trace element abundances, and oxygen isotopic composition with the proximal Lawlor Tuff. A coeval tephra from the Heise, Idaho, volcanic complex can be ruled out by the oxygen isotopic values. Age spectra for the two Bouse samples differ from the Lawlor in having different ages for pre-volcanic grains, suggesting locally incorporated zircon during deposition.

Bouse exposures are white limestone and calcareous sandstone, as well as stromatolite mounds; we interpret these as nearshore deposits. Ostracodes and diatoms commonly show evidence for reworking, and species indicate saline lake or estuarine environments with an admixture of freshwater forms. Along with wading bird tracks and a spine from a marine fish, these fossils suggest that the deposits formed in saline waters near a freshwater source such as a perennial stream. Although the Bouse outcrops of the Bristol basin lie within a long-lived tectonic zone, they are within the altitude range of Bouse outcrops in the Colorado River corridor. The Bristol basin arm of the Bouse probably was restricted from the main water body by narrow passages, but Bouse sediment of the Bristol basin arm is similar to that in the Blythe basin and does not include evaporites, from which we suggest that the Pliocene Bouse could have been deposited under estuarine conditions.