THE BOUSE FORMATION: PERSISTENT CONTROVERSIES AND PROGRESS TOWARDS POTENTIAL RESOLUTIONS

The Bouse Formation records the first arrival of the Colorado River (CR) to Gulf of California. This thin unit presents persistent research controversies in several subdisciplines, as follows. Stratigraphy: Do Bouse carbonates represent lacustrine, marine, or estuarine deposits, basin-to-basin? Tectonics: To what extent do present outcrop elevations reflect depositional positions? Paleontology: Do fossils in the Blythe basin record marine deposition or marine organisms established in coastal saline lakes? Isotope geochemistry: How do 87Sr/86Sr, stable isotopes, and other geochemical tracers constrain depositional and diagenetic history? Geochronology: when and where in the interval 6.0 - 4.8 Ma did Colorado River water and sediment reach the opening Gulf of California? Our ongoing progress is as follows. Stratigraphy: The basal carbonate in the Blythe basin is an encrustation draped on bedrock with bioherms, serpulid tubeworms colonies, porous tufa, and travertine mounds, all draped by marl. Tectonics: N-S syn- and post-Bouse normal faults likely down-dropped Bouse along the CR basin axis; epeirogenic uplift of the region at 100-m-scale, and similar-scale isostatic uplift of basin flanks plus loading of CR basin axis may account for ~ 5 degrees of tectonic tilt of the Bouse on basin flanks. Paleontology: Serpulids are of probably marine origin. Isotope geochemistry: Sr isotopes can be explained as mixtures of CR and marine waters in an estuarine environment and C-O isotopic covariation in basal carbonate facies is consistent with mixing of marine and CR water. Geochronology: Detrital sanidine (DS) in CR sand in SW Palo Verde valley has San Juan volcanic field ages, a major 17.25 Ma population similar to grains input along Silver Creek in the Mojave basin, and a single grain of 5.32±0.08 Ma that is assumed to be an accurate maximum depositional age (MDA) but requires verification with additional analyses. Bullhead gravel near Laughlin has 40-50 Ma grains from upper Green River (?), abundant San Juan volcanics- aged grains, and yields a DS-derived MDA of 4.62±0.01 Ma (n=3) consistent with ash geochronology that constrains the unit to between 4.5 and 3.5 Ma.