New directions in research on the Bouse Formation and the origins of the lower Colorado River

e goal of this paper is to pose research questions that may help resolve controversies about the depositional setting of the ~ 5–6 Ma Bouse Formation and paleohydrology of the earliest Colorado River system. We also summarize and update a recent Geosphere paper (Crossey et a., 2015) that discusses the application of multiple geochemical tracers (87Sr/86Sr, [Sr], δ13C, δ18O) for understanding carbonates of the Bouse Formation. e Bouse Formation, whose age is bracketed regionally between about 6.1 and 4.8 Ma, was deposited at least in part by rst-arriving waters of the Colorado River as it was integrated through several previously internally drained basins to the Gulf of California (e.g. House et al., 2008; Pearthree and House, 2014; McDougall and MirandaMartinez, this volume). e northern basins (C=Cottonwood, M=Mojave on Fig. 1) have Bouse outcrops that reach modern elevations of ~560 m that are interpreted as lake high stands. Downstream basins (Havasu and Blythe) have interpreted high stand carbonate deposits at ~ 360 and 330 m, respectively (Pearthree and House, 2014). Carbonate chemistry, especially 87Sr/86Sr, has been used to interpret that Bouse deposition took place in a lake isolated from the ocean (Spencer and Patchett, 1997) rather than a marine estuary (Buising, 1990). Figure 1 shows that the lower Colorado River region is transected by the San Andreas plate boundary, the eastern California shear zone, and the Stateline fault systems (among others). is raises the question of whether neotectonics may have modi ed outcrops relative to their depositional paleoelevations. Two alternative tectonic hypotheses need to be tested: (1) Highest Bouse carbonate outcrops