The upper Miocene to Pliocene Bouse Formation, exposed in the lower Colorado River trough (Arizona, California, Nevada), has variously been interpreted as a marine, estuarine, or freshwater deposit. The Bouse Formation is now commonly found at elevations of >300 m and up to 550 m above sea level. Deposition of the Bouse in a transgressive marine (or estuarine) environment requires rapid drowning of the lower Colorado River trough during Pliocene time and subsequent uplift of the nearby Colorado Plateau. Basal Bouse Formation carbonate samples were analyzed for stable isotope composition, minor and trace element concentrations, and mineralogy, in order to investigate the depositional environment.
Most of the data presented here are consistent with deposition of the basal Bouse Formation carbonate in a lacustrine environment, although some data are consistent with a possible estuarine origin, with additional evidence for diagenesis affecting individual samples. Our preferred interpretation is that deposition took place in a chain of freshwater lakes fed by the proto–Colorado River, which may have built a delta out into an estuarine environment now exposed as the southernmost part of the formation. This interpretation is in accord with that proposed by J.E. Spencer and P.J. Patchett and implies that the present-day elevations of basal Bouse Formation carbonate in the Colorado River trough place no constraints upon the rate and timing of uplift of the Colorado Plateau.