ISOTOPIC EVIDENCE FOR A LACUSTRINE ORIGIN FOR THE PLIOCENE BOUSE FORMATION IN THE LOWER COLORADO RIVER VALLEY

The ~5 Ma Bouse Formation in the lower Colorado River Valley consists of basal limestone and bedrock-coating tufa, and overlying siltstone and fine sandstone. It is overlain by, and locally interbedded with, Colorado River sand and gravel. The Bouse Formation has been interpreted as estuarine because it contains a mix of marine, estuarine, and fresh-water fossils, and as lacustrine for many reasons, including its high maximum elevations, areal distribution, and Sr-isotopic composition. In the lacustrine interpretation, it was deposited when Colorado River water first arrived in the eastern Mojave Desert region and filled closed basins inherited from Miocene extension. In the estuarine interpretation, it was deposited below sea level and then elevated by tectonic uplift. Measured ⁸⁷Sr/⁸⁶Sr from 50 samples of Bouse carbonates and shells provide strong support for a lacustrine interpretation, as follows: (1) ⁸⁷Sr/⁸⁶Sr is consistently between 0.7102 and 0.7114, with no evidence of mixing with Pliocene sea water (⁸⁷Sr/⁸⁶Sr = 0.7090) toward the south. This is especially significant because seawater has much higher Sr concentration than river water (7x relative to the modern Colorado River), and addition of even a few percent seawater should be detectable. Nor is there any evidence of a mixing trend between very high ⁸⁷Sr/⁸⁶Sr in the Lake Mead area to the north with seawater-level ⁸⁷Sr/⁸⁶Sr to the south. (2) The lowest-elevation Bouse samples in southernmost exposures, which might reflect a marine influence in the potentially stratified water column of an estuary, show no such influence. (3) Oxygen isotopic variation in Bouse carbonate samples is significant (δ ¹⁸O = 1 to -13), but this variation does not correspond to variation in ⁸⁷Sr/⁸⁶Sr. If varying degrees of diagenetic alteration were responsible for variations in oxygen isotopes, corresponding variation would be expected in Sr isotopes, with identifiable end-member compositions for both isotope ratios. (4) Thin-sections through barnacle shells from the Bouse Formation reveal primary banding. Oxygen and carbon isotopes from microsamples across bands show coordinated variation interpreted to reflect seasonal changes during growth. If original Sr in these shells had been replaced by Sr from groundwater, oxygen and carbon would have been homogenized.