SOME EVIDENCE FOR A LACUSTRINE ORIGIN FOR THE LOWER PLIOCENE BOUSE FORMATION, LOWER COLORADO RIVER VALLEY, SOUTHWESTERN U.S.A

The lowermost Pliocene Bouse Formation in the lower Colorado River valley (LCR) consists of basal marl and overlying siltstone, and includes sandstone and bedrock-coating tufa. Bouse deposits extend about 400 km up the LCR and are found up to 550 m asl. A mix of marine, brackish, and fresh-water fauna in the Bouse Formation supports an estuarine origin for the Bouse Formation. However, the ⁸⁷Sr/⁸⁶Sr values of Bouse marl, tufa, and calcareous shells are similar to those for Colorado River water but unlike those for sea water. The concentration of strontium in sea water is many times that in river water and the isotopic signature of sea water should be apparent in materials derived from mixed waters. This is especially problematic for an estuarine interpretation because ⁸⁷Sr/⁸⁶Sr from southern exposures near the mouth of the presumed Bouse estuary shows no marine influence. Most Bouse outcrops sampled for Sr isotope analysis were collected from areas below elevations reached during maximum aggradation of the Colorado River in the middle Pliocene and possibly were affected by Sr derived from river water after deposition, but some samples were collected from higher elevations and would not have been affected. There is no suggestion of 2 distinct populations in the ⁸⁷Sr/⁸⁶Sr data based on elevation, so it is unlikely that the Colorado River water systematically altered strontium values in Bouse carbonates. Numerical simulations of first arrival of Colorado River water to a sequence of closed basins along what is now the LCR indicate that salinity would have increased due to evaporation as the sequence of lakes filled and spilled, and that the lake furthest downstream always had the highest salinity. For slow incision rates of spillover divides, the lowest basin, centered on the town of Blythe, could have reached salinities comparable to sea water and could have become hospitable to marine organisms introduced by birds. Simulations also indicate that if gypsum or anhydrite were precipitated, such evaporites would likely be buried beneath the Blythe basin.