LAKE-ESTUARY HYPOTHESIS FOR THE BOUSE FORMATION: NEW LOOK AT AN OLD PROBLEM

The latest Miocene to early Pliocene Bouse Formation contains a debated record of conditions and processes that culminated in integration of the Colorado River (CR). Early studies proposed that the Bouse accumulated in a marine estuary at the north end of the Gulf of California rift, followed by progradation of the CR delta and post-depositional uplift of 500-1000 m. A second hypothesis postulates that the Bouse was deposited in a series of lakes that formed in a sequence of linked overflows, and thus places no constraint on post-Miocene uplift. A third hypothesis proposes that the Bouse Formation accumulated in a series of lakes in the north that were linked to a large marine estuary in the south with a narrow connection to the ocean. Existing and new data suggest that the lake-estuary hypothesis is viable and requires further testing: (1) Although ⁸⁷Sr/⁸⁶Sr ratios of Bouse carbonates are asserted to be the same as that of CR water, ⁸⁷Sr/⁸⁶Sr ratios of Bouse carbonates are typically higher and more variable than the CR. Young carbonates from lakes known to be fed by the CR are geochemically unlike the Bouse, and new data suggest a possible southward decrease in Bouse ⁸⁷Sr/⁸⁶Sr. (2) Southward increase in δ¹³C may have resulted from mixing of river water in the north with seawater in the south. Bouse carbonates display linear covariation of δ¹³C and δ¹⁸O that trends through the origin, common in marine estuaries and very rare in closed lake systems. (3) Sedimentologic evidence for a marine influence includes herringbone cross-bedding in well sorted calcarenite, flaser bedding, hummocky cross stratification, and Thalassinoides shrimp burrows. (4) Paleontological evidence includes marine and estuarine fossil fish, foraminifers, gastropods, bivalves, ostracodes, echinoids, coralline red algae, and bryozoans. While avian transport can explain occurrence of marine faunas in some modern lakes, no lake that is completely isolated from the ocean has the great abundance and moderate diversity of marine fossils seen in the Bouse Formation. (5) Mixing models show that adding just 1-8% radiogenic groundwater to CR water can produce the slightly elevated ⁸⁷Sr/⁸⁶Sr values of Bouse carbonates. Controversy over these issues remains unresolved. New data clearly are needed to test alternate hypotheses for this enigmatic formation.