Freshwater plumes and brackish lakes: Integrated microfossil and O-C-Sr isotopic evidence from the late Miocene and early Pliocene Bouse Formation (California-Arizona) supports a lake overflow model for the integration of the lower Colorado River corridor

Uncertainty over the depositional environment of the late Miocene and early Pliocene Bouse Formation hinders our understanding the evolution of the lower Colorado River corridor. Competing marine and lacustrine models for the origin of the southern Bouse Formation remain extremely difficult to reconcile after nearly 60 yr of study. This paper compares new microfossil data, inorganic and biologic carbonate δ¹⁸O and δ¹³C values (relative to Vienna Pee Dee Belemnite), and carbonate and fish bone ⁸⁷Sr/⁸⁶Sr ratios from northern and southern outcrops of the Bouse Formation. The lacustrine northern Bouse Formation and the contested southern Bouse Formation share a core Cyprideis (mixed marginal marine), Limnocythere (continental), and Candona (continental) ostracode assemblage, indicating similar environmental conditions. Micrite and ostracode valves from both areas yield nearly identical δ¹⁸O and δ¹³C values, suggesting similar origins. Ostracode valves from both areas document a large and abrupt shift from high δ¹⁸O values (-2‰) to low values (-10‰), consistent with fill-and-spill lacustrine origins. Tests of the planktic foraminifer Streptochilus from a southern outcrop yielded δ¹⁸O and δ¹³C values that are nearly identical to benthic ostracode δ¹⁸O and δ¹³C values. Recognition of benthic Streptochilus weakens a categorically marine interpretation for the southern Bouse Formation. Barnacle shell fragments at a key outcrop of the southern Bouse Formation that preserves sigmoidal bedding with possible spring-to-neap tidal bundling yielded low δ¹⁸O values (-8‰ ± 1‰) that are incompatible with calcification in seawater. The ⁸⁷Sr/⁸⁶Sr ratios from co-occurring fish bones (0.71104) and ostracode valves (0.71100) and the surrounding micrite (0.71086) reveal an isotopically complex lacustrine depositional environment for the southern Bouse Formation. A model invoking freshwater plumes from the early Colorado River into either a terminal or a tidally influenced, mildly brackish lake followed by an abrupt transition to a freshwater lake provides a comprehensive and internally consistent explanation for the microfossil and isotopic complexities observed in this southern Bouse Formation data set. A freshwater plume model is entirely consistent with filland- spill models for the downward integration of the early Colorado River.