OSTRACODE-BASED FAUNAL AND STABLE ISOTOPE DATA SUGGEST A MIXED MARGINAL MARINE TO STRATIFIED LACUSTRINE WATERBODY INTERPRETATION FOR THE SOUTHERN BOUSE FORMATION, AZ

Continued debate about the depositional environment of the southern Bouse Fm. centers on the presence of an unexpectedly diverse marginal marine fauna. Recent foraminifera-based work suggests that an outcrop in Hart Mine wash, south of Blythe, preserves a transition from marine or estuarine conditions to a freshwater lake (McDougall and Martinez, 2013). We collected 78 samples from 4 outcrops at the same locality, spanning 3 conformable and laterally interfingering units in the Bouse Fm: (1) ~ 4 m of fine-grained bedded marl; (2) ~ 1-3 m of interbedded marl and fissile clayey marl at the top of the basal Bouse carbonate; and (3) ~15 m of overlying green and red Colorado River-derived claystone (interbedded member of the Bouse Fm). We counted ostracode assemblages (n = 74) and other biologic remains, and measured d¹⁸O and d¹³C in fine-grained carbonate (n = 78) and co-occurring ostracode valves (n = 48). The near-shore marine ostracode Perissocytheridea cf. meyerabichi is identified for the first time in the Bouse. The brackish to saline-tolerant ostracodes Cyprideis and Cytheromorpha are common, and often co-occur with fresh-water ostracode genera (Candona, Darwinula, Heterocypris, Limnocythere). O-isotope data show that the brackish and fresh-water ostracodes calcified their valves in the same water mass; the fresh-water genera were not washed into the deposit as previously thought. Our new data show that the Bouse water body was isotopically stratified for at least part of its history. In unit 2, prior to appearance of the freshwater fauna, d¹⁸O of ostracode valves (Cyprideis, Cytheromorpha) varies from -6 to -2‰, several per mil higher than surrounding marl (-11.5 to -8‰). This interval also contains the planktic marine foraminifera Streptochilus (McDougall and Martinez, 2013). Freshwater ostracode genera appear in the transition to unit 3, and ostracode d¹⁸O values (including Cyprideis) drop to between -11 and -8‰, similar to the surrounding marl. These data are consistent with a transition from a brackish environment (units 1 & 2) to a well-mixed fresh-water lake (unit 3), as suggested by McDougall and Martinez. Continuing efforts are focused on determining if the isotopically stratified marl interval was deposited in a brackish lake or a salt-wedge estuary.