TRANSITION OF THE BOUSE FORMATION FROM A MARINE ESTUARY TO SALINE LAKE IN THE LATE MIOCENE TO EARLY PLIOCENE

Although there is considerable debate over whether the Bouse Formation in the Blythe Basin of California and Arizona represents deposition in a lake or in the northern terminus of the proto-Gulf of California, the presence of foraminifers argues in favor of a marine origin, and provides age and ecological constraints that indicate this basin was established before Colorado River water entered from the north. An increasingly diverse foraminiferal fauna in the bioclastic unit of the lower Bouse Formation and an age of ≥6.0 to 5.3 Ma based on planktic foraminifers, indicates that this unit predates the arrival of the Colorado River water, since this water did not arrive in the adjacent northern basin until after 5.6 Ma. A distinctive clay layer (DCL) and change from bioclastic to siliciclastic deposition in the Bouse Formation of the Blythe basin marks an abrupt environmental change. The decline of the foraminiferal population in the siliciclastic unit suggests a transition to a saline lake as river water entered the basin.

Paleoecology of the bioclastic Bouse Formation in the Blythe basin based on foraminifer analysis suggests deposition in a marine estuary. Five foraminiferal biofacies are recognized which document: a transition from nearshore low-energy wave-dominated shallow marine environment (Buliminella and Elphidium biofacies), to neritic depths in a well-oxygenated water mass with normal to slightly brackish salinities (Rosalina biofacies), to neritic depths but with decreased oxygen conditions and a restricted marine connection (Streptochilus biofacies), and a final transition to a siliclastic saline lake (Ammonia biofacies). Isotope values in the bioclastic member suggest deposition in a marine estuary thus supporting the interpretation based on foraminifers. The transition to a saline lake marked by the DCL may represent the breaching of the paleodam, followed by a decline in salinity, reduction of the foraminiferal faunas to a single, low salinity tolerate species, an increase in continental ostracodes, and a decline in isotope values.

The Bouse Formation in the San Luis and Fortuna basins and the Imperial Formation in the Salton Trough record a similar ecological transition as low salinity water was released after breaching the paleodam at the southern end of the Blythe basin in the early Pliocene.