BOUSE FORMATION ALONG THE LOWER COLORADO RIVER CORRIDOR: TRANSITION FROM MARINE ESTUARY TO SALINE LAKE

There is considerable debate over whether the Bouse Formation in the Blythe Basin represents deposition in a terminal lake in a series of fill and spill lakes, and is thus nonmarine or in the northern terminus of the proto-Gulf of California and is therefore marine. The presence of foraminifers argues in favor of a marine origin, and provides age constraints that indicate this marine basin was established before Colorado River water entered.

The age of the Bouse Formation in the Blythe Basin is bracketed by a lower age of 9.2 Ma in the underlying Osborne Wash strata, and an upper age of 4.83 Ma from ash beds correlated with the Lawlor Tuff. The presence of the age-diagnostic planktic foraminifer, Globorotalia lenguaensis, near the base of marl 2 in Hart Mine Wash, with a last occurrence datum of 6.0 Ma and the presencce of Streptochilus latus (ranges from 8.1 to 5.3 Ma) together with the presence of S. mcdougallae(local range of 6.0 to 5.3 Ma) in marl 2 and lower green claystone units constrains this marine interval to an age between >6.0 to 5.3 Ma. The age of the formation in the Blythe Basin is therefore refined to >6.0 Ma but <9.2 Ma at the base to 4.83 Ma at the top, and indicates that the bioclastic limestone and marl units of the Bouse Formation were deposited before Colorado River water entered the Blythe Basin.

The ecology of the Bouse Formation in this basin based on foraminifers also suggests deposition of these sediments in a marine estuary. Five foraminiferal biofacies are recognized which document a transition from nearshore (Buliminella and Elphidium biofacies) to neritic depths (Rosalina and Streptochilus biofacies), and a final transition to a saline lake or restrict marine embayment (Ammonia biofacies). The transition to a saline lake or restrict marine embayment is marked by a distinctive clay unit that may represent the breaching of the paleodam at Parker, 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.