TRANSITION FROM MARINE ESTUARY TO SALINE LAKE RECORDED BY THE BOUSE FORMATION IN THE BLYTHE BASIN

The late Miocene to early Pliocene Bouse Formation is discontinuously exposed along the lower Colorado River corridor. This formation is composed of a basal carbonate facies including travertine, bioclastic, and fine-grained marl units, and a siliciclastic facies, which includes green claystone, red mudstone, and Colorado River sandstone. The origin of the Bouse Formation is controversial and two hypotheses are commonly proposed. The lacustrine hypothesis concludes that the Bouse Formation accumulated in a series of inland lakes fed by the Colorado River. The estuary-lake hypothesis postulates the same for the northern basins but suggests that in the southernmost Blythe basin, most of the basal carbonate facies accumulated in a marine estuary, prior to the arrival of the Colorado River. In the Blythe basin, the carbonate and siliciclastic facies are separated by a distinctive clay layer (DCL). Planktic foraminifers found primarily below the DCL restrict the age of the basal carbonate to >6.0 to 5.3 Ma based on last occurrences of age diagnostic species. This age is consistent with the basal carbonate in the Blythe basin predating the arrival of the Colorado River water, since the water did not arrived in the adjacent Mohave and Cottonwood basins until after 5.6 Ma.

Foraminiferal assemblages in the Bouse Fm. of the Blythe basin indicate that deposition began in a relatively low-energy wave-dominated shallow marine environment. As water depths increased, a well-oxygenated water mass with normal to slightly brackish salinities developed. Although water depths continued to increase, a decline in oxygen conditions occurred as the marine connection was restricted. After deposition of the DCL, foraminifers and marginal marine ostracodes decline in abundance and are interspersed with samples that contain nonmarine ostracodes. Foraminiferal assemblages become monospecific in the siliciclastic facies likely due to a decline in salinity as Colorado River water was introduced. The number of juveniles declines and the number of abnormal tests increases up section, indicating increased variation in salinity or temperature. In summary, the data suggest deposition of the Bouse Fm. in the Blythe basin occurred in a restricted marine environment prior to a fresh water input from the Colorado River.