ENIGMATIC BOUSE FORMATION

The Bouse Formation is exposed in discontinuous outcrops along the lower Colorado River and in the subsurface of the Yuma Basin. The age was initially determined to be Pliocene, but in the Blythe Basin, deposition may have begun as early as 9.2 Ma and ended by 4.83 Ma. Origin of the Bouse Formation is controversial. Advocates of a lacustrine origin cite isotopes, no indication of uplift, and sediments as evidence that the Bouse Formation was deposited in a series of lakes that filled and spilled from one basin to the next until the Colorado River was a through flowing river. Proponents of a marine origin cite the presence of marine fossils as evidence that the unit was deposited in a marine estuary at the northern end of the proto-Gulf of California.

Recent studies of foraminiferal faunas suggest that the lower Bouse Formation in the Blythe Basin may be late Miocene in age and correlate with other sections in the proto-Gulf of California. The presence of the planktic foraminifers Streptochilus cf. S. subglobigerum and Streptochilus latus constrains the age to upper zone N17, 8.1 to 7.4 Ma. These faunas also suggest that the lower Bouse Formation was deposited in a marine environment at the northern end of proto-Gulf of California. The transition between the upper and lower Bouse occurs between elevations of 85 and 122 m. Evidence indicating the change are the highest occurrence of planktic foraminifers (~122 m), the change from low diversities to monospecific foraminiferal assemblages (~122 m), an increase in abundance of A. beccarii specimens (~104 m), increased number of deformed tests (~110 m), first appearance of Chara (~85 m), lowest occurrence of reworked Cretaceous coccoliths (~110 m), a decrease in strontium isotope values (70-120 m), and δ¹⁸O and δ¹³C values similar to sea water (70-100 m). Above 122 m, a monospecific foraminiferal assemblage indicates the presence of a saline lake. The age of the upper Bouse Formation in the Blythe Basin is constrained only by the Lawlor Tuff.

These data suggest that considerable study of the Bouse Formation is needed to better determine its stratigraphic succession, age, and environment of deposition as this formation is key to understanding the evolution of the Gulf of California, the integration of a major river system, and tectonic evolution of the San Andreas fault system.