EVIDENCE FOR A MARINE INCURSION ALONG THE LOWER COLORADO RIVER

The depositional setting of the Bouse Formation is controversial and is variously attributed to a lacustrine origin or a marine incursion. The lacustrine origin suggests deposition of the Bouse Formation in a series of lakes that filled and spilled until the system was integrated and the Colorado River was a through flowing river. Arguments in favor of a lacustrine origin for the Bouse Formation are isotopes, no indication of uplift, and paleogeomorphic and sedimentological evidence. An alternative origin suggests that the Bouse Formation accumulated in lakes along the northern part of the lower Colorado River but in the southern part, the unit was initially deposited in a marine environment at the northern end of the Proto-Gulf of California. Arguments in favor of the marine origin for the lower Bouse Formation are marine fossils.

Recent studies of the foraminiferal faunas indicate that the lower Bouse Formation in the Blythe Basin is marine whereas the upper part of the unit is nonmarine. The composition and distribution of benthic foraminiferal assemblages in the lower part of the Bouse Formation indicate deposition in inner neritic to lagoonal biofacies similar to those in the modern Gulf of California. Evidence suggesting a change from marine to nonmarine conditions are the highest occurrence of planktic foraminifers at an elevation of 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.

Planktic foraminifers in the lower Bouse Formation suggest a late Miocene age between 8.1 and 7.4 Ma which predates the development of the lower Colorado River by at least 3 Ma. This older age also eliminates the competition between the depositional models, but does necessitate a tectonic solution.