Marine foraminiferal, marine to fresh-water molluscan, and brackish- to fresh-water ostracode faunas occur in a thick section of limestone, silt, and clay of the Bouse Formation along the Colorado River from Parker to Yuma in an area now isolated from the sea. Faunas in the Parker-Blythe-Cibola area are limited in number of species but are remarkably persistent through the formation. The presence of marine Foraminifera, including Globigerina sp., is considered evidence that the area was continuously connected with the ocean.

Several thousand feet of similar sediments are found in the subsurface near Yuma and contain faunas which at shallow depths are similar to those to the north, but at greater depths contain bathyal assemblages with an abundance of globigerinids, which indicate a post-Miocene age for the section, and oceanic conditions.

It is likely that this marine embayment extended into the Imperial Valley, where the Imperial Formation of probable Pliocene age also contains marine faunas.

Evidence presented here strongly indicates a large long-lasting Pliocene marine embayment along the lower Colorado River, connected with the Imperial Valley.

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