Late Neogene marine incursions and the ancestral Gulf of California
The late Neogene section in the Salton Trough, California, along the lower Colorado River in Arizona, and around the Gulf of California in Mexico, is composed of a marine unit bracketed by nonmarine units. The marine strata are considered questionably latest Miocene to Pliocene in age and are believed to have been deposited as marine waters transgressed into the newly formed Gulf of California. Microfossils found in several sections throughout this region suggest that there were three separate marine incursions. The oldest of these incursions is recognized as far north as Isla Tibron where microfossils support a late Miocene age suggested by a K/Ar date (11.2 ± 1.3 Ma) on a tuff overlying the section. The next incursion, which was probably the most extensive, extende as far north as San Gorgonio Pass, California, and as far east as Yuma, Arizona. Although not well documented, microfossils from this incursion include benthic foraminifers (Amphistegina gibbosa, Cassidulina delicata, and Uvigerina peregrina), planktic foraminifers (Globigerinoides obliquus, G. extremus, and Globigerina nepenthes; zones N17-N19), and calcareous nannoplankton (Discoaster brouweri, D. aff. D. surculus, Sphenolithus abies, and S. neoabies; zones CN9-CN11) which coupled with published K/Ar dates (10.1 ± 1.2, 5.94 ± 0.18, and 6.04 ± 0.18 Ma) suggest a late Miocene age. The youngest marine incursion includes deposits assigned to the Imperial and Bouse Formations. The microfossils foraminifers suggest a Plioc ne age (planktic foraminiferal zone N19/20 and younger). The presence of three marine incursions indicates that the opening of the Gulf of California was the result of events which affected a fairly broad region and that the middle to late Miocene extension resulted in a reasonably broad seaway.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California