USING FOSSIL MOLLUSKS TO REFINE THE GEOLOGIC AGE OF THE EOCENE-OLIGOCENE SAN EMIGDIO FORMATION, KERN COUNTY, CALIFORNIA

The San Emigdio Formation in the San Emigdio Mountains, north of Los Angeles, is one of the few Eocene-Oligocene (40 to 30 m. y.) rock units in the southern California area. There have been no detailed investigations of the gastropod or bivalve taxa of this formation since 1923. New studies, based on collections housed at the Natural History Museum of Los Angeles County, as well as collections by the authors, revealed that the formation is temporally correlative to the uppermost Eocene to lowermost Oligocene provincial molluscan Galvinian Stage. This stage is equivalent to the Refugian Stage and the informal Gaviota stage of earlier workers.

San Emigdio Formation molluscan species diagnostic of the Galvinian Stage are the gastropods Turritella variata (Conrad), Bruclarkia columbianum (Anderson & Martin), Ficus gesteri (Wagner & Schilling), Olequahia lorenzana (Wagner & Schilling), Siphonalia merriami (Wagner &Schilling), and the bivalves Pseudocardium packardi Dickerson, Mactromeris rushi (Wagner &Schilling), Modiolus eugenensis Clark, and Tellina gibsonensis Van Winkle.

The San Emigdio Formation is temporally correlative to the Gaviota Formation in Santa Barbara County, California; the Keasey Formation of Oregon; and the Gries Ranch Formation of Washington.

Relative to older molluscan Eocene faunas, the San Emigdio fauna has a cooler water aspect that reflects the global cooling that took place during the Galvinian Stage.