Rediscovery of Type Locality of Turritella andersoni and Its Geologic Age Implications for Eocene Strata in Northeast Pacific

Richard L. Squires

Turritella andersoni is one of the most important index fossils from the early Eocene in the northeast Pacific region. Its geographic range is from Baja Sur, Mexico, to Victoria, British Columbia. Its holotype locality is in Urruttia Canyon, 26 km (16 mi) north of Coalinga, central California, but the exact topographic and stratigraphic positions of this locality have been a source of confusion and uncertainty for the past 76 years.

The holotype locality has been rediscovered, based on consultation of museum records, original field notes of early workers, and field checking. Numerous specimens were found only in the upper part of the Cerros Shale Member of the Lodo Formation in the center of Sec. 15, T18S, R14E, (1969, Joaquin Rocks, 7.5^prime quadrangle, California). The Cerros Shale Member is 26 m thick in this area. Microfossil samples were taken from greenish-gray siltstone in the immediate area of the holotype locality. Calcareous nannofossils (especially Discoaster lodoensis) from these strata indicate an early Eocene (CP10 Biozone) age, which would be equivalent to the provincial molluscan "Capay Stage" in the restricted sense of modern workers.

Previously, the molluscan stage of T. andersoni had been "Meganos"? (earliest Eocene) through "Capay," because of the possibility that the holotype locality (when eventually rediscovered) might be of "Meganos" age. This present study negates the "Meganos" age possibility, and T. andersoni can now be shown to be confined to the "Capay Stage." This age refinement will be a valuable asset to future biostratigraphic, paleogeographic, and paleobiogeographic work.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.