The name Monterey Formation or Monterey Shale has been applied to lithologically similar pelitic marine rocks of Miocene age exposed in the coastal regions of California. These rocks, characterized by unusually high proportions of silica, have been variously termed diatomite, porcelanite, porcelaeous shale and mudstone, chert, and cherty shale. The type area for the Monterey Formation was designated near Monterey, California.

Although the Monterey Formation in general contains numerous ash, tuff, and bentonite beds, only a few of these volcanic units have been dated. It has been necessary to make use of both the isotopically dated horizons and Kleinpell's Miocene benthonic foraminiferal stages to construct a time-stratigraphic framework for the Monterey Formation at different locations. Ages for the various stage and zone boundaries, based on published and unpublished data, are as follows:

Repettian-Stratotype B. oblique ("Delmontian"), ca. 5 m.y.B.P.; Stratotype B. oblique-Mohnian, ca. 7 m.y.B.P.; Mohnian-Luisian, ca. 12 m.y.B.P.; Luisian-Relizian, 14.0 to 14.9 m.y.B.P.; Relizian-Saucesian, 15.7 m.y.B.P.

At the type area, an ash from the Canyon del Rey Diatomite Member, the uppermost member of the Monterey Formation, was dated at 11.3 ± 0.9 m.y. (F-T on zircon). Coupled with a Luisian age for the oldest strata, the type Monterey Formation spans, at most, the time interval from 15 to 10 m.y.B.P. In contrast, the Monterey Shale in the Palos Verde Hills encompasses the time interval from 15 to 5 m.y.B.P. based on isotopic data from the Altamira Shale, Valmonte Diatomite, and Malaga Mudstone Members, which contain foraminiferal assemblages characteristic of the Relizian, Luisian, and Mohnian Stages, and the B. oblique zone ("Delmontian").

At other localities where biostratigraphic and isotopic data are available, the rocks termed Monterey are not exact time equivalents of the type foramation, an observation made earlier by both Kleinpell and Bramlette.

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