Eighty-six samples were collected from three stratigraphic sections in southwestern Santa Barbara County, California: Rodeo Canyon, Lyons Head, and Point Pedernales. In this area, the Monterey Formation is exposed in prominent cliffs along the coast from Rodeo Canyon on the south, to Lyons Head and Point Pedernales on the west. Because most of the opal-A in the Monterey diatomaceous shale in this area was converted to opal-CT and/or microcrystalline quartz, the diatoms are poorly preserved or obliterated. Therefore, the age of these rocks has been in question for a long time. Twenty-five of 86 samples yielded age diagnostic diatoms that range from the early Miocene, late Relizian, Annellus californicus zone, to the early Pliocene, late Delmontian, Thalassiosira oestrupii one. These ages can be directly related to those of diatomaceous shales, mudstones, and dolomites in the California offshore.

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