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Recent reconnaissance of the Tertiary rocks of the Coast Ranges north of San Francisco Bay, together with trace-element studies and K-Ar dating of the associated tuffs, has necessitated revision of the age assignments and correlations of some units. Previous workers generally have held that the Merced(?) Formation of Sonoma County interfingers laterally with the Sonoma Volcanics, which in turn unconformably overlie the Petaluma Formation. Age assignments have varied, but most recent workers assign the Merced(?) Formation and the Sonoma Volcanics to the late Pliocene and the Petaluma Formation to the middle Pliocene. We have determined that most of the molluscan collections from the Merced(?) Formation are late Pliocene, but at least 2 are older (early Pliocene in terms of a twofold provincial division of the epoch). Hemphillian and Blancan vertebrates reported from 7 localities in the Petaluma Formation, indicate an early or middle Pliocene and late Pliocene age, respectively, for this formation, in terms of the nonmarine chronology. A pumiceous, vitric-crystal tuff in the Merced(?) Formation occurs stratigraphically between the early and late Pliocene molluscan localities. This tuff has been dated by the K-Ar method at 5.9 m.y., and has been correlated by trace-element "fingerprinting" (using X-ray fluorescence on the glass) with a petrologically similar tuff in the Petaluma Formation. This dating confirms the correlation of early and late Pliocene mollusks with Hemphillian and Blancan mammals, and suggests a provincial early Pliocene-late Pliocene bound ry of about 6 m.y.
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