Geologic mapping in the Cambria and Black Mountain areas, western San Luis Obispo County, California, has led to recognition of the two stratigraphically and petrologically distinct extrusive felsic igneous units: (1) the Obispo Formation; and (2) a new unit, named here the Cambria Felsite. The Obispo Formation was deposited above the Rincon Shale and below the Monterey Formation during middle Miocene time. The Cambria Felsite rests with angular unconformity on Franciscan rocks of Jurassic and (or) Cretaceous age, and occurs as clasts in the nonmarine Lospe Formation of Oligocene age.
Textural and mineralogic intergradations exist between the Cambria Felsite and the nearby upper Oligocene hypabyssal volcanic necks, plugs, lava domes, and dikes of the Morro Rock–Islay Hill complex; this suggests that the Cambria Felsite represents an effusive equivalent of the Morro Rock–Islay Hill. Cambria tuffs are enriched in alkalis plus silica and are depleted in ferromagnesian constituents relative to samples of the Morro Rock type. Chemical contrasts in the two units probably arose either because of a winnowing of airborne crystals from the glassy shards or because of magma fractionation in the conduits followed by pyroclastic eruption of the upper portions of the melt columns.
No obvious correlation exists between the two-stage silicic volcanism, separated by a 10-m.y quiescent interval, and middle Tertiary plate tectonics. However, generation of these igneous rocks was probably a thermal response of the western margin of the continental crust-capped Americas plate to underflow of the Farallon plate, resulting in Morro Rock and Cambria units, followed by a complex encounter with the East Pacific Rise, resulting in the Obispo Formation.