Post–middle Miocene accretion of Franciscan rocks, northwestern California
Deformed sedimentary rocks assigned to the Franciscan assemblage in the King Range south of Cape Mendocino, northern California, are dominantly deep-water argillite and sandstone occurring as thick- to thin-bedded, locally channelized marine turbidites of arkosic to andesitic volcani-clastic composition. These rocks structurally overlie Late Cretaceous (Coniacian to Campanian) basaltic pillow flows, flow breccias, diabase sills, and arkosic sandstone turbidites that also are assignable to the Franciscan assemblage, and are well exposed along the coast at Point Delgada. Radiolarians and planktic and benthic foraminifers indicate that the oldest rocks in the King Range may be early Tertiary and that the youngest rocks are middle Miocene or younger, the youngest rocks yet reported from the Franciscan assemblage. Partly coeval Miocene (Relizian and Luisian) to Quaternary rocks, representing shallower sedimentary facies of the inner shelf to outer continental slope, occupy the Eel River basin and isolated fault-bounded slivers adjacent to the King Range on the north and east. Wide zones of penetrative shearing that may delineate a middle Miocene or younger suture separate the deep-water Franciscan rocks of the King Range from less severely deformed Neogene rocks of the Eel River basin. The King Range appears to be a displaced terrane of oceanic basement overlain by Paleogene(?) and Neogene sedimentary and igneous rocks of continental and oceanic derivation. This terrane may have been displaced north or eastward and obductively accreted to California ≤15 m.y. ago, before or perhaps during northward passage of the Pacific-Farallon-North American plate junction.