Sandstone Petrology and Tectonostratigraphic Terranes of the Northwest California and Southwest Oregon Coast Ranges

Abstract

Projection of Mesozoic terrane boundaries of Blake et al. (1985b) from Oregon into California permits the subdivision of the late Mesozoic Franciscan Complex into units correlative to the Eastern and Central Franciscan (CE) belts. Eastern belt rocks may be further divided into those correlative to the Pickett Peak terrane, and an eastern (EYB) and western (WYB) facies of the Yolla Bolly terrane. At Crescent City, the contact between these Yolla Bolly subterranes is depositional. All other terrane boundaries are faults. In southwest Oregon, the Upper Jurassic Otter Point Complex (OP) is faulted against the Yolla Bolly terrane. Detrital modes determined by the Gazzi-Dickinson point-count method for largely unfoliated Otter Point and Franciscan sandstones are as follows: % QtFL = 15-30-55 (OP), 36-36-28 (EYB), 31-37-32 (WYB), and 34-30-36 (CE); % QmFLt = 5-30-65 (OP), 22-36-42 (EYB), 20-37-43 (WYB), and 19-30-51 (CE); % QpLvLsm = 14-81-5 (OP), 30-60-10 (EYB), 26-66-8 (WYB), and 28-64-8 (CE); and % QmP = 13-87 (OP), 38-62 (EYB), 35-65 (WYB), and 41-59 (CE). No significant compositional, diagenetic or metamorphic differences exist among Franciscan sandstones counted. All are albitized and contain pumpellyite. Otter Point sandstones differ significantly in composition and alteration, and contain an average 3% detrital clinopyroxene. Although Franciscan sandstones were likely derived from the partially dissected Sierran-Klamath magmatic arc, Otter Point sandstones were derived from a less dissected arc which may have been allocthonous with respect to surrounding Franciscan rocks.