Wilbur Springs, Sacramento Valley
The Northern Coast Ranges (Fig. 61) are a composite geomorphic province consisting of structurally controlled, northwest trending, enechelon ridges and valleys that extend from the Klamath Mountains in the north to San Francisco Bay in the south. The core of the Coast Ranges consists of complexly deformed Upper Jurassic to Early Tertiary rocks of the Franciscan Complex. This core is structurally overlain to the east by less deformed, late Mesozoic marine, clastic rocks of the Great Valley Sequence. These two essentially coeval terranes are juxtaposed along a major low-angle fault, the Coast Range Thrust (Bailey et al., 1964), whose present geometry is extensively modified by Tertiary deformation of the Coast Ranges and extensive strike slip displacement along reactivated Mesozoic structural discontinuities (McLaughlin, 1974, 1981; Suppe and Foland, 1978; Suppe, 1979).
The Franciscan Complex is a highly deformed, lithologically heterogeneous assemblage. It consists predominately of graywacke and metagraywacke with subordinate shale, altered mafic volcanic rocks, radiolarian chert and minor limestones, which range from unmetamorphosed through zeolite, prehnite-pumpellyite, greenschist, and blueschist facies. It also includes minor blueschist and eclogite facies exotic blocks. The distribution and relationships of these rocks is complicated by numerous thrusts and chaotic melange zones. Vestiges of original stratigraphic relations are only locally preserved in large tectonic slabs and thrust sheets. Thrust sequences “top” eastward and “young” to the west and the regional tectonic fabric dips eastward at high angles. Overall, the high P, low T metamorphic grade increases west to east but the metamorphic facies progression is more complicated in detail.