On the basis of fossils, basement rocks at the northern end of the Sierra Pinta are thought to be Paleozoic in age. They are eugeosynclinal in association and consist of mica schists, slates, phyllites, thin marbles, cherts, and meta-volcanics. This is the first reported occurrence of eugeosynclinal Paleozoic rocks in the Peninsular Range Province of California and Mexico. Overlying Cenozoic volcanic rocks reach a maximum aggregate thickness of 4000 meters. The sequence consists of basal flow s of Laguna Dacite; tuffs, welded tuffs, and flows of Pinta Rhyolite; flows, flow breccias, and tuffs of La Ventana Andesite; and flows of Grande Dacite (new names). A major unconformity between the upper two formations has been radiometrically dated at 8 to 10 million years. Early faulting in the volcanic rocks occurred along northeast-trending normal faults. Offsetting these are later north-south normal and east-west reverse faults which produced the present horst. Fault types and directions are believed to be the result of forces imposed on the region during the opening of the northern Gulf of California
