Geology of the San Juan Canyon Area, Orange County, California

The area described in this report occupies a narrow strip along the San Juan River in the south-western portion of the Santa Ana Mountains in Southern California. This region includes a sedimentary series of rocks which forms a gentle westward-dipping homocline off the crystalline core of the Santa Ana Mountains. Rocks range in age from Triassic to Recent. The basement complex comprises a series of Triassic slates, quartzites, and other rock types, with later intrusive of granitic masses. A considerable thickness of Cretaceous rocks conformably overlie the crystalline complex and is composed of heavy coarse conglomerates, fine shales, and micaceous sansdstones. A section of Eocene clays and sandstones, 2500 feet thick of probable Martinez age, overlies the Cretaceous section. These rocks have been faulted up against the Lower Miocene San Onofre facies of the Temblor. The upper Miocene Monterey shales and Capistrano sands overlie these rocks. Local structure in the area includes faulting and some gentle folding. Structural and general topographic trends parallel the NW-SE structural trends of the Santa Ana Mountains