The Late Jurassic Peñasquitos Formation is a newly recognized formation in San Diego County distinct from the Cretaceous Santiago Peak Volcanics. The Peñasquitos Formation consists of low grade metamorphosed, volcanically derived, bedded sedimentary rocks that are exposed in five stream canyons throughout San Diego County: Los Penasquitos Canyon, La Zanja Canyon, Circo Digueno Canyon, Lusardi Canyon and San Dieguito Canyon. These are deep marine sedimentary and volcanic rocks based on the presence of the pelecypod Buchia piochii that were deposited by sediment gravity flows, including debris-flow, fluidized-flow, and grain-flow (Balch, et al.1984). Outcrops from three localities, Los Penasquitos Canyon, Circo Digueno and Lusardi Canyon were analyzed in this study.
Whole rock major and trace element concentrations of volcanic breccia and sandstone from the Peñasquitos Formation determined by X-ray fluorescence show that most of these rocks are basaltic andesites and andesites that range narrowly from ~52 to 56 wt% SiO2. The rocks are subalkaline and low potassium corresponding to the island arc tholeiite series of volcanic rocks based on major elements. Concentrations of relatively immobile trace elements including Ti, V, and Zr similarly yield coherent patterns consistent with an island arc tholeiite affinity on various trace element discrimination diagrams.