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Improved depositional age constraints and stratigraphic description of rocks in San Diego require designation of a new Upper Jurassic formation, herein named the Peñasquitos Formation after its exposures in Los Peñasquitos Canyon Preserve of the city of San Diego. The strata are dark-gray mudstone with interbedded first-cycle volcanogenic sandstone and conglomerate-breccia and contain the Tithonian marine pelecypod Buchia piochii. Laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon 206*Pb/238U ages of 147.9 ± 3.2 Ma, 145.6 ± 5.3 Ma, and 144.5 ± 3.0 Ma measured on volcaniclastic samples from Los Peñasquitos and Rancho Valencia Canyons are interpreted as magmatic crystallization ages and are consistent with the Tithonian depositional age indicated by fossils. Whole-rock geochemistry is consistent with an island-arc volcanic source for most of the rocks.

The strata of the Peñasquitos Formation have been assigned to the Santiago Peak volcanics by many workers, but there are major differences. The Peñasquitos Formation is marine; older (150–141 Ma); deformed everywhere and overturned in places; and locally is altered to pyrophyllite. In contrast, the Santiago Peak volcanics are nonmarine and contain paleosols in places; younger (128–110 Ma); undeformed and nearly flat lying in many places; and not altered to pyrophyllite. The Peñasquitos Formation rocks have also been assigned to the Bedford Canyon Formation by previous workers, but the Bedford Canyon is distinctly less volcanogenic and contains chert, pebbly mudstones, and limestone olistoliths(?) with Bajocian- to Callovian-age fossils.

Here, we interpret the Peñasquitos Formation as deep-water marine forearc basin sedimentary and volcanic strata deposited outboard of the Peninsular Ranges magmatic arc. The Upper Jurassic Mariposa Formation of the western Sierra Nevada Foothills is a good analog. Results of detrital zircon U/Pb dating from an exposure of continentally derived sandstone at Lusardi Creek are consistent with a mixed volcanic-continental provenance for the Peñasquitos Formation. A weighted mean U/Pb age of 144.9 ± 2.8 Ma from the youngest cluster of detrital grain ages is interpreted as the likely depositional age. Pre-Cordilleran arc zircon age distributions (>285 Ma) are similar to Jurassic deposits from the Colorado Plateau, with dominant Appalachian-derived Paleozoic (300–480 Ma), Pan African (531–641 Ma), and Grenville (950–1335 Ma) grains, consistent with derivation either directly, or through sediment recycling, from the Colorado Plateau Mesozoic basins and related fluvial transport systems. Appalachian- and Ouachita-like detrital zircon age distributions are characteristic of Jurassic Cordilleran forearc basins from northeast Oregon to west-central Baja California, indicating deposition within the same continent-fringing west-facing arc system.

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