Description
The Late Cretaceous-Tertiary stratigraphy of coastal San Diego County is separated into two distinct packages separated by an erosional unconformity. Campanian conglomerates of the Cabrillo Formation are dominated by silicic volcanic, plutonic and quartzite clasts that record a major pulse of forearc sedimentation derived from rapid unroofing of the adjacent Peninsular Ranges batholith (PRB). Unconformably overlying are mainly middle to upper Eocene fluvial-deltaic strata of the La Jolla-Poway Groups containing abundant conglomerate dominated by silicic volcanic clasts with minor granitic and quartzite clasts; volcanic clasts include the distinctive “Poway rhyolites” that have been recognized by southern California geologists for over a century and have been matched to Middle Jurassic bedrock sequences in Sonora, México by previous workers. This paper presents new whole rock, major- and trace-element chemistry (n=56) and laser ablation multicollector ICPMS U-Pb ages of zircons (n=28) from Cretaceous-Eocene conglomerate clasts as well as potential basement source regions in Sonora, México. The focus is on volcanic clasts but a granitoid from the apex of the Poway fan and a volcanic Poway clast from the Las Palmas Gravels of México were also analyzed. Cabrillo Formation rhyolite clasts (n = 10) yield U-Pb zircon ages that cluster narrowly from 96.6 ± 2.5 to 103.7 ± 2.5 Ma. In strong contrast, eleven Poway-La Jolla Group dacite-rhyolite clasts cluster from ~165.5 ± 2.7 to 175.0 ± 4.1 Ma; they overlap the ages of 167.8 ± 2.2 and 168.0 ± 2.3 Ma from potential Sonoran dacite and rhyolite source rock. Three Eocene clasts yielded younger ages of 150, 142, and 100 Ma. The Cretaceous and Eocene volcanic clasts all have similar high-K calc-alkaline geochemical characteristics. The Eocene clasts are clearly extraregional and can be matched to the Jurassic magmatic arc in northern Sonora, México and south-central Arizona on the basis of chemistry and age as proposed by others. The relationship of the ~100 Ma high-K calc alkaline rhyolite clasts of the Cabrillo Formation to the supracrustal volcanic cover of the Peninsular Ranges magmatic arc is less certain. The clasts have distinctly higher potassium content and are younger than any in situ ages reported from the Santiago Peak Volcanics (~ 110 to 135 Ma) in their western batholith exposures. Clast ages most closely match the eastern province PRB intrusions; however, the exhumation record of the PRB suggests the supracrustal volcanic cover above the eastern batholith was likely removed prior to late Campanian deposition of the Cabrillo Formation.
