Late Cretaceous-Eocene landscape evolution and drainage reorganization along the southwest edge of the Cordillera; insight from volcanic clasts in conglomerates of the Cabrillo Formation and Poway-La Jolla Groups, San Diego County, California -
Department of Geological Sciences,
San Diego State University,
Advisor Dr. David Kimbrough, Patrick Abbott
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 late Eocene fluvial-deltaic strata of the La Jolla-Poway Groups containing abundant conglomerate dominated by silicic volcanic clasts with minor granitic and quartzite clasts types; volcanic types include the distinctive “Poway-type” clasts 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 rocks major and trace element geochemistry (n=56) and laser ablation ICPMS zircon U-Pb ages (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-type 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, overlapping 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 ~142, 150 Ma, 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 and are younger than any in situ ages reported to date from the Santiago Peak Volcanics of western batholith exposures (~ 110 to 135 Ma). Clast ages most closely match the eastern province PRB intrusions; however, exhumation record of the PRB suggests the supracrustal volcanic cover above the eastern batholith was likely removed prior to upper Campanian deposition of the Cabrillo Formation.