Description
Clast counting of Maastrichtian Cabrillo Formation conglomerates between Bird Rock and Tourmaline Beach in San Diego, California defines distinctly different clast populations above and below a 20 m thick mudstone/turbidite interval in the middle of the section. Comparing lower and upper conglomerate means, proportions of volcanic clasts increase from 51.2 to 66.5% upsection, while metamorphic clast proportions decrease from 31.5 to 18.5%. Quartzites compose 86% of metamorphic clasts. The proportion of plutonic clasts in the section remains consistent throughout at 16.2%. Chisquare hypothesis testing confirms a statistical difference between upper and lower conglomerates. Monzogranites and garnet-muscovite leucogranites are abundant in the section while granitoids with rapakivi texture are a minor but widespread component. Previous workers have assumed a fully local Peninsular Ranges batholith provenance for the Cabrillo Formation. However, potassic and peraluminous granitoids are over-represented in the coastal conglomerates compared to the mainly tonalitic Peninsular Ranges batholith. The atypical granitoids, as well as abundant quartzite suggest that exotic sources may have supplied detritus to the Cabrillo Formation. Alternatively, the unusual granites may represent a lag deposit due to their superior durability compared with more calcic plutonics. In-situ magnetic susceptibility screening of plutonic clasts indicates 71.5% are consistent with an eastern-zone Peninsular Ranges batholith derivation. These measurements generally support an episode of uplift in the eastern Peninsular Ranges batholith in the Late Cretaceous proposed by others. The magnetic susceptibility data show no variation in granitoid provenance upsection.
