Description
The Cabrillo Formation represents an inner-submarine fan valley underlain conformably by the Point Loma Formation. Together the two formations formed as a westwardprograding submarine fan. Its provenance is thought to be the Peninsular Ranges batholith and metamorphic rock formations plus the Santiago Peak Volcanics. Counting clast lithologies can help to prove or disprove these sources. Clast counting of Cabrillo Formation conglomerate beds behind the San Diego Sewage Treatment Plant on Point Loma shows similar clasts in the lower and middle levels of the formation: volcanic clasts 51 vs. 46%, plutonic clasts are 18 vs. 24%, and metamorphic clasts at 28% vs. 23% respectively. All of the sedimentary clasts are rip-up clasts from the underlying mudstone. In situ magnetic susceptibility readings of all counted clasts at the two sites show that 86% of the clasts have less than 21 X 10-5 SI units, and 95% of the clasts have less than 100 X 10-5 SI units. High readings were found in aphanitic volcanics, dacites and some scarce quartz diorites. These primarily non-magnetic clasts match with values found today in the eastern-zone of the Peninsular Ranges batholith, helping to support other evidence that interprets an intense uplift in the eastern Peninsular Ranges.
