Description
The Cabrillo Formation, of Upper Cretaceous age, is exposed in San Diego County along the Point Loma Peninsula, the La Jolla sea cliff and in the Mount Soledad anticline in La Jolla. Rock types include gradational conglomerate, sandstone and mudstone. Each rock type is characterized by sedimentary structures whose origins are: 1) syn-depositional, 2) post-depositional, chiefly by gravity, 3) erosional and 4) biogenic. Interpretation of the petrology, stratigraphic relations and sedimentary structures suggests the Cabrillo Formation was deposited below wave base on a steep slope such as is now found off the coast of Southern California. The shoreline lay immediately west of the elevated granitic-metamorphic complex of what is now the Peninsular Ranges, which was the source of the sediments. The conglomerate clasts comprise a distinct assemblage of intermediate plutonic rocks (39%) and mildly metamorphosed andesitic volcanic rocks (38%) which belong to the Peninsular Ranges Suite. Small-scale cross-stratification is the most abundant sedimentary structure in the formation, but convolute bedding, flame structures, imbricated clasts and channels are common paleocurrent and paleoslope indicators. The structures indicate two well developed current systems: one from the southeast in the Point Loma area and one from the north in the La Jolla area. Coarse clastic debris was introduced by density currents and debris flows. Together the assemblage of rock types and sedimentary structures strongly suggests an environment very similar to the Recent La Jolla submarine fan and fan-valley.
