Submarine geology of the Santa Rosa-Cortes Ridge

The Santa Rosa-Cortes Ridge is located 60 mi. W. of San Pedro, California, in a submerged basin and range province known as the "Continental Borderland." The ridge is flanked by 2 steep slopes which are believed to be fault scarps, and it is cut by transverse depressions that probably were formed by erosion along fault lines or have a structural origin. Gravel composed of igneous, metamorphic, and sedimentary materials is present atop the ridge. Several lines of evidence suggest that it was deposited during the Pleistocene. Rocks obtained from the ridge range in age from Eocene to late Miocene. Middle and upper Eocene shales, sandstones, and siltstones are limited to the area S. of Begg Rock. Miocene shales were recovered from the rest of the ridge. The present distribution of bottom sediments is believed to be the result of 1) deposition during the Pleistocene of coarse detrital sediments at depths too great to be explained by present-day currents, and 2) post-Pleistocene accumulation of calcareous organic sediments over most of the ridge and medium to very fine sands immediately off the shores of the islands.