Submarine Geology off San Diego, California

The coast in the vicinity of San Diego is bordered by a shelf only 5-10 miles wide. Near the southern limit of the area, several small islands rise from the outer part of the shelf. These islands have a submarine extension to the north as a low ridge that is cut by a transverse canyon. North of the canyon the ridge forms an isolated, flattened bank that continues as a narrow ridge northward into deep water. Separating the bank and deep ridge from the shelf is a submarine valley having a "drainage" divide in the southern end. North of the divide the valley floor slopes northwestward into San Diego Trough. South of the divide the valley slopes southward, meeting the transverse canyon at a point east of the ridge. An escarpment just west of the islands and the bank forms one side of a deep graben-like depression known as San Diego Trough. A delta-like feature at the mouth of the canyon extends from the foot of the escarpment partway across the floor of San Diego Trough. The relationship of the transverse and longitudinal valleys and of the flat-topped "delta" may provide new information on the origin of submarine canyons. On the inner part of the shelf south of Point Loma is an area of outcropping Cretaceous shale and sandstone only partially covered with unconsolidated sediment. The bank top has another large tract of bedrock that consists of sandstone and shale of late Pliocene to late Pleistocene age, overlain in part by nodules of phosphorite which are Pleistocene to Recent in age. Rock cropping out on the shelf around the islands is lithologically similar to the Miocene rocks of the islands. Gravels are abundant in the various areas of bedrock outcrops and also are present in some areas where no bedrock is known. One tract of boulders and cobbles with only interstitial sand occurs a mile off the mouth of the Tia Juana River; similar gravels, but with much more sand, form an elongate area south of Point Loma. Most of the gravels are covered by delicate growths, showing that they are not being moved at the present time. Possibly they are residual deposits formed during times of glacially lowered sea-level. The finer sediments that partially blanket the sea floor have a distribution that is much more complex than the usual concept of marginal marine sediments. The chief factors that control the distribution are: (1) earlier deposition of coarse sediments at depths now below the effective reach of wave action, (2) present-day currents that by-pass fine detrital sediments and allow accumulation only of coarse calcareous organic sediments, and (3) present-day deposition of fine sands in areas adjoining beaches and of silts in deeper water. The mineralogy of the inner shelf and bank sediments is similar to that of the Pliocene-Pleistocene rocks that crop out on land and on the sea floor; hence they are considered as having been derived from Pliocene-Pleistocene rocks or from the same sources that supplied the Pliocene-Pleistocene sediments. Clastic sediments in the vicinity of Los Coronados Islands appear to be locally derived from the Miocene rocks of the islands.