Abstract

The Quaternary geology of four areas of the mainland shelf off San Diego County (Oceanside, La Jolla, Mission Beach, and San Diego Bay) was studied using high-resolution seismic profiles, side-scan sonar records, and textural and petrographic data from vibracore samples. The San Diego County shelf occurs in a tectonically active area, and is transected by a continuous, predominantly strike-slip structural zone marked by the Newport-Inglewood, South Coast, and Rose Canyon fault zones. Faulting, as well as Quaternary sea-level fluctuations, strongly affect the sedimentology and stratigraphy of the inner shelf.

The Quaternary strata along the San Diego County shelf consists of several discontinuous Pleistocene units, and sections as much as 40 m thick of Holocene nonmarine and marine sediment. The Pleistocene units include a prograded, outer shelf-slope marine facies, an estuarine sequence, marine and nonmarine deposits associated with marine terraces, stream terrace deposits, lag gravel deposits along the base of channels, and fault-ponded sediment. Fourteen river channels extend offshore from modern river mouths and lagoons, and are filled with thick sections of Pleistocene and Holocene nonmarine sediment which aggraded during the Flandrian transgression. A continuous deposit of very fine, Holocene, marine sand covers most of the shelf. Six Pleistocene and five Holocene marine terraces were mapped on the basis of their shoreline angle location. Terrace levels at 24 to 29 m and 38 to 45 m can be correlated both among the four areas examined and with oscillations in published sea-level curves from 11000 to 12000 years ago.

Sediment recovered in the vibracores was assigned to six types: Holocene Type I, a very fine-grained marine sand; Holocene Type II, nonmarine sand and gravel; Holocene Type III, a medium-grained marine sand; Pleistocene sediment; Miocene-Pliocene sediment; and Eocene sediment. Petrographic analysis revealed differences in values for the plagioclase/potassium feldspar ratios, total feldspar, and certain heavy minerals between sample sets north and south of La Jolla Submarine Canyon. Holocene samples between Carlsbad and La Jolla Canyons contain chert and intraclasts that were most probably derived from Eocene sedimentary rock exposed in adjacent sea cliffs. The change in lithology observed on either side of the submarine canyons is consistent with the concept that nearshore sediment transport is in the form of littoral cells bounded by submarine canyons.