Late Quaternary Seismic Stratigraphy and Shelf Deposits of the San Pedro to Oceanside Shelf

A study of the inner margin of the Southern California Continental Borderland between Point Conception , and the International Border with Mexico was recently completed, by Fischer and others in 1983. This report, a portion of the 1983 study, summarizes the Quaternary shelf sediment volumes between the Palos Verdes peninsula and Oceanside. The data set used for the study included an extensive set of high resolution seismic profiles, aerial photography and core hole information. Typically, the Late Quaternary (Holocene?) sediments of the mainland shelf form an elongate prism that parallels the shoreline and shelf break. The maximum thickness of the prism, which occurs near mid-shelf, is between 10 and 50 m. South of the Palos Verdes peninsula the shelf broadens between the Palos Verdes and Newport-Inglewood fault zones and sediment thickness is controlled by three active tectonic elements; the Huntington Beach fault , the Wilmington Graben and the Palos Verdes uplift. Sediments wedge-out over the faults and uplift and thicken locally to 50 m in the graben. The Quaternary sediment volume on the San Pedro shelf is estimated to be 4 X 109 m 3. South of the Newport submarine canyon the shelf narrows and the total sediment volume, (to the International Border) is 2.8 X 10 m 3. This sharp decrease in volume is attributed to the narrow shelf between Newport and La Jolla, estuaries which trap sediment, and a lack of major rivers. During the Holocene rise of sea level sedimentation rates on the margin increased and sediment was stored on the shelf. Local factors that control sediment distribution include; the major rivers which fed the San Pedro shelf, tectonic control (e.g. the Wilmington graben), submarine canyons which intercept lonqshore drift and shelf width.