The “Type” San Pedro Sand From the Palos Verdes Peninsula: How Characteristic Is It?

Daniel J. Ponti
U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, CA 94025

Outcrops of San Pedro Sand, Timms Point Silt, and Lomita Marl, described by Woodring and others (1946) in the town of San Pedro and along the northeast flank of the Palos Verdes Peninsula (PVP), represent some of the best exposures of Pleistocene marine deposits in southern California. These exposures comprise the type localities for the “San Pedro Formation” of Poland and others (1956), a term that has subsequently been used to describe much of the marine Pleistocene in the coastal basins of southern California. Although widespread use of the term “San Pedro Formation” implies a regional temporal correlation and genesis for these marine deposits, recent stratigraphic investigations near the PVP and elsewhere in the Los Angeles basin indicate that the “type” San Pedro is not typical of the “San Pedro Formation” as a whole. Instead, the type Timms Point Silt and San Pedro Sand comprise a >100 m thick deltaic sequence that prograded NW-SE along the NE flank of the Palos Verdes uplift about 400-200 k.y. ago. Rather than being laterally extensive, the deltaic facies is confined mostly to a narrow, syndepositional, structural basin formed between the Palos Verdes fault and the crest of the Wilmington anticline. West of the Newport-Inglewood (N-I) fault and NE of the Wilmington anticline, sequence-stratigraphic analyses from cores and well logs show that the sequence correlative with the “type” San Pedro is only ~20 m thick and reflects shallow marine and paralic sedimentary environments. This sequence overlies a much thicker deltaic complex, 500-600 k.y. old that is typically referred to as the “San Pedro Formation” in the western LA basin. Parts of this older sequence crop out along the N-I uplift.

In the central Los Angeles basin, preliminary interpretations of oil-industry seismic data suggest that the sequence correlative with the “type” San Pedro rarely exceeds 50 m thick and extends to depths of > 500 m. Across the central LA basin east of the N-I fault, this sequence occurs near the base of what commonly is referred to as “San Pedro Formation” from ground water studies. Toward the eastern margin of the LA basin, these strata appear to pinch out and/or change laterally into non-marine facies. Recent paleontological interpretations from “San Pedro Formation” exposures in the Coyote Hills (Powell and Stevens, 2000) indicate a much older lower Pleistocene to Pliocene age for the sediment, which is consistent with the seismic interpretation. Therefore, the “type” San Pedro exposures on the PVP appear not to be representative of the “San Pedro Formation” as a whole, but instead to reflect a restricted depositional environment, controlled by local tectonics, that existed near the PVP only during the latter part of the middle Pleistocene.

AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California