Quaternary evolution and seismic stratigraphy of the San Pedro Margin, Southern California

The San Pedro basin margin is a relatively wide platform located southeast of the Palos Verdes Peninsula and the Los Angeles-Long Beach Harbor. A departure from the normally narrow Southern California inner basin margins, the configuration of this area is the result of the Neogene evolution of two tectonic elements: the Palos Verdes uplift and the Wilmington graben. The interrelationships of tectonic events, sedimentation dominated by the Los Angeles, San Gabriel, and Santa Ana Rivers, and glacioeustatic sea level fluctuations, are critical factors in explaining the evolution of this area. Exposed and near-surface rocks on the outer San Pedro shelf (Palos Verdes uplift) include basement rocks (Catalina Schist?)m folded Miocene (Monterey Formation) and Pliocene (Repetto Formation) strata (Mohnian to Repettian benthic stages), Pleistocene San Pedro Formation (?) and slope deposits. These are partly covered by Holocene sediments. A thicker post-Miocene stratigraphic section in the Wilmington graben is inferred from oil fields onshore. The interpreted seismic reflection data reveals the presence, near the edges of the graben, of middle Pleistocene (?) deposits (“unnamed upper Pleistocene deposits” of Poland and others, 1956) and the San Pedro (?) Formation. The upper Quaternary section in the Wilmington graben is based on a seismic-stratigraphic model of an upper Pleistoscene slope unit (LP I), and the Holocene shelf sediments. Analogous older units (LP II-IV) are defined by unconformities and paleo-shelf breaks. They are termed seismic-stratigraphic sequences and as such have chronostratigraphic significance. These units are believed to reflect glacioeustatic sea level fluctuations, and are provisionally correlated with stages in the marine oxygen-isotope curve of Shackleton and Opdyke (1973). Periods of incision and aggradation of the Newport-Inglewood zone fluvial gaps are correlated with isotope stages 1 through 5, and with units LP III through Holocene. The lowest (first) marine terrace of the Palos Verdes Hills is correlated with unit LP II. LP III correlates with the lowest terrace in the town of San Pedro, and the fourth and possibly the third and second terraces of the Palos Verdes Hills. The high-angle reverse and right-lateral Palos Verdes fault separates the Palos Verdes uplift from the Wilmington graben. Slightly oblique-trending folds are suggestive of convergent dextral shear along the fault. Its late Quaternary seismic activity is shown by offsets of units LP IV through LP I. Activity appears to have diminished since the deposition of LP III (possibly starting about 140,000 years B.P.), the youngest horizon commonly disrupted on all splays. However, some seismic activity has continued until the present, as indicated by epicentral data and numerous offsets of the basal Holocene surface and the seafloor. An area off the Palos Verdes Peninsula is markedly bowed over the fault zone. The shoreward edge of the Wilmington graben is defined by another active fault zone, portions of which also offset the base of the Holocene interval. Abrupt upward tilting of LP IV and older strata, pinch out of LP III and LP II, and areas of no Holocene sediments on the upthrown side further delineate this fault.