The seafloor off greater Los Angeles, California, has been extensively studied for the past century. Terrain analysis of recently compiled multibeam bathymetry reveals the detailed seafloor morphology along the Los Angeles Margin and San Pedro Basin. The terrain analysis uses the multibeam bathymetry to calculate two seafloor indices, a seafloor slope, and a Topographic Position Index. The derived grids along with depth are analyzed in a hierarchical, decision-tree classification to delineate six seafloor provinces—high-relief shelf, low-relief shelf, steep-basin slope, gentle-basin slope, gullies and canyons, and basins. Rock outcrops protrude in places above the generally smooth continental shelf. Gullies incise the steep-basin slopes, and some submarine canyons extend from the coastline to the basin floor. San Pedro Basin is separated from the Santa Monica Basin to the north by a ridge consisting of the Redondo Knoll and the Redondo Submarine Canyon delta. An 865-m-deep sill separates the two basins. Water depths of San Pedro Basin are ~100 m deeper than those in the San Diego Trough to the south, and three passes breach a ridge that separates the San Pedro Basin from the San Diego Trough. Information gained from this study can be used as base maps for such future studies as tectonic reconstructions, identifying sedimentary processes, tracking pollution transport, and defining benthic habitats.

In the past decade, several large programs that monitor currents and transport patterns for periods from a few months to a few years were conducted by a consortium of university, federal, state, and municipal agencies in the central Southern California Bight, a heavily urbanized section of the coastal ocean off the west coast of the United States encompassing Santa Monica Bay, San Pedro Bay, and the Palos Verdes shelf. These programs were designed in part to determine how alongshelf and cross-shelf currents move sediments, pollutants, and suspended material through the region. Analysis of the data sets showed that the current patterns in this portion of the Bight have distinct changes in frequency and amplitude with location, in part because the topography of the shelf and upper slope varies rapidly over small spatial scales. However, because the mean, subtidal, and tidal-current patterns in any particular location were reasonably stable with time, one could determine a regional pattern for these current fields in the central Southern California Bight even though measurements at the various locations were obtained at different times. In particular, because the mean near-surface flows over the San Pedro and Palos Verdes shelves are divergent, near-surface waters from the upper slope tend to carry suspended material onto the shelf in the northwestern portion of San Pedro Bay. Water and suspended material are also carried off the shelf by the mean and subtidal flow fields in places where the orientation of the shelf break changes abruptly. The barotropic tidal currents in the central Southern California Bight flow primarily alongshore, but they have pronounced amplitude variations over relatively small changes in alongshelf location that are not totally predicted by numerical tidal models. Nonlinear internal tides and internal bores at tidal frequencies are oriented more across the shelf. They do not have a uniform transport direction, since they move fine sediment from the shelf to the slope in Santa Monica Bay, but carry suspended material from the mid-shelf to the beach in San Pedro Bay. It is clear that there are a large variety of processes that transport sediments and contaminants along and across the shelf in the central Southern California Bight. However, because these processes have a variety of frequencies and relatively small spatial scales, the dominant transport processes tend to be localized and have dissimilar characteristics even in adjacent regions of this small part of the coastal ocean.

Most groundwater produced within coastal Southern California occurs within three main types of siliciclastic basins: (1) deep (>600 m), elongate basins of the Transverse Ranges Physiographic Province, where basin axes and related fluvial systems strike parallel to tectonic structure, (2) deep (>6000 m), broad basins of the Los Angeles and Orange County coastal plains in the northern part of the Peninsular Ranges Physiographic Province, where fluvial systems cut across tectonic structure at high angles, and (3) shallow (75–350 m), relatively narrow fluvial valleys of the generally mountainous southern part of the Peninsular Ranges Physiographic Province in San Diego County. Groundwater pumped for agricultural, industrial, municipal, and private use from coastal aquifers within these basins increased with population growth since the mid-1850s. Despite a significant influx of imported water into the region in recent times, groundwater, although reduced as a component of total consumption, still constitutes a significant component of water supply. Historically, overdraft from the aquifers has caused land surface subsidence, flow between water basins with related migration of groundwater contaminants, as well as seawater intrusion into many shallow coastal aquifers. Although these effects have impacted water quality, most basins, particularly those with deeper aquifer systems, meet or exceed state and national primary and secondary drinking water standards. Municipalities, academicians, and local water and governmental agencies have studied the stratigraphy of these basins intensely since the early 1900s with the goals of understanding and better managing the important groundwater resource. Lack of a coordinated effort, due in part to jurisdictional issues, combined with the application of lithostratigraphic correlation techniques (based primarily on well cuttings coupled with limited borehole geophysics) have produced an often confusing, and occasionally conflicting, litany of names for the various formations, lithofacies, and aquifer systems identified within these basins. Despite these nomenclatural problems, available data show that most basins contain similar sequences of deposits and share similar geologic histories dominated by glacio-eustatic sea-level fluctuations, and overprinted by syndepositional and postdepositional tectonic deformation. Impermeable, indurated mid-Tertiary units typically form the base of each siliciclastic ground-water basin. These units are overlain by stacked sequences of Pliocene to Holocene interbedded marine, paralic, fluvial, and alluvial sediment (weakly indurated, folded, and fractured) that commonly contain the historically named “80-foot sand,” “200-foot sand,” and “400-foot gravel” in the upper part of the section. An unconformity, cut during the latest Pleistocene lowstand (δ 18 O stage 2; ca. 18 ka), forms a major sequence boundary that separates these units from the overlying Holocene fluvial sands and gravels. Unconfined aquifers occur in amalgamated coarse facies near the bounding mountains (forebay area). These units are inferred to become lithologically more complex toward the center of the basins and coast line, where interbedded permeable and low-permeability alluvial, fluvial, paralic, and marine facies contain confined aquifers (pressure area). Coastal bounding faults limit intrabasin and/or inter-basin flow in parts of many basins.

Abstract Palos Verdes Peninsula contains large landslides in areas that are otherwise desirable for residential development. The landslides can be developed providing the factor of safety is at least 1.5 or can be raised to 1.5 during development. The greatest uncertainty in the calculated factor of safety is the residual shear strength of bentonite that forms the bases of slides. The use of too high a shear strength inflates the calculated factor of safety and can result in landslide failure after development. Tests by various investigators yield a wide range of residual shear strengths for bentonite samples from Palos Verdes Peninsula. Most residual friction angles (4φr) range between 6° and 14° with one lower value (3.5°) and several higher values reported. Residual cohesions (C r ) range mostly between 0 and 36 kPa (0 to 750 lb/ft2). Data for various bentonite samples from Palos Verdes Peninsula indicate they have similar compositions and Atterberg limits. Calcium montmorillonite is the principal clay mineral; the liquid limit is generally between 80% and 110% with the plasticity index between 40% and 70%. The data suggest that much of the reported variation in residual shear strength is the result of differences in sample preparation, testing methods, and interpretation of results rather than true differences in strength. Bulk samples of bentonite from the base of the Portuguese Bend landslide were remolded and tested by conventional direct shear, long sample direct shear, and ring shear devices to determine the effect of testing method on residual shear strength measurements. The three devices gave similar results—for conventional direct shear, φ r = 6.9° and Cr = 33.0 kPa (690 lb/ft2); for long sample shear, tyr = 6.8° and Cr = 23.8 kPa (497 lb/ft2); for ring shear, tyr = 6.7° and Cr = 7.2 kPa (150 lb/ft2). Back calculations indicate the ring shear results most closely simulate the residual strength along the base of the Portuguese Bend landslide. For all those devices, the residual shear envelope is a curve whose slope reaches an asymptote near a confining pressure of 200 kPa (approximately 4,000 lb/ft2). Reported φ r angles are commonly too high because samples were not tested at sufficient confining pressure to define the asymptote and a best fit straight line was drawn through the data points.

Abstract The Abalone Cove landslide, in southern California, is an 80 acre (32 ha) landslide within an ~870 acre (348 ha) ancient landslide complex. The landslide has developed in seaward-dipping marine strata of the middle Miocene Monterey Formation. The lower part of the landslide began moving by February 1974; the upper part did not appear to start moving until the spring of 1978. Since 1980, landslide movement has been con-trolled by the removal of ground water from the landslide mass. During years of nearly normal rainfall, subsurface inflow was the major source of ground water, contributing 55%. Percolation of rainfall and of delivered water were second and third, contributing 22% and 19%, respectively. During years of nearly twice normal rainfall, percolation of rainfall was the major source of ground water, contributing 56%. Subsurface inflow, percolation of delivered water, and surface inflow contributed 27%, 9%, and 8%, respectively. Prior to the installation of seven dewatering wells, the major loss of ground water was discharge to the surface by seeps at the toe of the landslide. The seeps accounted for 81% of the ground-water disposal. During the last two years of the study when the dewatering system was fully operational, surface seeps accounted for 34% of the ground-water disposal and pumping accounted for 54%. Other sources of ground-water disposal were evapotranspiration and subsurface outflow.

Abstract Amino-acid and oxygen isotope data for fossils from terraces of the Palos Verdes Hills and San Pedro areas in Los Angeles County, California, shed new light on the ages of terraces, sea-level history, marine paleotemperatures, and late Quaternary tectonics in this region. Low terraces on the Palos Verdes peninsula correlate with the ∼80-ka and ∼125-ka sea-level highstands that are also recorded as terraces on other coasts. In San Pedro, the Palos Verdes sand (the deposit on what is mapped as the first terrace by Woodring and others, 1946) was previously thought to be a single deposit; amino-acid, oxygen isotope, U-series, and fauna] data indicate that deposits of two ages, representing the 80-ka and 125-ka highstands occur within this unit. Oxygen isotope data show that on open, exposed parts of the Palos Verdes peninsula, ocean waters during the 125-ka highstand were cooler than present (by about 2.3-2.6°C) similar to what has been reported for other exposed coastal areas in California. In contrast, in the protected embayment environment around San Pedro, water temperatures during the 125-ka highstand were as warm or warmer than present. During the 80-ka highstand, water temperatures were significantly cooler than present even in the relatively protected embayment environment of the San Pedro area. Late Quaternary tectonic-uplift rates can be calculated from terrace ages and elevations. Correlation of the lowest terraces around the Point Fermin area shows that the Cabrillo fault has a late Quaternary vertical-movement rate of 0.20 m/ka, based on the difference in uplift rates on the upthrown and downthrown sides of the fault. Elsewhere in the Palos Verdes Hills-San Pedro area, late Quaternary uplift rates vary from 0.32 m/ka to possibly as high as 0.72 m/ka. These rates, which reflect vertical movement on the Palos Verdes fault, are in broad agreement with estimated Holocene vertical rates of movement determined for offshore portions of the fault.