The geological setting of Santa Monica and San Pedro Basins, California Continental Borderland

Abstract

The California Continental Borderland's present configuration dates from about 4 to 5 × 10 6 years before present (BP) and is the most recent of several configuration of the southern California margin that have evolved after the North America plate over-rode the East Pacific Rise about 30 × 10 6 years ago. The present morphology is a series of two to three northwest-southeast trending rows of depressions separated by banks and insular ridges. Two inner basins, Santa Monica and San Pedro, have been the site for the California Basin Study (CaBs), funded by the US Department of Energy. The Santa Monica and San Pedro Basins contain post-Miocene sediment thicknesses of about 2.5 and 1.5 km respectively. During the Holocene (past 10,000 years) about 10–12m have accumulated. The sediment entered the basin by one or a combination of processes including particle infall (mainly as bioaggregates) from surface waters, from nepheloid plumes (surface, mid-depth and near-bottom), from turbidity currents, mass movements, and to a very minor degree direct precipitation. In Santa Monica Basin, during the last century, particle infall nepheloid plume transport have been the most common processes. The former process has been dominant in thecentral basin at water depths from 900–945m, where characteristic silt-clay is found with a typical mean particle diameter of about 0.0006mm ( φ standard deviation = 2; φ skewness near zero). Kurtosis is typically 2 (platykurtic); these values indicate broad, near-log normal particle size distributions. The calcium carbonate content averages about 10% and organic carbon about 4%. Surficial sediment bulk densities are 1.1–1.2 and accumulation rates range from 16–30mg cm −2Yr 1 in this central fine deposit. Bottom water oxygen contents are at or below 0.1 ml 1 −1 so that bioturbation is inhibited, thus preserving the primary sedimentary stratification. There appear to be annual varves, but the laminae couplets are not simple wet-dry season pairs. Their formation is related to longer period climatic events (El Niño cycle of 3 to 4 years is dominant), and is also affected by aperiodic bottom water flushing events. The “anoxic” zone of bottom water has expanded over the past 300 to 350 years from a samll area in the northeastern basin floor (ca 120km 2 at 300 years BP), and now covers most of the basin floor below depths of about 850m (1290km 2). This expansion may have been primarily relaed to ocean circulation rate changes following the Little Ice Age, but is also likely to have been strongly affected by subsequent major changes in the natural drainages post-European colonization. The bottom water oxygen contents must also reflect the introduction of oxidizable pollutants, coincident with the shift to agricultural (Mission era) and then to urban use (late 19th century to present) of the adjacent mainland. Such localized effects are also suggested by the contrasting very large increases in the area of low oxygen bottom water in Santa Monica Basin relative to the minimal changes in the area of anoxia in Santa Barbara Basin over the same period and the lack of such zone in San Pedro Basin. In the San Pedro Basin the bottom waters have remained dysaerobic so its sediments are bioturbated and preserve little or no evidence of primary stratification. Dumping of oil drilling muds and cuttings during the couple of decades prior to 1970 has left a surficial deposit ranging from 1 to 15cm in thickness, forming a bull's eye isopach pattern as seen in radiographs. Bottom water oxygen content is usually >0.2ml O 21 −1. Bottom infauna and epifauna are sparse. Central basin floor sediments (water depths>800m) of San Pedro Basin have textural characteristics similar to those of Santa Monica Basin with distal basin floor surficial sediments being deposited primarily by particle infall and nepheloid flow at the present time. (Mean diameters ≈0.006mm, φ standard deviation ≈ 2, φ skewness ≈ 0 and kurtosis ≈ 2 (platykurtic). Calcium carbonate content averages 8–10% and organic carbon is about 4%. An estimate of the contemporary sedimentation rate was determined for a single box core of 25–29mg cm −2 yr 1 from the central basin near the present distributary margin; it cannot be considered necessarily to be representative. Mixing is indicated by the data. An important climatic factor is the alternation of multi-year wet and dry periods. The California Basin Study was conducted during an extended dry period of low river discharge. This undoubtedly will have affected the conclusions of the study related to sediment-water interactions and pollutant transport by suspended clays.