Mainland Shelf Research Articles

Offshore oil production platforms as potential sources of larvae to coastal shelf regions off southern California

A diverse assemblage of adult reef fishes and invertebrates occurs at offshore oil production platforms in the Southern California Bight (SCB). Coincident with the initiation of the decommissioning of six platforms in the SCB, the goal of this study was to examine how a platform's geographical location plays a role in its potential contribution of larval recruits to natural areas. Using a three-dimensional biophysical model, we quantified the potential connectivity of larvae, particularly relevant to reef fishes, from three offshore platforms to four coastal shelf regions where the majority of rocky settlement habitat occurs in the SCB. The regions cover the shelves of the mainland coast and islands and offshore banks in the southern SCB. The main findings indicate that (1) the potential for larval subsidies from platforms in the southern SCB to populations in the northern SCB are greater than the potential for larval subsidies from platforms in the northern SCB to the southern SCB; (2) there is greater seasonal variability of potential connectivity from platforms to the mainland shelf region of the northern SCB than to the mainland shelf region of the southern SCB or shelves around islands and banks; and (3) there is consistency across years in the relative magnitude of potential connectivity from the platforms to the four shelf regions. We conclude that a platform's function as a larval source should be considered an ecological criterion when evaluating whether a platform is to be converted to an artificial reef and implementing marine spatial planning.

A Plio-Pleistocene sediment wedge on the continental shelf west of central Ireland: The Connemara Fan

Glacigenic sediment fans recording shelf edge deposition from marine-terminating ice sheets have previously been recognised along the NW European continental margin from Svalbard to as far south as Donegal, off north-west Ireland. Here we present evidence of a previously unrecognised partially glacially-fed Plio-Pleistocene sediment wedge on the continental shelf west of central Ireland using 2D and 3D seismic reflection data correlated to a commercial borehole. The ‘Connemara Fan’ covers a shelf area of approximately 9000 km2 in water depths of 125–310 m, extending westwards into the Porcupine Seabight from the Irish Mainland Shelf. The wedge comprises up to 160 m of sediment that culminates in a prominent moraininc ridge at seabed and contains two discordant reflection surfaces (R1 and 2) that subdivide it into three seismic units (SU1–3). Stratigraphic boreholes 27/24-2 and 2A located on the inner shelf show that the lower unit (SU1) is composed of Pliocene marine sediments, while SU2 and 3 comprise glacially influenced facies of Quaternary age. Extracts from a 3D seismic data volume within the central part of the fan show channels within the Pliocene succession, while iceberg scours are observed on the R1 and R2 reflectors. The Connemara Fan is inferred to record sediment supply from central western Ireland, with Quaternary units probably recording at least two glacial advance-retreat cycles with ice sheets repeatedly grounding across the inner (Irish Mainland) shelf. Our findings extend the range of glacially-influenced grounding line depocentres southwards along the NW European continental margin.

Nineteenth-century collapse of a benthic marine ecosystem on the open continental shelf

The soft-sediment seafloor of the open continental shelf is among the least-known biomes on Earth, despite its high diversity and importance to fisheries and biogeochemical cycling. Abundant dead shells of epifaunal suspension-feeding terebratulid brachiopods (Laqueus) and scallops on the now-muddy mainland continental shelf of southern California reveal the recent, previously unsuspected extirpation of an extensive offshore shell-gravel ecosystem, evidently driven by anthropogenic siltation. Living populations of attached epifauna, which formerly existed in a middle- and outer-shelf mosaic with patches of trophically diverse muds, are restricted today to rocky seafloor along the shelf edge and to the sandier shelves of offshore islands. Geological age-dating of 190 dead brachiopod shells shows that (i) no shells have been produced on the mainland shelf within the last 100 years, (ii) their shell production declined steeply during the nineteenth century, and (iii) they had formerly been present continuously for at least 4 kyr. This loss, sufficiently rapid (less than or equal to 100 years) and thorough to represent an ecosystem collapse, coincides with intensification of alluvial-plain land use in the nineteenth century, particularly livestock grazing. Extirpation was complete by the start of twentieth-century urbanization, warming, bottom fishing and scientific surveys. The loss of this filter-feeding fauna and the new spatial homogeneity and dominance of deposit- and detritus-feeders would have altered ecosystem functioning by reducing habitat heterogeneity and seawater filtering. This discovery, attesting to the power of this geological approach to recent ecological transitions, also strongly increases the spatial scope attributable to the negative effects of siltation, and suggests that it has been under-recognized on continental shelves elsewhere as a legacy of coastal land use.

Effect of coastal‐trapped waves and wind on currents and transport in the Gulf of California

AbstractSubsurface pressure (SsP) observations from stations inside and outside of the Gulf of California (GC) are used to analyze the relationship between low‐frequency currents, temperature, and transport inside the GC and intraseasonal coastal‐trapped waves (CTWs), which propagate poleward along the coast toward the GC. Correlation functions and coherences of SsP stations were consistent with intraseasonal CTWs splitting in two at the mouth of the gulf: one part enters the gulf, propagates around the gulf, and eventually, toward the mouth, and another part that appears to “jump” the mouth of the gulf and travels poleward along the west coast of the peninsula. The correlation and coherence estimates of SsP at Manzanillo with currents showed that downwelling CTWs generated along‐gulf current anomalies toward the head of the gulf at the mainland shelf of the mouth, whereas at Ballenas Channel sill (San Lorenzo sill) these waves generated current anomalies toward the mouth near the surface (bottom). At the San Lorenzo (SL) sill, downwelling CTWs increased the near‐bottom (∼400 m) temperature and reduced the bottom transport of deep, fresher, and colder water that flows toward the head of the gulf. Cross‐Calibrated Multiplatform winds were used to investigate their relationship with currents. The first empirical orthogonal function of the along‐gulf wind stress showed that wind blowing toward the head of the gulf generated a reduction of bottom transport toward the head of the gulf through the SL sill, and intensified surface geostrophic current fluctuations toward the head of the gulf. There was also significant correlation between inflow bottom transport and outflow surface geostrophic velocities averaged across the gulf, consistent with the exchange pattern for the Northern Gulf.

Genetic diversity and population structure of long-tailed macaque (Macaca fascicularis) populations in Peninsular Malaysia

The genetic diversity and structure of long-tailed macaques (Macaca fascicularis) in Peninsular Malaysia, a widely used non-human primate species in biomedical research, have not been thoroughly characterized. Thirteen sites of wild populations of long-tailed macaques representing six states were sampled and analyzed with 18 STR markers. The Sunggala and Penang Island populations showed the highest genetic diversity estimates, while the Jerejak Island population was the most genetically discrete due to isolation from the mainland shelf. Concordant with pairwise F(st) estimates, STRUCTURE analyses of the seven PCA-correlated clusters revealed low to moderate differentiation among the sampling sites. No association between geographic and genetic distances exists, suggesting that the study sites, including island study sites, are genetically if not geographically contiguous. The status of the genetic structure and composition of long-tailed macaque populations require further scrutiny to develop this species as an important animal model in biomedical research.

The effects of a tidal-mixing front on the distribution of larval fish habitats in a semi-enclosed sea during winter

We examined the effect of a tidal-mixing front on the three-dimensional distribution of larval fish habitats (LFHs) in the Midriff Archipelago Region in the Gulf of California during winter. Zooplankton and environmental variables were sampled from 0 to 200 m in 50 m strata. Four LFHs were defined in association with the front, two on the northern side and two on the southern side. The northern LFHs were: (1) the Mainland Shelf Habitat, located from the surface to 100 m depth on the north-east mainland shelf, characterized mainly by the presence of Citharichtys fragilis; and (2) the Wide Distribution Habitat, extending from north-west to south across the front from the surface to 200 m depth, dominated by the ubiquitous Engraulis mordax. The southern LFHs were: (3) the Eddy Zone Habitat, defined nearly on an anticyclonic eddy, with the highest larval abundance and richness from the surface to 100 m depth, dominated by Leuroglossus stilbius; and (4) the Southern Gulf Habitat, associated with low temperature waters from the southern Gulf of California, dominated by southern-gulf species (e.g. Scomber japonicus and Sardinops sagax). Despite the weak stratification and low thermal contrast (~1.5°C) across the south front compared to summer (~3°C), our results demonstrate that the frontal zone may influence the formation of planktonic habitats even during generally homogeneous periods, which may also be relevant in other regions of the world.

Seasonal changes in connectivity routes among larval fish assemblages in a semi-enclosed sea (Gulf of California)

Seasonal changes in connectivity routes among larval fish assemblages in the northern Gulf of California were studied with connectivity matrices from particletracking with a 3D baroclinic numerical model. Results show strong seasonality in connectivity routes among assemblages according to the seasonal circulation phases. In summer (cyclonic phase), when circulation is dominated by the central cyclonic eddy and the northwestward coastal current on the mainland shelf, particle retention among assemblages after 30 days is high (.55%), and particle dispersion toward the Southern Gulf is low (,17%). Thus recruitment of most fish species must occur near their spawning areas: in June, coastal–demersal species such as Gobulus crescentalis and Etropus crossotus, and in August, coastal-epipelagic species such as Opisthonema libertate and species of the genus Anchoa. In winter (anticyclonic phase), when the coastal current is southeastward, particle retention among assemblages is low (,35%), and particle dispersion toward the southern gulf is high (.54%). Therefore, recruitment may occur away from the spawning locations, as suggested by the wide distribution of Engraulis mordax (coastal-epipelagic) and Benthosema panamense (mesopelagic). Seasonal changes in connectivity routes show that ocean dynamics must be considered in management and conservation plans for marine ecosystems.

Benthic macrofaunal community condition in the Southern California Bight, 1994–2003

To assess benthic macrofaunal community condition in Southern California, 838 sites were sampled using spatially random designs in 1994, 1998, or 2003. Benthic community condition was assessed on a four-category scale and the area in each category estimated. Overall, benthic macrofauna in Southern California were in good condition during 2003, with 98% of the area in reference condition or deviating only marginally. There was no evidence of disturbance near Channel Islands or small wastewater discharges, and virtually none on the mainland shelf. In contrast, bay and estuary macrofaunal communities were more frequently disturbed with nearly 13% of the area supporting disturbed benthos. The condition of the mainland shelf did not change substantially over the 9-year period, with 1.6–2.8% of the area in poor benthic condition. Southern California benthic condition evaluations may be improved by extending the depth and salinity ranges of assessment tools, and improving trend detection methods.

Seasonal changes in larval fish assemblages in a semi-enclosed sea (Gulf of California)

The northern Gulf of California (NGC) is characterized by seasonal hydrography and circulation (cyclonic in summer and anticyclonic in winter), by intense tidal mixing in the midriff archipelago region (MAR), and by coastal upwelling on the eastern side from autumn to spring. We examined changes in larval fish assemblages (LFAs) in relation with hydrography and circulation during both phases of the seasonal circulation, as indicators of changes in the pelagic ecosystem. A canonical correspondence analysis defined LFAs ( r>0.70), which were related with: (i) the coastal current on the mainland shelf, (ii) the central eddy and (iii) the MAR. In the early cyclonic phase, when the temperature and stratification were increasing and the coastal current was starting, demersal ( Gobulus crescentalis, Lythrypnus dalli) and mesopelagic species ( Benthosema panamense) dominated the NGC. The highest larval abundance was in the Current LFA area and the lowest in the MAR LFA area. In the mature cyclonic phase, the larval abundance increased in the NGC and species characteristic of eastern boundary current systems such as Opisthonema libertate and Engraulis mordax displaced the demersal species and became dominant, together with B. panamense in the Current LFA area; the latter species dominated in the Eddy LFA area. In the early anticyclonic phase, the direction of the coastal current reversed and the temperature and larval abundance decreased. E. mordax and B. panamense larvae continued dominating the NGC with higher abundance in the MAR than in the Current and Eddy LFA areas. In the mature anticyclonic phase, E. mordax larvae dominated in the Current and the Eddy LFA areas with the highest abundance in the former, while M. productus larvae (an eastern boundary current species) dominated in the Eddy LFA area. Results showed that in the NGC, the dramatically seasonal and predictable hydrographic and circulation features trigger the seasonal spawning of the dominant species. The biological richness of the coastal current area, in both circulation phases, suggested that this area has an important role in the pelagic ecosystem functionality of the NGC.

SST, thermohaline structure, and circulation in the southern Gulf of California in June 2004 during the North American Monsoon Experiment

The thermohaline structure, circulation, and heat fluxes in the Gulf of California entrance during June 2004 are described based on conductivity‐temperature‐depth and Lowering Acoustic Doppler Current Profiler data collected in a 14‐day survey, supported by satellite data. The AVHRR images show extensive mesoscale structures in the region, the most striking being (1) a cool filament extending from the California Current domain and (2) a warm intrusion along the mainland shelf. On the warm side of the thermal front created by the cool filament there was a strong current flowing into the Gulf, with speeds up to 0.70 ms−1 in the surface; this current, which the SST images suggest was associated with a decaying eddy, carried 6 Sv into the Gulf. Associated with the second structure, there was an ingoing coastal current on the mainland shelf, with weak surface currents but with speeds ∼0.25 ms−1 at its core, between 70 and 200 m; this coastal current transported 2 Sv into the Gulf. The two ingoing currents appear to join inside the Gulf, forming a very strong (speeds 0.40–0.80 ms−1) narrow (∼30 km) coastal current between the surface and 500 m depth. Changes in the thermohaline structure of the upper layers observed by repeat sampling of three cross sections were dominated by advection. However, it was found that the advective heat flux is very variable in space and time. For the period of observation it was estimated that the lateral heat input was 4.8 ± 3.0 × 105 Wm−2 as estimated with LADCP currents and 5.7 ± 2.20 × 105 Wm−2 with geostrophic velocities.

Rissoidae (Mollusca: Gastropoda) from northeast Atlantic seamounts

The gastropod family Rissoidae is revised at the species level for the Lusitanian seamounts, situated between Portugal and Madeira, and the Meteor group of seamounts, situated south of the Azores in the northeast Atlantic Ocean. Based on material obtained by dredging and trawling, 48 species are reported, of which 30 are described as new. There is very little overlap between the assemblages found on both groups of seamounts, with only two shared species. On the Lusitanian seamounts, 24 species were collected. Seven species (six with planktotrophic development) are shared with the mainland shelf or slope and are represented in low numbers. Eleven species are endemic to this seamount group as a whole and of these, three account for 75% of individuals. Of the species not shared with the mainland, only one is found on the four seamounts, eight (three new) are found on two or three seamounts and/or neighbouring islands, and six (all new) are endemic to a single seamount. On the Meteor group, 26 species were found, of which five are shared with the Azores and 20 (all new) are endemic to the seamount group as a whole. Most species are found on only two or three seamounts, whereas nine species are endemic to only one of the seamounts, and of these five are concentrated on Atlantis seamount. Eight endemic species of the Meteor group included in Porosalvania n. gen. have very different shapes and occupy discrete bathymetric intervals, but are best interpreted as a local radiation originating from a relatively old colonization of this seamount group. The endemic species, and among them the successful ones, all have a paucispiral protoconch denoting non‐planktotrophic development. As in the Macaronesian archipelagos, the Rissoidae are the most species‐rich molluscan family on the northeast Atlantic seamounts.

Estimating the dispersal potential of polychaete species in the Southern California Bight: implications for designing marine reserves

Using known and inferred life-history information, we estimated the dispersal potential of 501 polychaete species sampled during a 1998 monitoring study in the Southern California Bight. We tested the hypothesis that species having life-history traits that suggest long-distance dispersal will be encountered more frequently throughout the region than will species having life histories that suggest limited dispersal. When all 501 species and all 200 sampling sites were analyzed, occurrence frequency (percentage of sites at which a species was collected) was not significantly related to dis- persal potential. When data from 53 shelf sites in the Channel Islands were analyzed separately from collections at 147 mainland-shelf sites, there was a significant positive relationship between disper- sal potential and occurrence frequency at the island sites but not at the mainland sites. Of the 501 species, 119 were collected only at island sites, 98 were found only at mainland sites, and 284 were found at both island and mainland sites. The majority of the 'island only' species had life-history traits indicating low dispersal potential. In contrast, only 13% of the 'mainland only' species were catego- rized as having low dispersal potential. The 'cosmopolitan' species had a broad range of dispersal potential. Models indicate that efforts to conserve biodiversity by establishing Marine Protected Areas (MPAs) must consider species' dispersal. In the shelf communities of the Southern California Bight, networks of small reserves that are located in existing areas of high diversity should succeed in the Channel Islands, where the majority of polychaete species tend to have limited dispersal poten- tial. On the mainland shelf, however, designing effective MPAs will be more challenging due to the prevalence of species that have a greater potential for long-distance dispersal to or from unprotected sites.

Characterizing benthic substrates of Santa Monica Bay with seafloor photography and multibeam sonar imagery.

Seafloor photography from three cruises is combined with multibeam sonar imagery to characterize benthic substrates and associated fauna of Santa Monica Bay, California. The multibeam EM1000 imagery was collected in 1996. Two sampling cruises (in 1998 and 1999) provided photographs at 142 sites throughout the Bay; a final cruise (in 2000) collected still photographs and continuous video along nine transects on the mainland shelf from Pt. Dume to the Palos Verdes peninsula. Muddy substrates (typically low backscatter) were the predominant habitat throughout the Santa Monica Bay, from the 20 m isobath to the adjacent Santa Monica basin floor (780 m). Bioturbation was pervasive as evidenced by abundant open burrows, mounds, and faunal tracks and trails. Sandy substrates (typically intermediate to high backscatter) were restricted to the innermost mainland shelf and a narrow outer shelf band north of Santa Monica Canyon. Cobble and gravel substrates (high backscatter) were restricted to the innermost shelf south of El Segundo and limited parts of the shelf edge. Rocky substrates (high backscatter) with interspersed patches of sand and gravel occurred on the high-relief marginal plateau and along parts of the shelf break offshore of Malibu.

Benthic Response Index for Assessing Infaunal Communities on the Southern California Mainland Shelf

Although benthic infaunal communities are commonly measured to assess the effectiveness of environmental management in protecting biological resources, the tools used to interpret the resulting data are often subjective or site specific. We present an objective, quantitative index for application throughout the southern California coastal shelf environment that measures the condition of a benthic assemblage, with defined thresholds for levels of environmental disturbance. The index was calculated using a two-step process in which ordination analysis was employed to quantify a pollution gradient within a 717-sample calibration data set. The pollution tolerance of each species was determined based upon its distribution of abundance along the gradient. The index is calculated as the abundance-weighted average pollution tolerance of species in a sample. Thresholds were established for reference condition as well as for four levels of biological response. Reference condition was established as the index value in samples taken distant from areas of anthropogenic activity and for which no contaminants exceeded the effects range low (ERL) screening levels. The four response levels were established as the index values at which key community attributes were lost. Independent data sets were used to validate the index in three ways. First, index sensitivity to a spatial gradient of exposure to a discharge from a point source was tested. Second, index response to a temporal gradient of exposure to a discharge from a point source was examined, testing index robustness to natural temporal variation. Third, the effect of changes in natural habitat (e.g., substrate, depth, and latitude) on index sensitivity was tested by evaluating the ability of the index to segregate samples taken in areas with high and low chemical exposure, across a gradient of physical habitats.

Relationship between depth, sediment, latitude, and the structure of benthic infaunal assemblages on the mainland shelf of southern California

A regional benthic survey was conducted in 1994, and the data were used to assess the relationship among three habitat factors (depth, sediment grain size, and latitude) and the distribution of benthic infaunal assemblages on the southern California coastal shelf. Benthic samples were collected with a 0.1 m2 Van Veen grab from 251 sites on the continental shelf (10–200 m deep) from Point Conception, California, to the United States–Mexico international border. The relationship between habitat and assemblages was investigated by conducting a Q-mode cluster analysis to define groups of stations with similar species composition and then examining whether differences were present in physical habitat attributes among those groups of stations. Analysis of data from 175 uncontaminated sites yielded four habitat-related benthic infaunal assemblages along the southern California coastal shelf: a shallow-water assemblage from 10–32 m, a mid-depth assemblage between 32 and 115 m, and two deep-water (115–200 m) assemblages, one in fine and one in coarse sediments. These empirically defined points in the depth and sediment grain size gradients can be used to define reference habitats for the development of biocriteria. Benthic abundance and diversity were greatest in the mid-depth assemblage, conforming to predictions for benthic assemblages in regions of upwelling. Within the 500 km of coastline examined, latitude was not an important factor in defining assemblages.

Sediment Chemistry on the Mainland Shelf of the Southern California Bight

To understand large-scale environmental changes and the cumulative effects of multiple discharges, 248 sites on the mainland shelf of the Southern California Bight (SCB) were sampled using a stratified-random study design. Eighty-nine per cent of SCB sediments were found to contain evidence of anthropogenic contamination. Eighty-nine per cent of the SCB was contaminated with chlorinated hydrocarbons including total DDT (82%) and total PCB (46%). Using iron as a conservative tracer of natural contributions for eight metals of interest, 50% of the SCB was estimated to be anthropogenically enriched in at least one trace metal. Sediments near discharges from publicly owned treatment works (POTWs) and urbanized watersheds had a greater extent and magnitude of contamination than other SCB sediments. Santa Monica Bay, where numerous anthropogenic sources commingle, had significantly higher concentrations of every constituent examined relative to other regions of the SCB.

Distribution of Anthropogenic and Natural Debris on the Mainland Shelf of the Southern California Bight

Various studies have been conducted to quantify debris found along beaches; however, little information has been compiled about debris found on the seafloor. This study describes the distribution, types, and amounts of marine debris found in the Southern California Bight (SCB) in July and August of 1994. Anthropogenic debris was most common in the central region, on the outer shelf, and in areas near publicly owned treatment works (POTWs). Fishing gear was the most common type of anthropogenic debris in the central region and in the outer shelf zone, whereas glass bottles and plastic were most common in POTW areas. Natural debris was more common close to shore in the inner shelf zone than anthropogenic debris. The deeper distribution of anthropogenic debris relative to natural debris, as well as the types of debris, suggest that the primary source of anthropogenic debris is marine vessel and fishing activity.

Biological Monitoring: The Dilemma of Data Analysis

Biological Monitoring: The Dilemma of Data Analysis

The transition between the Erris and the Rockall basins: new evidence from wide-angle seismic data

The Rockall Trough is a deep-water basin west of Britain and Ireland. The origin of the basin and the nature of the crust beneath it is controversial. A series of seismic wide-angle experiments carried out during 1988 and 1990, help to clarify the crustal and upper mantle structure in the region. Results for the crustal structure from a profile which straddles the shelf break between the basin and the Irish Shelf are discussed here. These data together with the available geological information indicate that the basin probably formed in the late Palaeozoic to early Mesozoic as part of a regionally linked basin assemblage which includes the Hatton Basin and the shallow sea basins surrounding Ireland and Britain. Good data quality has allowed the transition between the relatively unstretched crust of the Irish and British mainland, defined by previous onshore seismic refraction experiments, to be well resolved. The Erris Trough, one of the small late Palaeozoic to early Mesozoic basins which fringe the mainland shelf region north of the Porcupine Basin is a half-graben. Its major bounding fault is located on its western margin and the basin is divided from the Rockall Trough by a narrow horst, the Erris Ridge. The shelf trajectory along the western flank of the horst deepens smoothly towards the trough centre, where the crust thins to 5 km near the trough margin below a sedimentary and water column 8 km thick. Surprisingly, the crustal thickness is slightly greater over a 150 km broad zone at the trough centre (i.e., ca. 6 km). This change in crustal thickness may be due to lateral strain migration to the warmer basin margins as its centre cooled during the deformation. The crustal structure beneath the sediment pile at the trough centre is two layered, as opposed to the three-layered seismic refraction structure found onshore. However, the basic character of the lower crust present in onshore Ireland, in particular the presence of a gradient zone defining the crust/mantle transition, is still preserved in the Trough. This similarity in structure precludes the presence of magmatic underplating. The crustal structure observed in the Rockall Trough can be formed by differential stretching of the lithosphere. In this model the lower ductile crust and mantle lithosphere are stretched over a wide region byβ 2 = 2−3. Strain focusing into a much narrower region of brittle upper crust generates severe amounts of crustal thinning (β 1 = 8−10), and is responsible for the fusing of the upper and mid-crustal seismic refraction layers found beneath onshore Ireland and Britain. Mediating detachment surfaces, sited at the brittle/ductile transition at any time, served to relay the strain from the lower lithosphere into the upper crust. Syntectonic heat loss plays an important role in controlling the deformation pattern.

Late Quaternary history of the Ventura mainland shelf, California

The Ventura mainland shelf off California provides an interesting example of sedimentary response to tectonic activity and eustatic sea-level fluctuations during the late Quaternary. A network of 370 km of high-resolution seismic-reflection profiles provides the basis for the study. The high local rates of uplift, as well as local basin subsidence, have resulted in two distinct styles of sedimentation. A late Pleistocene angular unconformity separates folded Plio-Pleistocene strata from undeformed upper Pleistocene and Holocene strata on the northern shelf. In contrast, the southern shelf is characterized by a thick section of stacked, progradational deltaic sequences containing a well-preserved record of late Quaternary sedimentation. Sediment overlying the unconformity north of the Pitas Point fault ranges from 10 to 40 m in thickness, whereas the equivalent sequence in the adjacent southern shelf is 80–85 m thick. The distribution of upper Pleistocene and Holocene deposits is controlled by structural features including the Pitas Point and Oak Ridge faults, and the Ventura Avenue anticline, as well as proximity to fluvial input from the Santa Clara and Ventura Rivers. Within upper Pleistocene deposits, a pair of buried marine terraces occur at depths of 33 and 50 m below present sea level (b.p.s.l.). Terrace deposits consist of a basal, coarse-grained, transgressive deposit which is transitional into a finer grained marine facies. These deposits are associated with a period of aggradation during minor late Wisconsinan sea-level cycles. Deltaic deposits beneath the southern shelf 4 km north of Hueneme submarine canyon suggest that the Santa Clara River debouched into the sea at this location during the late Pleistocene. The Santa Clara River shifted southward during the late Wisconsinan and cut a large channel, now buried, near Hueneme canyon. This channel indicates that relative sea level was at least 113 m b.p.s.l. during the late Wisconsinan. A smaller channel indicates a fall in sea level to 46 m b.p.s.l. during the Holocene. The Oak Ridge fault displays a general increase in offset seaward across the shelf, with a maximum vertical displacement of 17 m on the Saugus/upper Pleistocene contact. Displacements on the Pitas Point fault increase shoreward, attaining as much as 40 m of vertical displacement of the late Pleistocene unconformity. Nearshore, the Pitas Point fault vertically displaces the seafloor by 4 m.