Central California Continental Margin Research Articles

Late Quaternary pollen record from the central California continental margin

A pollen record obtained from a deep-sea core from the continental margin off central California contains evidence of the terrestrial floral adaptation to changing climatic conditions over the last ca. 20,000 calBP. The pollen record is divided into three pollen zones, two of which are late Pleistocene in age (Glacial and Transitional) and the third is Holocene (Interglacial).The Glacial Pollen Zone (ca. 20,000–17,000 calBP) is characterized by Pinus and Abies pollen, reflecting environmental conditions during the last glacial maximum. Overlying this (ca. 17,000–11,600 calBP) is the Transitional Pollen Zone containing a pollen assemblage indicative of a transitional climatic regime, with species that reflect more arid environmental conditions (Pinus and Artemisia) decreasing in abundance and those reflecting moister conditions (Sequoia and Alnus) rising in abundance. Pediastrum and dinoflagellates are common and are attributable to increased nutrient-rich coastal runoff. The Bølling–Allerød (295–256 cm; ca. 14,600–12,900BP) and Younger Dryas (256–243 cm; ca. 12,900–11,600BP) events occur within the Transitional Pollen Zone as well. The Interglacial Pollen Zone (ca. 11,600 calBP to present) is characterized by decreasing Pinus and increasing Sequoia, Quercus, and Asteraceae, reflecting changes in vegetation during the climatic warming of the Holocene. Two brief dry periods are represented in this pollen zone, one from 150 to 125 cm (ca. 8000–6400 calBP) and the other at 22–20 cm (ca. 800–700 calBP). They are correlated with the middle Holocene dry period and the Medieval Climate Anomaly, respectively.

The influence of El Niño-Southern Oscillation (ENSO) cycles on wave-driven sea-floor sediment mobility along the central California continental margin

Ocean surface waves are the dominant temporally and spatially variable process influencing sea floor sediment resuspension along most continental shelves. Wave-induced sediment mobility on the continental shelf and upper continental slope off central California for different phases of El Niño-Southern Oscillation (ENSO) events was modeled using monthly statistics derived from more than 14 years of concurrent hourly oceanographic and meteorologic data as boundary input for the Delft SWAN wave model, gridded sea floor grain-size data from the usSEABED database, and regional bathymetry. Differences as small as 0.5 m in wave height, 1 s in wave period, and 10° in wave direction, in conjunction with the spatially heterogeneous unconsolidated sea-floor sedimentary cover, result in significant changes in the predicted mobility of continental shelf surficial sediment in the study area. El Niño events result in more frequent mobilization on the inner shelf in the summer and winter than during La Niña events and on the outer shelf and upper slope in the winter months, while La Niña events result in more frequent mobilization on the mid-shelf during spring and summer months than during El Niño events. The timing and patterns of seabed mobility are addressed in context of geologic and biologic processes. By understanding the spatial and temporal variability in the disturbance of the sea floor, scientists can better interpret sedimentary patterns and ecosystem structure, while providing managers and planners an understanding of natural impacts when considering the permitting of offshore activities that disturb the sea floor such as trawling, dredging, and the emplacement of sea-floor engineering structures.

ATOC/Pioneer Seamount cable after 8 years on the seafloor: Observations, environmental impact

A study was conducted on the impacts of the presence of the Acoustic Thermometry of Ocean Climate (ATOC)/Pioneer Seamount cable on the benthos from nearshore waters adjacent to its origin at Pillar Point Air Force Station in Half Moon Bay, California to its terminus 95 km along its length on Pioneer Seamount. The coaxial Type SD cable was installed, unburied on the seafloor in 1995. Thirteen sites along the cable route were surveyed using the Monterey Bay Aquarium Research Institute (MBARI) ROVs Ventana and Tiburon equipped with cable-tracking tools. Quantitative comparisons of biological communities and seafloor features between cable and control sites were performed at nine stations. Forty-two hours of video footage and 138 push cores were collected over 15.1 km of seafloor. Approximately 12.1 km of the cable was observed (13% of the cable route). This study documents the appearance and condition of the cable and the underlying seafloor, and the effects of the cable on biological communities along its route. Limited self-burial of the cable has occurred during the 8-year deployment, particularly over the continental shelf and upper slope. Cable strumming by nearshore wave action has incised rocky siltstone outcrops. Several observations of kinks and snags in the cable on the upper slope (∼240 m depth) suggest contact with trawling gear. Few changes in the abundance or distribution of benthic fauna were detectable from video observations (epifaunal) and sediment core samples (infauna). Of 17 megafaunal groups and 19 infaunal taxa, no tests evaluating the overall effect of the cable were statistically significant. While these results indicate that the biological impacts of the cable are minor at most, three megafaunal groups exhibited cable-related changes at one or more stations. Actiniarians (sea anemones) colonized the cable when it was exposed on the seafloor, and were therefore generally more abundant on the cable than in surrounding, sediment-dominated seafloor habitats. Some fishes were also more abundant near the cable, apparently due to the higher habitat complexity provided by the cable. The study also documents general changes in the benthos across the Central California continental margin.

Natural supply of oil-derived hydrocarbons into marine sediments along the California continental margin during the late Quaternary

Sediment samples from three Ocean Drilling Program (ODP) Leg 167 sites, representing a south–north transect along the California continental margin and covering the last 160 ky—with selected samples extending in age to 950 ka—were analyzed for their bulk and molecular biomarker compositions. The aliphatic/alicyclic hydrocarbon fractions revealed clear evidence of a largely uniform natural supply of petroleum hydrocarbons during the entire time period. The mature fossil hydrocarbons in the sediments from the southern-central and central California continental margin can be related to the oil-prone rocks of the Monterey Formation in that area. The petroleum hydrocarbon signal on the northern California continental margin differs significantly from that of the two southern sites indicating another, presumably more northern source of the oil-type hydrocarbons. Their distribution pattern matches those of Eel River basin oils and, thus, suggests these oils or their source rocks to be the most likely origin of the hydrocarbons.

Crustal structure of a transform plate boundary: San Francisco Bay and the central California continental margin

Wide‐angle seismic data collected during the Bay Area Seismic Imaging Experiment provide new glimpses of the deep structure of the San Francisco Bay Area Block and across the offshore continental margin. San Francisco Bay is underlain by a veneer (<300 m) of sediments, beneath which P wave velocities increase rapidly from 5.2 km/s to 6.0 km/s at 7 km depth, consistent with rocks of the Franciscan subduction assemblage. The base of the Franciscan at 15–18 km depth is marked by a strong wide‐angle reflector, beneath which lies an 8‐ to 10‐km‐thick lower crust with an average velocity of 6.75 ± 0.15 km/s. The lower crust of the Bay Area Block may be oceanic in origin, but its structure and reflectivity indicate that it has been modified by shearing and/or magmatic intrusion. Wide‐angle reflections define two layers within the lower crust, with velocities of 6.4–6.6 km/s and 6.9–7.3 km/s. Prominent subhorizontal reflectivity observed at near‐vertical incidence resides principally in the lowermost layer, the top of which corresponds to the “6‐s reflector” of Brocher et al. [1994]. Rheological modeling suggests that the lower crust beneath the 6‐s reflector is the weakest part of the lithosphere; the horizontal shear zone suggested by Furlong et al. [1989] to link the San Andreas and Hayward/Calaveras fault systems may actually be a broad zone of shear deformation occupying the lowermost crust. A transect across the continental margin from the paleotrench to the Hayward fault shows a deep crustal structure that is more complex than previously realized. Strong lateral variability in seismic velocity and wide‐angle reflectivity suggests that crustal composition changes across major transcurrent fault systems. Pacific oceanic crust extends 40–50 km landward of the paleotrench but, contrary to prior models, probably does not continue beneath the Salinian Block, a Cretaceous arc complex that lies west of the San Andreas fault in the Bay Area. The thickness (10 km) and high lower‐crustal velocity of Pacific oceanic crust suggest that it was underplated by magmatism associated with the nearby Pioneer seamount. The Salinian Block consists of a 15‐km‐thick layer of velocity 6.0–6.2 km/s overlying a 5‐km‐thick, high‐velocity (7.0 km/s) lower crust that may be oceanic crust, Cretaceous arc‐derived lower crust, or a magmatically underplated layer. The strong structural variability across the margin attests to the activity of strike‐slip faulting prior to and during development of the transcurrent Pacific/North American plate boundary around 29 Ma.

Hydrothermal palygorskite and ferromanganese mineralization at a central California margin fracture zone

Ferromanganese oxyhydroxide crusts and nodules associated with palygorskite were recovered from the Santa Lucia Escarpment where the Morro Fracture Zone intersects the central California continental margin. Palygorskite was found in pure, high-Mg, low-Al, boxwork-textured veins, and disseminated in poorly consolidated palygorskite-rich mudstone. The purity of the palygorskite boxwork blades and the boxwork structure suggest formation by direct precipitation rather than by diagenetic or detrital processes. Interaction of hydrothermal fluids with oceanic basalt and/or deeper ultramafic rocks produced a Mg-Si enriched fluid supersaturated with respect to palygorskite that precipitated directly from the fluid at or near the seafloor. The close association of Fe-Mn crusts and nodules with both the palygorskite-rich mudstone and boxwork-vein palygorskite suggests a genetic link between the three types of mineralization. Mixed origin hydrothermal-hydrogenetic Fe-Mn crusts, with up to 50% hydrothermal input, formed contemporaneously with and subsequent to palygorskite formation. Fe-Mn nodules collected in the same dredge are of combined hydrogenetic and diagenetic origin and appear to be unrelated to hydrothermal mineralization that produced the crusts and palygorskite. The thickness of the Fe-Mn crusts and rare diatom fragments within the mudstone suggest an age of formation between 13 and 5 Ma.

Tracing the subducted oceanic crust beneath the Central California Continental Margin: Results from ocean bottom seismometers deployed during the 1986 Pacific Gas and Electric EDGE Experiment

Large aperture seismic data were collected on several ocean bottom seismometers (OBS) deployed along a deep crustal seismic profile that was shot across the central California continental margin. The line of shots extends from the oceanic crust seaward of the Santa Lucia Escarpment to the California coast near Morro Bay and crosses the Santa Lucia Basin, Santa Lucia Bank, Santa Maria Basin, and Hosgri fault zone. The OBS data permit us to trace the subducted oceanic crust from seaward of the Santa Lucia Escarpment to beneath the central part of the Santa Maria Basin. A complementary study using onshore recordings of the same shots extends the crustal model onshore. Just seaward of the Santa Lucia Escarpment, the oceanic crust is subhorizontal and covered by a thin layer of low‐velocity sediment. The velocity (4.5 km/s) and gradient (1.20–1.25 km/s/km) of the upper oceanic crust in this region are well determined and agree with earlier determinations of the crustal structure of the eastern Pacific. Beneath the Santa Lucia Escarpment and Santa Lucia Basin, the oceanic crust dips approximately 16° to the east. It is overlain by material with a velocity that increases from 4.8 to 6.4 km/s at a depth of 1.7–5.5 km below the seafloor beneath the Santa Lucia Basin. A low‐velocity zone may be sandwiched between the subducted crust and this shallow high‐velocity material, which we interpret to represent obducted oceanic crustal material. Beneath the eastern edge of Santa Lucia Basin, the dip of the subducted oceanic crust decreases to less than 2°. The configuration of the subducted crust in this region is consistent with imbrication of the subducted crust, as suggested by others based on common midpoint reflection data. Beneath the central Santa Maria Basin, the top of the subducted oceanic crust is at a depth of about 14–16 km and the Moho is at 19–21 km.

Deep structure and structural inversion along the central California Continental Margin from edge seismic profile RU‐3

Deep‐penetration seismic reflection profile RU‐3 crosses the central California continental margin revealing a subducted oceanic plate, a modified accretionary prism, and complex structures of the overlying sedimentary basins. This structural framework was established by subduction processes during Paleogene and earlier time and subsequently was modified by Neogene transform motion combined with apparent components of extension and compression. Subducted rocks are indicated by deep, gently dipping reflectors that extend beneath the continental margin for at least 38 km at a depth of about 15 km. We interpret the subducted crust as either a part of the Pacific plate or, more likely, a subducted fragment derived from the Farallon plate. A set of more steeply dipping, deep events may indicate faulting within the subducted plate or its boundary with a no‐slab zone. The overlying, largely nonreflective layer of accreted material rapidly reaches 10 km in thickness landward of the paleotrench and increases to 15 km in thickness near the coast. The Santa Lucia Basin, landward of the steep continental slope, originated as a slope basin during Paleogene subduction. The lower strata of this basin were deposited onto and partially incorporated into the accretionary complex. The offshore Santa Maria Basin exhibits a variety of compressional structures that formed in the last 3–5 m.y. and whose locations correspond to an earlier framework of extensional faults. Structural inversion has occurred in Miocene depocenters adjacent to the Santa Lucia Bank fault and at the Queenie structure. Miocene and lower Pliocene strata also thicken toward the Hosgri fault zone where subsequent compression is characterized by low‐angle thrusts and folding.

Seismic stratigraphy and late Quaternary shelf history, south-central Monterey Bay, California

The south-central Monterey Bay shelf is a high-energy, wave-dominated, tectonically active coastal region on the central California continental margin. A prominent feature of this shelf is a sediment lobe off the mouth of the Salinas River that has surface expression.High-resolution seismic-reflection profiles reveal that an angular unconformity (Quaternary?) underlies the entire shelf and separates undeformed strata above it from deformed strata below it. The Salinas River lobe is a convex bulge on the shelf covering an area of approximately 72 km2 in water depths from 10 to 90 m. It reaches a maximum thickness of 35 m about 2.5 km seaward of the river mouth and thins in all directions away from this point. Adjacent shelf areas are characterized by only a thin (2 to 5 m thick) and uniform veneer of sediment. Acoustic stratigraphy of the lobe is complex and is characterized by at least three unconformity-bounded depositional sequences. Acoustically, these sequences are relatively well bedded. Acoustic foresets occur within the intermediate sequence and dip seaward at 0.7° to 2.0°.Comparison with sedimentary sequences in uplifted onshore Pleistocene marine-terrace deposits of the Monterey Bay area, which were presumably formed in a similar setting under similar processes, suggests that a general interpretation can be formulated for seismic stratigraphic patterns. Depositional sequences are interpreted to represent shallowing-upwards progradational sequences of marine to nonmarine coastal deposits formed during interglacial highstands and/or during early stages of falling sea level. Acoustic foresets within the intermediate sequence are evidence of seaward progradation. Acoustic unconformities that separate depositional sequences are interpreted as having formed largely by shoreface planation and may be the only record of the intervening transgressions.The internal stratigraphy of the Salinas River lobe thus suggests that at least several late Quaternary regressions and transgressions may be recorded under the present shelf. This record may represent the last major eustatic cycle of sea level, an interval not observed in uplifted onshore Pleistocene marine terraces.

Sedimentological and micropaleontological detection of turbidite muds in hemipelagic sequences: An example from the late Pleistocene levee of Monterey Fan, central California continental margin

Undetected turbidite muds in hemipelagic sequences confound biostratigraphic and paleo-oceanographic interpretation of continental margins. We show that turbiditic muds can be distinguished from hemipelagic muds sedimentologically based on recognition of grading in mean silt particle-size, coarseness of silt mean size, and particle-size frequency distributions of the silt fraction. Further we show that the fossil assemblages in turbidite muds are hydraulically sorted and subtly altered. Lastly, we demonstrate that inclusion of sorted fossil faunas in high resolution biostratigraphy and paleo-oceanography leads to incorrect interpretation. Ten-mg samples were taken at one-cm intervals from beneath to above the megascopic portion of the five turbidites from the western levee of Monterey Fan of the central Californian continental margin. The silt fraction was analyzed in an electronic particle counter to determine the particle-size distribution. Four turbidites grade upward from visually distinct, well-sorted silt layers (Bouma T d), through megascopically unrecognizable, fining-upward turbiditic muds ranging from 5.2 to 6.4 ϕ in mean diameter, to an ungraded hemipelagic interval that ranges in mean size from 6.4 to 6.7 ϕ. Hemipelagic mud directly overlies the turbiditic silt of one turbidite. Foraminifers were picked from three samples taken from bottom to top of each turbidite unit. The specimens were measured for length of long, intermediate, and short axes to determine shape and size. Results indicate that benthonic and planktonic foraminifers are hydraulically sorted in turbiditic muds. Hemipelagic muds contain benthonic tests that are large and variable in size, whereas turbiditic muds and silts contain specimens that are small and similar in size. Planktonic foraminifers also are sorted by size in turbiditic samples. The sorting subtly alters the relative species frequencies in turbiditic assemblages compared to those in hemipelagic muds. Biostratigraphic samples in a piston core from the fan levee were tested using silt size parameters to determine if any samples lay within undetected turbiditic muds. We recognized seven such turbiditic samples and successfully resolved two latest Pleistocene datums after rejecting the sorted fossil faunas.

Source Rock Potential of Upper Cretaceous-Lower Tertiary (Maestrichtian-Danian) Moreno Formation, West-Central San Joaquin Valley, California: ABSTRACT

The Moreno Formation represents a base-of-slope to shelf-edge sedimentary sequence deposited along the central California continental margin during the late Maestrichtian to Danian (about 69-63 Ma). The formation consists of shales, mudstones, and siltstones with irregularly interbedded sandstones. The Marca Shale member of the Moreno Formation is particularly attractive as a potential petroleum source rock. The Marca contains sedimentologic, faunal, and geochemical evidence suggesting that it represents an anoxic depositional facies within the upper Moreno Formation. Submillimeter-scale depositional laminations are well preserved within the siliceous shale and diatomite of the Marca. Benthic foraminiferal biofacies are strikingly similar in patterns of abundance, dominance, and morphology to those found in Tertiary and Holocene sediments known or inferred to have been deposited under anoxic conditions. Also, total organic carbon (TOC) is as much as 7.25 wt.%. Laminated sediments contain kerogens of marine origin (Type II), whereas adjacent massive sediments contain organic matter dominated by kerogens of terrestrial origin (Type III). Data derived from this study indicate that the Moreno Formation has a significantly greater potential as a source of hydrocarbons than heretofore recognized. Rock-Eval pyrolysis and vitrinite reflectance data derived from core samples from a well near Coalinga show that the Marca shalemore » is marginally mature at relatively shallow depths. To the northeast, the Moreno is more deeply buried beneath late Tertiary and Quaternary basin-fill sediments, and has probably reached generative levels of maturity. Sands within the Moreno Formation are productive in at least two small fields in the western San Joaquin Valley. Thus, the Cretaceous of the San Joaquin basin may represent an exploration frontier in an otherwise mature basin.« less

Sea-floor phosphorites along the Central California continental margin

Early to middle Miocene phosphorites are patchily distributed along the continental margin of central California, at water depths of 200 to 2,000 m, between Cape San Martin and Point Reyes. Greater concentrations of phosphorites are present along the lower continental slope off Pescadero Point, on Sur Knoll, and on Twin Knolls, at water depths of 1,000 to 1,400 m. No phosphorites were found on Cordell Bank or in Monterey Bay, nor along the walls of adjacent submarine canyons.The dominant mineral of these Miocene phosphorites is carbonate fluorapatite (francolite). Associated minerals include glauconite, dolomite, detrital silicates, and minor pyrite. Most of the central California margin phosphorites are conglomeratic and contain a multitude of microdiscontinuity surfaces implying repeated alternations of deposition, erosion, reworking, and cementation. Both orthochemical and allochemical phosphorites are recognized petrographically; samples from offshore Pescadero are mostly oolitic phosphorites whereas those collected from Sur and Twin Knolls are predominantly microsphorites. Many samples also contain volcanic glass shards as well as opaque organics; they have been extensively silicified and are intensely bioturbated. SEM observations reveal micron-size, euhedral crystals of apatite that are distinctly hexagonal, and commonly are intergrown or stacked indicating in situ precipitation and/or recrystallization of an amorphous precursor. Bacterialike structures were also observed suggesting the possibility of microbially mediated precipitation of apatite.Geochemically, the central California margin samples are phosphorites. P 2 O 5 content ranges from 11.5 to 31.0 percent and averages 24.4 percent. Major element (Si, Al, Fe, Ca, Mg, K, C) analyses are consistent with published values for phosphorites but do indicate slight, relative enrichments. Trace element concentrations are also typical for marine phosphorites with the exception of Mn which is significantly depleted and Sr which is greatly enriched, whereas uranium values range from 47 to 170 ppm, averaging 90 ppm. Organic carbon values range from 0.45 to 2.65 percent with darker colored phosphorites enriched in organic carbon relative to lighter colored samples.The Miocene central California margin phosphorites originated primarily by direct physio- and/or biochemical precipitation of apatite within oxygen-depleted, near-surface sediments, although subsequent recrystallization during burial may also have occurred. Polytaxic, paleoceanographic conditions appear to have dominated during phosphogenesis but may have been repeatedly interrupted by episodes of increased oceanic circulation during glacial events. Times of transition from interglacial to glacial modes are believed to have been critical to phosphogenesis. Diagenetic formation of these phosphorites occurred along impinging dysaerobic (0.1-1.0 ml/l O 2 ) oxygen minimum zones where internal nutrient recycling may have provided sufficient in situ organic material (phosphorous) and sufficiently high pH values to allow apatite precipitation.Central California margin phosphorites are a potential natural resource. However, their patchy distribution and occurrence at relatively great water depths make them economically less attractive than well-known, on-land accumulations in central California, as well as submarine deposits off the southern California shore that occur at much shallower (<330 m) water depths and in higher concentrations.