Monterey Fan Research Articles

Benthic megafauna assemblage change over three decades in the abyss: Variations from species to functional groups

Megafaunal seafloor assemblages on the Monterey Fan in the NE Pacific (Station M, 4000 m depth) were studied between 2006–2018 using remotely operated vehicles (ROVs) as part of a continuing time-series study that began in 1989. Since 2006 we identified nearly 120,000 individual animals representing over 142 morphospecies, and observed continuous changes in the megafaunal assemblage. This study, which tracked variation in observed morphospecies over a 13-year period, is one of the most detailed long-term records of megafaunal change for abyssal depths. Our investigation shows that new variations continued to emerge, reinforcing the concept that the deep-sea is dynamic over short time scales, rather than static over long periods. Some species were uncommon, but later observed in high numbers, then decreased to very low or undetectable levels (e.g. Elpidia sp. A), while others (e.g. Psychropotes longicauda) exhibited a relatively persistent presence with less fluctuation in abundance. Decreasing total echinoderm density from 2013–2018 did not correspond with the continued occurrence of large episodic POC flux events between 2016–2018. This may be attributed to the quality of food supply arriving at the seafloor and the varied ability of organisms to utilize it. Long-term tracking (30 years) of 10 specific epibenthic echinoderm species originally quantified from camera-sled images shows a pattern of assemblage structure, perhaps returning toward the composition observed in the 1990s and early 2000s. Many questions remain as to how this abyssal site and others will change with continued, and potentially increasing, anthropogenic change in the upper ocean. For example, the marine heat anomaly known as the ‘Warm Blob’ may have influenced major ecological processes at the abyssal seafloor in terms of morphospecies and functional group composition due to changes in POC flux. The degree of dynamism continues to indicate that ad hoc or short-term investigations provide a limited perspective for assessing community structure in conservation or resource exploitation impact assessment studies in the deep sea.

Correlation of marine and coastal terrestrial records of central California: Response to paleoceanographic and paleoclimatic change during the past 19,000 years

New benthic foraminiferal census data combined with previously published planktic foraminiferal and pollen data from the continental margin off central California provide a unique opportunity to document concurrent paleoceanographic and paleoclimatic changes in the region during the late Quaternary. All three datasets were evaluated in gravity core S3-15G, collected at a depth of 3491 on the western levy of the Monterey Fan (36°23.53′N, 123°20.52′W). Accelerator mass spectrometry radiocarbon dates and the ratio of the planktic foraminiferal species Neogloboquardrina pachyderma (Ehrenberg) to Neogloboquadrina incompta (Cifelli) provide a good age-depth model for the last 19,000 years, covering the last glacial, Bølling–Allerød, Younger Dryas, and Holocene intervals. Separate Q-mode cluster analyses of the hemipelagic as well as mixed (combined hemipelagic and turbiditic) mud samples grouped the benthic foraminiferal fauna into two clusters reflecting faunal adaptation to changing climatic conditions during the Pleistocene and Holocene. R-mode cluster analysis also differentiated glacial (Uvigerina senticosa and Globobulimina auriculata) and interglacial (Melonis pompilioides and Gyroidina planulata) faunas. A general trend of slightly increasing oxygen in the deep sea is suggested from the Pleistocene to Holocene based on the reduction in abundance of G. auriculata and increased frequency of M. pompilioides. Q-mode cluster analysis of the planktic foraminifera illustrates a change in the surface water from a glacial subpolar fauna in the Pleistocene to a transitional fauna in the Holocene, whereas the pollen record separated into three clusters, two of Pleistocene age (glacial and transitional) and one in the Holocene (interglacial), reflecting the terrestrial floral adaptation in the California Coast Ranges of central California to the warmer climate in the Holocene. Decoupling is evident between the benthic and planktic foraminiferal and terrestrial floral responses to changing oceanographic and climatic conditions. The floral response leads the surface-dwelling fauna by several millennia, and is followed by the deep-dwelling benthic foraminiferal fauna a millennium later.

Evidence of modern fine-grained sediment accumulation in the Monterey Fan from measurements of the pesticide DDT and its metabolites

The documented presence of the pesticide DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane) and its metabolites DDE (1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene), and DDD (1,1-dichloro-2,2-bis(p-chlorophenyl)ethane), collectively referred to as ΣDDT, in sediments from the Monterey Submarine Canyon, Monterey Fan Channel and Monterey Fan indicates that during the present sea-level highstand the Monterey Fan has been an active depositional system. The presence of ΣDDT in marine sediment is an indication of the recent arrival of terrigenous fine-grained sediment because DDT was only applied on land in California between 1944 and 1972, and is being transported to the ocean in the fine-grained sediment fraction of rivers. ΣDDT was found in 81 out of 82 cores collected in water depths deeper than 3000 m in the Monterey Fan Channel and in the Monterey Fan, within a geographical extent of 250 km to the south and 210 km to the west of the Monterey Canyon head, with surface concentrations averaging 6 and 3 ng g− 1, in the upper and lower Monterey Fan, respectively. This is the first documentation of ΣDDT presence in ocean sediments below 3000 m water depth worldwide. The estimated total amount of ΣDDT in the fan is consistent with the amount of ΣDDT discharged by the Salinas and Pajaro Rivers since DDT started being used in California, suggesting that current fine-grained sediment discharge from these rivers has been accumulating in the fan. The ratio DDE/ΣDDT, an indication of the extent of DDT degradation, provides a first-order proxy for the relative age of the sediments. DDT is found along the thalweg of the Monterey Fan channel as far as 215 km from the canyon head, indicating recent fine-grained sediment transport along the channel. In contrast, only DDE was detected in sediments on the fan, which suggests that the fan does not accumulate fine-grained sediments at a constant rate.

Channel formation by flow stripping: large‐scale scour features along the Monterey East Channel and their relation to sediment waves

AbstractThe Monterey East system is formed by large‐scale sediment waves deposited as a result of flows stripped from the deeply incised Monterey fan valley (Monterey Channel) at the apex of the Shepard Meander. The system is dissected by a linear series of steps that take the form of scour‐shaped depressions ranging from 3·5 to 4·5 km in width, 3 to 6 km in length and from 80 to 200 m in depth. These giant scours are aligned downstream from a breech in the levee on the southern side of the Shepard Meander. The floor of the breech is only 150 m above the floor of the Monterey fan valley but more than 100 m below the levee crests resulting in significant flow stripping. Numerical modeling suggests that the steps in the Monterey East system were created by Froude‐supercritical turbidity currents stripped from the main flow in the Monterey channel itself. Froude‐supercritical flow over an erodible bed can be subject to an instability that gives rise to the formation of cyclic steps, i.e. trains of upstream‐migrating steps bounded upstream and downstream by hydraulic jumps in the flow above them. The flow that creates these steps may be net‐erosional or net‐depositional. In the former case it gives rise to trains of scours such as those in the Monterey East system, and in the latter case it gives rise to the familiar trains of upstream‐migrating sediment waves commonly seen on submarine levees. The Monterey East system provides a unique opportunity to introduce the concept of cyclic steps in the submarine environment to study processes that might result in channel initiation on modern submarine fans.

Late Quaternary evolution of channel and lobe complexes of Monterey Fan

The modern Monterey submarine fan, one of the largest deep-water deposits off the western US, is composed of two major turbidite systems: the Neogene Lower Turbidite System (LTS) and the late Quaternary Upper Turbidite System (UTS). The areally extensive LTS is a distal deposit with low-relief, poorly defined channels, overbank, and lower-fan elements. The younger UTS comprises almost half of the total fan volume and was initiated in the late Pleistocene from canyons in the Monterey Bay area. Rapidly prograding high-relief, channel-levee complexes dominated deposition early in the UTS with periodic avulsion events. In the last few 100 ka, much of the sediment bypassed the northern fan as a result of allocyclic controls, and deposition is simultaneously occurring on a sandy lobe with low-relief channels and on an adjacent detached muddier lobe built from reconfinement of overbank flow from the northern high-relief channels. During the relatively short-lived UTS deposition, at least seven different channel types and two lobe types were formed. This study provides a significant reinterpretation of the depositional history of Monterey Fan by incorporating all available unpublished geophysical data.

Sedimentary processes of the lower Monterey Fan channel and channel-mouth lobe

The distribution of deposits, sediment transport pathways and processes on the lower Monterey Fan channel and channel-mouth lobe (CML) are studied through the integration of GLORIA and TOBI sidescan sonar data with 7-kHz subbottom profiler records and sediment cores for ground-truthing. The lower Monterey channel is characterised by an up to 30-m-deep channel with poorly developed levees and alternating muddy and silty muddy overbank deposits. The channel is discontinuous, disappearing where gradients are less than about 1:350. Ground-truthing of the large CML shows that the entire CML is characterised by widespread deposits of generally fine sand, with coarser sand at the base of turbidites. Sand is particularly concentrated in finger-like areas of low-backscatter intensity and is interpreted as the result of non-turbulent sediment-gravity flows depositing metres thick massive, fine sand. TOBI sidescan sonar data reveal recent erosional features in the form of scours, secondary channels, large flow slides, and trains of blocks at the distal end of the CML. Erosion is probably related to increasing gradient as the CML approaches Murray Fracture zone and to differential loading of sandy submarine fan deposits onto pelagic clays. Reworking of older flow slides by sediment transport processes on the lobe produces trains of blocks that are several metres in diameter and aligned parallel to the flow direction.

Sedimentary features associated with channel overbank flow: examples from the Monterey Fan

Abstract Overbank flow of turbidity currents sweeping through the entrenched Monterey Fan Channel has generated erosional and depositional features along the channel walls and across the adjacent levees. These features, investigated with side-scan sonar, SeaBeam bathymetry, submersibles, and a towed camera sled include fields of sediment waves, overbank channels, gullied walls and terraces, and slump scars. Overbank flow occurs where the channel path is straight, but is accentuated along sharp outside bends. Along straight channel segments sediment waves trend oblique to the channel and are commonly present on the right-hand (when looking downstream) levee adjacent to the channel. Along curved segments of the channel path, large sediment waves, with crests sub-parallel to the channel and with wavelengths reaching 3 km, are present on the outside levee as far as 15 km from the channel. Commonly, a gully is present on the lee side of the sediment wave, near the mid-point of the wave crest. Gullies are the heads of a tributary system of overbank channels, which feed a larger trunk channel on the fan surface.

Late Pleistocene channel–levee development on Monterey submarine fan, central California

Much of the modern upper (proximal) Monterey fan is a channel–levee complex, the Upper Turbidite Sequence (UTS), that was deeply eroded after the channel breached a volcanic ridge to reach a deeper base level. Ages of sediment samples collected with the ALVIN submersible from the deepest outcrop within the channel–levee system, 390 m below the adjacent western levee crest, indicate that the UTS deposits accumulated at ≥1 m ka−1 during the last 500 ka. Neogene and Early Pleistocene sediment accumulation on the fan prior to the UTS was much slower (<0.03 m ka−1), and underlying turbidite systems(?) had substantially different morphologic expression(s).

Acoustic scattering from the seafloor: Modeling and data comparison

An existing model of seafloor backscattering [D. R. Jackson, D. P. Winebrenner, and A. Ishimaru, J. Acoust. Soc. Am. 79, 1410–1422 (1986)] was implemented with the additions of volume scattering from a random inhomogeneous continuum and scattering from subbottom interfaces. Results of computer simulations with the extended model were compared with values of scattering strength obtained from processed GLORIA data from the Monterey Fan off the coast of California. Regions of well-delineated high and low backscatter are seen in the GLORIA imagery. The geoacoustic input parameters for the simulation runs were either taken directly from or estimated from core data obtained by ground truth sampling in the image area. From the model simulation results it appears that the main factor causing the observed dichotomous character of the strength of returns in the Fingers Area is the presence or absence of a topmost, inhomogeneous silt-clay layer (with a thickness of around 1–1.5 m). In the high backscatter areas, inhomogeneous continuum volume scattering from the silt-clay layer predominates. In the low backscatter areas interface scattering from an acoustically hard, shallow subbottom sand interface (0–10 cm below the water–sediment interface) predominates. These two additions to the scattering model allow use of the ground truth measurements to constrain the input parameter values.

Modeling acoustic backscatter from the seafloor by long-range side-scan sonar.

An existing model of seafloor backscattering [Jackson etal., J. Acoust. Soc. Am. 79, 1410–1422 (1986)] was extended to include volume scattering from a random homogeneous continuum and scattering from subbottom interfaces. Results of computer simulations with the extended model were compared with values of scattering strength obtained from processed gloria data from the Monterey Fan off the coast of California. Regions of well delineated high and low backscatter are seen in the gloria imagery. The geoacoustic input parameters for the simulation runs were either taken directly from or estimated from core data obtained by ground truth sampling in the image area. From the model simulation results, it was found that the low backscatter region is dominated by interface scattering from a single subbottom interface over a thick homogeneous sand layer. The high return regions are dominated by scattering from the random inhomogeneous continuum. These two additions to the scattering model allow use of the ground truth measurements to constrain the input parameter values. No free parameters are required to fit the scattering strength data. [Work supported by Naval Research Laboratory, Code 5100.]

Ground‐truthing 6.5‐kHz side scan sonographs: What are we really imaging?

A 1000‐km2 area on the distal lobe of Monterey Fan shows a digitate pattern of juxtaposed high and low backscatter on GLORIA side scan sonographs. This area was investigated using stereo photography, high‐resolution seismic profiles, and measurements of physical properties of cores to quantitatively evaluate the causes of backscatter from the 6.5‐kHz side scan sonar. Stereo photography and bottom video were used to determine that the sediment‐water interface typically has a bed roughness less than 10 cm over the entire ground truth area; consequently, bed roughness is not a significant contributor to the sonar backscatter. Vertical‐incidence 3.5‐kHz profiles reveal that high‐backscatter areas allow less penetration and have slightly more relief than low‐backscatter areas. Closely spaced measurements of p wave velocity, density, and grain size made on transponder‐navigated cores are used to investigate the geoacoustic properties of the sediment with the aid of a numerical model. The model results demonstrate that the sediment‐water interface is, in most cases, acoustically transparent to the sonar energy and that most or all of the energy is refracted into the sediment to depths of at least a few meters rather than scattered from the surface. The variability of the dominant lithofacies (sands with minor silty clays or silty clay with minor sands) accounts for a regional correlation with backscatter when viewed over hundreds to thousands of square kilometers, but at the detailed scale of hundreds to a few square meters, we have not yet been able to identify a correlation of backscatter intensity with the angle of incidence, surface lithology, measured acoustic impedance, or attenuation. However, there is a qualitative, but unexpected, correlation with lithostratigraphic variability. In this area, thick (up to 50 cm) sand deposits with thin interbeds of silty clay correlate with lower backscatter than do silty clay deposits with thin interbeds of sand. This suggests that volume inhomogeneities and complex constructive and destructive interferences caused by the subsurface volume inhomogeneities within the top few meters of the sediment ultimately modulate the intensity of backscatter. The ground‐truthing effort was further complicated by trying to compare the very large sonar footprint of a pixel (∼70,000 m2) relative to the area of an individual core (0.006 m2), bottom photograph (∼5 m2), or seismic profile. Although 6.5‐kHz sonographs appear easy to interpret in a conventional and simplistic manner, caution should be used when interpreting lithofacies from backscatter intensities.

Geological setting of chemosynthetic communities in the Monterey Fan Valley system

Alvin dives and camera tows within the “meander area” of the Monterey and Ascension Fan Valleys have located nine chemosynthetic communities over depths ranging from 3000 to 3600 m over a distance of 55 km. Most of the observed communities consist largely of Calyptogena phaseoliformis, but Solemya (species unknown) and a pogonophoran (genus Polybrachia), have also been identified. The δ 13C values (-35.0 to -33.6 per mil) and the presence of APS reductase and ATP sulfurylase in the C. phaseoliformis tissue is consistent with sulfur chemoautotrophy. Two reduced organic matter sources for the H 2S are proposed: (1) older beds exposed by the deep erosion (up to 400 m) of the fan valleys and (2) concentrations of anaerobically decomposd organic matter buried in the valley floor.

Quantitative estimation of seafloor backscattering strength using long-range sidescan sonar data

In ocean water depths of a few kilometers, long-range sidescan sonar systems generate “acoustic brightness” images of seafloor swaths 10 km or more in width using energy acoustic energy returned from the sea floor by monostatic scattering. When simultaneously acquired seafloor bathymetry is available, as it is for some modern systems, the acoustic received level versus time (imagery) data from the towed sonar “fish” can be inverted to extract estimates of seafloor backscattering strength and associated seafloor grazing angle. These estimates can be made for the entire “swath” of seafloor for which imagery is usually generated. Such inversion has been performed for sidescan imagery data from a portion of the Monterey Fan off the coast of California. Published sidescan imagery for the same region [Atlas of the Exclusive Economic Zone, Western Conterminous United States (U. S. Geological Survey Miscellaneous Investigations Series 1–1792, 1986)] indicates a strongly scattering seafloor for much of the region investigated. Consistent with this, the backscattering strength values estimated from data inversion are higher than typical values from the literature in the bright image regions, though consistent with typical values over much of the local seafloor. [Work supported in part by the Geodynamics Research Inst., Texas A&amp;M Univ.]

Sur submarine slide, Monterey Fan, central California

ABSTRACTThe Sur debris slide and associated debris flow cover more than 1000 km2 of the eastern part of Monterey submarine fan and extend from the base of the continental slope near the apex of the fan to the Monterey fan valley. The flow is generally confined between the continental slope and remnants of an older channel (Monterey East fan valley). The hummocky surface of the debris slide and diffuse echo returns from the surface of the nearly acoustically transparent debris flow, as seen in 3.5 kHz profiles, are common to large submarine slides described from passive‐margin continental‐slope and rise deposits. Piston cores from the Sur slide recovered contorted and sheared fan turbidite units as well as clast‐ and matrix‐supported mudlump debris flows. The cores show that the slide debris is overlain by 0.23–0.87 m of mud and turbidite sand that limit the age of the slide to latest Holocene, although more than one stage of emplacement is possible.

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.

Biostratigraphic implications for the turbidite depositional processes on the Monterey deep-sea fan, Central California

ABSTRACT Recognition of the Pleistocene/Holocene boundary and biohorizons on the continental margin of California is complicated by turbidite deposition. The coiling ratio of Neogloboquadrina pachyderma (Ehrenberg), the relative frequencies of subarctic and transitional planktonic foraminifers, radiolarian number, and the radiolarian-foraminiferal ratio all show variability in upper Quaternary sections that appear related to downslope transport processes, in addition to the environment of surface waters and carbonate dissolution. We examined the distribution of microfossils in ten turbidites from the western levee of the Monterey Fan to determine 1) how turbidite microfossil assemblages differ from those of hemipelagic intervals, 2) if turbiditic muds can be distinguished from hemipelagic muds based on fossil content, and 3) if microfossils used to define biostratigraphic zones are significantly affected by turbiditic processes. Using cluster analysis, samples were successfully divided into hemipelagic and turbiditic groups based on the relationship among five of 16 variables (cluster A): sand content, the relative frequency of shallow-water benthonic Foraminifera, benthonic foraminiferal number, adjusted benthonic foraminiferal number, and the relative frequency of dextral Neogloboquadrina pachyderma (Ehrenberg). The variables jointly characterize a transitional assemblage transported from a bathyal or neritic source. Both Radiolaria and planktonic Foraminifera are used to mark biohorizons on the California continental margin. In this study, it was found that both fossil groups vary significantly in concentration and relative frequency between turbiditic and hemipelagic muds of the Monterey Fan, and microfossil assemblages are measurably different between tops and bases of turbidites. The coiling ratio of Neogloboquadrina pachyderma (Ehrenberg) varies the least and may be most resistant to bias by hydraulic sorting and differential carbonate preservation related to transport processes.

Morphology of Central California Continental Margin, Revealed by Long-Range Side-Scan Sonar (GLORIA): ABSTRACT

Leg 2 of the 4-leg USGS EEZ-SCAN 84 program used GLORIA long-range side-scan sonar to survey the region from Pt. Conception to just south of Pt. Arena, from the shelf break to the 200-nmi coverage. The overlapping digital sonographs were slant-range and anamorphically corrected, and a photomosaic of the sonographs was constructed at a scale of 1:375,000 (1 in. = 11.1 km). The underlying bed rock appears to be an important control in shaping the morphology of this margin. Several faults have sea-floor expression and lie subparallel to the margin. The density of canyons and gullies on the slope varies from south to north, probably because of variations in the characteristics of the bed rock. The slope west of San Francisco is the most dissected segment of the central California slope. Monterey Fan is covered by large-scale bed forms (5-15 m amplitude and 1.5-2.0 km wavelength) over much of its surface. Monterey channel crosses southwestward across the fan, but abruptly turns south along a 40-km long surface fault that coincides with a well-mapped meander loop. The channel loops to the north then turns southward crossing the entire Monterey Fan and, at its distal reach, changes to a broad, braided pattern. Major slumps on the margin have long (> 30 km) scarps, some have slump folds, and one has a debris-flow deposit that can be acoustically traced for more than 75 km. Seventeen new seamounts were mapped. Taney Seamounts are large, rimmed, calderas with diameters of about 15 km each; these appear to be very large explosive or summit-collapse features. End_of_Article - Last_Page 257------------

Sedimentologic and Tectonic Factors Controlling Growth and Dimensions of Submarine Fans: ABSTRACT

The morphology, development, composition, and structure of submarine fans reflect five controlling factors. (1) Grain-size distribution--sandy debris results in radial fans, whereas mud-dominated debris promotes elongate fans. (2) Eustacy--during high sea-level stands, fan growth is curtailed as sediment, especially coarse sediment, is trapped in paralic settings, whereas during low stands, fan growth is accelerated owing to direct access of sediment to canyon heads and the flushing of debris from shelf areas. (3) Basin architecture--confined or unconfined. (4) Penecontemporaneous tectonics from seismic triggering to migrating source terranes. (5) Postdepositional tectonics that bias or alter lithofacies preservation--bias in sediment subduction results in the preferentia loss of distal facies. Some examples that demonstrate these five controlling factors include the following. (1a) The Navy and Arguello fans of the California continental borderland are sandy, radial fans, whereas (1b) the Bengal and Indus fans are muddy elongate fans. (2) The Eocene Farello fan, southern California borderland, is composed of thick intervals of sand and conglomerate lithofacies, corresponding to low sea level stands, and thin fine-grained intervals, corresponding to high sea level stands. (3) Relatively unconfined fans are most typical of passive or trailing continental margins, e.g., the Bengal, Amazon, and Mississippi fans, although even here diapir fields, seamounts, and fracture zones offer local impediments. In contrast, fans in confined basins are characteristic of convergent or transf rm continental margins, e.g., borderland, forearc, and backarc basins, although passive margins may also have confined basins. The Laurentian fan, for example, is blocked by the Kelvin Seamounts. (4) Large infrequent earthquakes (one to several per millenium) may trigger large depositional events, e.g., the 1929 Grand Banks shock. Shingling of depositional units occurs in pull-apart basins of strike-slip fault systems; such sequences may have stratigraphic thicknesses (not vertical accumulations) of 10 km/m.y. (33,000 ft/m.y.). Examples include the Neogene Ridge basin in California and the Cretaceous Izumi basin in Japan. Where a canyon head lies seaward of a transform fault, a large fan develops with multiple sources, e.g., the Monterey and Delgada fan systems. Where a canyon lies landw rd of a transform boundary, multiple small fans of one source terrane are strung out along a continental margin, e.g., the Baranoff fans of southeastern Alaska. (5) Inferred subduction complexes, e.g., the Torlesse of New Zealand, are composed principally of thousands of meters of tectonically thickened (thrust faulted) thick-bedded sandstone lithofacies. This composition may be due to decoupling of the more distal, fine-grained facies from the proximal facies and its transportation into the mantle along with the underlying oceanic crust. End_of_Article - Last_Page 488------------

Monterey Fan: Growth pattern control by basin morphology and changing sea levels

Monterey Fan is the largest modern fan off the California shore. Two main submarine canyon systems feed it via a complex pattern of fan valleys and channels. The northern Ascension Canyon system is relatively inactive during high sea-level periods. In contrast, Monterey Canyon and its tributaries to the south cut across the shelf and remain active during high sea level. Deposition on the upper fan is controlled primarily by the relative activity within these two canyon systems. Deposition over the rest of the fan is controlled by the oceanic crust topography, resulting in an irregular fan shape and periodic major shifts in the locus of deposition.

Sediment waves on the monterey fan levee: A preliminary physical interpretation

Sediment waves on the deep ocean floor occur mostly on the lower continental rise on slopes of 1° or less. Previous studies show that their amplitude and wavelength vary greatly, but little is known about their shape in plan. A detailed survey of a 30-km 2 area of abyssal-depth sediment waves associated with the levee of the Monterey fan valley shows a pattern of sinuous crests and troughs with parallel, well-bedded internal structure. Material in the upper 1 m of sediment consists predominantly of bioturbated, muddy coccolith ooze. A single thin, silty horizon can be correlated between adjoining waves. On the basis of measured wave dimensions and an estimate of flow velocity we use a simple two-layer model for water movement to infer approximate flow parameters. The sediment waves are considered to be formed most likely by low-velocity (10 cm/s), low-concentration turbidity flows approximately 100–800 m thick. This interpretation emphasizes the role of low-speed, low-concentration turbidity currents in the downslope movement of fine-grained material. This type of transport—deposition regime explains the formation of sediment waves very well except for certain waves occurring on depositional ridges in the deep ocean.