In 1992, a multi-disciplinary project was initiated to investigate and understand the processes that resuspend and transport sediment on the Palos Verdes shelf. One goal was to collect environmental data that could be used by numerical models to predict the resuspension and movement of the actual shelf material. Hence, an extensive field program was developed to collect current, surface wave, wind stress and sea level data for an entire year. Instruments were deployed simultaneously along and across the shelf to characterize the spatial and temporal structures of the environmental parameters. Both tidal and subtidal currents flowed primarily along, rather than across the shelf, with similar amplitudes at common depths below the surface. In particular, near-bed alongshore currents had similar amplitudes and common temporal patterns. Hence, it was possible to use a modified modal analysis to create a single record to represent the regional flow pattern for sediment-transport models. This synthetheic record accounted for over 70% of the variability in near-bed alongshore currents and had none of the data gaps found in individual current records. The temporal portion of the mode was scaled to have a mean and standard deviation similar to the measured current field. A synthetic record to statistically characterize near-bed, cross-shore currents was created using a scaled-average, rather than modified-modal, analysis because cross-shelf currents had similar amplitudes but no common temporal structure. Wind-driven currents were weak, usually less than 5 cm/s, and accounted for less than 15% of the variability in the subtidal current field. The presence or absence of surface waves, which are a combination of local, wind-driven waves and non-local swell, was not related to the amplitude of the near-bed currents. Forty-five percent of the larger wave velocities were associated with current speeds less than 10 cm/s; only 3% with current speeds greater then than 25 cm/s. This lack of correlation between currents and wind, or currents and waves, suggest that these fields can be input independently into erosion models for the Palos Verdes shelf.

Measurements of currents and light transmission were made at bottom tripods and moorings arrayed across the northern California continental shelf along the Coastal Ocean Dynamics Experiment (CODE) "C" transect as part of the 1990-1991 Sediment Transport Events on Shelves and Slopes (STRESS) experiment. In combination with meteorological and wave data from the National Data Buoy Center Buoy 46013, these measurements provide information about the physical forcing and resultant resuspension and transport of bottom material between 21 November and 8 March. Sixteen events were identified in the wave, wind and current-meter records for this period. Only two were local storms with southerly winds, but they caused about half of the seasonal net transport. Seven were swell events that combined long-period waves generated by distant storms with local currents. At the 90-m site, swells interacted with the mean northward flow to produce northward transport. During six northerly wind events, upwelling-favorable winds often were sufficient to slow or reverse the mean northward flow and thus caused southward transport. A single current event, which produced moderate southward transport, was observed at the 130-m site. Net transport during the winter experiment was offshore at all sites, northward at the inner- and mid-shelf sites, but southward at the outer-shelf site. The results suggest that local storms with southerly winds may dominate seasonal transport, as on the Washington shelf, but significant transport also can occur during fair weather and during periods of northerly winds.

The oceans have been and will continue to be disposal sites for a wide variety of waste products. Often these wastes are not dumped at the designated sites or transport occurs during or after dumping, and, subsequent attempts to monitor the effects the waste products have on the environment are inadequate because the actual location of the waste is not known. Acoustic mapping of the seafloor with sidescan sonar is a very effective technique for locating and monitoring dredge-spoil material and other debris. Sidescan sonar provides an acoustic image or sonograph of the sea floor that is similar to a satellite image of the Earth's land surface. In effect sidescan sonar allows the water column to be stripped from the sea floor, thereby providing a clear, unobstructed view of the sea bed.An example of the potential of this technique is summarized herein for the Gulf of the Farallones region. More than 47 800 drums (55 gallon) and other containers of low-level radioactive waste were dumped on the continental margin offshore the San Francisco Bay between 1946 and 1970. These drums now litter a large area (1200 km2) of the sea floor within the Gulf of the Farallones National Marine Sanctuary (GFNMS). The exact location of the drums and the potential hazard the drums pose to the environment are unknown. To evaluate the risk, samples of the sediment, biota and water must be collected near and distant from the concentrations of barrels. To do this the exact location of the barrels must be known prior to sampling. The USGS, through a cooperative research agreement with GFNMS, used sidescan sonar to map two areas within the sanctuary. Total sea-floor coverage was obtained and computer-processed sonographic mosaics were constructed on board ship. Many small nongeologic targets were distributed throughout the survey areas that covered about 70 km2 on the shelf and 120 kM2 on the slope. Analysis of the sidescan data suggests that the targets are 55-gallon drums. This interpretation was confirmed at one site with an underwater video and 35-mm camera system. Data were collected with both a 30-kHz and a 120-kHz sidescan system within a 15-km2 area on the shelf. We found that the barrels were more easily detected with the mid-range 30-kHz system than with the higher resolution 120-kHz system. Maps of barrel distribution derived from the sonographs are being used to design sampling schemes to evaluate the risk that the radioactivity may have on the biota and environment.

Two sequences of bottom photographs taken every two or four hours for two months during the Coastal Ocean Dynamics Experiment (CODE) off the Russian River, California, reveal the dynamic nature of interations between the water column, the sediments, and benthic organisms in the mid-shelf silt deposit.

Gorda Ridge is the southern segment of the Juan de Fuca Ridge complex, in the north-east Pacific. Along-strike spreading-rate variation on Gorda Ridge and deformation of Gorda Plate are evidence for compression between the Pacific and Gorda Plates. GLORIA sidescan sonographs allow the spreading fabric associated with Gorda Ridge to be mapped in detail. Between 5 and 2 Ma, a pair of propagating rifts re-orientated the northern segment of Gorda Ridge by about 10° clockwise, accommodating a clockwise shift in Pacific-Juan de Fuca plate motion that occurred around 5 Ma. Deformation of Gorda Plate, associated with southward decreasing spreading rates along southern Gorda Ridge, is accommodated by a combination of clockwise rotation of Gorda Plate crust, coupled with left-lateral motion on the original normal faults of the ocean crust. Segments of Gorda Plate which have rotated by different amounts are separated by narrow deformation zones across which sharp changes in ocean fabric trend are seen. Although minor lateral movement may occur on these NW to WNW structures, no major right-lateral movement, as predicted by previous models, is observed.

Research Article| December 01, 1987 Crescentic dunes on the inner continental shelf off northern California D. A. Cacchione; D. A. Cacchione 1Branch of Pacific Marine Geology, U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar M. E. Field; M. E. Field 1Branch of Pacific Marine Geology, U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar D. E. Drake; D. E. Drake 1Branch of Pacific Marine Geology, U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar G. B. Tate G. B. Tate 1Branch of Pacific Marine Geology, U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Geology (1987) 15 (12): 1134–1137. https://doi.org/10.1130/0091-7613(1987)15<1134:CDOTIC>2.0.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation D. A. Cacchione, M. E. Field, D. E. Drake, G. B. Tate; Crescentic dunes on the inner continental shelf off northern California. Geology 1987;; 15 (12): 1134–1137. doi: https://doi.org/10.1130/0091-7613(1987)15<1134:CDOTIC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Large crescentic dunes that resemble barchans have been discovered within elongate scour depressions on the northern California inner continental shelf by using side-scan sonar. These dunes appear to be migrating obliquely to the regional shelf gradient; a preferred offshore direction of transport is indicated by the extended southern wings of many dunes. The isolated dunes and the scour depressions that contain them are located seaward of sea-floor outcrops off rocky capes and sea stacks. Repeated side-scan sonar records spanning four years (1981–1985) indicate that the dunes and bounding sidewalls of the scour depressions were somewhat modified, but the overall alteration to the bottom morphology during this period was only moderate to undetectable. The apparent low height of the dunes is consistent with a low migration speed. Over longer time periods (decades), the seaward transport of fine to medium sand in the crescentic dunes is probably an important way by which sand escapes the shallow part of the continental shelf in this region and mixes with the muddy deposits of the central shelf. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Theoretical and laboratory results indicate that bottom velocities within shoaling internal gravity waves intensify upslope approximately inversely proportional to the water depth. The elevated velocities (and bottom stresses) caused by shoaling and, possibly, breaking internal waves might explain the generation and maintenance of near-bottom nepheloid zones and attached turbid plumes that have been observed over certain continental shelves and slopes. This process is proposed as an explanation of zones of relatively low transmissibility that emanate from the upper continental slope near Newport submarine canyon off southern California.

In 1983 the United States declared sovereign rights and jurisdiction over living and nonliving resources in an area extending 200 nautical miles (370 km) seaward from its shores. In response to the establishment of this Exclusive Economic Zone (EEZ), the U.S. Geological Survey (USGS) has implemented a program, called EEZ‐Scan, to systematically map the EEZ, using the Geological Long‐ Range Inclined ASDIC (GLORIA) II longrange side scan sonar system developed by the Institute of Oceanographic Sciences (IOS) of Great Britain [Somers et al, 1978]. The first part of the EEZ‐Scan field program was completed in the summer of 1984, when USGS and IOS scientists surveyed the EEZ off the western conterminous United States aboard the British research vessel Farnella (Figure 1). The west coast survey, requiring 96 days of ship time and four separate legs, has resulted in virtually total sonograph coverage of the sea floor from the continental shelf break to the 200‐nautical mile limit between the Mexican and Canadian borders, an area of about 850,000 km2 . Other data collected on the cruises included two‐channel digital seismic reflection and 3.5‐kHz highresolution and 10‐kHz bathymetric profiles, as well as towed magnetometer data along approximately 20,000 km of trackline spaced nominally at 30‐km intervals.

Summary The California Continental Borderland contains a series of margin basins which are sediment traps at increasing distance from the principal terrigenous and biological sediment sources located in the northern coastal areas. Terrigenous sediments are contributed during a short winter season from small steep coastal drainages. Sand content can be as much as 30–40% of the total contributed load. Silts and clays are sorted out of the river input by coastal wave action and distributed offshore by the seasonal ocean circulation systems. Suspended sediment moves to the basin floors via nepheloid plumes that are generated in the well mixed coastal waters and move offshore with the currents as surface, mid-depth and bottom turbid plumes. During times of low suspensate content in the dry season, lateral transport is dominant. At times of high sediment input and higher concentrations of fine suspensates in the coastal waters, cascading to successively deeper pycnoclines becomes an important process. Resuspension of fine sediments from the outer shelf substrate and the axes of submarine canyons by internal waves and tidally generated currents also contributes to the load of the bottom nepheloid plumes. Large semi-fixed seasonal eddies in the Borderland surface circulation hold the suspended load in the northern sector of the borderland for several days to weeks. Zooplankters have ample opportunity to filter out this material and aggregate it into faecal pellets which rapidly fall to the local basin floors. The removal is sufficiently effective so as to deposit a minimum of 60% of the continental contribution within the northern inner zone of the borderland. High sedimentation rates in these basins promote mass-movement and also decrease the already low oxygen contents of basin floor water as a result of oxidation of the organic fraction of the suspended load. Primary sedimentation structures are preserved in these anaerobic basin floor environments. Mass-movements initiate mud turbidity currents that move to the central basin floors. Mass-movements include creep and sliding of thin sheets from low angle slopes. These effects are amplified following exceptional floods at intervals of around 30 years.

Surface currents influenced by a wind-driven upwelling event in San Pedro Bay moved total suspended matter (TSM) confined to the inner shelf on 19 April 1978 seaward, so that by 27 April surface TSM had increased over the outer shelf. Near-bottom concentrations of TSM also increased across the shelf during this time. This is explained by sediment resuspended by large surface waves being advected from the inner shelf seaward by the mean flow after this flow had turned from southeasterly to southerly when upwelling ceased on 26 April. These complex shelf dynamics contribute to the off-shelf transport of mud to the slope and deep basins.

ABSTRACT The central part of lower Cook Inlet, Alaska, is covered by marine bedforms of varying size and type. Swift tidal currents sweep the area, and assessment of the effects of these currents on the sandy bottom materials is essential to the industrial development of lower Cook Inlet. As part of the summer 1978 field program, we selected a small area for detailed observations. In addition to high resolution seismic profiling and side-scan sonar, we conducted bottom television/camera observations and vertical current-velocity profiling. Time-series data on bottom currents, bottom pressure, near-bottom suspended matter, and bottom photography were obtained with the Geological Processes Bottom Environmental (GElJPROBE) system during August and September 1978 in the study area. An anchor station was occupied for 26 hours over 9-m-high sand waves after the site was surveyed in detail. While the vessel was anchored, current-meter profiles and bottom television/camera observations were made. In addition suspended sediment samples were obtained during deployment of the GEOPROBEs. Surface currents showed a rotary tidal-current pattern with peak velocities of about 100 cm/s. Near the bottom we observed a trimodal current-direction distribution and measured unexpectedly low velocities that were possibly due to obstruction by the bedforms. We observed sand transport along the bottom only during the last hour of both the ebb-tidal and flood-tidal cycles of the spring tide, when velocities exceeded 30 cm/s at 1 m above bottom. The complex bathymetry of the large sand waves, characterized by rapid variations in the elevation of crests, complicates comparison of the anchor station results. Short-term deployments of GEOPROBEs in July 1978 showed that most of the transport of bottom material, with a mean grain size of fine to medium sand, occurs at threshold current velocities of at least 20 cm/s at 1 m above bottom. The highest transport rate in the study area occurred during ebb tide near the crest of a large sand wave. GEOPROBE bottom protographs show that ripples near the crests of large sand waves reverse orientation from ebb to the next flood. Suspended-sediment samples and light-beam transmission data also show active resuspension during peak tidal currents in both the crest and trough area. During neap tides, however, a negligible amount of bottom sediment moves in the sand-wave trough, despite bottom current velocities of more than 20 cm/so an admixture of silt and organic detritus may inhibit sediment response in the troughs. INTRODUCTION Cook Inlet is a large estuary in south-central Alaska that extends from Anchorage to the Pacific Ocean. Tidally dominated currents control the hydrographic regime and may have a significant influence on surficial bottom sediments. Response of the sediments to strong tidal currents was part of the environmental geologic studies conducted by the U.S. Geological Survey during 1976-78 on board the research vessel Sea Sounder. Surveys were conducted utilizing high-resolution seismic reflection profiling and side-scan sonar. Several stations were occupied for bottom sampling, underwater television and camera observations, vertical current-velocity profiles, and deployment of GEOPROBEs (Geological Processes Bottom Environmental system; Cacchione and Drake, 1979).