Point-sampled suspended sediment concentration profiles (grain size largely<32 μm) were measured in August 1978, April 1990, and May 1990 in the Changjiang Estuary. They were selectively fit with the Rouse equation in order to calculate the median settling velocities ws,50 of fine suspended particles in the Changjiang Estuary, East China Sea. Calculated settling velocities ranged from 0.4 to 4.1 mm s−1. Furthermore, ws values increased with the mean concentration C̄ of sediment in suspension. The best-fit equation for the field settling velocity of fine particles in the Changjiang Estuary can be expressed by the power law: ws=2.37 C̄0.84 (r2<0.3).

Kgr The continental shelf of the greater Monterey Bay area has been surveyed using various techniques to give seafloor acoustic imagery from Point Año Nuevo on the north to Point Sur on the south. This map is an interpretation of this acoustic imagery in terms of bottom type. The acoustic imagery can be accessed via http://TerraWeb.wr.usgs.gov/TRS/projects/MontereySonar. Where bedrock is exposed on the seafloor, we have identified the outcropping rock type. Since the acoustic techniques cannot resolve sediment thicknesses less than about 1 m, it is possible that some of the rock outcrops are, in fact, covered by as much as 1 m of modern sediment. However, where photographic evidence is available, our interpreted outcrops are found to be uncovered. To produce the seafloor imagery on the continental shelf north of Monterey Canyon, towed-fish side-scan sonar systems were deployed from the R/V David Johnston of the U.S. Geological Survey and the R/V McArthur of the National Oceanic and Atmospheric Administration. To aid in the interpretation of seafloor materials on the northern shelf, we also collected highresolution seismic-reflection profiles using a surfacetowed 1-kHz boomer as sound source and a 10-m streamer as receiver. Lines were spaced 150 m apart on the inner Santa Cruz shelf and 400 m apart on the midto outer-shelf. Differential global positioning system (GPS) navigation was used on all acoustic survey lines, providing boat-position accuracy better than 10 m. On the northern shelf from the sheltered area around Santa Cruz northwest to Point Año Nuevo, the innermost survey lines were run into water depths as shallow as 5 m. However in most other areas, shallowest lines were at about the 10-m isobath, leaving a significant data gap to the shoreline. On the shelf south of Monterey Canyon, two surveys were conducted with hull-mounted multibeam swath-bathymetry/backscatter systems. The first survey used a Simrad EM1000 system on the R/V Pacific Hunter of California State University, Humbolt. The second survey used a Simrad EM300 system on the R/V Ocean Alert (contracted through the University of New Brunswick and C&C Technologies in cooperation with the Monterey Bay Aquarium Research Institute). These systems recorded both water depth and acoustic backscatter from the seafloor. Line spacing varied to accommodate the variation in seafloor coverage afforded by the systems that image swaths averaging about 5 times water depth. Nearshore coverage extended shoreward to the 20-m isobath along the coast between Monterey and Moss Landing and to approximately the 50-m isobath around and south of the Monterey Peninsula. The side-scan sonar mosaics on the shelf north of Monterey Canyon have a pixel resolution of 0.4 m for the inner shelf and 0.8 m for the outer shelf. Although this pixel resolution allows us to resolve relatively small features on the seafloor, the accuracy of pixel location is considerably poorer than this due to inaccuracies in navigating the fish towed behind the ship for the northern surveys. The southern-shelf data, which were collected with hull-mounted systems, were processed at pixel resolutions of 2.5 m for backscatter and 5 m for bathymetry. The estimated navigational accuracy in pixel location is 50 m for the northern shelf data and 10 m for the southern shelf data. See Eittreim and others (in press) for a fuller discussion of the navigational accuracy and acoustic systems. Based on the acoustic imagery, boundaries were drawn between areas interpreted as different bottom types. In general, mud produces a low-reflectivity bottom, whereas outcropping rocks and coarse sand have high reflectivity (fig. D). Sands show uniform reflectivity whereas rock outcrops display geometric patterns that are associated with layering, jointing, faulting and folding (figs. A, B). Granitic rocks show non-layered knobby patterns associated with weathering and jointing style (fig. E). Coarse sands usually occur in shallow troughs (fig. F) and display 1-m wavelength ripples where highresolution data (better than 1-m per pixel) are available. Interpretation was done at the scale of 2.4 m per pixel. At an image resolution of 72 dpi, common to most video screens, this represents a scale of 1:6,800. We emphasize that the distributions of bottom types are based largely on the acoustic imagery and not on direct samples. However, numerous sample identifications, mostly from rocks dredged from the upper continental slope and canyon walls (McCulloch and others, 1985; Stakes and others, 1999), seafloor sediment samples and published descriptions of the seafloor environment (Edwards, in press; Galliher, 1932), and samples collected by ROV (remotely operated vehicles; H.G. Greene and D. Stakes, MBARI, personal communication) support the interpretations. Most importantly, outcrops directly onshore were often used to identify the innershelf outcrops (Clark, 1981; Brabb, 1989; Dibblee, 1999; Clark and others, 1997). Our interpretations are largely in concert with those of McCulloch and Greene (1989), who presented a regional interpretation of the geology of the California continental margin.

A side-scan sonar survey was conducted of Monterey Canyon and the San Gregorio fault zone, off shore of Monterey Bay. The acoustic character and morphology of the sonar images, enhanced by SeaBeam bathymetry, show the path of the San Gregorio fault zone across the shelf, upper slope, and Monterey Canyon. High backscatter linear features a few kilometers long and 100 to 200 m wide delineate the sea-floor expression of the fault zone on the shelf. Previous studies have shown that brachiopod pavements and carbonate crusts are the source of the lineations backscatter. In Monterey Canyon, the fault zone occurs where the path of the canyon makes a sharp bend from WNW to SSW (1800 m). Here, the fault is marked by NW–SE-trending, high reflectivity lineations that cross the canyon floor between 1850 m and 1900 m. The lineations can be traced to ridges on the northwestern canyon wall where they have ∼15 m of relief. Above the low-relief ridges, bowl-shaped features have been excavated on the canyon wall contributing to the widening of the canyon. We suggest that shear along the San Gregorio fault has led to the formation of the low-relief ridges near the canyon wall and that carbonate crusts, as along the shelf, may be the source of the high backscatter features on the canyon floor. The path of the fault zone across the upper slope is marked by elongated tributary canyons with high backscatter floors and `U'-shaped cross-sectional profiles. Linear features and stepped scarps suggestive of recent crustal movement and mass-wasting, occur on the walls and floors of these canyons. Three magnitude-4 earthquakes have occurred within the last 30 years in the vicinity of the canyons that may have contributed to the observed features. As shown by others, motion along the fault zone has juxtaposed diverse lithologies that outcrop on the canyon walls. Gully morphology and the canyon's drainage patterns have been influenced by the substrate into which the gullies have formed.

Seismic reflection data along a flow line of crustal generation in the central Pacific that spans ages of zero to 85 Ma and spreading half rates of 30 to 100 km m.y.−1 shows a nearly constant travel time of 2 s through igneous crust to reflection Moho. The highest‐amplitude and most laterally continuous Moho reflections were recorded over 20–30 Ma crust that was emplaced at “superfast” (75–95 km m.y.−1) spreading rates. The superfast spread portion also records the lowest scatter about the 2‐s average travel time to reflection Moho. Seismic images show that lower crustal reflectors dip consistently eastward toward the ridge crest. These dipping reflectors are truncated by the reflection Moho.

The Wilkes‐Adelie margin of East Antarctica, a passive margin rifted in the Early Cretaceous, has an unusually reflective Moho which can be traced seismically across the continent‐ocean transition. Velocity models and depth sections were constructed from a combined set of U.S. and French multichannel seismic reflection lines to investigate the transition from continental to oceanic crust. These data show that the boundary between oldest oceanic crust and transitional continental crust is marked by a minimum in subsediment crustal thickness and, in places, by a shoaling of Moho. The Moho reflection is continuous across the edge of oceanic crust, and gradually deepens landward under the continental edge. A marginal rift basin, some tens of kilometers in width, lies in the transition between continental and oceanic crust, contains an average of about 4 km of synrift sediment that is prograded in places, and has characteristics of a former rift valley, now subsided to about 10 km. Three types of reflections in the seismic data are interpreted as volcanic deposits: (1) high‐amplitude reflections that floor the marginal rift basin, (2) irregularly seaward dipping sequences that comprise an anomalously thick edge of oceanic crust, and (3) highly irregular and diffractive reflections from oceanic crustal basalts that cap a normal‐thickness ocean crust. The present depth to the prerift surface of continental crust is compatible with passive margin subsidence since 95 Ma, corrected for its load of synrift and postrift sediment and mechanically stretched by factors of β = 1.8 or higher. Comparison of seismic crustal thickness measurements with inferred crustal thinning from subsidence analysis shows agreement for areas where β < 4. In areas where β > 4, measured thickness is greater than that inferred from subsidence analysis, a result that could be explained by underplating the crust beneath the marginal rift basin.

Magnetic anomalies of 52–80 Ma age have been mapped in the equatorial Pacific in a region where they were previously unrecognized. These anomalies, now at 7° N are best modeled with crustal rocks of negative inclination, apparently because the crust was formed at a spreading ridge that was south of the magnetic equator in Late Cretaceous‐Early Tertiary time. Magnetic anomalies recorded along a 5900‐km trackline that follows a flowline of crustal generation show one long period, from 12 to 31 Ma, of apparent constant half‐spreading rate over the 0 to 80 Ma represented.

The continental margins of the southern and central Red Sea and most of Wilkes Land, Antarctica have bulk crustal configurations and detailed structures that are best explained by a prolonged history of magmatic expansion that followed a brief, but intense period of mechanical extension. Extension on the Red Sea margins was spatially confined to a rift that was 20-30 km in width. The rifting phase along the Arabian margin of the central and southern Red Sea occurred 25-32 Ma ago, primarily by detachment faulting at upper crustal levels and ductile uniform stretching at depth. Rifting was followed by an early magmatic phase during which the margin was invaded by dikes and plutons, primarily of gabbro and diorite, at 20-24 Ma, after the crust was mechanically thinned from 40 km to almost-equal-to 20 km. We infer continued spreading after that in which broad shelves were formed by a process of magmatic expansion, because the offshore crust is only 8-15 km thick, including sediment, and seismic reflection data do not depict horst and graben or half graben structures from which mechanical extension might be inferred. The Wilkes Land margin is similar to the Arabian example. The margin is about 150 km in width, the amount of upper crustal extension is too low to explain the change in sub-sediment crustal thickness from almost-equal-to 35 km on the mainland to < 10 km beneath the margin, and reflectors in the deepest seismic sequence are nearly flat lying. Our model requires large volumes of melt in the early stages of continental rifting. The voluminous melt might be partly a product of nearby hot spots, such as Afar, and partly the result of an initial period of partial fusion in the deep continental lithosphere under lower temperatures than ordinarily required by dry soildus conditions.

In January 1984, the U. S. Geological Survey research vessel S. P. Lee carried out investigations of the Antarctic continental margin in the Wilkes Land and Victoria Land areas, using 24-channel and high-resolution seismic, sonobuoy refraction, gravity, magnetic, and bottom-sampling methods. This investigation augmented previous surveys of the Dumont d'Urville area by the French Petroleum Institute and explored new areas west and east to the boundary between the onshore Wilkes basin and the Victoria Land highlands. These surveys defined sediment thickness distribution and seismic stratigraphy in this frontier area. The tectonic style of the boundary between the East Antarctic craton and the younger crust of West Antarctica in the Ross Sea is revealed by one multichannel s ismic line across this important boundary. The initial breakup of Antarctica from Australia occurred as a slowly spreading phase during the middle Cretaceous. According to Deep Sea Drilling Project results on the Tasman Rise, conditions of restricted circulation existed in the growing basin between the continents before the late Eocene. After the late Eocene, the major oceanic circulation pattern was established. Before that time, conditions were favorable for preservation of organic-carbon deposits on the sea floor. Among the questions to be addressed with this data are the following. (1) How do apparent subsidence rates of this passive margin compare with others around the world? (2) Does the onshore subglacial Wilkes basin continue onto the continental shelf? (3) Do Antarctic counterpart basins to the Otway and Ceduna basin of Australia exist? (4) What is the effect of the icecap on the stratigraphy of this margin? (5) Do the two major Tertiary ice advances have conspicuous seismic-stratigraphic signatures? End_of_Article - Last_Page 474------------

The Vema fracture zone trough, at 11°N between 41° and 45°E, is open to the west at the 5000‐m level but is silled at the 4650‐m level on the east where it intersects the axis of the Mid‐Atlantic Ridge. The trough is filled with Antarctic Bottom Water (AABW) with a potential temperature of 1.32°C and salinity of 34.82 ppt. The bottom water is thermally well mixed in a nearly homogeneous layer about 700 m thick. The great thickness of this bottom layer, as compared with the bottom‐water structure of the western Atlantic basin, may result from enhanced mixing induced by topographic constriction at the west end of the fracture zone trough. A benthic thermocline, with potential temperature gradients of about 1.2 mdeg m−1, is associated with an abrupt increase in turbidity with depth at about 1200 m above bottom. A transitional layer of more moderate temperature gradients, about 0.4 mdeg m−1, lies between the benthic thermocline above and the AABW below. The AABW layer whose depth‐averaged suspended paniculate concentrations range from 8 to 19 μg L−1, is consistently higher in turbidity than the overlying waters. At the eastern end of the trough, 140 m below sill depth, very low northeastward current velocities, with maximums of 3 cm s−1, were recorded for an 11‐day period.

Circulation patterns on the shelf and at the shelf break appear to dominate the Barrow Canyon system. The canyon's shelf portion underlies and is maintained by the Alaska Coastal Current (A.C.C.), which flows northeastward along the coast toward the northeast corner of the broad Chukchi Sea. Offshelf and onshelf advective processes are indicated by oceanographic measurements of other workers. These advective processes may play an important role in the production of bedforms that are found near the canyon head as well as in processes of erosion or non-deposition in the deeper canyon itself. Coarse sediments recovered from the canyon axis at 400 to 570 m indicate that there is presently significant flow along the canyon. The canyon hooks left at a point north of Point Barrow where the A.C.C. loses its coastal constriction. The left hook, as well as preferential west-wall erosion, continues down to the abyssal plain of the Canada Basin at 3800 m. A possible explanation for the preferential west-wall erosion along the canyon, at least for the upper few hundred meters, is that the occasional upwelling events, which cause nutrient-rich water to flow along the west wall would in turn cause larger populations of burrowing organisms to live there than on the east wall, and that these organisms cause high rates of bioerosion. This hypothesis assumes that the dominant factor in the canyon's erosion is biological activity, not current velocity. Sedimentary bedforms consisting of waves and furrows are formed in soft mud in a region on the shelf west of the canyon head; their presence there perhaps reflects: (a) the supply of fine suspended sediments delivered by the A.C.C. from sources to the south, probably the Yukon and other rivers draining northwestern Alaska; and (b) the westward transport of these suspended sediments by the prevailing Beaufort Gyre which flows along the outer shelf.

The continental margin north of Alaska is of Atlantic type. It began to form probably in Early Jurassic time but possibly in middle Early Cretaceous time, when the oceanic Canada Basin of the Arctic Ocean is thought to have opened by rifting about a pole of rotation near the Mackenzie Delta. Offsets of the rift along two fracture zones are thought to have divided the Alaskan margin into three sectors of contrasting structure and stratigraphy. In the Barter Island sector on the east and the Chukchi sector on the west the rift was closer to the present northern Alaska mainland than in the Barrow sector, which lies between them. In the Barter Island and Chukchi sectors the continental shelf is underlain by prisms of clastic sedimentary rocks that are inferred to include thick sections of Jurassic and Neocomian (lower Lower Cretaceous) strata of southern provenance. In the intervening Barrow sector the shelf is underlain by relatively thin sections of Jurassic and Neocomian strata derived from northern sources that now lie beneath the outer continental shelf. The rifted continental margin is overlain by a prograded prism of Albian (upper Lower Cretaceous) to Tertiary clastic sedimentary rocks that comprises the continental terrace of the western Beaufort and northern Chukchi Seas. On the south the prism is bounded by Barrow arch, which is a hingeline between the northward-tilted basement surface beneath the continental shelf of the western Beaufort Sea and the southward-tilted Arctic Platform of northern Alaska. The Arctic platform is overlain by shelf clastic and carbonate strata of Mississippian to Cretaceous age, and by Jurassic and Cretaceous clastic strata of the Colville foredeep. Both the Arctic platform and Colville foredeep sequences extend from northern Alaska beneath the northern Chukchi Sea. At Herald fault zone in the central Chukchi Sea they are overthrust by more strongly deformed Cretaceous to Paleozoic sedimentary rocks of Herald arch, which trends northwest from Cape Lisburne. Hope basin, an extensional intracontinental sedimentary basin of Tertiary age, underlies the Chukchi Sea south of Herald arch.

More than 100 airgun‐sonobuoy records from the northern Alaska shelves have been reduced to yield an average of six layers from each record. Seafloor sound velocities, regional velocity functions, and high‐resolution velocity‐depth inversions were computed. Low seafloor velocities in the northwest part of the otherwise Cretaceous seafloor of the Colville foredeep represent thin Tertiary sediments that overstep the Barrow arch from the north. Anticlinal cores of ‘mid‐shelf arches’ (Grantz et al., 1981) appear as 200‐m/s velocity increases in the Neogene seafloor of the western Beaufort shelf. Very recent or currently, active sedimentation in the central Hope basin is indicated by low seafloor sound velocities. The value of K in the least squares determination of V = V0 + Kt (t is one‐way vertical travel time) decreases toward the east from 1.74 km/s2 just east of Point Barrow of 1.67 km/s2 around Prudhoe Bay, to 1.53 km/s2 near the Mackenzie River. This decrease in K corresponds with an eastward increase in the thickness of the clastic rocks within the youngest part of the Brookian sequence (Early Cretaceous to Quaternary). A 2.1‐km/s2 value of K is the Brookian sequence of the north Chukchi basin suggests a change in the relative proportions of Cretaceous and Tertiary sediments between the basin and the Beaufort shelf. High‐resolution velocity‐depth inversions, using closely digitized travel time data, yield velocities at about 40‐m vertical spacings. Calculated values of near‐surface velocity gradients determined by velocity‐depth plots of these data are generally not in good agreement with the values predicted from the velocity functions, which are statistical generalizations based on conventional refraction picks that resolve layers no thinner than about 250 m. The discrepancies are minimized by use of independently determined seafloor velocities.

The Antarctic Bottom Water (AABW) activity and the variations in the abundance and grain size of the terrigenous sediments, derived from Africa and Madagascar land masses, are reflected in different types of microtopography in the Mozambique Basin. In southerly areas, where the sediment supply is much less, the bottom-current activity has resulted in the presence of manganese nodules, a thin veneer of sediments, and the absence of sediment waves. Farther north, along the marginal areas of the basin where the fine-grained sediments from the Africa—Madagascar source have been supplied in abundance, wavy bedforms have been generated by AABW. Wavy bedforms do not exist even in the northerly areas if coarse-grained, turbidite sediments are present on the sea floor. The continuation of acoustic reflectors from the zone of turbidites in the central areas of the basin into the zone of sediment waves along the margins, and the lithology and structures in sediment cores from these zones suggest that the turbidity-current-fed, fine-grained sediments were deposited as wavy bedforms by AABW flow. Thus, sediment waves formed readily during Pleistocene times. The enrichment of quartz and displaced Antarctic diatoms, and the relatively low kaolinite/chlorite ratios in the sediments, the north-pointing current lineations on the sea floor, the lack of any perceptible sedimentary fill in the troughs of waves, and the dense nepheloid layer in the westerly areas of the Mozambique Basin, attest to the current-controlled sedimentation and generation of wavy bedforms during Holocene time also. The formation of sediment waves in the Mozambique Basin can be modeled after a fluvial antidune mechanism. This model envisages that internal waves, focussed on a benthic boundary layer cap, have been locked in phase with sediment waves in the presence of an 8–10 cm/sec current in the Mozambique Basin. A density contrast of 2·10−6 g/cm3 appears to exist at the tops of benthic boundary layers in the Mozambique Basin and is quite sufficient for supporting the internal waves. The densiometric Froude number calculated for a 60–280 m thick boundary layer in the basin is close to unity or greater, and is compatible with the antidune model.