Shelf Channels Research Articles

An oceanographic flip‐flop: deep water exchange in Fortune Bay, Newfoundland

Observations are presented of deep‐water exchange in a large coastal embayment on the south coast of Newfoundland. The bay has several outer sills not unlike those found in fjords. Outside the bay, two submerged channels on the continental shelf are reservoirs for two different deep‐water masses which may flow over separate sills to replace deep water within the bay. The deep water is renewed approximately twice a year. On average, exchange occurs both during the half‐year period centered on midwinter and during the half‐year period centered on late summer, with the water properties of the inflow in each period being dominated by one of the two source water masses in the shelf channels. The system therefore occupies one of two renewal states for much of the year. The transition between states appears to be induced largely by seasonal variations in the wind forcing over the shelf.

Channels across continental shelves: Corequisites of canyon-fan systems and potential petroleum conduits

Abstract Channels on continental shelves supply much of the large volume of sediments found in submarine fans. Modern and ancient examples are known, but few have been studied in detail. Shelf channels may form from retrogressive mass failures initiated on the continental slope or on the shelf itself, from erosion by semi-permanent or intermittent shelf currents, or from erosion initiated at sites of sediment spillover along the shelf break. Channel-filling sediments are generally coarser than laterally contiguous facies and commonly include mud-deficient permeable rocks which are capable of acting as reservoirs. Because the shelf channel-submarine canyon-submarine fan system taps a wide range of potential hydrocarbon source rocks—muds of the basin floor, slope, and distal shelf—the shelf channels are highly likely petroleum conduits and may form traps.

Geomorphology of the Goto shelf channels off northern Kyusyu, Japan : Katsura, Tadahiko and Manao Nagano, 1982 Special number. Data Rept hydrogr. Obsns, Tokyo, 17:17–92 (In Japanese, English abstract.) Maritime Safety Agency, Tokyo, Japan.

Geomorphology of the Goto shelf channels off northern Kyusyu, Japan : Katsura, Tadahiko and Manao Nagano, 1982 Special number. Data Rept hydrogr. Obsns, Tokyo, 17:17–92 (In Japanese, English abstract.) Maritime Safety Agency, Tokyo, Japan.

対馬海峡大陸だなの地形発達 対馬陸橋に関連して

The Tsushima Strait are consists of broad continental shelves, connecting the Japanese Islands with the Asian Continent. Systematic sea bottom surveys of this area with two-mile spacing soundings were carried out by Hydrographic Department of Japan from 1973 to 1976. From the seismic profiling, it was revealed that the center of broad continental shelves are surfaced with thick Quaternary sediments where longitudinal depressions lie, trending in the NE-SW direction.In 1977 and 1978, with 0.5-mile-spacing soundings more detailed surveys were conducted by Hydrographic Department in the areas from the southern coast of Tsushima Island to the north of Iki Island and near Shiro-se off the west coast of the Goto Islands, respectively. The distinct depositional terraces were detected at depths of -90m to -110m and -110m to -130m in the Shiro-se area. Along these terrace edges there are sand bars, suggesting the ancient shore lines, and inward shallow depressions.It is noticeable that the basement of the longitudinal depression is buried by sediments which form the depositional terrace at the depths of -110m to -130m. This fact means that the longitudinal depression represents an uncovered portion of the basin reclaimed by shelf sediments. The present topography of the longitudinal depression is modified, however, by currents indicated by a pair of caldrons excavated from the bottom of the depression on the west of Korai-sone.On the other hand, a lot of submarine sand ridges were found in the Tsushima East Channel. These submarine sand ridges are classified into two types: one is a U-type, developing in the narrowest part of the channel between the southern coast of Tsushima Island and Iki Island, and the other is a longitudinal type with a NE trend, developing abundantly in the vast area behind the U-type sand ridge area. From the relations between regular distribution patterns of the sand ridges, and distributions of current velocity and grain size of sediments, the author concluded that these sand ridges are not growing at the present with the Tsushima Current, but have been made by tidal currents in the past when sea level lowered to a depth of -80m. These sand ridges cover the submarine terraces at depths of -90m to -120m. Accordingly, the abundant submarine sand ridges are formed at a certain stage of the sea level rising after the Würm glacial stage when the -110m to -120m terrace was presumably cut.A more detailed survey on a scale of 1/10000 was carried out by Hydrographic Department of Japan in the area off the west coast of Tsushima Island. From the survery, distinct five submarine terraces and fifteen buried shelf channels were discovered. The second terrace, -20m to -35m deep, is a depositional plain covered by Alluvium of 5m to 20m thickness. On the other hand, the fourth terrace at -60m to -70m depths is an erosional plain with a veneer of coarse sediments. The buried shelf channels start from the land river mouths and continue to a depth of about 100m. It is noticeable that the course of the buried shelf channels are interrupted at the fourth terrace, and arc crossed by the second terrace.As a result, the author summarized evolution of the continental shelves as follows. During the Würm glacial stage, the coast line regressed to the present depth of about -100m, and the marine terrace was formed by wave abrasion at that level. Many rivers extended to that coast and cut new dried land. And then sea level rose toward the present sea level.At a certain stage of the sea level rising the sea level stayed at the present depth of about -80m, and many submarine sand ridges were formed in the ancient Tsushima East Channel as in the case of the present Malacca Strait. After the rising of sea level from -80m, the sea level stayed again at the present depth of about -60m. At that time, wave abrasion made the fourth terrace and scratched the submarine valleys out on it.

Suspended Particulate Matter in the New York Bight Apex, Fall 1973

ABSTRACT Four surveys of suspended particulate matter in the New York Bight apex from September through November 1973 revealed consistent distributions that reflect the water circulation. Two major currents were dominant during the fall season of limited river flow and gradually weakening stratification of the water column: (1) low-salinity (29 to 31) surface water, containing between 1 to 4 mg/l of suspended particles, flows from Hudson estuary southward along the New Jersey coast within 5 to 10 km from shore; and (2) northward flow along Hudson shelf channel occurred during all surveys. Low concentrations of total suspended matter and ash fractions dominated by diatoms indicate a central shelf origin for the shelf-channel current. The distribution of total suspended matter and the dispersion patterns of iron particles released at the acid-waste dumpsite support the existence of a clockwise gyre in the central part of the area during autumn; the shelf-channel current forms the western limb of this gyre. Dredge spoil and sewage sludge dumped near the head of the shelf channel settle into this depression to form mud lenses rich in organic matter. Some of this material is entrained into a turbid bottom layer by the northward current and transported from the depression to the northeast over Cholera Bank. The bank crest does not accumulate fine sediment owing to relatively vigorous wave surge. As the turbid current turns south along the east side of Cholera Bank, fine-grained, mineral and organic material begins to s ttle, forming two disconnected lenses of mud on either side of the sand bank. Contaminated sediments are widely dispersed throughout the water column in the bight apex. Major dispersion appears to be centered on the Hudson shelf channel, and there is good evidence that material dumped near the channel head is transported up and down the channel in substantial quantities.

Geomorphology and tectonic movement of the sea floor, northwest off Kyushu, Japan

As a result of the sea bottom survey of continental shelf northwest off Kyushu including Tsushima, Iki and Goto Islands which has been carried out by the Hydrographic Department of Japan, the following features of the area are revealed. Topographically, three groups of shelf channels, trending NE-SW, N-S and NW-SE respectively, are discovered and several small banks are found. Sand wave-like shapes are also seen in a seafloor about 200 m deep in the vicinity of Tsushima Straits. Geologically, remarkable tectonic lines characterized by faults and folding axes are confirmed and named Tsushima, Iki, Hirato and Goto Tectonic Lines. Several tectonic provinces can be defined by these tectonic lines. Each province consists of small geological blocks formed by indivisual tectonic movement such as tilting, uplifting and sinking. Such differential tectonic movement is often seen on the continental shelves around active island arc. In this area, these tectonics might be caused by activity of the Ryukyu Arc. This area may thus be a metastable portion existing between the northwestern Kyushu and the Asiatic continental block. The present submarine topography is probably closely related to these tectonics in addition to the eustatic sea-level changes in Quaternary age.

Possible bottom current response to surface winds in the Hudson Shelf Channel

Current measurements made in the Hudson Shelf Channel during the summer of 1973 show essentially channel axial bottom current even though the channel aspect ratio is small in the area of measurement. Although the current record is of short duration, correlation of water movement with surface winds is suggested by the data. The sense of summertime nontidal bottom flow in the channel (up or down channel) would appear to be controlled by the surface wind direction (offshore or onshore). These results would suggest the likelihood of net down-channel flow during the summer months.

Structure and morphology of the west Reykjanes basin and the southeast Greenland continental margin

The continental-shelf morphology is dominated by glacial erosion and deposition. Erosion is prominent on the near-shore shelf and deposition along the outer shelf edge. The continental slope is characterized by delta-shaped progradations (glaciomarine-sediment fans) seaward of the shelf channels. Canyons cross the continental slope only in the region southeast of Cape Farewell. The continental rise is incised by a number of submarine canyons. Broad sediment ridges on the upper continental rise are probably canyon-eroded remains of extensive Plio-Pleistocene fans. A mid-ocean channel which crosses the continental rise is possibly related to the axis of maximum depth of Denmark Strait. Despite the presence of strong bottom currents, there is no indication of depositional sediment drifts along the continental margin of Greenland between Cape Farewell and Denmark Strait. This may be a function of high current velocity or low sediment load. Sea floor older than 60 m.y. B.P. is present just seaward of the Greenland continental margin implying either downwarped continental material or an early rift formed prior to the separation of Greenland from the European plate. A left lateral offset of anomalies 20 and 21 at 65°N indicates a major fracture zone related to the Greenland continental margin offset nearby.

Construction in the coastal zone: A potential use of waste materials

The Inner New York Bight, at the head of the Hudson shelf channel, has been the site for ocean disposal of various waste products since at least 1888. Natural channel-like bathymetry expressed in 1845 is today a series of hills rising to within 12 m (40 ft.) of the water surface superimposed upon a broad lobate mound. This topographic inversion created over the past nine decades is attributable to disposal of materials (soil, sand, and stone) of varying composition generated during construction in the New York metropolitan area. Data indicate approximately 765 · 10 6 m 3 (1 · 10 9 yd 3) of waste has been dumped in that region from 1888 to 1934. Isopach maps, sea-floor profiles, seismic records, and vibratory cores show much of the fill has remained in place in spite of bottom currents of approximately 25 cm/sec (0.5 knot) and a wave climate of H s = 0.76 m (2.5 ft.); T = 5−15 sec. Man-made islands proposed for the inner continental shelf for siting power, port, or recreational facilities will use large volumes of stable material for core fill, which could be waste materials such as those described. Effective regional coastal-zone planning should recognize uses for past and future waste material as such practices would conserve sand and gravel resources for other high-volume needs (shoreline nourishment and protection and construction aggregate) and alleviate some of the site-selection problems in land disposal of waste.