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32-604: The North West Shelf is a continental shelf region of Western Australia . It includes an extensive oil and gas region off the North West Australia coast in the Pilbara region. Considerable parts of the region are the highest prospective gas and oil areas of Australia. The main sedimentary basin providing the opportunity is the Northern Carnarvon Basin - however it is only one part of

64-455: A continental shelf differs significantly from the geological definition. UNCLOS states that the shelf extends to the limit of the continental margin , but no less than 200 nmi (370 km; 230 mi) and no more than 350 nmi (650 km; 400 mi) from the baseline . Thus inhabited volcanic islands such as the Canaries , which have no actual continental shelf, nonetheless have

96-832: A distance where the depth of waters admitted of resource exploitation were claimed by the marine nations that signed the Convention on the Continental Shelf drawn up by the UN's International Law Commission in 1958. This was partly superseded by the 1982 United Nations Convention on the Law of the Sea (UNCLOS). The 1982 convention created the 200 nautical miles (370 km; 230 mi) exclusive economic zone, plus continental shelf rights for states with physical continental shelves that extend beyond that distance. The legal definition of

128-458: A hillslope) observed in submarine canyons. Mass wasting is the term used for the slower and smaller action of material moving downhill. Slumping is generally used for rotational movement of masses on a hillside. Landslides, or slides, generally comprise the detachment and displacement of sediment masses. It is now understood that many mechanisms of submarine canyon creation have had effect to greater or lesser degree in different places, even within

160-538: A legal continental shelf, whereas uninhabitable islands have no shelf. Submarine canyon About 3% of submarine canyons include shelf valleys that have cut transversely across continental shelves, and which begin with their upstream ends in alignment with and sometimes within the mouths of large rivers , such as the Congo River and the Hudson Canyon . About 28.5% of submarine canyons cut back into

192-426: A sea with a bed significantly below sea level is cut off from the larger ocean to which it is usually connected. The sea which is normally repleted by contact and inflow from the ocean is now no longer replenished and hence dries up over a period of time, which can be very short if the local climate is arid. In this scenario, rivers that previously flowed into the sea at a sea level elevation now can cut far deeper into

224-593: Is also minimal, at less than 20 m (66 ft). Though the continental shelf is treated as a physiographic province of the ocean , it is not part of the deep ocean basin proper, but the flooded margins of the continent. Passive continental margins such as most of the Atlantic coasts have wide and shallow shelves, made of thick sedimentary wedges derived from long erosion of a neighboring continent. Active continental margins have narrow, relatively steep shelves, due to frequent earthquakes that move sediment to

256-469: Is evidence that changing wind, rainfall, and regional ocean currents in a warming ocean are having an effect on some shelf seas. Improved data collection via Integrated Ocean Observing Systems in shelf sea regions is making identification of these changes possible. Continental shelves teem with life because of the sunlight available in shallow waters, in contrast to the biotic desert of the oceans' abyssal plain . The pelagic (water column) environment of

288-462: Is known as an insular shelf . The continental margin , between the continental shelf and the abyssal plain , comprises a steep continental slope, surrounded by the flatter continental rise , in which sediment from the continent above cascades down the slope and accumulates as a pile of sediment at the base of the slope. Extending as far as 500 km (310 mi) from the slope, it consists of thick sediments deposited by turbidity currents from

320-414: Is often cut with submarine canyons . The physical mechanisms involved in forming these canyons were not well understood until the 1960s. Continental shelves cover an area of about 27 million km (10 million sq mi), equal to about 7% of the surface area of the oceans. The width of the continental shelf varies considerably—it is not uncommon for an area to have virtually no shelf at all, particularly where

352-597: The Burrup Peninsula , Barrow Island and other locations. This article about a location in Western Australia is a stub . You can help Misplaced Pages by expanding it . Continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea . Much of these shelves were exposed by drops in sea level during glacial periods . The shelf surrounding an island

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384-484: The shelf break ). The sea floor below the break is the continental slope . Below the slope is the continental rise , which finally merges into the deep ocean floor, the abyssal plain . The continental shelf and the slope are part of the continental margin . The shelf area is commonly subdivided into the inner continental shelf , mid continental shelf , and outer continental shelf , each with their specific geomorphology and marine biology . The character of

416-444: The water depths as great as 3,000 meters (9,800 ft) where canyons have been mapped, as it is well established (by many lines of evidence) that sea levels did not fall to those depths. The major mechanism of canyon erosion is thought to be turbidity currents and underwater landslides . Turbidity currents are dense , sediment-laden currents which flow downslope when an unstable mass of sediment that has been rapidly deposited on

448-567: The Asian mainland. Other familiar bodies of water that overlie continental shelves are the North Sea and the Persian Gulf . The average width of continental shelves is about 80 km (50 mi). The depth of the shelf also varies, but is generally limited to water shallower than 100 m (330 ft). The slope of the shelf is usually quite low, on the order of 0.5°; vertical relief

480-554: The Thevenard Production Area close to Onslow , and the Varanus Production Area west of Dampier. The North West Shelf oil extraction is negligible compared to the volume of gas produced. The North West Shelf Venture is a project to extract resources within the region from various gas fields - however it is considered to be only in part of the whole shelf region. It involves developments on

512-837: The abyssal plain. Ancient examples have been found in rocks dating back to the Neoproterozoic . Turbidites are deposited at the downstream mouths or ends of canyons, building an abyssal fan . Submarine canyons are more common on the steep slopes found on active margins compared to those on the gentler slopes found on passive margins . They show erosion through all substrates, from unlithified sediment to crystalline rock . Canyons are steeper, shorter, more dendritic and more closely spaced on active than on passive continental margins. The walls are generally very steep and can be near vertical. The walls are subject to erosion by bioerosion , or slumping . There are an estimated 9,477 submarine canyons on Earth, covering about 11% of

544-597: The bottom of the bed now exposed. The Messinian salinity crisis is an example of this phenomenon; between five and six million years ago, the Mediterranean Sea became isolated from the Atlantic Ocean and evaporated away in roughly a thousand years. During this time, the Nile River delta, among other rivers, extended far beyond its present location, both in depth and length. In a cataclysmic event,

576-567: The coast; sand is limited to shallow, wave-agitated waters, while silt and clays are deposited in quieter, deep water far offshore. These accumulate 15–40 centimetres (5.9–15.7 in) every millennium, much faster than deep-sea pelagic sediments . "Shelf seas" are the ocean waters on the continental shelf. Their motion is controlled by the combined influences of the tides , wind-forcing and brackish water formed from river inflows ( Regions of Freshwater Influence ). These regions can often be biologically highly productive due to mixing caused by

608-420: The continental shelf constitutes the neritic zone , and the benthic (sea floor) province of the shelf is the sublittoral zone . The shelves make up less than 10% of the ocean, and a rough estimate suggests that only about 30% of the continental shelf sea floor receives enough sunlight to allow benthic photosynthesis. Though the shelves are usually fertile, if anoxic conditions prevail during sedimentation,

640-541: The continental slope. Different mechanisms have been proposed for the formation of submarine canyons. Their primary causes have been subject to debate since the early 1930s. An early and obvious theory was that the canyons present today were carved during glacial times, when sea level was about 125 meters (410 ft) below present sea level, and rivers flowed to the edge of the continental shelf. However, while many (but not all) canyons are found offshore from major rivers, subaerial river erosion cannot have been active to

672-427: The continental slope. While at first glance the erosion patterns of submarine canyons may appear to mimic those of river-canyons on land, several markedly different processes have been found to take place at the soil/water interface. Many canyons have been found at depths greater than 2 km (1 mi) below sea level . Some may extend seawards across continental shelves for hundreds of kilometres before reaching

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704-418: The deep sea. The continental shelves are covered by terrigenous sediments ; that is, those derived from erosion of the continents. However, little of the sediment is from current rivers ; some 60–70% of the sediment on the world's shelves is relict sediment , deposited during the last ice age, when sea level was 100–120 m lower than it is now. Sediments usually become increasingly fine with distance from

736-433: The deposits may over geologic time become sources for fossil fuels . The continental shelf is the best understood part of the ocean floor, as it is relatively accessible. Most commercial exploitation of the sea, such as extraction of metallic ore, non-metallic ore, and hydrocarbons , takes place on the continental shelf. Sovereign rights over their continental shelves down to a depth of 100 m (330 ft) or to

768-458: The edge of the continental shelf, whereas the majority (about 68.5%) of submarine canyons have not managed at all to cut significantly across their continental shelves, having their upstream beginnings or "heads" on the continental slope, below the edge of continental shelves. The formation of submarine canyons is believed to occur as the result of at least two main process: 1) erosion by turbidity current erosion; and 2) slumping and mass wasting of

800-618: The forward edge of an advancing oceanic plate dives beneath continental crust in an offshore subduction zone such as off the coast of Chile or the west coast of Sumatra . The largest shelf—the Siberian Shelf in the Arctic Ocean —stretches to 1,500 kilometers (930 mi) in width. The South China Sea lies over another extensive area of continental shelf, the Sunda Shelf , which joins Borneo , Sumatra, and Java to

832-576: The regional complex. It has a considerable number of oil and gas wells, pipelines, production areas and support facilities. As an area it is located in the Indian Ocean between North West Cape and Dampier . Dampier is usually considered the main administrative locality for the shelf. The production areas are located offshore and within the jurisdiction of the Western Australian state government. The two main production areas are

864-507: The same canyon, or at different times during a canyon's development. However, if a primary mechanism must be selected, the downslope lineal morphology of canyons and channels and the transportation of excavated or loose materials of the continental slope over extensive distances require that various kinds of turbidity or density currents act as major participants. In addition to the processes described above, submarine canyons that are especially deep may form by another method. In certain cases,

896-400: The shallower waters and the enhanced current speeds. Despite covering only about 8% of Earth's ocean surface area, shelf seas support 15–20% of global primary productivity . In temperate continental shelf seas, three distinctive oceanographic regimes are found, as a consequence of the interplay between surface heating, lateral buoyancy gradients (due to river inflow), and turbulent mixing by

928-485: The shelf and slope. The continental rise 's gradient is intermediate between the gradients of the slope and the shelf. Under the United Nations Convention on the Law of the Sea , the name continental shelf was given a legal definition as the stretch of the seabed adjacent to the shores of a particular country to which it belongs. The shelf usually ends at a point of increasing slope (called

960-428: The shelf changes dramatically at the shelf break, where the continental slope begins. With a few exceptions, the shelf break is located at a remarkably uniform depth of roughly 140 m (460 ft); this is likely a hallmark of past ice ages, when sea level was lower than it is now. The continental slope is much steeper than the shelf; the average angle is 3°, but it can be as low as 1° or as high as 10°. The slope

992-459: The tides and to a lesser extent the wind. Indian Ocean shelf seas are dominated by major river systems, including the Ganges and Indus rivers. The shelf seas around New Zealand are complicated because the submerged continent of Zealandia creates wide plateaus. Shelf seas around Antarctica and the shores of the Arctic Ocean are influenced by sea ice production and polynya . There

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1024-471: The upper slope fails, perhaps triggered by earthquakes. There is a spectrum of turbidity- or density-current types ranging from " muddy water" to massive mudflow, and evidence of both these end members can be observed in deposits associated with the deeper parts of submarine canyons and channels, such as lobate deposits (mudflow) and levees along channels. Mass wasting , slumping, and submarine landslides are forms of slope failures (the effect of gravity on

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