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off the continental shelf (Figs. 5-33, 5-34) The continental shelf is the shallow ocean surrounding the continent. The depth at the edge of the shelf is usually not more than 100 to 150 meters (the length of one to one-and-a-half football fields). Some sand and mud are carried to the edge of the continental shelf via submarine canyons which are like undersea river valleys. Sediments build up at the edge of the shelf and when too much has accumulated these flow down the continental slope and rise as turbidity currents (like underwater mud flows). The resulting deposits, called turbidites, contain some chaotic, poorly sorted coarse layers at their base and then finer layers on top. Repeated sequences of turbidites indicate deposition on the continental slope and continental rise.
deep abyssal plains (Fig. 5-35) In the deep sea, out on the abyssal plains, the depth to the seafloor varies from about 2.5 to 6 km (2500 to 6000 meters) or more below sea level. The abyssal plains receive very little sediment from the continents. Pelagic clays from windblown dust from the continents and oceanic volcanoes form finely laminated (layered) shales. Biogenic oozes: Calcareous oozes from deposits of single-cell, microscopic organisms with calcite shells result in finely laminated limestone. Siliceous oozes from single-cell, microscopic organisms with silica shells form finely laminated chert (silica) layers. Furthermore, the limestones indicate warm water; limestone dissolves in cold water. Chert indicates high biological productivity and cool water.
fluvio-lacustrine (stream and lake) deposition (Figs. 5-13, 5-15, 5-16) In humid climates on the continents, streams are the primary medium for transport and deposition of sediments. Streams carry sediments from uplifted (mountains) source areas, eventually to the sea. Lowlands are often sites of stream deposition. Meandering streams deposit point bar sands that may be preserved as a sheet of sandstone as a result of the migration of the point bar across the stream valley. Occasional flooding carries suspended silt and clay out of the stream channel and onto the flood plain. As the flood waters recede, the fine sediments are deposited in sheets in the backswamp area. These so-called overbank deposits may be preserved as layers of shale. The sandstones and shales formed in a stream valley will contain terrestrial and fresh water fossils, not marine fossils. The overbank shales often contain mudcracks that form when the floods recede and the clay dries out (Fig. 5-10) and raindrop impressions. Assymetric or current ripple marks also indicate deposition in a stream. Lakes normally have muddy bottoms and perhaps a narrow shoreline of sand and gravel. Shales with fossils of fresh-water organisms are commonly formed in lakes.
Fine particles, clay and silt are picked up as windblown dust (analogous to the suspended load in stream transport). The dust will eventually settle in an area adjacent to the desert in a more humid area with sufficient vegetation to protect sediments from further wind transport. The resulting deposits are called loess.
All along a coast, sediments derived from longshore drift and sediments formed in place from wave action are distributed by wave energy. Wave action is strongest at the ocean surface and decreases with depth in the water down to a depth of half the wavelength (L/2). Because of this, in shallow water near the shore the fine sediments are washed away as suspended load. Only coarse sediments are deposited in shallow water. As the depth to the bottom increases, the bottom is stirred less and less by wave action; progressively finer sediments can be deposited in increasingly deeper water. Deposited sediments progress from sands near the shore to silts and clays farther offshore. In cool or turbid (murky) water, fine sediments will dominate to the edge of the continental shelf.