America and from the Ganges and Indus rivers that drain the Himalaya and Tibet. Accumulations of rock as much as 15 km thick can develop along Atlantic-type margins. These, the greatest thicknesses of sediments in the world, are accommodated by subsidence as the lithosphere—thin and hot at continental rupture—cools and thickens. The load of accumulating sediments depresses the lithosphere farther and amplifies this subsidence.

Broad continental shelves extending for many tens of kilometers from the edge of the ocean at depths of only a few tens of meters below sea level are characteristic of Atlantic-type oceanic margins, because of deposition by powerful river systems and perhaps because of extensive lithospheric thinning at the time of continental rupture. Those off the coast of New England and the Alaskan coast of the Bering Sea are typical. The recent sea level changes that mark responses to glaciation and deglaciation have led to repeated erosive episodes of the unconsolidated sediments of continental shelves. Large masses of such sediments have been off-loaded through submarine canyons onto deep-sea fans and abyssal plains.

Limestone shelves develop where there is little sediment eroded from the land and in areas of abundant biological activity. Sediments originate mainly from the calcareous skeletons of shallow-water, bottom-dwelling marine organisms. These limestone shelves respond to sea level change in a way very different from sand and mud shelves. When sea level falls, exposure to fresh water as rain or runoff produces cementation that binds the loose skeletal sediment. As a result, carbonate-dominated banks and shelves reflect conditions of deposition during high stands of the sea and of subsequent cementation during low stands. Much of Florida and all of the Bahama banks were produced by this process.

Submarine canyons carry sediments from the continental shelves to the deep oceans and the submarine fans, where that sediment settles. When they were first recognized about 50 years ago, the prime question asked was how such enormous features could form. Computation of the huge volume of sediments in the fans showed that turbid sediment-laden flows pouring from the continental shelves in times of glacially controlled low stands of the sea could have readily carved even the greatest of submarine canyons, many of which are much larger than the Grand Canyon. Modern research on both submarine fans and canyons is accelerating because of the availability of new instrumental capabilities. Deep-sea drilling, multibeam echo sounders, side-look scanning sonars, and manned and remotely controlled submersibles are providing a much more detailed picture than was formerly obtainable.

Research on the sedimentary development of the Atlantic-type margins has expanded enormously during the past decade, largely in response to two stimuli: an appreciation, following the plate tectonic revolution, of how continental rupture happens and an understanding of how the sediment wedge at the continental margin evolves through time. The latter owes much to oil exploration, which led to the development of the technique of sequence stratigraphy, where coherent packages of distinctive strata in reflection seismic data—calibrated against the record of local oil wells—can be used to establish a detailed history of the transgression and regression of the sea. Lively controversy persists as to exactly how and whether the seismic stratigraphic records can be linked to global sea level fluctuations.

Deposition at Convergent Plate Boundaries

The greatest variations in topographic relief are produced at convergent plate boundaries. The giant peaks of the Himalaya, 8 km high, result from the collision of India with Asia, and the 11-km extreme of oceanic depth is found in the Marianas Trench, where subduction is carrying the Pacific plate into the mantle. Two dominant sedimentary depositional environments are associated with these immense topographic contrasts: trenches and foreland basins.

Subduction-zone trenches contain substantial sediment accumulations only where the supply of sediments is large enough to exceed the rate of removal by subduction (Figure 3.6). This type of accumulation is occurring, for example, at the eastern end of the Aleutian Trench close to sediment sources in the North American continent and at the southern end of the Lesser Antilles Trench close to the South American continent at the mouth of the Orinoco River. Sediments accumulate at the front of the related arc system to form a thick wedge, or accretionary prism, that extends along the trench. An exciting challenge—currently being addressed by deep-sea drilling, multibeam echo sounding, and other new techniques—is to establish exactly how unconsolidated sediment entering a deep-sea trench becomes solid rock in the accretionary prism, a process that involves both intense deformation and the expulsion of vast quantities of water.

The other sites of substantial sediment accumulation in convergent plate boundary zones are the



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