FIGURE 3.6 Accretionary (top) and erosional (bottom) end-members of convergent margins. Accretionary prisms form only where sufficient sediment is supplied to a trench.

foreland basins that develop next to continental margin mountain belts in areas where the load of the mountains depresses the lithosphere profoundly. Sedimentary thicknesses in the foreland basin depressions are huge and may rival those of the Atlantic-type margins, especially in places where the mountains are being actively thrust over the basin. At present, many of the best-developed foreland basins are accumulating sediments above sea level. This is the case in the finest examples of all, the foreland basins lying to the east of the 5,000-km-long Andean chain, which has been rising for the past 3-million-years.

During most of the geological past, when sea level was higher, foreland basins accumulated marine sediments. Recent research in foreland basins emphasizes an integrated approach that models how episodes of uplift in the mountain belts modify sediment supply and interact with sea level changes and with thrusting of the mountain load over the basin. The methods of sequence stratigraphy address these problems. Further pursuits in oil exploration have led to studies of how fluids migrate through the foreland basins for distances of up to hundreds of kilometers.

Deposition in the Deep Ocean

Very little of the material eroded from the continents reaches the central areas of the oceans, and much of what does is in the form of windblown dust. In these remote regions, far from the continents, accumulation of the skeletons of microorganisms that live in the oceanic waters dominates the depositional process. While calcareous skeletons are most important at shallower depths, they dissolve in the deepest and coldest waters faster than they can accumulate. Beneath the deepest waters siliceous skeletons form a significant part of the sediment pile.

The calcareous sediments located around the ocean's abyssal plains are proving to contain an astonishingly informative record of the history of the water masses of the world ocean. The oxygen-isotopic compositions of the skeletons of foraminifera, which make up most of the calcareous oozes, reflect the isotopic composition of the water in which they lived. The ratios between different isotopes reflect the size of the world's ice sheets and the temperature and salinity of the water in which the organisms grew. Cores from the deep seafloor show

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