Sedimentary Rocks


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Sedimentary Rocks

 

rocks that have developed through the deposition of matter in water or sometimes from the air or through glacial action on the earth’s surface and in sea and ocean basins. Sedimentation may occur mechanically under the influence of gravity and changes in the dynamics of the environment, chemically in water solutions when they reach the saturation point and as a result of exchange reactions, or biogenically through the action of living organisms. Sedimentary rocks are subdivided into detrital, chemical, and biogenic rocks, depending on the type of sedimentation.

The sources of matter for the formation of sedimentary rocks include the products of weathering of the magmatic, meta-morphic, and more ancient sedimentary rocks that make up the earth’s crust; components dissolved in water; atmospheric gases; products resulting from the vital activity of organisms; and volcanic material, both solid particles blown from volcanoes and hot water solutions and gases brought to the earth’s surface and into water basins by volcanic eruptions. Material from outer space, such as fine grains of plessite, silicate grains, and magnetite crystals, are also found in present-day oceanic sediment (deep-water red clay, silt) and in ancient sedimentary rocks. In addition, the composition of sedimentary rocks usually includes plant and animal remains that were accumulated at the time of the rock formation or that originated in more ancient times and were redeposited. Such sedimentary rocks as limestone, coal, and diatomite are composed entirely of organic sediments. The size, shape, and combination of the particles (grains) determine the structure of sedimentary rocks.

Sedimentary rocks form strata, layers, lenses, and other geological bodies of various shapes and sizes that occur in a horizontal position, on an incline, or as complex folds. The internal structure of these bodies, determined by the orientation and arrangement of the grains and by the way in which the space is filled, is called the texture of the sedimentary rock. Most of these rocks have a layered texture, and the texture depends on the conditions of formation, primarily on the dynamics of the environment.

Sedimentary rocks are formed when the initial products derived from the decomposition of the parent rock are transported by water, wind, or glaciers and are deposited on land or in water basins. A loose and porous water-saturated sediment made up of various components is formed. It constitutes an unbalanced, complex physicochemical (and partially biological) system that gradually, over time, becomes sedimentary rock.

The classification of sedimentary rocks is based on their composition and origin. Because most of the rocks are polygenic, that is, they may be formed by several different processes (for example, limestones may be detrital, chemogenic, or organogenic), their composition is taken into account in establishing the basic groups. More than ten groups of sedimentary rocks have been identified, including detrital, clay, glauconitic, aluminous, ferruginous, magnesian, phosphate, silicate, and carbonate rocks; salts; and caustobioliths. In addition to the basic groups, there are rocks of mixed composition that are intermediate between detrital and carbonate rocks, carbonate and silicate rocks, and so on, as well as volcanic-sedimentary rocks, consisting of a mixture of detrital-sedimentary material and the solid products of volcanic eruptions. Each group may be further broken down according to structure (size of grains), mineral composition, and origin.

In terms of chemical composition sedimentary rocks differ from magmatic rocks in being more differentiated, in having a greater variety of rock-forming components, in containing more water, carbon dioxide, organic carbon, calcium, sulfur, and haloids, and in having high ratios of ferric iron to ferrous iron.

The most extensive sedimentary rocks are argillaceous rocks (clays, argillites, and clay shales), sandy rocks (sands and sandstones), and carbonate rocks (limestones, dolomites). Argillaceous rocks account for about 50 percent of all sedimentary rocks and sandy and carbonate rocks for about 45 percent, equally divided; the other types of sedimentary rock account for less than 5 percent.

The formation and location of sedimentary rocks on the earth’s surface are determined chiefly by climatic and tectonic conditions. In regions with a humid and warm climate, aluminous, ferruginous, and magnesian rocks and various caustobioliths are formed. Deposits of dolomite, gypsum, halite, potassium salts, and red rocks are typical in arid regions, and various detrital rocks—the products of physical weathering— are common in polar and high-mountain areas.

The influence of tectonic conditions is equally important. The thick strata of sedimentary rocks that accumulate in geosyn-clines usually show spatial variability, are composed of various kinds of detrital and other material, and include layers of volcanic-sedimentary rock. In contrast, platforms have thin strata of sedimentary rock, often with layers that extend over large areas and with homogeneous detrital material.

Because the conditions under which sediment accumulation occurred in past geological ages, particularly in the Phanerozoic, were similar to those of today, the present location of various types of rock on the earth’s surface enables us to reconstruct the earth’s paleogeographic and paleotectonic history.

Sediment and rock formation is a periodic process; the formation of similar rocks and their paragenetic associations (formations) recurs over time. This is related to periodic (long-term) climatic changes and geotectonic movements. Moreover, there has been a gradual change in the conditions of sediment accumulation in the course of the development of the earth’s crust. The evolution of sediment accumulation is associated with a change in the composition of the ocean water and the atmosphere, with the evolution of the organic world, with transformations in the structure of the earth’s crust, and with an increase in the total amount of sedimentary rock on the earth’s surface.

Sedimentary rocks constitute about 10 percent of the earth’s crust by volume but cover 75 percent of the earth’s surface. Most of the rocks are concentrated on continents (500 million cu km) and continental slopes (190 million cu km); the ocean floor accounts for 250 million cu km. On continents about 75 percent by volume of all sedimentary rock is confined to geosynclinal regions, and about 25 percent is associated with platforms. More than 75 percent of the useful minerals extracted from the earth’s interior are contained in sedimentary rock. These include coal, petroleum, salts, ores of iron, manganese, and aluminum, placers of gold and platinum, phosphorites, and building materials.

REFERENCES

Rosenbusch, G. Opisatel’naia petrografiia. Moscow, 1934. (Translated from German.)
Strakhov, N. M. Osnovy teorii litogeneza, parts 1–2. Moscow, 1960–62. Ronov, A. B., and A. A. Iaroshevskii. “Khimicheskoe stroenie zemnoi kory.” Geokhimiia, 1967, no. 11.
Rukhin, L. B. Osnovy litologii, 3rd ed. Leningrad, 1969. Logvinenko, N. V. Petrografiia osadochnykh porod, 2nd ed. Moscow, 1974.

N. V. LOGVINENKO

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