Lithology(redirected from lithologically)
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the science of present-day sediments and sedimentary rocks, their composition, structure, origin, and laws governing their spatial distribution.
Many sedimentary rocks are useful minerals. They include ores of iron, manganese, aluminum (bauxite), phosphorus (phosphorites), strontium, and nickel; peat, brown and hard coals, graphite, and combustible shales; sands, such as placer gold, platinum, tin, titanium, diamonds, and other valuable minerals; quartz, glass, foundry, and building sands; and refractory clay, brick earth, bentonite, adsorbents, raw material for cement, gypsum, and natural salts. Moreover, sedimentary rocks contain deposits of oil and combustible gas; copper, lead, zinc, and mercury ores; and artesian waters—drinking water, therapeutic mineral waters, and brine. The study of sedimentary rocks makes it possible to determine their stability as a foundation for buildings and other structures. Present-day sediments on the floors of the seas and oceans contain placers of valuable minerals; manganese, nickel, and phosphorus ores; and oil and gas deposits.
Lithology as a separate branch of geology developed in the late 19th and early 20th centuries as a result of the stratigraphic and paleogeographic research related to the study of the material composition of sedimentary rocks and the minerals associated with them. The material collected by the English oceanographic expedition on the ship Challenger (the English scientist J. Murray and the Belgian A. Renard, 1891) and the research of the German geologist J. Walther (1893–94) on questions of sedimentary rock formation were very important for the development of lithology. Owing to the work of Russian (P. A. Zemiatchenskii, la. V. Samoilov, V. N. Chirvinskii, A. P. Karpinskii, A. P. Pavlov) and foreign (the English scientist H. Sorby, the American J. Barrell, the Frenchmen L. Cayeux and W. Vaughan, the German H. Potonié) scientists, lithology emerged as an independent science in the second decade of the 20th century. Major contributions to its subsequent development were made by both Soviet (A. D. Arkhangel’skii, A. N. Zavaritskii, D. V. Nalivkin, M. S. Shvetsov, V. P. Baturin, L. V. Pus-tovalov, N. M. Strakhov, L. B. Rukhin) and foreign (the Americans W. Twenhofel, W. Krumbein, and F. Pettijohn) scientists. Lithology developed with particular intensity in the USSR after the First Lithological Conference (1952), at which the achievements of lithology during the entire preceding period were discussed and a program of further research was outlined.
Modern lithology is closely connected to stratigraphy, tectonics, paleogeography, geochemistry, paleontology, climatology, and the group of physicochemical and mathematical sciences. Lithology’s chief task is to identify the laws governing the distribution of various types of sedimentary rocks and minerals in the general course of the processes of rock formation during the earth’s geological history. This task is accomplished mainly by using genetic (facies) analysis of sedimentary rocks, their natural paragenetic combinations (sedimentary formations), and the paleogeographic conditions of their accumulation. The significant advances made by Soviet scientists in this area have been reflected in a series of atlases of lithological-paleogeographic maps compiled for the USSR as a whole and for particular regions of the country (1960–72). Such atlases have also been prepared for a number of other countries. Their significance is that they give a general prediction of the location of many types of minerals that are associated with sedimentary rocks.
One of the most important functions of lithology is to work out the theory of lithogenesis. A leading place in developing this theory belongs to the Soviet scientist N. M. Strakhov, who developed (1956–63) the theory of the four chief types of lithogenesis (glacial, humid, arid, and volcanogenic-sedimentary) and of their evolution throughout the earth’s history. Important for the development of the theories on the sedimentary process in general were studies of weathering mantles (I. I. Ginzburg, for example) and the conditions of formation and the laws governing the distribution of deposits of sedimentary minerals. Lithology extensively studies present-day sediments and conditions of their formation both on land and on the sea floor (P. L. Bezrukov, A. P. Lisitsyn). The largest number of works on lithology have been devoted to investigating the composition, structure, and texture of sedimentary rocks, the laws governing their distribution, and changes in particular regions. A relatively new area of lithology is the study of ancient (Precambrian) deeply metamorphosed sedimentary rocks (A. V. Sidorenko, for example). These rocks preserve some initial sedimentary features, and identifying them makes it possible to reconstruct the conditions of ancient sediment accumulation in order to clarify the laws governing the history of development of the earth’s crust in the Precambrian and the processes of ore formation related to these laws. One of the very timely tasks of lithology is identifying, using “stage analysis,” secondary changes in sedimentary rocks when they become buried in deeper horizons of the lithosphere. In the 1960’s and 1970’s study of the lithology of volcanogenic-sedimentary rocks, which are distributed primarily in geosynclinal areas, became very widespread (G. S. Dzotsenidze, I. V. Khvorova). Determining the role of volcanogenic-sedimentary lithogenesis in overall rock formation is another important task of lithology.
Lithology employs various methods of investigation. Among them are field research methods, laboratory methods, and generalization. Lithological field work is characterized by more detailed description of the composition and structure of sedimentary rocks, careful observation of organic remains and their degree of preservation, replacement by other minerals, distribution, quantitative ratios, and conditions of burial. Analyses of the composition and other properties of sedimentary rocks are made in boreholes on the basis of an identification of the physical parameters of the rocks of the section (for example, logging diagrams).
Laboratory work includes analytical research and experiments: modeling and investigating the physicochemical conditions of the environment of sediment formation. One of the chief laboratory methods is microscopic analysis employing the polarizing microscope, which has been in use since the 19th century. The immersion method, which is also based on the optical properties of minerals, is used to analyze the mineral composition of rock in grains. Granulometric analysis is used to classify the grains of loose rocks according to granulomere composition. Thermal analysis makes it possible to determine the behavior of minerals composing various sedimentary rocks when they are heated to 1000°C and higher. The changes observed in the process make it possible to determine the composition of these rocks. Electron microscopy and roentgenostructural analysis are used to study finely dispersed rocks (for example, argillaceous, siliceous, and carbonate rocks). The last two methods are particularly important for obtaining a precise diagnosis of the mineral composition of rocks and for investigating changes in certain mineral types at different stages of lithogenesis.
The principal methods of generalization in the genetic study of sedimentary rocks are the methods of facies and formation analyses and the comparative lithological method, which have been worked out especially fully in the USSR using coal-bearing strata as a model (lu. A. Zhemchuzhnikov, G. F. Krashenin-nikov, P. P. Timofeev). For generalizing the material from laboratory research there are a number of methods of expressing the results graphically (various diagrams, cumulative curves, and the like), as well as mathematical statistical techniques that make it possible to use modern computer technology. Lithological columns and facies profiles as well as lithological-facies maps of various scales are compiled on the basis of the results of field and laboratory research.
Lithology is developing intensively in all countries that have geological surveys. In the USSR, all-Union lithological conferences are called periodically. Between 1952 and 1973 there were ten such conferences devoted to the most varied problems of lithology. The Trudy (Proceedings) of the conferences and topical seminars are published regularly. In 1956 the Commission on Sedimentary Rocks was formed in the Division of Geology, Geophysics, and Geochemistry of the Academy of Sciences of the USSR. The journal Litologiia i poleznye iskopaemye (Lithology and Mineral Resources) has been published in the USSR since 1963. Problems in lithology are discussed at sessions of the International Geological Congress, which meets once every three or four years. In 1952 the International Association of Sedimentologists was formed. It organizes congresses on a regular basis, and delegations of Soviet lithologists have attended them since 1958. Among the foreign journals that publish articles on lithology the best known are Journal of Sedimentary Petrology (Tulsa, since 1931), Sedimentary Geology (Amsterdam, since 1967), and Marine Geology (Amsterdam, since 1964).
REFERENCESShvetsov, M. S. Petrografia osadochnykh porod. Moscow, 1958.
Pustovalov, L. V. Petrograflia osadochnykh porod, parts 1–2. Moscow-Leningrad, 1940.
Strakhov, N. M. Osnovy istoricheskoigeologii, parts 1–2. Moscow-Leningrad, 1948.
Strakhov, N. M. Osnovy teorii litogeneza, vols. 1–3. Moscow, 1960–62.
Strakhov, N. M. Tipy litogeneza i ikh evoliutsiia v istorii Zemli. Moscow, 1963.
Strakhov, N. M. Razvitie litogeneticheskikh idei v Rossii i SSSR. Moscow, 1971.
Sostoianie i zadachi sovetskoi litologii, fascs. 1–3. Moscow, 1970.
Krasheninnikov, G. F. Uchenie o fatsiiakh. Moscow, 1971.
Pettijohn, F. J. Sedimentary Rocks, 2nd ed. New York, 1957.
Twenhofel, W. H. Principles of Sedimentation. New York, 1950.
G. F. KRASHENINNIKOV