Lithogenesis(redirected from lithogeny)
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the complex of natural processes in the formation and subsequent changes of sedimentary rocks. The principal factors in lithogenesis are tectonic movements and climate. The concept of lithogenesis was introduced in 1893–94 by J. Walther, who identified five principal phases in the process of the formation of sedimentary rocks: weathering of rocks, denudation (including transportation of the raw material of the sediments), deposition, diagenesis, and metamorphism.
The following stages are distinguished in the cycle of lithogenesis: (1) superficial hypergenesis—the formation and concentration of the raw material of the sediments during the process of mechanical and chemical weathering of parent rocks and the transportation of this material to its burial place; (2) sedimento-génesis—the entry of sediments into terminal runoff basins and final deposition; (3) diagenesis—the physical and chemical stabilizing of the water-saturated sediment, concluding with its conversion into sedimentary rock; (4) catagenesis (sometimes this stage is imprecisely called epigénesis)—further changes in the rock as the depth of its burial increases, as a result of rising temperature and pressure and, in some cases, the effect of water solutions and gases; and (5) metagenesis, or metamorphism proper—further transformation of the composition of rocks, especially clayey rocks, as they are buried deeper. Metagenesis manifests itself most often in geosynclines.
Some investigators (the Soviet geologists N. M. Strakhov and N. V. Logvinenko, for example) include only hypergenesis, sedi-mentogenesis, and diagenesis in lithogenesis and consider metagenesis to be an independent stage between catagenesis and metamorphism.
When a particular sector of the earth’s crust stops subsiding and is uplifted instead, progressive lithogenesis is halted in one of its stages and regressive lithogenesis begins. Regressive lithogenesis concludes with hypergenesis, which is at first concealed or takes place underground (under anaerobic conditions) and, later, on the surface, when rocks are subjected to denudation, which ends one cycle of lithogenesis and begins a new one.
The Soviet geologist N. M. Strakhov was the first (1956) to identify the basic types of lithogenesis: glacial, humid, arid, and volcanogenic-sedimentary, which appear to have existed since post-Riphean times. In glacial lithogenesis the processes of sediment formation take place in sectors of the continents that are covered by ice; lithogenesis occurs in the form of mechanical rock formation with undefined differentiation of the material. Humid lithogenesis is typical of rock formation on land and in seas under conditions of a moist climate. In arid lithogenesis rock formation takes place on the continents and in seas under conditions of an arid climate. Volcanogenic-sedimentary lithogenesis is characterized by rock formation in regions with terrestrial and underwater volcanic activity and in the areas adjacent to them. The first three types of lithogenesis are caused by climate, and therefore their distribution on the earth’s surface is zonal and they are most pronounced on the platforms. Volcanogenic-sedimentary lithogenesis does not depend on climate and occurs throughout the zones, primarily in geosynclinal areas— that is, in the most tectonically active areas. Each type of lithogenesis has a typical combination of sedimentary rocks, which reflects the specific course of mechanical and chemical sedimentary differentiation and also of biogenic processes and volcanic activity.
Since lithogenesis is the process of sedimentary rock formation, the formation of a very large number of the most diverse mineral products, including coal, oil, combustible natural gases, iron and manganese ores, bauxites, phosphorites, and placer deposits, is associated with it.
REFERENCESStrakhov, N. M. Tipy litogeneza i ikh evoliutsiia v istorii Zemli. Moscow, 1963.
Diagenez i katagenez osadochnykh obrazovanii. Moscow, 1971. (Translated from English.)
Vassoevich, N. B. “Eshche o terminakh dlia oboznacheniia stadii i eta-pov litogeneza.” Tr. Vses. neftianogo nauchno-issledovatel’skogo geolo-go-razvedochnogo instituta, 1962, no. 190.
N. B. VASSOEVICH