Catagenesis


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Catagenesis

 

(in geology), the natural processes that alter sedimentary rock after it has developed from sediments by dia-genesis and before it has become metamorphic rock. Some geologists, such as N. M. Strakhov, distinguish metagenesis, a stage between catagenesis and metamorphism. The term “catagenesis” was introduced in 1922 by A. E. Fersman to designate the chemical transformations in rock after it has been covered by layers of new sediment. Since then it has gradually come to replace other names for postdiagenetic processes, for example, epigenesis.

The chief factors in catagenesis (including the stage of metagenesis as interpreted by M. N. Strakhov) are temperatures rising to 300°-350°C at a depth of 8–12 km, at the border of the metamorphic zone; pressure increasing to 180–290 meganewtons per sq m (1, 800–2, 900 atmospheres) at these depths; and pore water (solutions), which interacts with the rock saturated with it.

An important result of catagenesis is rock consolidation, which initially occurs without the destruction of its structure and later with its destruction. By the end of the stage the porosity of sandstones, aleurolites, and argillites usually does not exceed 1–2 percent. Initially free, and later bound, water is squeezed out and removed. The mineral composition of terrigenous rock undergoes changes that intensify with depth and age. Some minerals are dissolved, while others precipitate. There is extensive regenerative growth of quartz granules, chloritization, albitiza-tion, and zeolitization. New structures arise, such as microstylo-litic seams, and the forcing of some sandstone granules into others also occurs. Organic matter, losing C02 and hydrocarbons, is transformed until it reaches the semianthracite stages.

A knowledge of the principles of catagenesis is of great practical significance, for example, in evaluating the probability of discovering petroleum in sedimentary strata and in forecasting the properties (grades) of fossil coal and building materials.

REFERENCES

Fersman, A. E. Geokhimiia Rossii. Petrograd, 1922.
Diagenez i katagenez osadochnykh obrazovanii. Moscow, 1971. (Translated from English.)

N. B. VASSOEVICH