Diagenesis

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diagenesis

[‚dī·ə′jen·ə·səs]
(geology)
Chemical and physical changes occurring in sediments during and after their deposition but before consolidation.

Diagenesis

 

the natural processes by which loose sediments on the floor of water basins are transformed into sedimentary rock under the conditions of the upper zone of the earth’s crust. The concept of diagenesis was introduced by the German geologist W. von Gümbel (1888), who included within the concept all the changes in the sediment from its initial form to its conversion into metamorphic rock. Later the concept of diagenesis was narrowed so as to denote only the conversion of sediment into sedimentary rock proper (the German geologist J. Walther, the Soviet geologist A. E. Fersman, and others). Subsequent transformations of sedimentary rock belong to the stages of katagenesis and metagenesis.

Diagenesis is thought to be a stage in the physicochemical balancing of sediment, which initially is an unbalanced physicochemical system that is highly inundated and rich in both living (bacteria) and dead organic matter. In the very earliest stage of the balancing process the organisms absorb the free oxygen of the silty water; this is followed by the reduction of the oxides of Fe3+, Mn4+, V5+, and others, as well as of SO42-. The environment changes from an oxidizing one to a reducing one. The solid biogenic phases of SiO2, CaCO3, MgCO3, SrCO3, and other substances present in the sediment, which have been in prolonged contact with water not saturated with them, are gradually dissolved until the stage of full saturation of the solution is reached. At the same time exchange takes place between the positively charged ions found in an absorbed state on the micelles of argillaceous minerals and the positively charged ions of the silty water. As it decomposes, the organic matter is partially converted into gases (CO2, NH3, H2S, N2, and CH4), water-soluble compounds, which accumulate in the water, and more stable compounds, which are preserved in the solid phase of the sediment. As a result of these processes the water solution saturating the sediment, especially argillaceous sediment, changes its composition. The sulfate content in the water solution is sharply decreased, alkalinity is increased, and enrichment by Fe2+, Mn2+, SiO2 and organic matter occurs. H2S, CH4, CO2, NH3, H2, and others accumulate in place of O2. In this way a geochemical world arises that differs sharply from the geochemical world of the water above the floor.

Two processes accompany the formation of specific silt solutions. The first is an exchange of substances between the water above the floor and silt solution. The O2 and SO42- (and together with the sulfate ion Ca2+ and Mg2-) that disappear from the sediment during diagenesis diffuse intensively into the silt solution from the water above the floor and are absorbed by the silt. The gases (CO2, NH3, and others) that have accumulated in the silts diffuse slowly into the water above the floor together with the Fe2+, Mn2+, SiO2, CaCO3 and other components. The exchange of substances encompasses the sediment to a depth of 2 to 4 m. As a result of the second process, which occurs only in the silts, those combinations of ions that supersaturate the silty water are precipitated from it. Authigenic minerals are formed, such as glauconite, phosphorites, siderite, rhodochrosite, sulfides of Fe, Pb, Zn, and Cu, vivianite, and zeolites. Two stages of diagenetic mineral formation may be distinguished: the oxidizing stage, which refers to the uppermost film of the sediment still containing free O2, and the reducing stage, which encompasses the deeper layers containing no O2 and characterized by reduction processes.

The process of balancing in sediments does not, however, end with the formation of diagenetic minerals. Variations in the physicochemical condition (in terms of pH, Eh, and concentration of ions) of different parts of the sediment lead to a redistribution of newly formed diagenetic minerals. Nodules of them form: patches, lenses, concretions, strata-like bodies, and so on. This later stage of diagenesis has been named the stage of redistribution of matter. It is very important in the formation of the ore deposits of many elements, including P, Mn, Pb, Cu, and Zn. Simultaneously with the formation of diagenetic minerals the sediment loses its free water and becomes somewhat more compact, at first locally and in patches. Subsequently, in the later stages of katagenesis and metagenesis the sediment becomes completely compact.

REFERENCES

Strakhov, N. M. “Diagenez osadkov i ego znachenie dlia osadochnogo rudoobrazovaniia.” Izv. AN SSSR. Seriia geologicheskaya, 1953, no. 5.
Strakhov, N. M. Osnovy teorii litogeneza, 2nd ed., vol. 2. Moscow, 1962.

N. M. STRAKHOV

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