Diagenesis is a collection of physical and chemical processes that act on sedimentary grains below the surface of the sediment.
In the diagenesis stage, the sediment is compacted, its moisture content decreases, the formation of new minerals from the silt solutions, the redistribution of the substance in the sediment. The newly formed minerals are called authigenic , unlike allotigenic, ie, brought from the outside.
The composition of authigenic minerals formed in early diagenesis depends on the physicochemical conditions of the medium in the sediment, the concentration of ions in the silt or pore waters, the quantity and nature of the organic matter.
In the clay sediments of marine basins enriched with organic matter, pyrites ReB are formed: in the form of small crystals.
<?> Glauconites, chlorites, phosphorites are formed in shallow clastic and clay sediments of seas of normal salinity containing an insignificant amount of organic matter.
oxide and hydroxide mineralsaccumulate in coastal marine porous sediments
In the sediments of freshened inland seas and land basins , siderites FeCO3 are formed in the form of oolites and spherulites. Especially often siderites are formed in the thickness of delta swamp sediments.
In the terrigenous sediments of the humid zone (deluvium, proluvium, alluvium), , which often overlap with water, scattered clay minerals are deposited in the pore space, and various ions are transferred from the condensed silts. They, together with clay minerals form cement precipitation.
In the sandy sediments of the arid zone , in the presence of a large amount of Ee and oxidizing conditions, oxides of Fe - limonites and hematites are formed.
In the late stage of diagenesis in terrigenous and clay sediments, due to the uneven concentration of mud solutions in different parts of the sediment, a redistribution of diagenetic minerals occurs: their dissolution in some places and deposition in others. Mineral concretions are formed - concretions - minerals strata that have a composition that differs from the host rock, formed as a result of precipitation from aqueous solutions within the rock.
Stage of catagenesis
Catagenesis is a long-term stage of secondary changes in sedimentary rock following the stage of diagenesis.
The main acting factors of catagenesis are:
• dissolved mineral and gaseous substances;
• the pH and Eh of the reservoir waters;
• Radioactive radiation;
• duration of action of these factors, i.e. geological time.
At the stage of catagenesis, condensation occurs, the dissolution of composite
parts of the rock, various mineral formation processes, recrystallization and other changes in sedimentary rocks.
Metagenesis is the stage of a deep mineral and structural transformation of rocks occurring at a considerable depth, preceding the transformation of the sedimentary rock into a metamorphic rock.
The main factors of metagenesis are the same as for catagenesis, but with large numerical values:
• Groundwater with dissolved salts and gases;
• Other pH values (the environment is more acidic) and Eb (the medium is more reducing).
In the course of metagenesis, the rocks are compacted , their porosity becomes minimal. The movement of fluids becomes possible only through cracks or diffusion. With tectonic movements in rocks, there is a thin fracturing, thanks to this new ways are created for the migration of solutions.
At this stage, clays are transformed into mudstones; Regeneration of quartz is maximally developed; the structures of adaptation (conformal) and the introduction of grains into each other (incorporation), as well as the processes of recrystallization, are widely developed.