The diagenesis is (Greek δια "through" and γένεσις "origin") of the geological process the consolidation of sediments , and the other change in the resulting characterized sedimentary rocks under relatively low pressures and temperatures up to its removal . It is part of the rock cycle .
Diagenesis, in the sense of converting loose sediment into solid rock, essentially comprises two processes: compaction and cementation . Early diagnostic processes exclusively involve cementation and take place shortly after the deposition. During cementation, crystals form in the pore space of the sediment through the separation of salts from the water in the pore space. The crystals can consist of minerals that are already present in the sediment, or of new, previously non-existent minerals ( authigenic minerals ). A typical cement mineral that is often formed early in the diagnosis is calcite . Typical minerals newly formed early in the diagnosis are glauconite and francolite (a carbonate apatite ). Usually, however, diagenesis only begins after the sediment has subsided to a certain depth. The increasing lithostatic pressure compacts and drains the sediment to a greater or lesser extent, depending on the material and the grain size. Clay sediments are compacted much more strongly than sands . The pressure causes a pressure solution (Riecke's principle) at the grain boundaries , which mobilizes additional material for cementation. It is not uncommon for this material to precipitate again immediately in the immediately adjacent pore space. Loose sand gradually becomes solid sandstone , and clay becomes mudstone . The diagenesis story of a rock does not end there. As long as a rock body is outside the influence of weathering and erosion on the one hand and metamorphic processes (see below ) on the other hand, diagenetic processes can start over and over again at any time and change the rock ( late diagnosis ).
Examples of special diagenetic phenomena are the formation of flint lumps from dissolved silica in certain limestones or the formation of fossils from animal and plant remains ( fossilization , also known as fossil diagenesis ).
Diagenesis is not limited to mineral rock components. The conversion of organic matter into kerogens and the subsequent conversion into crude oil and natural gas or the conversion of vegetable matter into peat and further into coal are among the diagenetic processes for which the increasing temperatures (→ geothermal gradient ) of Meaning are.
Difference to metamorphosis
During metamorphosis , a rock transformation also takes place depending on pressure and temperature, but it takes place at a significantly greater crust depth at significantly higher pressures and temperatures. As a result, during metamorphosis there are usually significant changes in the mineral inventory and often also in the rock structure that do not take place during diagenesis. The so-called anchimetamorphosis , which takes place at relatively low pressures and temperatures, can be understood as a transition form between diagenesis and "real" metamorphosis.
- Hans Murawski, Wilhelm Meyer: Geological dictionary . 10th edition. MVS Medizinverlage, Stuttgart 1998, ISBN 3-432-84100-0 .