Imperial College Rock Library

Glossary: Diagenesis

Diagenesis is the process by which sediments are lithified into sedimentary rocks and represents the sum of physical and chemical changes that occur during burial. Diagenesis involves physical compaction of components due to pressure increase on burial, the precipitation of mineral cements from pore fluids and phase transformations of mineral components.
Compaction of sediments leads to decreases in porosity, and can result in pressure solution and interpenetration of grains. Fragmentation of component grains can also occur during compaction. Burial results in compaction due to increase in lithostatic pressure, however, syn-depositional tectonic stresses can also lead to compaction.
Precipitation and dissolution of phases by pore fluids also occurs during diagenesis. Precipitation leads to the formation of a cement that lithifies the sediment and binds together components. Quartz, feldspar, clay-minerals, zeolites and carbonate minerals all form common cements. Dolomitisation is a diagenetic process occuring in carbonate rocks involving the transformation of calcite to dolomite.
The transformation of phases in response to changes in pressure and temperature alters the mineralogy of sediments. Clay minerals undergo transformations during burial, often with the formation of illite. Simultaneous alignment of clay minerals in mudrocks leads to the formation of a fissile fabric and produles shale. Aragonite undergoes transformation to calcite and organics undergo maturation to more refractory organic compounds.
Changes in rocks due to metamorphism are not considered to be diagenetic, however, in reality there is no distinct boundary between diagenesis and burial metamorphism.

Related Terms

metamorphism, pressure solution

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