Diagenen

Diagenen, or diagenesis, refers to the physical, chemical, and biological changes that sediments experience after deposition and before metamorphism. During diagenen, loose sediments are compacted, cemented, and chemically altered, eventually becoming solid sedimentary rock if cementation and lithification proceed.

Main diagenetic processes include compaction reducing porosity; cementation by precipitation of minerals from pore fluids (commonly calcite, silica, or phosphate); dissolution and replacement of grains; authigenesis, especially the formation of new clay minerals such as illite; and recrystallization leading to changes in mineral structure. Fluid composition, pressure, temperature, and microbial activity influence these changes.

Diagenen operates at relatively low temperatures and shallow depths, typically in burial zones up to roughly 150–200°C, driven by overburden pressure, pore-water chemistry, and biological activity. Over time, deeper burial drives further diagenetic change and may transition toward low-grade metamorphism if conditions rise sufficiently.

The effects of diagenen are critical for sedimentary rock properties, notably porosity and permeability, which control reservoirs for groundwater and hydrocarbons. Common diagenetic alterations include calcite and silica cementation, dolomitization, illitization of clays, pyritization, and dissolution features. Diagenetic textures and mineral assemblages provide clues to burial history, fluid flow, and the chemistry of ancient environments.