What Is Diagenesis in Geology?

Diagenesis is the name for a wide range of changes that affect sediments during their progress to become sedimentary rocks: after they are laid down, while they are becoming rock, and before they first undergo metamorphism. It does not include weathering, the processes that turn all kinds of rock into sediment. Diagenesis is sometimes divided into early and late phases. 

Examples of Early Phase Diagenesis

Early diagenesis covers everything that may happen after sediment is laid down (deposition) until it first becomes rock (consolidation). Processes in this stage are mechanical (reworking, compaction), chemical (dissolution/precipitation, cementation), and organic (soil formation, bioturbation, bacterial action). Lithification takes place during early diagenesis. Russian geologists and some American geologists restrict the term "diagenesis" to this early stage.

Examples of Late Phase Diagenesis

Late diagenesis, or epigenesis, covers everything that may happen to sedimentary rock between consolidation and the lowest stage of metamorphism. Emplacement of sedimentary dikes, growth of new minerals (authigenesis), and various low-temperature chemical changes (hydration, dolomitization) mark this stage.

What's the Difference Between Diagenesis and Metamorphism?

There isn't an official boundary between diagenesis and metamorphism, but many geologists set the line at about 1-kilobar pressure, corresponding to depths of a few kilometers, or temperatures over 100 C. Processes such as petroleum generation, hydrothermal activity, and vein emplacement occur in this borderline region.