Diagenesis is the sum of all chemical, physical, and biological changes that occur to sedimentary materials after deposition but before lithification (conversion to sedimentary rocks). Diagenesis occurs at pressures and temperatures lower than those required for the formation of metamorphic rocks and can be broken down into early and late diagenesis, although some workers restrict the term solely to early diagenesis.
Early diagenesis occurs in the upper portions of sediments (upper ~1 m to several 100 m) where temperatures are less than ~50 °C. Sediment pore spaces are water-saturated, although in some cases gas (methane) bubbles may occur. During diagenesis, compaction with burial decreases sediment porosity from ~50% to 90–95% in surface sediments to 20% (or less) within 1–2 km of burial (Burdige 2006).
The lithogenic and biogenic materials initially deposited in sediments are involved in a number of early diagenetic reactions. In the latter case, this...
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