The term diagenesis covers the whole array of physical and chemical processes modifying a freshly deposited sediment, and transforming it into a solid rock. Being highly diverse in extent and nature, diagenetic phenomena depend mainly on composition and original grain size of the sediment, on the relationship along the water-sediment interface, on the porosity and particularly on the permeability of the material subjected to lithification. Another important factor is the evolution of temperature and pressure during burial or tectonic activity. Water, expelled at different rates during compaction, plays an important role as a storage or transporting medium for dissolved matter. The various mechanisms of diagenetic transformation can be identified from petrographic, mineralogical, and geochemical modifications of the original paragenetic assemblage, from chemical transformation within the cement as reconstructed from stable-isotope data, and by the localization of the sediment within stable ranges of Eh and pH. The presence of HS-, HCO 3 - , and metal cations in the pore waters is also of indicative value, as are thermodynamic considerations. The various constituents of a sedimentary rock — carbonates, silica, clay, organic matter, etc. — are modified at quite different rates under the influence of rather diverse physico-chemical factors. Many rocks have experienced incomplete diagenetic evolution, in some cases even without notable lithification. There are very old rocks which have been subjected to only very small postdepositional modification, such as the plastic Leningrad clays of Precambrian age.
KeywordsZeolite Hydrocarbon Compaction Olivine Devonian
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