Editing Limestone (section)

===Diagenesis===
[[Diagenesis]] is the process in which sediments are compacted and [[lithification|turned into solid rock]]. During diagenesis of carbonate sediments, significant chemical and textural changes take place. For example, aragonite is converted to low-magnesium calcite. Diagenesis is the likely origin of ''pisoliths'', concentrically layered particles ranging from {{convert|1|to|10|mm|in|abbr=in}} in diameter found in some limestones. Pisoliths superficially resemble ooids but have no nucleus of foreign matter, fit together tightly, and show other signs that they formed after the original deposition of the sediments.{{sfn|Blatt|Middleton|Murray|1980|pp=497–501}}

[[File:Çört yumrusu, Chert.2.jpg|thumb|Chert nodule within soft limestone at [[Akçakoca]], Turkey]]
[[File:Stylolites mcr1.jpg|thumb|right|[[Stylolite]]s in limestone]]
Silicification occurs early in diagenesis, at low pH and temperature, and contributes to fossil preservation.<ref name="Götz-2017">{{Cite journal |last1=Götz |first1=Annette E. |last2=Montenari |first2=Michael |last3=Costin |first3=Gelu |date=2017 |title=Silicification and organic matter preservation in the Anisian Muschelkalk: Implications for the basin dynamics of the central European Muschelkalk Sea |url=https://akjournals.com/view/journals/24/60/1/article-p35.xml |journal=Central European Geology |volume=60 |issue=1 |pages=35–52 |doi=10.1556/24.60.2017.002 |bibcode=2017CEJGl..60...35G |issn=1788-2281 |doi-access=free}}</ref> Silicification takes place through the reaction:<ref name="Götz-2017" />

:{{chem2|CaCO3 + H2O + CO2 + H4SiO4 -> SiO2 + Ca(2+) + 2 HCO3- + 2 H2O }}

Fossils are often preserved in exquisite detail as chert.<ref name="Götz-2017" />{{sfn|Blatt|Middleton|Murray|1980|p=497–503}}

Cementing takes place rapidly in carbonate sediments, typically within less than a million years of deposition. Some cementing occurs while the sediments are still under water, forming [[hardground]]s. Cementing accelerates after the retreat of the sea from the depositional environment, as rainwater infiltrates the sediment beds, often within just a few thousand years. As rainwater mixes with groundwater, aragonite and high-magnesium calcite are converted to low-calcium calcite. Cementing of thick carbonate deposits by rainwater may commence even before the retreat of the sea, as rainwater can infiltrate over {{convert|100|km|mi|abbr=in|sigfig=1}} into sediments beneath the continental shelf.{{sfn|Blatt|Tracy|1996|p=312}}

As carbonate sediments are increasingly deeply buried under younger sediments, chemical and mechanical compaction of the sediments increases. Chemical compaction takes place by ''[[pressure solution]]'' of the sediments. This process dissolves minerals from points of contact between grains and redeposits it in pore space, reducing the porosity of the limestone from an initial high value of 40% to 80% to less than 10%.{{sfn|Blatt|Middleton|Murray|1980|pp=507-509}} Pressure solution produces distinctive [[stylolite]]s, irregular surfaces within the limestone at which silica-rich sediments accumulate. These may reflect dissolution and loss of a considerable fraction of the limestone bed. At depths greater than {{convert|1|km|mi|abbr=in}}, burial cementation completes the lithification process. Burial cementation does not produce stylolites.{{sfn|Blatt|Tracy|1996|p=312–316}}

When overlying beds are eroded, bringing limestone closer to the surface, the final stage of diagenesis takes place. This produces ''secondary porosity'' as some of the cement is dissolved by rainwater infiltrating the beds. This may include the formation of [[vug]]s, which are crystal-lined cavities within the limestone.{{sfn|Blatt|Tracy|1996|p=312–316}}

Diagenesis may include conversion of limestone to dolomite by magnesium-rich fluids. There is considerable evidence of replacement of limestone by dolomite, including sharp replacement boundaries that cut across bedding.{{sfn|Boggs|2006|pp=186-187}} The process of [[dolomitization]] remains an area of active research,<ref name="machel-2004">{{cite journal |last1=Machel |first1=Hans G. |title=Concepts and models of dolomitization: a critical reappraisal |journal=Geological Society, London, Special Publications |date=2004 |volume=235 |issue=1 |pages=7–63 |doi=10.1144/GSL.SP.2004.235.01.02|bibcode=2004GSLSP.235....7M |s2cid=131159219 }}</ref> but possible mechanisms include exposure to concentrated brines in hot environments (''evaporative reflux'') or exposure to diluted seawater in delta or estuary environments (''Dorag dolomitization'').{{sfn|Blatt|Middleton|Murray|1980|pp=512-528}} However, Dorag dolomitization has fallen into disfavor as a mechanism for dolomitization,<ref>{{cite journal |last1=Luczaj |first1=John A. |title=Evidence against the Dorag (mixing-zone) model for dolomitization along the Wisconsin arch ― A case for hydrothermal diagenesis |journal=AAPG Bulletin |date=November 2006 |volume=90 |issue=11 |pages=1719–1738 |doi=10.1306/01130605077|bibcode=2006BAAPG..90.1719L }}</ref> with one 2004 review paper describing it bluntly as "a myth".<ref name="machel-2004"/> Ordinary seawater is capable of converting calcite to dolomite, if the seawater is regularly flushed through the rock, as by the ebb and flow of tides (tidal pumping).{{sfn|Boggs|2006|pp=186-187}} Once dolomitization begins, it proceeds rapidly, so that there is very little carbonate rock containing mixed calcite and dolomite. Carbonate rock tends to be either almost all calcite/aragonite or almost all dolomite.{{sfn|Blatt|Middleton|Murray|1980|pp=512-528}}