Editing Chalk (section)

==Formation==
In Western Europe, chalk was formed in the Late [[Cretaceous]] Epoch and the early [[Palaeocene]] Epoch (between 100 and 61 million years ago).<ref name=chalkeast>{{cite web|url=http://www.geo-east.org.uk/special_projects/chalk.htm|archive-date=3 June 2012|archive-url= https://web.archive.org/web/20120603234302/http://www.geo-east.org.uk/special_projects/chalk.htm|title=Introducing the Chalk|website=Chalk East}}</ref><ref name="VanDerVoert_etal_2018">{{cite journal | title=Geological evolution of the Chalk Group in the northern Dutch North Sea: inversion, sedimentation and redeposition | first1=E. | last1=Van Der Voert | first2=L. | last2=Heijnen | first3=J.J.G. | last3=Reijmer | journal=[[Geological Magazine]] | year=2019 | volume=156 | issue=7 | pages=1265–1284 | doi=10.1017/S0016756818000572| bibcode=2019GeoM..156.1265V | hdl=1871.1/76ca3535-823f-483f-9a38-cb070ab65e32 | s2cid=134798076 | url=https://research.vu.nl/en/publications/76ca3535-823f-483f-9a38-cb070ab65e32 | hdl-access=free }}</ref> It was deposited on extensive [[continental shelves]] at depths between {{convert|100 and 600|m||}}, during a time of nonseasonal (likely arid) climate that reduced the amount of erosion from nearby exposed rock. The lack of nearby erosion explains the high purity of chalk. The coccolithophores, foraminifera, and other microscopic organisms from which the chalk came mostly form low-magnesium calcite skeletons, so the sediments were already in the form of highly stable low-magnesium calcite when deposited. This is in contrast with most other limestones, which formed from high-magnesium calcite or aragonite that rapidly converted to the more stable low-magnesium calcite after deposition, resulting in the early [[Cementation (geology)|cementation]] of such limestones. In chalk, the absence of calcium carbonate conversion process prevented early cementation, and it accounts for chalk's high porosity.<ref name="hancock-1975"/> Additionally, chalk is the only form of limestone that commonly shows signs of compaction.<ref>{{cite book |last1=Blatt |first1=Harvey |last2=Middleton |first2=Gerard |last3=Murray |first3=Raymond |title=Origin of sedimentary rocks |date=1980 |publisher=Prentice-Hall |location=Englewood Cliffs, N.J. |isbn=0136427103 |edition=2d |page=508}}</ref>

[[Flint]] (a type of [[chert]]) is very common as bands parallel to the [[Bed (geology)|bedding]] or as [[Nodule (geology)|nodule]]s in [[seam (geology)|seams]], or linings to [[fracture (geology)|fractures]], embedded in chalk. It is probably derived from [[Spicule (sponge)|sponge spicule]]s<ref name=craven/> or other [[Silica|siliceous]] organisms as water is expelled upwards during compaction. Flint is often deposited around larger [[fossil]]s such as [[Echinoidea]] which may be [[Silicification|silicified]] (i.e. replaced molecule by molecule by flint).{{sfn|Blatt|Middleton|Murray|1980|p=576}}