Chalk Group

{{Short description|Stratigraphic unit in England}} {{Infobox rockunit | name = Chalk Group | image = Brass Point (Seven Sister) - geograph.org.uk - 1272999.jpg | caption = Brass Point, one of the [[Seven Sisters, Sussex|Seven Sisters]] | type = [[Group (stratigraphy)|Group]] | age = [[Cenomanian]] to [[Maastrichtian]], {{Fossil range|100|66}} | period = Late Cretaceous | prilithology = [[limestone]] | otherlithology = [[mudstone]], [[flint]] | namedfor = | namedby = | region = | country = [[England]] | coordinates = | unitof = | subunits = [[Cambridge Greensand]], Grey Chalk Subgroup, White Chalk Subgroup | underlies = [[unconformity]], [[Thanet Formation]], [[Lambeth Group]] | overlies = [[Selborne Group]], [[Hunstanton Formation]], [[Cambridge Greensand]] | thickness = over {{convert|1500|m|abbr=on}} | extent = southern and eastern England | area = | map = | map_caption = }}

The '''Chalk Group''' (often just called '''the Chalk''') is the [[lithostratigraphy|lithostratigraphic unit]] (a certain number of [[Stratum|rock strata]]) which contains the [[Upper Cretaceous]] [[limestone]] succession in southern and eastern England. The same or similar rock sequences occur across the wider northwest [[Europe]]an chalk '[[Physiographic province|province]]'. It is characterised by thick deposits of [[chalk]], a soft [[Porosity|porous]] white limestone, deposited in a marine environment.

Chalk is a limestone that consists of [[coccolith]] biomicrite.<ref>As discussed in [http://www.geologyshop.co.uk/chalk.htm Chalk Facts] by C. S. Harris and Scholle ''et al.'' (1983)</ref> A biomicrite is a limestone composed of [[fossil]] debris ("bio") and [[calcium carbonate]] mud ("[[micrite]]"). Most of the fossil debris in chalk consists of the microscopic plates, which are called coccoliths, of microscopic green algae known as [[coccolithophore]]s. In addition to the coccoliths, the fossil debris includes a variable, but minor, percentage of the fragments of [[foraminifera]], [[ostracod]]s and [[Mollusca|mollusks]]. The coccolithophores lived in the upper part of the water column. When they died, the microscopic calcium carbonate plates, which formed their shells settled downward through the ocean water and accumulated on the ocean bottom to form a thick layer of [[calcareous]] [[pelagic sediment|ooze]], which eventually became the Chalk Group.

The Chalk Group usually shows few signs of bedding, other than lines of [[flint]] nodules which become common in the upper part. [[Nodule (geology)|Nodules]] of the mineral [[pyrite]] also occur and are usually [[Redox|oxidized]] to brown [[iron oxide]] on exposed surfaces.

Well-known outcrops include the [[White Cliffs of Dover]], [[Beachy Head]], the southern coastal cliffs of the [[Isle of Wight]], [[Flamborough Head]] East Yorkshire, the [[quarry|quarries]] and motorway cuttings at [[Blue Bell Hill]], [[Kent]], (which has been classified as a [[Site of Special Scientific Interest]]) and at the [[Stokenchurch Gap]] on the [[Oxfordshire]]/[[Buckinghamshire]] border where the [[M40 motorway]] cuts through the [[Aston Rowant National Nature Reserve]].

[[File:The Needles.jpg|thumb|[[The Needles]], (Isle of Wight); part of southern England's extensive chalk outcrop.]] [[File:Fossil Echinoid Echinocorys.jpg|thumb|Fossil [[echinoid]] ''[[Echinocorys]]'' from the Chalk Group of England]]

==Subdivisions== The Chalk Group is now divided into a ''White Chalk Subgroup'' and a ''Grey Chalk Subgroup'', both of which are further subdivided into [[Geological formation|formations]]. These modern divisions replace numerous earlier divisions, references to which occur widely on geological maps and in other geological literature. Previously no subgroups were defined but three formations were identified; the Upper Chalk, Middle Chalk and Lower Chalk. Different formations are defined within the 'northern' and 'southern' provinces, from Norfolk northwards and south of the Thames valley respectively. A 'transitional province' between the two and covering much of [[East Anglia]] and the [[Chiltern Hills]] is also recognised. A different approach again is taken as regards the succession beneath the [[North Sea]].<ref>Hopson, P.M., 2005 ''A Stratigraphical Framework for the Upper Cretaceous Chalk of England and Scotland with Statements on the Chalk of Northern Ireland and the UK Offshore Sector'', British Geological Survey Research Report RR/05/01 (downloaded from www.bgs.ac.uk)</ref>

===Grey Chalk Subgroup=== {{anchor|Glauconitic Marl Member}} The Grey Chalk Subgroup (formerly the Lower Chalk minus the Plenus Marls) is usually relatively soft and greyish in colour. It is also the most fossiliferous (especially for [[Ammonoidea|ammonite]] fossils). The [[stratum|strata]] of this subgroup usually begin with the ''''Glauconitic Marl Member'''' (formerly known as the Glauconitic or Chloritic Marl), named after the grains of the green [[mineral]]s [[glauconite]] and [[Chlorite group|chlorite]] which it contains. The remainder of the subgroup is [[argillaceous minerals|argillaceous]] in its lower part (the West Melbury Marly Chalk Formation (formerly the 'Chalk Marl') and becomes progressively purer in the 'Zig-zag Chalk Formation' (the former 'Grey Chalk'). In the central [[Chiltern Hills|Chilterns]] the two parts are separated by the hard [[Totternhoe Stone]], which forms a prominent scarp in some places. There are few, if any, flint nodules present.

These two formations are not recognised within the northern province i.e. the outcrop north from [[East Anglia]] to [[Yorkshire]], where the entire sequence is now referred to as the 'Ferriby Chalk Formation'. The thickness of the Grey Chalk Subgroup strata varies, averaging around {{convert|200|ft|abbr=on}}, depending upon the location. They often contains fossils such as the ammonites ''[[Schloenbachia]]'', ''[[Scaphites]]'', and ''[[Mantelliceras]]'', the [[Belemnoidea|belemnite]] ''[[Actinocamax]]'', and the [[Bivalvia|bivalves]] ''[[Inoceramus]]'' and ''[[Ostrea]]''. [[File:Chalk & greensand at Lulworth.jpg|thumb|center|500px|Contact between two units of the [[lithostratigraphy]] of South England: the Chalk Group (left, white, upper unit) and the [[Greensand|Greensand Formation]] (right, green, lower unit). Location: [[Lulworth Cove]], near [[West Lulworth]], [[Dorset]], England.]]

===White Chalk Subgroup=== The White Chalk Subgroup includes what were formerly designated the Middle Chalk and Upper Chalk Formations, together with the Plenus Marls (topmost part of the former Lower Chalk Formation). In the southern province it is divided in the following way (youngest/uppermost at top): * Portsdown Chalk Formation (formerly part of 'Upper Chalk' and the equivalent of Rowe Chalk Formation, below) * Culver Chalk Formation (formerly part of 'Upper Chalk') ** Spetisbury Chalk Member (formerly part of 'Upper Chalk') ** Tarrant Chalk Member (formerly part of 'Upper Chalk') * Newhaven Chalk Formation (formerly part of 'Upper Chalk') * Seaford Chalk Formation (formerly part of 'Upper Chalk') * Lewes Nodular Chalk Formation (formerly part of 'Upper Chalk') * New Pit Chalk Formation (formerly part of 'Middle Chalk') * Holywell Nodular Chalk Formation (formerly part of 'Middle Chalk') ** Plenus Marls Member

In the northern province the sequence is divided thus: * Rowe Chalk Formation (formerly part of 'Upper Chalk' and the equivalent of Portsdown Chalk Formation, above) * Flamborough Chalk Formation (formerly part of 'Upper Chalk') * Burnham Chalk Formation (formerly part of 'Upper Chalk') * Welton Chalk Formation (formerly 'Middle Chalk') ** Plenus Marls Member

In the southern province, the former Middle Chalk, now the Holywell Nodular Chalk Formation and overlying New Pit Formation, averages about {{convert|200|ft|abbr=on}} in thickness. The sparse fossils found in this sequence include the [[brachiopod]] ''[[Terebratulina gracilis|Terebratulina]]'' and the [[Sea urchin|echinoid]] ''[[Conulus]]''.

The former Upper Chalk by comparison is softer than the underlying sequence and the flint nodules it contains are far more abundant in the [[Southern England|South of England]], although in [[Yorkshire]] the underlying strata have the highest concentration of flints. It may contain ammonite and [[Gastropoda|gastropod]] fossils in some nodular layers. The thickness of this sequence varies greatly, often averaging around {{convert|300|ft|abbr=on}}. Fossils may be abundant and include the [[Bivalvia|bivalve]] ''[[Spondylus]]'', the [[brachiopod]]s ''Terebratulina'' and ''Gibbithyris'', the [[Sea urchin|echinoids]] ''Sternotaxis'', ''[[Micraster]]'', ''[[Echinocorys]]'', and ''[[Tylocidaris]]'', the [[crinoid]] ''[[Marsupites]]'', and the small [[sponge]] ''[[Porosphaera (sponge)|Porosphaera]]''. A possible [[azhdarchoid]] [[pterosaur]] is known from [[Coniacian]]-aged rocks that form part of the Upper Chalk, making it the youngest known pterosaur discovered to date in [[England]].<ref>Martill DM, Witton MP, Gale A (2008) Possible azhdarchoid pterosaur remains from the Coniacian (Late Cretaceous) of England. ''Zitteliana'' B 28: 209–218.</ref>

The youngest beds of the sequence are found on the coast of [[Norfolk]]. Other fossils commonly found in this formation include: solitary [[coral]]s (such as ''[[Parasmilia]]''), [[Polychaete|marine worm]] tubes (such as ''[[Rotularia]]''), [[Bryozoa|bryozoans]], scattered fragments of [[starfish]] and fish remains (including [[shark]] teeth such as ''[[Cretolamna]]'' and ''[[Squalicorax]]'').

== Chalk landscapes of England == [[File:KentGeologyWealdenDomeSimple.svg|thumb|alt=Cross-sectional diagram of eroded layers of geological anticline with locations of towns indicated|400px|The [[Weald]]en [[Anticline]].]] The Chalk outcrops across large parts of southern and eastern England and forms a significant number of the major physiographical features. Whilst it has been postulated that a chalk cover was laid down across just about all of England and Wales during [[Cretaceous]] times, subsequent uplift and erosion has resulted in it remaining only southeast of a line drawn roughly between [[The Wash]] and [[Lyme Bay]] in [[Dorset]] and eastwards from the [[escarpment|scarp]]s of the [[Lincolnshire Wolds|Lincolnshire]] and [[Yorkshire Wolds]]. Gentle [[Fold (geology)|folding]] of the [[Mesozoic]] rocks of this region during the [[Alpine orogeny]] has produced the [[London Basin]] and the [[Weald–Artois Anticline]], the [[Hampshire Basin]] and the less gentle [[Purbeck Monocline|Purbeck-Wight monocline]].

The broadly western margin of the Chalk outcrop is marked, from northeast to southwest, to south by the Chalk downlands of the Yorkshire Wolds, the Lincolnshire Wolds, a subdued feature through western [[Norfolk]], including [[Breckland]], the [[Chiltern Hills]], the [[Berkshire Downs]], [[North Wessex Downs|Marlborough Downs]] and the western margins of [[Salisbury Plain]] and [[Cranborne Chase]] and the [[Dorset Downs|North and South Dorset Downs]].<ref>Ordnance Survey 1:625,000 scale Physical Map of Great Britain sheet 2</ref> In parts of the Thames Basin and eastern East Anglia the Chalk is concealed by later deposits, as is the case too within the Hampshire Basin.

[[File:Ivinghoe Beacon seen from The Ridgeway.jpg|left|thumb|[[Ivinghoe Beacon]], Chiltern Hills]]

Only where the Weald–Artois Anticline has been 'unroofed' by erosion i.e. within the [[Weald]] is the Chalk entirely absent. In this area the long north-facing scarp of the [[South Downs]] and the longer south-facing scarp of the [[North Downs]] face one another across the Weald. For similar reasons, the Chalk is largely absent from the rather smaller area to the south of the Purbeck-Wight Monocline, save for [[St Boniface Down|the downs]] immediately north of [[Ventnor]] on the [[Isle of Wight]]. [[File:Litlington White Horse 2025.png|thumb|The [[Litlington White Horse]], created by exposing the underlying strata of the Chalk Group.]] Some of the best exposures of the Chalk are where these ranges intersect the coast to produce dramatic, often vertical cliffs as at [[Flamborough Head]], the [[White Cliffs of Dover]], [[Seven Sisters, Sussex|Seven Sisters]], [[Old Harry Rocks]] (Purbeck) and [[The Needles]] on the [[Isle of Wight]]. The Chalk, which once extended across the [[English Channel]], gives rise to similar cliff features on the French coast.

The distinctive white color and relative softness of the Chalk Group have also been utilized for the creation of [[hill figure]]s. By 'scouring' away the overlying turf and topsoil, the bright white bedrock is exposed to create monumental images. A 20th-century example is the [[Litlington White Horse]] in [[East Sussex]]. Located in the [[South Downs]], the figure is cut into the Seaford Chalk Formation, utilising the high purity of the limestone to maintain its visibility against the green downland.

==Offshore and elsewhere==

===Northern Ireland=== In the 'Ulster Cretaceous Province' of [[Northern Ireland]] the [[Clastic rock|clastic]]-dominated [[Hibernian Greensands Group]] and the overlying [[Ulster White Limestone Group]] are the stratigraphical equivalents of the Chalk Group of England. They are best exposed near the [[County Antrim]] coast.

===Scotland=== In the '[[Scotland|Scottish]] Chalk Province' (extending from [[Isle of Mull|Mull]] to [[Skye]]) the [[Inner Hebrides Group]] is the stratigraphical equivalent of England's Chalk Group. It comprises largely [[sandstone]]s and [[mudstone]]s though the [[Santonian]] age Gribun Chalk Formation of Mull and nearby [[Morvern]] is recognised.

===The Low Countries=== The Dutch ({{langx|nl|Krijtkalk-Groep}}) and [[Belgium|Belgian]] ({{langx|nl|Krijt-Groep}}) equivalents of the Chalk Group are basically continuous and crop out as a slightly northwest dipping [[monocline]] in a belt from the [[Germany|German]] city of [[Aachen]] to the city of [[Mons, Belgium|Mons]], where they join Cretaceous deposits of the [[Paris Basin]]. North of [[Namur]] the Cretaceous is overlain by younger [[Paleocene]] and [[Eocene]] deposits of the [[Landen Group]].

In the [[Low Countries]], the Chalk Group succession is divided into five formations, from top to base:<ref>{{aut|Hack, Robert; Azzam, Rafig; Charlier, Robert}}; '''2004''': ''Engineering Geology for Infrastructure Planning in Europe'', Lecture Notes in Earth Sciences '''104''', Springer, {{ISBN|978-3-540-21075-7}}, p. 491</ref> *the [[Houthem Formation]], consisting of Paleocene calcareous arenites; *the [[Maastricht Formation]], consisting of [[Maastrichtian]] chalk and calcareous [[arenite]]s; *the [[Gulpen Formation]], consisting of [[Campanian]] to Maastrichtian chalk; *the [[Vaals Formation]], consisting of alternating Campanian clays, glauconiferous sands and silts; *the [[Aachen Formation]], consisting of [[Santonian]] glauconiferous sands and silts. In Belgium, the Houthem Formation is sometimes not included in the Chalk Group because it is not a Cretaceous formation. Some stratigraphers therefore prefer to put it in the lower [[Paleogene]] [[Hesbaye Group]].

===The English Channel=== The [[Channel Tunnel]] linking [[England]] and [[France]] was constructed by tunnelling through the West Melbury Marly Chalk (formerly the 'Chalk Marl' - a prominent sub-unit of the Grey Chalk Subgroup).

===The North Sea=== {{see also|Geology of the North Sea}} The chalk is also an important [[petroleum reservoir]] in the North Sea Central [[Graben]], mainly in [[Norway|Norwegian]] and [[Denmark|Danish]] sectors and to a lesser extent in the [[United Kingdom Continental Shelf]] sector (UKCS).<ref name="DECC">{{cite web | url=https://www.og.decc.gov.uk/UKpromote/posters/The_Chalk_play_of_the_UK_Central_Graben.pdf | title=The Chalk play of the UK Central Graben | publisher=[[Department of Energy and Climate Change]] | date=2006 | access-date=18 April 2015 | url-status=dead | archive-url=https://web.archive.org/web/20150418152855/https://www.og.decc.gov.uk/UKpromote/posters/The_Chalk_play_of_the_UK_Central_Graben.pdf | archive-date=18 April 2015 }}</ref>

Across the north central and northern North Sea, the Chalk Group is a major seal unit, overlying a number of blocks of reservoir rocks and preventing their fluid contents from migrating upwards. North of the line of the Mid-North Sea - Ringkobing - Fyn structural high, the Chalk Group is still recognisable in drilled samples, but becomes increasingly muddy northwards. North of the Beryl Embayment (59°30' N 01°30'E), the Chalk Group is a series of slightly to moderately calcareous mudstones grouped under the name of the [[Shetland]] Group. With the exception of some thin sandy units in the inner [[Moray Firth]], this sequence has neither source potential nor reservoir capacity and is not generally considered a drilling target. Its thickness and homogeneity does make it a common target for carrying out [[directional drilling]] manoeuvers.

In the Shearwater and [[Eastern Trough Area Project]] areas (around 56°30' N 02°30'E, UKCS quadrants 22,23,29 and 30), the Chalk Group can be significantly overpressured. Further south in UKCS quadrant 30 and Norwegian quadrants 1 and 2, this overpressure helps preserve porosity and enables the Chalk to be an effective reservoir.

===Reservoir stratigraphy===

*Chalk Unit<ref>{{cite book | last = Evans | first = D. | title = The Millennium Atlas of the North Sea | year = 2003 | publisher = Millennium Atlas Co. | isbn = 1-86239-119-X | pages = Chapter 13, Upper Cretaceous, Chalk Group | no-pp = true |display-authors=etal}} </ref> 6 - [[Ekofisk Formation]] - [[Danian]] age (major reservoir in the [[Ekofisk oil field]] and others) *Chalk Unit 5 - [[Tor Formation]] - [[Maastrichtian]] age, (major reservoir in many fields including Joanne oil field (UKCS), Valhall oil field (NCS)) *Chalk Unit 4 - [[Hod Formation]] - Late [[Campanian]] age (mostly non reservoir) *Chalk Unit 3 - Hod Formation - [[Santonian]] to Middle Campanian *Chalk Unit 2 - Hod Formation - Middle [[Turonian]] to [[Coniacian]] *Turonian shale - Early Turonian (non reservoir) *Plenus Marl - Late [[Cenomanian]] (non reservoir) *Chalk Unit 1 - Hidra Formation - Cenomanian (non reservoir)

===Reservoir geology=== The majority of Chalk [[Petroleum reservoir|reservoirs]] are redeposited [[allochthon]]ous beds. These include debris flows and [[turbidite]] flows. [[Porosity|Porosities]] can be very high when preserved from [[diagenesis]] by early [[hydrocarbon]] charge. However, when these hydrocarbons are produced, diagenesis and compaction can restart which has led to several metres of subsidence at seabed, the collapse of a number of wells, and some ''extremely'' expensive remedial work to lift the platforms and re-position them.<ref name="DECC"/>

== Fossils == Fossils of the [[echinoid]] ''[[Micraster]]'' from the Chalk Group have been studied for their continuous morphogical variation throughout the record.<ref>{{Cite journal|date=1959-05-28|title=Changes in the chalk heart-urchin Micraster Interpreted in relation to living forms|journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences|language=en|volume=242|issue=693|pages=347–437|doi=10.1098/rstb.1959.0007|issn=2054-0280|doi-access=free}}</ref> [[Mosasaur]] remains referred to "''Mosasaurus''" ''gracillis'' from the [[Turonian]] aged Chalk Group deposits actually are more closely allied to the [[Russellosaurina]].<ref>{{Cite journal|last1=Street|first1=Hallie P.|last2=Caldwell|first2=Michael W.|date=2014-07-29|title=Reassessment of Turonian mosasaur material from the 'Middle Chalk' (England, U.K.), and the status of Mosasaurus gracilis Owen, 1849|url=http://www.tandfonline.com/doi/abs/10.1080/02724634.2014.846263|journal=Journal of Vertebrate Paleontology|language=en|volume=34|issue=5|pages=1072–1079|doi=10.1080/02724634.2014.846263|s2cid=84392980 |issn=0272-4634|url-access=subscription}}</ref> A single partial maxillar tooth from Cenomanian aged Chalk Group described as "''Iguanodon hilli''" belongs to a non-Hadrosaurid [[Hadrosauroid]].<ref>Dalla Vecchia FM. (2009b) [https://www.academia.edu/1079026/European_hadrosauroids European hadrosauroids]. Actas de las IV Jornadas Internacionales sobre Paleontología de Dinosaurios y su Entorno. Salas de los Infantes, Burgos, 45–74.</ref>

==See also== * [[Chalk stream]] * [[Downland]] * [[Geology of Great Britain]] * [[Southern England Chalk Formation]]

== References == {{reflist|2}}

== Further reading == <!-- Old references for backup only ----------------------------------- Please, use the WP template for citations and references. Thank You.

* Hancock, J.M. (1975) ''The petrology of the Chalk'' Proc. Geologists Assoc. v. 86, p. 499-535 * Harris, C.S. (nd) [http://www.geologyshop.co.uk/chalk.htm Chalk Facts] - website on everything Chalk * Lousley, J.E. (1969) ''Wild Flowers of Chalk & Limestone''. London: Collins. * Scholle, P.A., Bedout, D.G., and Moore, C.H. (1983) ''Carbonate Depositional Environments'' American Association of Petroleum Geologist Memoir 33. * Smith, A.B., and Batten, D.J. (2002) ''Fossils of the Chalk (Second Edition)'' The Palaeontological Association. -------------------------------- Old references for backup only -->

<!-- Citation template for books ------------------------------------ Please, use the WP template for citations and references. Thank You.

* {{cite book | title = | publisher = | location = | url = | doi = | pages = | isbn= }} -------------------------------- Citation template for books -->

* {{cite book | last = Hancock | first = J.M. | title = The Petrology of the Chalk | year = 1975 | publisher = Proceedings of the Geologists Association, vol 86 | location = London | pages = 449–535 }}

* {{cite web | last = Harris | first = C.S. | title = Chalk Facts – website on everything Chalk | year = 2007 | url = http://www.geologyshop.co.uk/chalk.htm }}

* {{cite book | last = Lousley | first = J.E. | title = Wild Flowers of Chalk and Limestone | year = 1969 | publisher = Collins | location = London }}

* {{cite book | last = Scholle | first = P.A. |author2=Bedout, D.G. |author3=Moore, C.H. | title = Carbonate Depositional Environments | year = 1983 | publisher = American Association of Petroleum Geologist Memoir 33 }}

* {{cite book | last = Smith | first = A.B. |author2=Batten, D.J. | title = Fossils of the Chalk | edition = Second | year = 2002 | publisher = The Palaeontological Association }}

[[Category:Geological groups of the United Kingdom]] [[Category:Geologic formations of England]] [[Category:Upper Cretaceous Series of Europe]] [[Category:Cretaceous England]] [[Category:Chalk formations]] [[Category:Petroleum geology]] [[Category:Lithostratigraphy of England]]