Chalk Group: Difference between revisions

| image = Brass Point (Seven Sister) - geograph.org.uk - 1272999.jpg

| caption = Brass Point, one of the [[Seven Sisters, Sussex|Seven Sisters]]

| age = [[Cenomanian]] to [[Maastrichtian]], ([[Cretaceous]]){{Fossil range|100|66}}

| subunits = [[Cambridge Greensand]] Grey Chalk Subgroup, White Chalk Subgroup

| underlies = [[unconformity]], [[Thanet Formation]], [[Lambeth Group]]

| overlies = [[GreensandSelborne (geology)|Upper Greensand FormationGroup]], [[GaultHunstanton Formation]], [[HunstantonCambridge FormationGreensand]]

| thickness = over 1500m{{convert|1500|m|abbr=on}}

The '''Chalk Group''' (often just called '''the Chalk''') is the [[lithostratigraphy|lithostratigraphic unit]] (a certain number of [[Stratum|rock strata]]) which contains the late [[Upper Cretaceous]] [[limestone]] succession in southern and eastern England. The same or similar rock sequences occur across the wider northwest [[Europe]]an chalk '[[Province#GeologyPhysiographic 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 [[coccoliths|coccolithophorescoccolithophore]]s. In addition to the coccoliths, the fossil debris includes a variable, but minor, percentage of the fragments of [[foraminifera]], [[ostracod]]s and [[molluskMollusca|mollusks]]s. The [[coccolithophore]]scoccolithophores 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 [[OxidationRedox|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]] and 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 NNRNational Nature Reserve]].

[[ImageFile:The Needles.jpg|thumb|[[The Needles]], (Isle of Wight); part of southern England's extensive chalk outcrop.]]

The Chalk Group is now divided into a [[''White Chalk Subgroup'']] and a ''Grey Chalk Subgroup'', both of which are further subdivided into [[formationGeological (stratigraphy)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>

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 beginsbegin 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 feet (60 m)|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]]''.

In the southern province, the former '''Middle Chalk''', now the Holywell Nodular Chalk Formation and overlying New Pit Formation, averages about {{convert|200 feet (60 m)|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 feet (95 m)|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 [[seasponge]] ''[[Porosphaera (sponge)|spongePorosphaera]]''. 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. ''PorosphaeraZitteliana'' 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]]''), [[bryozoanBryozoa|bryozoans]]s, scattered fragments of [[starfish]] and fish remains (including [[shark]] teeth such as ''[[Cretolamna]]'' and ''[[Squalicorax]]'').

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|Antrim]] coast.

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 sandstones[[sandstone]]s and mudstones[[mudstone]]s though the [[Santonian]] age Gribun Chalk Formation of Mull and nearby [[Morvern]] is recognised.

The Dutch ({{lang-nl|Krijtkalk-Groep}}) and [[Belgium|Belgian]] ({{lang-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 (city)|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 [[Gulpen Formation]], consisting of [[Campanian]] to Maastrichtian chalk;

*the [[Vaals Formation]], consisting of alternating Campanian clays, glauconiferous sands and silts;

The chalk is also an important [[Oil reservoir|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|British 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 | accessdateaccess-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 ETAP[[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.

| noppno-pp = true

</ref> 6 - [[Ekofisk formationFormation]] - [[Danian]] age (major reservoir in the [[Ekofisk oil field]] Field and others)

*Chalk Unit 5 - [[Tor formationFormation]] - [[Maastrichtian]] age, (major reservoir in many fields including Joanne oil field (UKCS), Valhall oil field (NCS))

*Chalk Unit 4 - [[Hod formationFormation]] - Late [[Campanian]] age (mostly non reservoir)

*Chalk Unit 3 - Hod formationFormation - [[Santonian]] to Middle Campanian

*Chalk Unit 2 - Hod formationFormation - Middle [[Turonian]] to [[Coniacian]]

The majority of Chalk [[OilPetroleum 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"/>

== References and further reading==

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