| Chalk Group | |
|---|---|
| Stratigraphic range:Cenomanian toMaastrichtian,100–66 Ma | |
_-_geograph.org.uk_-_1272999.jpg) Brass Point, one of theSeven Sisters | |
| Type | Group |
| Sub-units | Cambridge Greensand Grey Chalk Subgroup, White Chalk Subgroup |
| Underlies | unconformity,Thanet Formation,Lambeth Group |
| Overlies | Selborne Group,Hunstanton Formation,Cambridge Greensand |
| Thickness | over 1,500 m (4,900 ft) |
| Lithology | |
| Primary | limestone |
| Other | mudstone,flint |
| Location | |
| Country | England |
| Extent | southern and eastern England |
TheChalk Group (often just calledthe Chalk) is thelithostratigraphic unit (a certain number ofrock strata) which contains theUpper Cretaceouslimestone succession in southern and eastern England. The same or similar rock sequences occur across the wider northwestEuropean chalk 'province'. It is characterised by thick deposits ofchalk, a softporous white limestone, deposited in a marine environment.
Chalk is a limestone that consists ofcoccolith biomicrite.[1] A biomicrite is a limestone composed offossil debris ("bio") andcalcium carbonate mud ("micrite"). Most of the fossil debris in chalk consists of the microscopic plates, which are called coccoliths, of microscopic green algae known ascoccolithophores. In addition to the coccoliths, the fossil debris includes a variable, but minor, percentage of the fragments offoraminifera,ostracods andmollusks. 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 ofcalcareousooze, which eventually became the Chalk Group.
The Chalk Group usually shows few signs of bedding, other than lines offlint nodules which become common in the upper part.Nodules of the mineralpyrite also occur and are usuallyoxidized to browniron oxide on exposed surfaces.
Well-known outcrops include theWhite Cliffs of Dover,Beachy Head, the southern coastal cliffs of theIsle of Wight,Flamborough Head East Yorkshire, thequarries and motorway cuttings atBlue Bell Hill,Kent, (which has been classified as aSite of Special Scientific Interest) and at theStokenchurch Gap on theOxfordshire/Buckinghamshire border where theM40 motorway cuts through theAston Rowant National Nature Reserve.
The Chalk Group is now divided into aWhite Chalk Subgroup and aGrey Chalk Subgroup, both of which are further subdivided intoformations. 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 ofEast Anglia and theChiltern Hills is also recognised. A different approach again is taken as regards the succession beneath theNorth Sea.[2]
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 forammonite fossils). Thestrata of this subgroup usually begin with the 'Glauconitic Marl Member' (formerly known as the Glauconitic or Chloritic Marl), named after the grains of the greenmineralsglauconite andchlorite which it contains. The remainder of the subgroup isargillaceous 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 centralChilterns the two parts are separated by the hardTotternhoe 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 fromEast Anglia toYorkshire, where the entire sequence is now referred to as the 'Ferriby Chalk Formation'. The thickness of the Grey Chalk Subgroup strata varies, averaging around 200 ft (61 m), depending upon the location. They often contains fossils such as the ammonitesSchloenbachia,Scaphites, andMantelliceras, thebelemniteActinocamax, and thebivalvesInoceramus andOstrea.
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):
In the northern province the sequence is divided thus:
In the southern province, the former Middle Chalk, now the Holywell Nodular Chalk Formation and overlying New Pit Formation, averages about 200 ft (61 m) in thickness. The sparse fossils found in this sequence include thebrachiopodTerebratulina and theechinoidConulus.
The former Upper Chalk by comparison is softer than the underlying sequence and the flint nodules it contains are far more abundant in theSouth of England, although inYorkshire the underlying strata have the highest concentration of flints. It may contain ammonite andgastropod fossils in some nodular layers. The thickness of this sequence varies greatly, often averaging around 300 ft (91 m). Fossils may be abundant and include thebivalveSpondylus, thebrachiopodsTerebratulina andGibbithyris, theechinoidsSternotaxis,Micraster,Echinocorys, andTylocidaris, thecrinoidMarsupites, and the smallspongePorosphaera. A possibleazhdarchoidpterosaur is known fromConiacian-aged rocks that form part of the Upper Chalk, making it the youngest known pterosaur discovered to date inEngland.[3]
The youngest beds of the sequence are found on the coast ofNorfolk. Other fossils commonly found in this formation include: solitarycorals (such asParasmilia),marine worm tubes (such asRotularia),bryozoans, scattered fragments ofstarfish and fish remains (includingshark teeth such asCretolamna andSqualicorax).
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 duringCretaceous times, subsequent uplift and erosion has resulted in it remaining only southeast of a line drawn roughly betweenThe Wash andLyme Bay inDorset and eastwards from thescarps of theLincolnshire andYorkshire Wolds. Gentlefolding of theMesozoic rocks of this region during theAlpine orogeny has produced theLondon Basin and theWeald–Artois Anticline, theHampshire Basin and the less gentlePurbeck-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 westernNorfolk, includingBreckland, theChiltern Hills, theBerkshire Downs,Marlborough Downs and the western margins ofSalisbury Plain andCranborne Chase and theNorth and South Dorset Downs.[4] 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.
Only where the Weald–Artois Anticline has been 'unroofed' by erosion i.e. within theWeald is the Chalk entirely absent. In this area the long north-facing scarp of theSouth Downs and the longer south-facing scarp of theNorth 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 forthe downs immediately north ofVentnor on theIsle of Wight.
Some of the best exposures of the Chalk are where these ranges intersect the coast to produce dramatic, often vertical cliffs as atFlamborough Head, theWhite Cliffs of Dover,Seven Sisters,Old Harry Rocks (Purbeck) andThe Needles on theIsle of Wight. The Chalk, which once extended across theEnglish Channel, gives rise to similar cliff features on the French coast.
In the 'Ulster Cretaceous Province' ofNorthern Ireland theclastic-dominatedHibernian Greensands Group and the overlyingUlster White Limestone Group are the stratigraphical equivalents of the Chalk Group of England. They are best exposed near theAntrim coast.
In the 'Scottish Chalk Province' (extending fromMull toSkye) theInner Hebrides Group is the stratigraphical equivalent of England's Chalk Group. It comprises largelysandstones andmudstones though theSantonian age Gribun Chalk Formation of Mull and nearbyMorvern is recognised.
The Dutch (Dutch:Krijtkalk-Groep) andBelgian (Dutch:Krijt-Groep) equivalents of the Chalk Group are basically continuous and crop out as a slightly northwest dippingmonocline in a belt from theGerman city ofAachen to the city ofMons, where they join Cretaceous deposits of theParis Basin. North ofNamur the Cretaceous is overlain by youngerPaleocene andEocene deposits of theLanden Group.
In theLow Countries, the Chalk Group succession is divided into five formations, from top to base:[5]
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 lowerPaleogeneHesbaye Group.
TheChannel Tunnel linkingEngland andFrance was constructed by tunnelling through the West Melbury Marly Chalk (formerly the 'Chalk Marl' - a prominent sub-unit of the Grey Chalk Subgroup).
The chalk is also an importantpetroleum reservoir in the North Sea CentralGraben, mainly inNorwegian andDanish sectors and to a lesser extent in theUnited Kingdom Continental Shelf sector (UKCS).[6]
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 theShetland Group. With the exception of some thin sandy units in the innerMoray 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 outdirectional drilling manoeuvers.
In the Shearwater andEastern 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.
The majority of Chalkreservoirs are redepositedallochthonous beds. These include debris flows andturbidite flows.Porosities can be very high when preserved fromdiagenesis by earlyhydrocarbon 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 someextremely expensive remedial work to lift the platforms and re-position them.[6]
Fossils of theechinoidMicraster from the Chalk Group have been studied for their continuous morphogical variation throughout the record.[8]Mosasaur remains referred to "Mosasaurus"gracillis from theTuronian aged Chalk Group deposits actually are more closely allied to theRussellosaurina.[9] A single partial maxillar tooth from Cenomanian aged Chalk Group described as "Iguanodon hilli" belongs to a non-HadrosauridHadrosauroid.[10]