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The Chalk Group (often just called the Chalk) is the lithostratigraphic unit (a certain number of Stratum) which contains the Upper Cretaceous limestone succession in southern and eastern England. The same or similar rock sequences occur across the wider northwest chalk 'province'. It is characterised by thick deposits of chalk, a soft Porosity white limestone, deposited in a marine environment.
Chalk is a limestone that consists of coccolith biomicrite.As discussed in Chalk Facts by C. S. Harris and Scholle et al. (1983) 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 . In addition to the coccoliths, the fossil debris includes a variable, but minor, percentage of the fragments of foraminifera, and Mollusca. 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, 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. Nodules of the mineral pyrite also occur and are usually Redox 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 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.
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 , depending upon the location. They often contains fossils such as the ammonites Schloenbachia, Scaphites, and Mantelliceras, the Belemnoidea Actinocamax, and the Bivalvia Inoceramus and Ostrea.
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 in thickness. The sparse fossils found in this sequence include the brachiopod Terebratulina and the Sea urchin 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, although in Yorkshire the underlying strata have the highest concentration of flints. It may contain ammonite and Gastropoda fossils in some nodular layers. The thickness of this sequence varies greatly, often averaging around . Fossils may be abundant and include the Bivalvia Spondylus, the Terebratulina and Gibbithyris, the Sea urchin Sternotaxis, Micraster, Echinocorys, and Tylocidaris, the crinoid Marsupites, and the small 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.Martill DM, Witton MP, Gale A (2008) Possible azhdarchoid pterosaur remains from the Coniacian (Late Cretaceous) of England. Zitteliana B 28: 209–218.
The youngest beds of the sequence are found on the coast of Norfolk. Other fossils commonly found in this formation include: solitary (such as Parasmilia), Polychaete tubes (such as Rotularia), Bryozoa, scattered fragments of starfish and fish remains (including shark teeth such as Cretolamna and Squalicorax).
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, Marlborough Downs and the western margins of Salisbury Plain and Cranborne Chase and the Dorset Downs.Ordnance Survey 1:625,000 scale Physical Map of Great Britain sheet 2 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 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 the downs immediately north of Ventnor on the Isle of Wight. 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, 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 . 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.
In the Low Countries, the Chalk Group succession is divided into five formations, from top to base:; 2004: Engineering Geology for Infrastructure Planning in Europe, Lecture Notes in Earth Sciences 104, Springer, , p. 491
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.
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