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50-419: Chalk is a limestone that consists of coccolith biomicrite. 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 coccolithophores . In addition to the coccoliths, the fossil debris includes a variable, but minor, percentage of

100-410: A coccosphere . Coccolithophores are spherical cells about 5–100 micrometres across, enclosed by calcareous plates called coccoliths , which are about 2–25 micrometres across. Coccolithophores are an important group of about 200 marine phytoplankton species  which cover themselves with a calcium carbonate shell called a "coccosphere". They are ecologically and biogeochemically important but

150-408: A radial array of elaborately shaped crystal units. Holococcoliths are formed of minute (~0.1 micrometre) calcite rhombohedra , arranged in continuous arrays. The two coccolith types were originally thought to be produced by different families of coccolithophores. Now, however, it is known through a mix of observations on field samples and laboratory cultures, that the two coccolith types are produced by

200-545: 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 scarps of the Lincolnshire and Yorkshire Wolds . Gentle folding of

250-470: A structure of radiating calcite fibers and may also display concentric growth rings . Diagenetic modifications of the shells are complex. Radiating calcitic crystals are thin, or very large, with a shape indicative of a strong alteration. In other samples, the aragonite - calcite boundary is not dependent on growth lines. In a given fossil sites, some specimens are calcite, others are aragonite. The guard, phragmocone and pro-ostracum were all internal to

300-762: Is found in Europe and Asia , can measure up to 46 centimetres (18 in) in length, giving the living animal an estimated length of 3 metres (10 ft). Very exceptional belemnoid specimens have been found showing the preserved soft parts of the animal. Elsewhere in the fossil record, bullet-shaped belemnite guards are locally found in such profusion that such deposits are referred to semi-formally as "belemnite battlefields" (cf. "orthocone orgies" ). It remains unclear whether these deposits represent post-mating mass death events, as are common among modern cephalopods and other semelparous creatures. The name " thunderbolt " or "thunderstone" has also been traditionally applied to

350-760: 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

400-828: The Chiltern Hills , the Berkshire Downs , Marlborough Downs and the western margins of Salisbury Plain and Cranborne Chase and the North and South Dorset Downs . 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

450-633: The Jurassic and Cretaceous periods, and their fossils are abundant in Mesozoic marine rocks, often accompanying their cousins the ammonites . The belemnoids become extinct at the end of the Cretaceous period along with the ammonites. The belemnoids' origin lies within the bactritoid nautiloids, which date from the Devonian period; well-formed belemnoid guards can be found in rocks dating from

500-811: 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-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 ,

550-485: The Mississippian (or Early Carboniferous ) onward through the Cretaceous. Other fossil cephalopods include baculites , nautiloids and goniatites . Belemnoids possessed a central rostrom/guard made of aragonite and with negative buoyancy. To the rear of the creature was a heavy calcite guard whose main role appears to have been to counterbalance the front (towards the head) of the organism; it positions

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600-487: The Russellosaurina . A single partial maxillar tooth from Cenomanian aged Chalk Group described as " Iguanodon hilli " belongs to a non-Hadrosaurid Hadrosauroid . Coccolith Coccoliths are individual plates or scales of calcium carbonate formed by coccolithophores (single-celled phytoplankton such as Emiliania huxleyi ) and cover the cell surface arranged in the form of a spherical shell, called

650-652: The Santonian age Gribun Chalk Formation of Mull and nearby Morvern is recognised. The Dutch ( Dutch : Krijtkalk-Groep ) and Belgian ( Dutch : Krijt-Groep ) equivalents of the Chalk Group are basically continuous and crop out as a slightly northwest dipping monocline in a belt from the German city of Aachen to the city of Mons , where they join Cretaceous deposits of the Paris Basin . North of Namur

700-587: The South of England , although in Yorkshire the underlying strata have the highest concentration of flints. It may contain ammonite and gastropod 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 the bivalve Spondylus , the brachiopods Terebratulina and Gibbithyris , the echinoids Sternotaxis , Micraster , Echinocorys , and Tylocidaris ,

750-785: The Stokenchurch Gap on the Oxfordshire / Buckinghamshire border where the M40 motorway cuts through the Aston Rowant National Nature Reserve . The Chalk Group is now divided into a White Chalk Subgroup and a Grey Chalk Subgroup , both of which are further subdivided into 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;

800-535: The bivalves Inoceramus and Ostrea . 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,

850-553: The 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 Antrim coast. In the ' Scottish Chalk Province' (extending from Mull to Skye ) the Inner Hebrides Group is the stratigraphical equivalent of England's Chalk Group. It comprises largely sandstones and mudstones though

900-798: 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 . The youngest beds of the sequence are found on the coast of Norfolk . Other fossils commonly found in this formation include: solitary corals (such as Parasmilia ), marine worm tubes (such as Rotularia ), bryozoans , scattered fragments of starfish and fish remains (including shark teeth such as Cretolamna and Squalicorax ). The Chalk outcrops across large parts of southern and eastern England and forms

950-420: The golgi body . When the coccolith is complete these vesicles fuse with the cell wall and the coccolith is exocytosed and incorporated in the coccosphere. The coccoliths are either dispersed following death and breakup of the coccosphere, or are shed continually by some species. They sink through the water column to form an important part of the deep-sea sediments (depending on the water depth). Thomas Huxley

1000-624: 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 Chalk reservoirs are redeposited allochthonous beds. These include debris flows and turbidite flows. 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,

1050-587: 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

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1100-592: 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. In the 'Ulster Cretaceous Province' of Northern Ireland

1150-653: 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: 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 Channel Tunnel linking England and France

1200-589: The Lower Chalk minus the Plenus Marls) is usually relatively soft and greyish in colour. It is also the most fossiliferous (especially for ammonite fossils). The 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 minerals glauconite and chlorite which it contains. The remainder of

1250-639: The Mediterranean. Coccoliths are also classified depending on shape. Common shapes include: Although coccoliths are remarkably elaborate structures whose formation is a complex product of cellular processes, their function is unclear. Hypotheses include defence against grazing by zooplankton or infection by bacteria or viruses ; maintenance of buoyancy ; release of carbon dioxide for photosynthesis ; to filter out harmful UV light; or in deep-dwelling species, to concentrate light for photosynthesis. Because coccoliths are formed of low-Mg calcite,

1300-551: 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 . The Grey Chalk Subgroup (formerly

1350-402: The body for streamlining. Normally with fossil belemnoids only the back part of the shell (called the guard or rostrum ) is found. The guard is usually elongated and bullet-shaped (though in some subgroups the rostrum may only exist as a thin layer coating the phragmocone). The hollow region at the front of the guard is termed the alveolus , and this houses a chambered conical-shaped part of

1400-504: The centre of mass below the centre of buoyancy, increasing the stability of the swimming organism. The guard would account for between a third and a fifth of the length of the complete organism, arms included. Like some modern squid, belemnoid arms carried a series of small hooks for grabbing prey . Belemnoids were efficient carnivores that caught small fish and other marine animals with their arms and ate them with their beak-like jaws . In turn, belemnites appear to have formed part of

1450-428: The coccosphere reduces the grazing efficiency by making it more difficult for the predator to utilise the organic content of coccolithophores. Heterotrophic protists are able to selectively choose prey on the basis of its size or shape and through chemical signals  and may thus favor other prey that is available and not protected by coccoliths. Coccoliths are formed within the cell in vesicles derived from

1500-465: The collapse of a number of wells, and some extremely expensive remedial work to lift the platforms and re-position them. Fossils of the echinoid Micraster from the Chalk Group have been studied for their continuous morphogical variation throughout the record. Mosasaur remains referred to " Mosasaurus " gracillis from the Turonian aged Chalk Group deposits actually are more closely allied to

1550-503: The diet of marine reptiles such as Ichthyosaurs , whose fossilized stomachs frequently contain phosphatic hooks from the arms of cephalopods. Belemnoids were effectively neutrally buoyant, and swam in near-shore to mid-shelf oceans. Their fins could be used to their advantage in all water speeds; in a gentle current they could be flapped for propulsion; in a stronger current they could be held erect to generate lift; and when swimming rapidly by jet propulsion they could be tucked in to

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1600-493: 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 the brachiopod Terebratulina and the 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

1650-650: The fossilised rostra of belemnoids. The origin of these bullet-shaped stones was not understood, and thus a mythological explanation of stones created where lightning struck has arisen. The stable isotope composition of a belemnoid rostrum from the Peedee Formation ( Cretaceous , southeast USA) has long been used as a global standard (Peedee Belemnite, "PDB") against which other isotope geochemistry samples are measured, for both carbon isotopes and oxygen isotopes . Some belemnoids (such as Belemnites of Belemnitida ) serve as index fossils , particularly in

1700-487: The fragments of foraminifera , ostracods and 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 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

1750-414: The geologic past. Coccoliths and related fossils are referred to as calcareous nanofossils or calcareous nannoplankton (nanoplankton) . Belemnoidea Aulacocerida Phragmoteuthida Belemnitida Diplobelida Belemnoids are an extinct group of marine cephalopod , very similar in many ways to the modern squid . Like them, the belemnoids possessed an ink sac , but, unlike

1800-424: The interaction between a coccolithophore and its virus. The major predators of marine phytoplankton are microzooplankton like ciliates and dinoflagellates . These are estimated to consume about two-thirds of the primary production in the ocean  and microzooplankton can exert a strong grazing pressure on coccolithophore populations. Although calcification does not prevent predation, it has been argued that

1850-436: The living creature, forming a skeleton which was enclosed entirely by soft muscular tissue. The original living creature would have been larger than the fossilized shell, with a long streamlined body and prominent eyes. The guard would have been in place toward the rear of the creature, with the phragmocone behind the head and the pointed end of the guard facing backward. The guard of the belemnoid Megateuthis gigantea , which

1900-474: The most stable form of calcium carbonate, they are readily fossilised. They are found in sediments together with similar microfossils of uncertain affinities (nanoliths) from the Upper Triassic to recent. They are widely used as biostratigraphic markers and as paleoclimatic proxies. Stable oxygen and carbon isotope data from coccoliths is used to reconstruct estimates of oceanic CO 2 concentrations in

1950-574: 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),

2000-500: 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 200 ft (61 m), depending upon the location. They often contains fossils such as the ammonites Schloenbachia , Scaphites , and Mantelliceras , the belemnite Actinocamax , and

2050-445: The oceanic uptake of atmospheric CO 2 . As of 2021, it is not known why coccolithophores calcify and how their ability to produce coccoliths is associated with their ecological success. The most plausible benefit of having a coccosphere seems to be a protection against predators or viruses. Viral infection is an important cause of phytoplankton death in the oceans, and it has recently been shown that calcification can influence

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2100-490: The reason why they calcify remains elusive. One key function may be that the coccosphere offers protection against microzooplankton predation, which is one of the main causes of phytoplankton death in the ocean. Coccolithophores have been an integral part of marine plankton communities since the Jurassic . Today, coccolithophores contribute ~1–10% to primary production in the surface ocean  and ~50% to pelagic CaCO 3 sediments. Their calcareous shell increases

2150-493: The same species but at different life cycle phases. Heterococcoliths are produced in the diploid life-cycle phase and holococcoliths in the haploid phase. Both in field samples and laboratory cultures, there is the possibility of observing a cell covered by a combination of heterococcoliths and holococcoliths. This indicates the transition from the diploid to the haploid phase of the species. Such combination of coccoliths has been observed in field samples, with many of them coming from

2200-463: The shell (called the phragmocone ). The phragmocone is usually only found with the better preserved specimens. Projecting forwards from one side of the phragmocone is the thin pro-ostracum . While belemnoid phragmocones are homologous with the shells of other cephalopods and are similarly composed of aragonite , belemnoid guards are evolutionarily novel and are composed of calcite or aragonite , thus tending to preserve well. Broken guards show

2250-411: The sinking velocity of photosynthetically fixed CO 2 into the deep ocean by ballasting organic matter. At the same time, the biogenic precipitation of calcium carbonate during coccolith formation reduces the total alkalinity of seawater and releases CO 2 . Thus, coccolithophores play an important role in the marine carbon cycle by influencing the efficiency of the biological carbon pump and

2300-580: The squid, they possessed ten arms of roughly equal length, and no tentacles . The name "belemnoid" comes from the Greek word βέλεμνον, belemnon meaning "a dart or arrow" and the Greek word είδος, eidos meaning "form". Belemnoids include belemnites (which belong to the order Belemnitida ), aulacocerids (order Aulacocerida ), phragmoteuthids (order Phragmoteuthida ), and diplobelids (order Diplobelida ). Belemnoidea has been suggested to be paraphyletic by some authors. Belemnoids were numerous during

2350-527: The subgroup is 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 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

2400-405: The upper part. Nodules of the mineral pyrite also occur and are usually 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 quarries and motorway cuttings at Blue Bell Hill , Kent , (which has been classified as a Site of Special Scientific Interest ) and at

2450-777: 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 important petroleum reservoir in the North Sea Central Graben , mainly in Norwegian and Danish sectors and to a lesser extent in the United Kingdom Continental Shelf sector (UKCS). Across the north central and northern North Sea,

2500-574: Was the first person to observe these forms in modern marine sediments and he gave them the name 'coccoliths' in a report published in 1858. Coccoliths are composed of calcium carbonate as the mineral calcite and are the main constituent of chalk deposits such as the white cliffs of Dover (deposited in Cretaceous times), in which they were first described by Henry Clifton Sorby in 1861. There are two main types of coccoliths, heterococcoliths and holococcoliths . Heterococcoliths are formed of

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