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As discussed in Chalk Facts by C. S. Harris and Scholle et al. (1983), the Chalk Formation consists mostly of coccolith biomicrite. A biomicrite is a Limestone composed of Fossil debris ("bio") and Calcium Carbonate mud ("micrite"). The majority of the fossil debris comprising this 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 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 Formation. The Chalk Formation 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 Oxidized to brown Iron Oxide on exposed surfaces. STRATIGRAPHY AND PALEONTOLOGY The Lower Chalk is usually relatively soft and greyish in colour, it is also the most fossiliferous (especially for Ammonite fossils). The Lower Chalk Strata usually begins with marl called the Glauconitic or (Chloritic) Marl, named after the grains of the green Mineral s Glauconite and Chlorite which it contains. The remainder of the Lower Chalk is normal chalk with few, if any, flint nodules present. The thickness of the Lower Chalk strata varies, averaging around 200 feet (320 m), depending upon the location. The Lower Chalk often contains fossils such as the ammonites ''Schloenbachia'', ''Scaphites'', and ''Mantelliceras'', the Belemnite ''Actinocamax'', and the Bivalves ''Inoceramus'' and ''Ostrea''. The Middle Chalk averages about 200 feet (60 m) in thickness. Fossils found in the Middle Chalk include the Brachiopod ''Terebratulina'' and the Echinoid ''Conulus''. However, though fossils have been found, they are generally sparce. The Upper Chalk by comparison is softer than the Middle Chalk and the flint nodules it contains are far more abundant, and may contain ammonite and Gastropod fossils in some nodular layers. The thickness of the Upper Chalk strata varies greatly, often averaging around 300 feet (480 m). In the Upper Chalk fossils may be abundant and include the Bivalve ''Spondylus'', the Brachiopod s ''Terebratulina'' and ''Gibbithyris'', the Echinoids ''Sternotaxis'', ''Micraster'', ''Echinocorys'', and ''Tylocidaris'', the Crinoid ''Marsupites'', and the small Sponge ''Porosphaera''. The youngest beds of the Upper Chalk formation in s (such as ''Parasmilia''), marine worm tubes (such as ''Rotularia''), Bryozoan s, scattered fragments of Starfish , and fish remains (including Shark teeth such as ''Cretolamna'' and '' Squalicorax ''). CHALK PETROLEUM RESERVOIRS The chalk is also an important Petroleum Reservoir in the North Sea , mainly in Norwegian and Danish sectors and to a lesser extent in the United Kingdom sector (UKCS). Reservoir stratigraphy
Reservoir geology 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. SEE ALSO REFERENCES AND FURTHER READING
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