![]() ![]() The Chalk Group usually shows few signs of bedding, other than lines of flint nodules which become common in the upper part. 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 coccolithophores lived in the upper part of the water column. In addition to the coccoliths, the fossil debris includes a variable, but minor, percentage of the fragments of foraminifera, ostracods and mollusks. Most of the fossil debris in chalk consists of the microscopic plates, which are called coccoliths, of microscopic green algae known as coccolithophores. A biomicrite is a limestone composed of fossil debris ("bio") and calcium carbonate mud (" micrite"). It is characterised by thick deposits of chalk, a soft porous white limestone, deposited in a marine environment.Ĭhalk is a limestone that consists of coccolith biomicrite. The same or similar rock sequences occur across the wider northwest European chalk ' province'. The Chalk Group (often just called the Chalk) is the lithostratigraphic unit (a certain number of rock strata) which contains the Upper Cretaceous limestone succession in southern and eastern England. Selborne Group, Hunstanton Formation, Cambridge Greensand Unconformity, Thanet Formation, Lambeth Group Cambridge Greensand Grey Chalk Subgroup, White Chalk Subgroup ![]()
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