Chalk formation

Chalk Group Stratigraphic range: Cenomanian to Maastrichtian (Cretaceous)
Type	Group
Sub-units	Grey Chalk Subgroup, White Chalk Subgroup
Underlies	unconformity
Overlies	Upper Greensand Formation, Gault Formation, Hunstanton Formation
Thickness	over 1500m
Lithology
Primary	limestone
Other	mudstone, flint
Location
Country	England
Extent	southern and eastern England

The Chalk Group (often just called the Chalk) is the lithostratigraphic unit (a certain number of rock strata) which contains the late Cretaceous limestone succession in southern and eastern England. The same or similar rock sequences occur across the wider northwest European chalk 'province'. It is characterised by thick deposits of chalk, a soft porous white limestone, deposited in a marine environment.

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 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 the upper part. Nodules of the mineral pyrite also occur and are usually oxidized to brown iron oxide on exposed surfaces.

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