Category:Southern Province Chalk nomenclature - Grey Chalk Subgroup

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Name

First proposed in Rawson et al. (2001) as part of the agreed standard for the Chalk Group of England.

Type section

None defined for the whole subgroup. Full succession visible under favourable conditions in the Isle of Wight and on the Kent coast around Folkestone in the Southern Province where the constituent formations have their type sections. Can be considered as defined by reference to the type sections of the Ferriby Chalk Formation in the Northern Province.

Primary reference section

The coastal section between Copt Point, Folkestone [TR 242 365] and Hay Cliff [TR 301 394] including Abbots Cliff path [TR 268 385] in the Southern Province as discussed in Robinson (1986).

Speeton Cliffs [TA 162 752 to TA 192 744] (Wright, 1968; Mitchell, 1995a) in Northern Province.

Formal subdivisions

Divided in the Southern Province into the west Melbury Marly Chalk Formation and the Zig Zag Chalk Formation as defined herein. Is coextensive with the Ferriby Chalk Formation of the Northern Province.

Lithology

Clayey (‘marly’) chalk without flint. The Lower part comprises limestone/marl ‘couplets’ equivalent to the West Melbury Marly Chalk Formation in the Southern Province. Upper part distinctly less ‘marly’ with notable calc-arenite beds and is equivalent to the Zig Zag Chalk Formation of the Southern Province. The Ferriby Formation of the Northern Province is the lateral equivalent of the Grey Chalk Subgroup and comprises grey, soft, marly, flint-free chalk, typically weathering buff in exposures; locally includes pinkish bands; some harder, gritty, shell-debris-rich beds, and thin discrete marl seams.

Definition of upper boundary

Conformable at the highest bedding plane beneath the lowest bed of the Plenus Marls Member of the Holywell Nodular Chalk Formation in the Southern Province and the Welton Chalk Formation in the Northern Province. (Note that the Plenus Marls Member is now considered as part of the overlying subgroup thus providing a consistent datum throughout the Chalk Group of England and the North Sea).

Definition of lower boundary

Unconformable, set at the burrowed erosion surface marking the base of the Cenomanian. In the Southern Province this marks the distinct change from chalk-free to chalk-rich sediment. In the Northern Province the boundary is at an erosion surface between the Ferriby and Hunstanton formations.

Thickness

Variable, generally between 45 and 90 m in Southern Province. Is equivalent to the 30m or so of the Ferriby Chalk Formation of the Northern Province.

Distribution

The Subgroup is known throughout the onshore outcrops in England and offshore in the Southern, Central and Northern North Sea areas. In the Northern Province the term Ferriby Chalk Formation is analogous.

Previous names

Equivalent in part to the Lower Chalk (with the exception of the Plenus Marls Member) of the traditional scheme in the Southern and Northern Provinces; to the Ferriby Chalk Formation of Woods and Smith (1978), and most of the Lower Chalk Formation of Bristow et al. (1997). Equivalent to the Hydra or Swarte Formations in the Central and Northern North Sea Basins respectively.

Parent

Chalk Group.

Age and biostratigraphy

Upper Cretaceous, Cenomanian. Mantelliceras mantelli to Calycoceras guerangeri Zones.

References

Subgroup first defined herein, with the term-only published in Rawson et al., 2001.