The BGS Lexicon of Named Rock Units — Result Details

Grey Chalk Subgroup

Computer Code: GYCK Preferred Map Code: notEntered
Status Code: Full
Age range: Cenomanian Age (KE) — Cenomanian Age (KE)
Lithological Description: Clayey ("marly") chalk without flint. The Lower part comprises limestone/marl "couplets" equivalent to the West Melbury Marly Chalk Formation in the Southern Province. The upper part is 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 Lower Boundary: The lower boundary is 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.
Definition of Upper Boundary: The upper boundary is 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).
Thickness: Variable, generally between 45 and 90m in the Southern Province. Is equivalent to the 30m or so of the Ferriby Chalk Formation of the Northern Province.
Geographical Limits: 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.
Parent Unit: Chalk Group (CK)
Previous Name(s): Lower Chalk (Part) (-285)
Alternative Name(s): Ferriby Chalk Formation
Stratotypes:
Reference Section  Speeton Cliffs [TA 162 752 to TA 192 744] (Wright, 1968; Mitchell, 1995a) in the Northern Province (Ferriby Chalk Formation). 
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). Where the most complete succession of the West Melbury Marly Chalk and Zig Zag Chalk formations are present. 
Reference(s):
Hopson, P M. 2005. A stratigraphical framework for the Upper Cretaceous Chalk of England and Scotland, with statements on the Chalk of Northern Ireland and the UK Offshore Sector. British Geological Survey Research Report RR/05/01 102pp. ISBN 0 852725175 
Rawson, P F, Allen, P M and Gale, A. 2001. A revised lithostratigraphy for the Chalk Group. Geoscientist, Vol.11, p.21. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable