The BGS Lexicon of Named Rock Units — Result Details

Holywell Nodular Chalk Formation
Computer Code: HCK Preferred Map Code: HCk
Status Code: Full
Age range: Cenomanian Age (KE) — Turonian Age (KT)
Lithological Description: Generally hard nodular chalks with thin flaser marls and significant proportions of shell debris in part. Base marked by the interbedded coloured marl and chalk succession characteristic of the Plenus Marls Member (a term applicable in both the Southern and Northern Provinces). The Melbourn Rock Member above the base can be distinguished by its lack of shell material.
Definition of Lower Boundary: The lower boundary is conformable with the base of the formation taken at the bedding plane immediately below the lower marl bed in the Plenus Marls Member. (Note that the Plenus Marls Member is now considered as part of this formation thus providing a consistent datum throughout the Chalk Group of England and the North Sea).
Definition of Upper Boundary: The upper boundary is conformable and is taken at the base of the Gun Gardens Main Marl in the expanded sequences of the Southern Province and at the highest shell detrital bed in mapping terms.
Thickness: In thicker successions this formation is generally 25 to 35m thick. It thins rapidly into the "Transitional" Province of the North Downs, Berkshire Downs and the Chilterns (10 to 15m) and is almost absent in the southwest of Dorset and in Devon.
Geographical Limits: Known throughout the Southern Province, within the Chilterns and northward into East Anglia in the Transitional Province.
Parent Unit: White Chalk Subgroup (WHCK)
Previous Name(s): Holywell Chalk Member (-1589)
Lower And Middle Holywell Beds (-953)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  Shillingstone Pit. Working quarry on the north side of Shillingstone Hill. About 25m of the entire formation is exposed in the quarry, See Mortimore et al. (2001, p.143, Figure 3.35) and Bristow (1995). 
Reference Section  Chinor quarry [SU 754 994] in Oxfordshire. 13m of the formation preseved in the quarry 3 face with a correlation to other standard successions that suggests the uppermost preserved chalk is near to the top of the formation. See Mortimore et al. (2001) and Sumbler and Woods (1992). 
Reference Section  Ashwell Quarry, near Biggleswade. The Melbourn Rock was originally defined at Melbourn in Cambridgeshire where the type site is now poorly exposed in a car park beneath a building. A replacement type section is taken at Ashwell Quarry [TL 2687 3945] on the Biggleswade sheet (see Hopson et al., 1996). 
Reference Section  Shakespear Cliff, Kent. "Plenus Marls Formation" and lower part of the "Shakespear Cliff Member" of the "Dover Chalk Formation" (Robinson, 1996). 
Type Section  Cliff section,Eastbourne to Beachy Head, Sussex. Also the type Section of the Holywell Beds of Mortimore, 1986. The Melbourn Rock Bed or Beds were defined by Mortimore (1986) at Gun Gardens, Eastbourne.  
Type Section  The Plenus Marls Member type site at Merstham [TQ 295 542] of Jefferies (1963) is now obscured. 
Reference Section  Akers Steps, including Melbourn Rock Bed. Full Succession visible along cliff face path. See Robinson (1986) where the formation is equivalent to the lower part of his Shakespear Cliff Member (Dover Chalk Formation) and a separate underlying Plenus Marl Formation. 
Type Section  Holywell [TV 603 973] section west of Eastbourne, Sussex. The thickest onshore sequence is seen at Eastbourne adjacent to the Holywell Cafe [TV 602 967]. 
Reference(s):
Bristow, C R. 1989. Geology of the East Stour - Shaftesbury district (Dorset) British Geological Survey Technical Report, WA/89/58. 
Mortimore, R N. 1986. Stratigraphy of the Upper Cretaceous White Chalk of Sussex. Proceedings of the Geologists' Association, Vol.97(2), 97-139. 
Bristow, C R, Mortimore, R N and Wood, C J. 1999. Lithostratigraphy for mapping the chalk in southern England by Bristow et al. (1997): Discussion. Proceedings of the Geologists' Association, Vol.110, 68-72 
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. 
Robinson, N D. 1986. Lithostratigraphy of the Chalk Group, North Downs, Southeast England. Proceedings of the Geologists' Association, Vol.97, 141-170. 
Bristow, C R, Barton, C M, Freshney, E C, Wood, C J, Evans, D J, Cox, B M, Ivimey-Cook, H and Taylor, R. 1995. Geology of the country around Shaftesbury. Memoir of the British Geological Survey, Sheet 313 (England and Wales). 
Mortimore, R N, Wood, C J, and Gallois, R W. 2001. British Upper Cretaceous Stratigraphy. Geological Conservation Review Series, No. 23. (Peterborough: Joint Nature Conservation Committee.) 
Sumbler, M G and Woods, C J. 1992. The stratigraphy of the lower and Middle Chalk at Chinnor, Oxfordshire. Proceedings of the Geologists' Association, Vol.103, 111-118. 
Bristow, C R. 1991. Geology of Sheet ST80NW and Sheet ST80SW (Turnworth-Milton Abbas, Dorset). British Geological Survey Technical Report, WA/92/20. 
Bristow, C R, Barton, C M, Freshney, E C, Wood, C J, Evans, D J, Cox, B M, Ivimey-Cook, H and Taylor, R T. 1993. Geology of the country around Shaftesbury, Sheet 313 (England and Wales). 
Bristow, C R, Mortimore, R N and Wood C J. 1997. Lithostratigraphy for mapping the Chalk of southern England. Proceedings of the Geologists' Association, Vol.108(4), 293-315. 
Rawson, P F, Allen, P M and Gale, A S. 2001. The Chalk Group - a revised lithostratigraphy. Geoscientist, Vol.11, p.21. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E281 E300 E313 E314 E327 E341 E342 E343
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