The BGS Lexicon of Named Rock Units — Result Details

Zig Zag Chalk Formation
Computer Code: ZZCH Preferred Map Code: ZCk
Status Code: Full
Age range: Cenomanian Age (KE) — Cenomanian Age (KE)
Lithological Description: Mostly firm, pale grey to off-white blocky chalk with a lower part characterised by rhythmic alternations of marls and marly chalks with firm white chalk. Thin gritty, silty chalk beds act as markers in the sequence.
Definition of Lower Boundary: The lower boundary is conformable in full sequences. The lower boundary is placed at the erosional contact at the base of the "Cast Bed" (Southern Province) or the Totternhoe Stone (Transitional Province) in southern England. The former is the direct equivalent of Bed C1 of Gale (1995). The formation oversteps onto older formations over the Mid-Dorset Swell where the preceding West Melbury Marly Chalk Formation is absent. The basal unit here is termed the Cenomanian Basement Bed (Drummond, 1970).
Definition of Upper Boundary: The upper surface is conformable and is redefined as the bedding plane beneath the lowest of the marls in the Plenus Marls Member in the overlying Holywell Nodular Chalk Formation. (Note that the Plenus Marls Member is now considered as part of the overlying formation, thus providing a consistent datum throughout the Chalk Group of England and the North Sea).
Thickness: Generally in the range 35 to 50m in the Southern Province with the most expanded sequences (up to 75m exceptionally) in the area of West Sussex, East Hampshire and the Isle of Wight. Thins markedly to the west and is at its least over the Mid-Dorset Swell where as little as 10m may be preserved.
Geographical Limits: Throughout the Southern Province and the Transitional Province of England.
Parent Unit: Grey Chalk Subgroup (GYCK)
Previous Name(s): East Wear Bay And Abbots Cliff Formations (-4415)
Zig Zag Chalk Member (-2551)
Grey Chalk Member (-1938)
Grey Chalk Member [Obsolete Usage: Part Of ZZCH] (GECK)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  Old quarry on the north side of Hambledon Hill (Kennedy, 1970; Bristow, 1991). 
Partial Type Section  Old quarry at foot of Zig Zag Hill. Poorly exposed of the basal beds of the formation. 
Reference Section  Old quarry at the foot of White Sheet Hill (Bristow, 1990). 
Reference Section  Cliff section in Compton Bay, Isle of Wight. 
Reference Section  Chinor Quarry, Oxfordshire Sumbler and Woods (1992). 
Reference Section  Exposures in the Southerham Grey Pit and adjacent Southerham Machine Bottom Pit, Sussex. 
Reference Section  Hay Cliff Cliff section. Between Copt Point and Hay Cliff (including Abbotts Cliff and Path), Kent. 
Reference(s):
Bristow, C R. 1991. Geology of Sheet ST 81 (Shillingstone-Compton Abbas, Dorset). British Geological Survey Technical Report WA/91/03. 
Rawson, P F, Allen, P M and Gale, A S. 2001. The Chalk Group - a revised lithostratigraphy. Geoscientist, Vol.11, p.21. 
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. 
Bristow, C R. 1989. Geology of the East Stour - Shaftesbury district (Dorset) British Geological Survey Technical Report WA/89/58. 
Bristow, C R. 1990. Geology of Sheet ST92SW (Berwick St John, Wiltshire). British Geological Survey Technical Report WA/90/49. 
Kennedy, W J. 1970. A correlation of the uppermost Albian and the Cenomanian of south west England. Proceedings of the Geologists' Association, Vol.81, 613-677. 
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 
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. 
Sumbler, M G and Woods, M A. 1992. The stratigraphy of the Lower and Middle Chalk at Chinnor, Oxfordshire. Proceedings of the Geologists' Association, Vol.103, 111-118. 
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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