The BGS Lexicon of Named Rock Units — Result Details

West Melbury Marly Chalk Formation
Computer Code: WMCH Preferred Map Code: WMCk
Status Code: Full
Age range: Cenomanian Age (KE) — Cenomanian Age (KE)
Lithological Description: Buff, grey and off-white, soft, marly chalk and hard grey limestone arranged in couplets.
Definition of Lower Boundary: The lower boundary is unconformable. Base marked by an erosion surface, commonly bored, at the base of the Glauconitic Marl Member (Southern Province), Melbury Sandstone Member (western Southern Province) or Cambridge Greensand Member (Transitional Province).
Definition of Upper Boundary: The upper boundary is conformable in "full" sequences at the erosional base of the overlying Cast Bed ["Member"] (bed C1 of Gale, 1995) of the Zig Zag Chalk Formation in the Southern Province and by the base of the Totternhoe Stone Member, its lateral equivalent, in the Transitional Province north of the Thames. This mid-Cenomanian erosional break marks the change from well-defined limestone marl couplets below to chalks with marls above, and the boundary between the "B" and "C" Chalk Marl couplets of Gale (1995). In full sequences the "Tenuis Limestone" is the bed immediately beneath the mid-Cenomanian break. Elsewhere a variable number of the upper "B" beds of Gale (1995) are absent through erosion.
Thickness: In general between 15 and 25m of these beds are preserved but they are absent over the Mid-Dorset Swell (Drummond, 1970; Kennedy, 1970; Bristow et al., 1995) and thin over other syndepositional structural "highs".
Geographical Limits: Throughout the Southern Province and the Transitional Province of England. Is known to be absent or attenuated over structural highs. Most notably the mid-Dorset Swell in Dorset.
Parent Unit: Grey Chalk Subgroup (GYCK)
Previous Name(s): West Melbury Chalk Member (-3139)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  Former river cliff section, 900m northeast of Gain's Cross. Section recorded by Mottram, 1957 and Kennedy, 1970; see Bristow, 1991. 
Type Section  Old quarry, now infilled, on the north side of Melbury Hill. Section recorded by Jukes-Browne and Hill (1900, 1903) but no longer visible in full. 
Reference Section  Quarry faces in the Chinor Quarry [SU 754 994] in Oxfordshire. The quarry exposes a part of the uppermost West Melbury Marly Chalk Formation to the top of the formation below the Totternhoe Stone Member that is here developed in the "shelf facies" and is itself the lateral equivalent of the Cast Bed (basal Zig Zag Chalk Formation) of Sussex and Kent, see Sumbler and Woods (1996). 
Reference Section  Quarry faces in the Southerham Grey Pit, Sussex [TQ 427 090]. The succession from a point about 12m above the base up to the surface between the Tenuis Limestone and the Cast Bed equivalent is visible in the quarry faces. 
Reference Section  Cliff sections at Copt Point [TR 242 365] to Hay Cliff [TR 301 394] (including Abbots Cliff and path [TR 268 385]), Folkestone To Dover, Kent. The principal full exposure of this formation and the type section of the traditional "Lower Chalk". The base rests on the Gault Formation and the top is marked by the surface beneath the Cast Bed. The section is minutely studied and provides one of the standard sections for the orbitally controlled cyclostratigraphy of the whole Cenomanian 
Reference Section  Cliff section in Compton Bay, Isle of Wight [SZ 350 855]. The full succession is visible in favourable tide and landslide conditions when the base, resting on the upper greensand and the Cast Bed at the top of the succession can be identified. 
Reference(s):
Bristow, C R. 1991. Geology of Sheet ST81 (Shillingstone - Compton Abbas, Dorset). British Geological Survey Technical Report WA/90/49. 
Jukes-Browne, A J and Hill, W. 1903. The Cretaceous Rocks of Britain. 2. The Lower and Middle Chalk of England. Memoir of the Geological Survey of Great Britain. 
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. 
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 
Bristow, C R. 1989. Geology of the East Stour - Shaftesbury district (Dorset). British Geological Survey Technical Report WA/89/59. 
Bristow, C R. 1990. Geology of ST92SW (Berwick St. John, Wiltshire). British Geological Survey Technical Report, WA/90/49. 
Jukes-Browne, A J and Hill, W. 1900. The Cretaceous Rocks of Britain. 1. Gault and Upper Greensand. Memoir of the Geological Survey of Great Britain. 
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. 
Mottram, B H. 1957. Whitsun Field Meeting at Shaftesbury. Proceedings of the Geologists' Association, Vol.67, 160-167, for 1956. 
Rawson, P F, Allen, P M and Gale, A S. 2001. The Chalk Group - a revised lithostratigraphy. Geoscientist, Vol.11, p.21. 
Bristow, C R, Barton, C M, Westhead, R K, Freshney, E C, Cox, B M and Woods, M A. 1999. The Wincanton district - a concise account of the geology. Memoir of the British Geological Survey, Sheet 297 (England and Wales). 
Sumbler, M G and Woods, M A. 1996. 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
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