The BGS Lexicon of Named Rock Units — Result Details

Combe Down Oolite Member

Computer Code: CODO Preferred Map Code: CDO
Status Code: Full
Age range: Bathonian Age (JN) — Bathonian Age (JN)
Lithological Description: Calcareous ooid-grainstone, white, grey and yellow, fine- to coarse-grained, generally moderately bioclastic, in medium to thick beds, and cross-bedded, with interbedded calcareous mudstone beds in the lower part. Subordinate, impersistent bioclastic limestone beds. Coralliferous, clayey and burrowed limestone locally developed at at least one level, but otherwise whole fossils are uncommon. The basal beds from Tormarton north to Hawkesbury Upton comprise a unit of coarse-grained shelly ooidal limestone ("Grickstone Beds"). Wyatt and Cave (2002) recognise lower and upper units of very similar gross lithology separated by a bored hardground.
Definition of Lower Boundary: South of Badminton [ST 78 83] the base is a sharp upward change from grey calcareous silicate mudstone with subordinate argillaceous limestone beds of the Fuller's Earth Formation into ooid-limestone, locally with limestone pebbles, or coarse-grained shelly ooidal limestone ("Grickstone Beds"), locally with limestone pebbles. North of Badminton, the base rests sharply on fine-grained ooid-limestone of the Athelstan Oolite Formation, which is commonly capped by a hardground (separated on sheet 251 (1970 edition) and known as "chinastone" or "Coppice Limestone" (Cave, 1977)), or locally where that formation is absent, on the underlying fine-grained slightly argillaceous limestone of the Tresham Rock Formation.
Definition of Upper Boundary: Disconformable: moderately bioclastic ooid-limestone of Combe Down Oolite Member, surmounted by planar, locally irregular, bored and oyster-encrusted surface, overlain by generally non-ooidal, pisoidal and shelly limestone of Twinhoe Member. Where latter absent north of Corsham, Combe Down Oolite may be difficult to distinguish from the Bath Oolite Member, which is more sparsely bioclastic. In places in the north of the member's range, its top may be a non-sequence, taken above the highest pale coloured generally moderately bioclastic ooid-grainstone, either erosive or marked by a bored and/or oyster-encrusted hardground, overlain by grey or brown variably sandy bioclastic limestone, variously ooidal or shelly, or locally by calcareous silicate-mudstone (all of the Forest Marble Formation).
Thickness: 8 to 18m through the Bath to Chippenham area, thinning northwards to zero near Didmarton, and passing into mudstone southwards. 7.83m in Chalfield No.1 Borehole.
Geographical Limits: Outcrop: Wellow, 7km south-west of Bath, Somerset, where the Combe Down Oolite passes laterally into part of the Frome Clay Formation, north-east through Bath and Corsham, Wiltshire, into south Gloucestershire, where it wedges out above the Athelstan Oolite Formation near Didmarton. Subcrop: may be present at depth in the Weald (Wyatt and Cave, 2002; Wyatt, 2011).
Parent Unit: Chalfield Oolite Formation (CFDO)
Previous Name(s): Combe Down Oolite (-5066)
Lower Rags (Lower Part) (-5118)
Lower Ragstones (Lower Part) (-5119)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  Chalfield No.1A borehole (ST86SE3), full thickness of 7.83m between 46.25 and 54.08m depth. Samples held by BGS. Wyatt and Cave, 2002 
Type Area  Odd Down to Combe Down area of Bath, Somerset, extensively quarried and mined. 
Reference(s):
Arkell W J, 1933. The Jurassic System in Great Britain [Oxford: Clarendon Press.] 
Wyatt, R J and Cave, R. 2002. The Chalfield Oolite Formation (Bathonian, Middle Jurassic) and the Forest Marble overstep in the south Cotswolds, and the stratigraphical position of the Fairford Coral Bed. Proceedings of the Geologists' Association, Vol.113, 139-152. 
Green, G W and Donovan, D T. 1969. The Great Oolite of the Bath area. Bulletin of the Geological Survey of Great Britain, Vol.30, 1-63. 
Sumbler, M G. 2003. Comments on Wyatt and Cave's 'The Chalfield Oolite Formation (Bathonian, Middle Jurassic) and the Forest Marble overstep in the South Cotswolds and the stratigraphical position of the Fairford Coral Bed'. Proceedings of the Geologists' Association, Vol.114, 181-184. 
Lonsdale, W. 1832. On the oolitic district of Bath. Transactions of the Geological Society of London, Series 2, Vol.3, 241-276. 
Woodward, H B, 1894. The Jurassic Rocks of Britain, Vol.4. The Lower Oolitic Rocks of England (Yorkshire excepted). Memoir of the Geological Survey of the United Kingdom. 
Cave R. 1977. Geology of the Malmesbury District. Memoir of the Geological Survey of Great Britain, Sheet 251 (England and Wales). 
Barron, A J M, Sheppard, T H, Gallois, R W, Hobbs, P R N, and Smith, N J P. 2011. Geology of the Bath district. British Geological Survey Sheet Explanation, Sheet 265 (England and Wales). 
Penn, I E, and Wyatt, R J. 1979. The stratigraphy and correlation of the Bathonian strata in the Bath-Frome area. Institute of Geological Sciences, London, Vol. 78/22, 23-88. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E265 E251