The BGS Lexicon of Named Rock Units — Result Details
Chalfield Oolite Formation
| Computer Code: | CFDO | Preferred Map Code: | ChO |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Bathonian Age (JN) — Bathonian Age (JN) | ||
| Lithological Description: | Calc-ooidal grainstone, predominantly pale grey and pale yellow to white fine- to coarse-grained, with variable bioclastic content, in medium to thick beds; cross-bedded in parts; bioturbated in parts. A median unit of pisoidal, bioclastic limestone (Twinhoe Member) is developed south of Corsham. The generally moderately bioclastic ooid-limestones below (Combe Down Oolite Member) are fine- to coarse-grained, and those above (Bath Oolite Member) are medium- to coarse-grained ooid-limestones. Where the Twinhoe Member is not recognisable, the Combe Down and Bath Oolite members may not be distinguishable. Lenses or partings of coral debris are locally seen in the Bath Oolite. The basal beds from Tormarton north to Hawkesbury Upton comprise a unit of coarse-grained shelly ooidal limestone ("Grickstone Beds"). These resemble the basal limestone of the Forest Marble Formation and north of Little Badminton where the overlying Combe Down Oolite beds are absent they cannot be reliably distinguished. | ||
| 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 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 in Malmesbury area: BGS 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: | 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) or in places by white or brown variably ooidal and bioclastic limestone with one or more lenticular coralliferous beds of the Corsham Limestone Formation (termed the 'Upper Rags' by Green and Donovan (1969)). | ||
| Thickness: | 25 to perhaps 31m in the type area, thins northwards to zero. 30.6m in the Biddestone Borehole. | ||
| Geographical Limits: | Outcrop: Wellow, 7km southwest of Bath, where the Chalfield Oolite passes laterally into the Frome Clay, northeast to Kemble, Gloucestershire, where it passes into the Signet Member (White Limestone Formation). The formation thins progressively and substantially northeast of Badminton, mainly due to erosion of the upper beds beneath the Forest Marble Formation, but also possibly through lateral passage into the Athelstan Oolite Formation. Subcrop: might be present at depth in the Weald (Wyatt and Cave, 2002). | ||
| Parent Unit: | Great Oolite Group (GOG) | ||
| Previous Name(s): | Great Oolite [Obsolete Name and Code: Use CFDO, GOG]
(-4534)
Great Oolite Formation [Obsolete: See CFDO] (GOF) Great Oolite Limestone [Obsolete Name And Code: Use BWL, GOG or CFDO] (GOL) |
||
| Alternative Name(s): | none recorded or not applicable | ||
| Stratotypes: | |||
| Reference Section | Mural exposures at Brown's Folly, Bathford. About 15m of strata seen from top, but base not seen. Cox and Sumbler, 2002, pp.125-127. | ||
| Type Section | Chalfield No.1A borehole, ST86SE/3, full thickness between 28.73 to 54.08m depth. Samples held by BGS. Wyatt and Cave, 2002. | ||
| Reference Section | Biddestone borehole, ST87SE/1, full thickness between 16.96 to 52.19m depth. Samples held by BGS. Wyatt and Cave, 2002. | ||
| Reference(s): | |||
| 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). | |||
| Sumbler, M G, 1991. The Fairford Coral Bed: new data on the White Limestone Formation (Bathonian) of the Gloucestershire Cotswolds. Proceedings of the Geologists' Association, Vol.102, 55-62. | |||
| Sumbler, M G, Barron, A J M and Morigi, A N, 2000. Geology of the Cirencester district. Memoir of the British Geological Survey, Sheet 235 (England and Wales). | |||
| Green, G W and Donovan, D T. 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, 184-185. | |||
| 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. | |||
| Arkell, W J, and Donovan, D T. 1952. The Fuller's Earth of the Cotswolds and its relation to the Great Oolite. Quarterly Journal of the Geological Society of London, Vol. 107, 227-253. | |||
| Cave R. 1977. Geology of the Malmesbury District. Memoir of the Geological Survey of Great Britain, Sheet 251 (England and Wales). | |||
| 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. | |||
| Lonsdale, W. 1832. On the oolitic district of Bath. Transactions of the Geological Society of London, Series 2, Vol.3, 241-276. | |||
| Welch, F B A. 1957. District report, south-west England. Summary of Progress Geological Survey of Great Britain for 1956. | |||
| Warner, R. 1811. A new guide through Bath and its environs. (Bath.) | |||
| Wyatt, R J, and Cave, R. 2003. Reply to Sumbler's and Green and Donovan's 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, 185-191. | |||
| Cox, B M, and Sumbler, M G. in prep. Bathonian-Callovian Correlation Chart. A correlation of Jurassic rocks in the British Isles: Middle and Upper Jurassic. Cope, J C W (editor). Special report of the Geological Society of London,. | |||
| Waters, C N, Smith, K, Hopson, P M, Wilson, D, Bridge, D M, Carney, J N, Cooper, A H, Crofts, R G, Ellison, R A, Mathers, S J, Moorlock, B S P, Scrivener, R C, McMillan, A A, Ambrose, K, Barclay, W J, and Barron, A J M. 2007. Stratigraphical Chart of the United Kingdom: Southern Britain. British Geological Survey, 1 poster. | |||
| Wyatt, R J. 1996. A correlation of the Bathonian (Middle Jurassic) succession between Bath and Burford, and its relation to that near Oxford. Proceedings of the Geologists' Association, Vol. 107, 299-322. | |||
| Cox, B M, and Sumbler, M G. 2002. British Middle Jurassic Stratigraphy. Geological Conservation Review Series No.26. [Peterborough: Joint Nature Conservation Committee.] | |||
| 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. | |||
| 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. | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| none recorded or not applicable | |||