The BGS Lexicon of Named Rock Units — Result Details

Twinhoe Member

Computer Code: TW Preferred Map Code: Tw
Status Code: Full
Age range: Bathonian Age (JN) — Bathonian Age (JN)
Lithological Description: The member comprises three facies in a generally ascending sequence: Twinhoe Ironshot, Freshford and Winsley. The Twinhoe Ironshot consists of brown, rubbly, shelly, bioclastic, clayey, ferruginous, ooidal and pisoidal limestone and is the most impersistent, the Freshford facies is cream, lime-muddy, variably ferruginous, pisoidal, bioclastic limestone and is the most variable and laterally extensive, the Winsley facies is grey and cream, massive, non-ooidal, bioclastic, fine-grained limestone. Fossils are common to abundant apart from in the Winsley facies, bivalves are particularly numerous and diverse.
Definition of Lower Boundary: Disconformable: generally non-ooidal, pisoidal and shelly limestone of Twinhoe Member overlying planar, locally irregular, bored and oyster-encrusted surface of ooid-limestone of Combe Down Oolite Member.
Definition of Upper Boundary: Sharp change up from generally non-ooidal, pisoidal and shelly limestone of Twinhoe Member into ooid-limestone of Bath Oolite Member. Diachronous (younging southwards), with lateral facies passage (Penn and Wyatt, 1979).
Thickness: 0 to 12.5m, thickening southwards (Penn and Wyatt, 1979, fig. 11A). 11.6m in Chalfield No.1 Borehole.
Geographical Limits: Bath district: from Baggridge, Somerset, and Trowbridge, Wiltshire, where it passes south into mudstone with limestone beds of the Frome Clay Formation, north to Box, near Corsham, Wiltshire, where it passes into the basal beds of the Bath Oolite Member.
Parent Unit: Chalfield Oolite Formation (CFDO)
Previous Name(s): Lower Rags (Upper Part) (-5120)
Lower Ragstone (Upper Part) (-5121)
Twinhoe Beds (-5064)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Chalfield No.1A borehole (ST86SE3), full thickness of 11.60m between 34.65 and 46.25m depth. Samples held by BGS Wyatt and Cave, 2002 
Type Section  Twinhoe Ironshot facies best developed on Twinhoe Ridge [ST 74 59], the Freshford facies at Freshford [ST 78 60], and the Winsley facies thickest at Winsley [ST 80 61]. Green and Donovan, 1969 
Reference(s):
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. 
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. 
Lonsdale, W. 1832. On the oolitic district of Bath. Transactions of the Geological Society of London, Series 2, Vol.3, 241-276. 
Arkell W J, 1933. The Jurassic System in Great Britain [Oxford: Clarendon Press.] 
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. 
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). 
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:
E265