The BGS Lexicon of Named Rock Units — Result Details

Bideford Formation

Computer Code: BFF Preferred Map Code: notEntered
Status Code: Full
Age range: Langsettian Substage (CA) — Langsettian Substage (CA)
Lithological Description: Mudstones with abundant thin to medium-bedded sandstones and siltstones with several thick units of thick-bedded sandstone. The upper two thirds of the sequence consists of nine coarsening upwards cycles (individually up to 180m thick) comprising in upwards order, black mudstone, silty mudstone, silt and sand-streaked mudstones, interbedded thin sandstones and mudstones, thick-bedded channelised cross-bedded sandstones capped by burrowed horizons and locally rootlets. The lower parts of some cycles contain units of thin-bedded 'Bourma-type' sandstone turbidites. Three of the thick cross-bedded sandstones capping the cycles are named, the topmost one being the Cornborough Sandstone. The lower third of the sequence (Westward Ho! Member) is not cyclic but comprises thin-bedded mudstones, siltstones and sandstones, some of which are turbiditic and others cross laminated and wave rippled. The Westward Ho! Member includes several channels filled by slumped and disturbed strata or, more rarely, cross-bedded or cross laminated sandstones.
Definition of Lower Boundary: The lower boundary is taken at the incoming of channels overlain by syndepositionally disturbed units and thick, locally cross-bedded sandstones of the Westward Ho! Member above the regularly interbedded mudstones and turbidite sandstones of the Crackington Formation. The boundary is not exposed at the type section and has never been mapped, as the Westward Ho! Member was previously included in the Crackington Formation. Further work is needed on this boundary.
Definition of Upper Boundary: The upper boundary is taken at the sharp incoming of over 100m of grey mudstones and siltstones with two thin anthracitic coals (culm) of the overlying Bude Formation above the thick cross-bedded Cornborough Sandstone that caps the topmost cycle of the Bideford Formation.
Thickness: 1220m+ seen at the type section.
Geographical Limits: Bideford area (SS 45 27) eastwards to 6km west of South Molton (SS 71 26), mid Devon.
Parent Unit: Holsworthy Group (HOWY)
Previous Name(s): Bideford Group [Obsolete Name and Code: Use BFF] (-3208)
Abbotsham Beds [Obsolete Name and Code: Use BFF] (-3844)
Abbotsham Formation [Obsolete Name and Code: Use BFF] (-3845)
Crackington Formation (CKF)
Northam Beds (-691)
Northam Formation [Obsolete Name and Code: Use BFF] (-1347)
Westward Ho Formation [Obsolete Name And Code: See WDHO] (WHF)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Coastal cliffs from Westward Ho!, Bideford west and southwest for 3.5km to Greencliff Rock, north Devon. All but the lowermost part of the Formation is exposed, including the upper contact with the overlying Bude Formation. Some of the Formation is repeated by folding. De Raaf et al (1965); Edmonds et al (1979); Walker (1970); Xu Li (1990). 
Reference(s):
Waters, C N, Barclay, W J, Davies, J R and Waters, R A. In press. Stratigraphical framework for Carboniferous successions of Southern Great Britain (Onshore). British Geological Survey Research Report, RR/05/06. 
De Raaf, J F M, Reading, H G and Walker, R G. 1965. Cyclic sedimentation in the lower Westphalian of north Devon, England. Sedimentology, Vol.4, 1-52. 
Xu Li. 1990. Changes in the Deltaic Sedimentation in the Upper Carboniferous Westward Ho! Formation and Bideford Group of southwest England. Proceedings of the Ussher Society, Vol.7, 232-236. 
Edmonds, E A, Williams, B J and Taylor, R T. 1979. Geology of Bideford and Lundy Island. Memoir of the Geological Survey of Great Britain, Sheets 292 with 275, 276, 291, and part of 308 (England and Wales). 
Prentice, J E. 1960. The stratigraphy of the Upper Carboniferous rocks of the Bideford region, north Devon. Quarterly Journal of the Geographical Society of London, Vol.116, 397-408. 
Walker, R G. 1970. Deposition of turbidites and agitated-water siltstones: a study of the Upper Carboniferous Westward Ho! Formation, north Devon. Proceedings of the Geologists' Association, Vol.18, 43-67. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable