The BGS Lexicon of Named Rock Units — Result Details

Bude Formation

Computer Code: BF Preferred Map Code: BF
Status Code: Full
Age range: Langsettian Substage (CA) — Bolsovian Substage (CC)
Lithological Description: Grey thick-bedded, somewhat argillaceous and silty sandstones, in laterally discontinuous internally massive beds 1-5m thick and commonly amalgamated into units up to 10m thick. When weathered the sandstones become buff and friable. Very thick beds of slumped and destratified strata are also present. Grey mudstones occur as interbeds up to 1m thick but locally packets of darker mudstone up to 20m thick with thin ironstone beds and bundles of thin sandstones are present, especially in the upper part of the Formation. Five named beds of black sulphurous "shales" with goniatite-bearing calcareous nodules occur within the Formation. Thin units of thin- to medium-bedded siltstones with Xithosurid trails are also present. Deposited in a large-scale freshwater lake situated on a broad shelf. Sandstone deposition was within storm wave base and by river-fed turbidity currents. Also, there was probably deltaic progradation across part of the basin late in its development. The goniatite-bearing mudstones represent periodic marine invasions.
Definition of Lower Boundary: On the north Cornwall and north Devon coast, the base is taken at the top of the Hartland Quay Shale, the uppermost of the named goniatite-bearing mudstones in the underlying Crackington Formation. Inland, where the Hartland Quay Shale cannot be mapped in pervasively folded terrain, as it is too thin, the base is taken at the incoming of softer, siltier thick-bedded sandstones of the Bude Formation above the dominantly thin- to medium-bedded sandstones and mudstones of the underlying Crackington Formation. In north Devon, south of Bideford, the base is taken at the sharp incoming of over 100 m of grey mudstones and siltstones with two thin anthracitic coals (culm) above the cross-bedded thick-bedded Cornborough Sandstone that caps the top of the underlying Bideford Formation. Thick-bedded sandstone with internally massive beds typical of the Bude Formation, appear above these mudstones and siltstones.
Definition of Upper Boundary: The upper boundary is taken at an angular unconformity, where breccias of the Exeter Group rest upon sandstone and subordinate mudstone of the Bude Formation.
Thickness: At least 1290 m calculated on the north Cornwall and north Devon coast.
Geographical Limits: Occupies large area in mid Devon between the coast at Bude [SS 21 06] and the area west of Tiverton [SS 96 13].
Parent Unit: Holsworthy Group (HOWY)
Previous Name(s): Middle Culm [Obsolete Name And Code: See SME] (MCU)
Cockington Beds [Obsolete Name and Code: Use CKF, BF] (-3842)
Morchard-Type Culm [Obsolete Name and Code: Use BF] (-4456)
Bude Sandstones (-688)
Greencliff Beds (-689)
Alternative Name(s): Bideford Formation
Stratotypes:
Type Area  Named from the coastal resort of Bude, north Cornwall. See Freshney and Taylor, 1972. 
Type Section  Coast section, north of Bude, between Duckpool, near Steeple Point, Combe, and Gull Rock, 6 km to the north. The section exposes the entire known sequence (1290 m) including the base at Gull Rock. The sequence is affected by pervasive upright folding. Freshney et al., 1979. 
Reference(s):
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. 
Waters, C N, Waters, R A, Barclay, W J, and Davies, J R. 2009. Lithostratigraphical framework for Carboniferous successions of Southern Great Britain (Onshore). British Geological Survey Research Report, RR/09/01. 184pp. 
Ussher, W A E. 1892. The British Culm Measures. Proceedings of the Somerset Archeological and Natural History Society, Vol.38, 111-219. 
Freshney, E C and Taylor, R T. 1972. The Upper Carboniferous stratigraphy of North Cornwall and West Devon. Proceedings of the Ussher Society, Vol.2, 464-471 
Ussher, W A E. 1887. The Culm of Devonshire. Geological Magazine, Vol.24, 10-17. 
Owen, D E. 1934. The Carboniferous Rocks of the north Cornish coast and their structures. Proceedings of the Geologists' Association, Vol.45, 451-471. 
Edmonds, E A, Wright, J E, Beer, K E, Hawkes, J R, Fenning, P J, Freshney, E C, Lovelock, P E R, McKeown, M C, Ramsbottom, W H C and Williams, M. 1968. Geology of the country around Okehampton. Memoir of the Geological Survey of Great Britain, Sheet 324 (England and Wales). 
Freshney, E C, Edmonds, E A, Taylor, R T and Williams, B J. 1979. Geology of the country around Bude and Bradworthy. Memoir of the Geological Survey of Great Britain, sheets 307 and 308 (England and Wales). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E307 E308 E309 E293 E339 E324 E322 E323 E292 E310