The BGS Lexicon of Named Rock Units — Result Details

Crackington Formation

Computer Code: CKF Preferred Map Code: CKF
Status Code: Full
Age range: Arnsbergian Substage (CG) — Langsettian Substage (CA)
Lithological Description: Rhythmically bedded, dark blue-grey mudstones and subordinate predominantly grey sandstones and siltstones. Sandstone percentage varies from 20-75%, both vertically and geographically. The sandstones are parallel-sided "Bouma-type" turbidites with abundant well-developed sole structures. Individual beds are from a few centimetres to 0.4m thick and rarely up to 1m thick. Thicker sandstones are locally present, mainly in the uppermost part, but tend to be massive and lack sole structures. Packets with abundant sandstones are commonly developed and mappable. On the north Cornwall and Devon coast, six named units of dark grey to black pyritic shale with goniatite- and fish-bearing calcareous nodules are present in the upper part of the Formation. The thickest of these units is up to 7m thick. A few thick units of slumped or destratified beds are also present in the uppermost part of the Formation, as are scattered ironstone nodules.
Definition of Lower Boundary: Due to thrusting along the south crop of the Formation, the base is seen only between Okehampton (SX 59 94) and the Teign Valley (SX 84 84), east Dartmoor, Devon. The base is gradational and taken at the incoming of more abundant and relatively thicker sandstones of the Crackington Formation above the mudstones with less than 10% sandstone in the underlying Dowhills Mudstones Formation. Between the Bideford area (SX 45 27) north Devon and the Westleigh area (ST 06 17), west Somerset, the base is taken at a similar level. However, in the Bideford area, the base needs further investigation, as the Dowhills Mudstone Formation (Limekiln Beds of Prentice 1960a) was included in the Crackington Formation by Edmonds et al. (1979).
Definition of Upper Boundary: On the north Cornwall and north Devon coast, the top is taken at the top of the Hartland Quay Shale, the uppermost of the named goniatite bearing mudstones in the Crackington Formation. Inland, where the Hartland Quay Shale cannot be mapped in pervasively folded terrain as it is too thin, the top is taken at the incoming of softer, more silty and argillaceous thick-bedded sandstones of the overlying Bude Formation above the dominantly thin- to medium-bedded sandstones and mudstones of the Crackington Formation. In the Bideford area the top is taken at the incoming of channels overlain by syndepositionally disturbed units and thick, locally cross-bedded, sandstones of the Westward Ho! Member of the overlying Bideford Formation above the regularly interbedded mudstones and turbidite sandstones of the Crackington Formation.
Thickness: >c.1000m in the Exeter district; 250m in the Bideford area.
Geographical Limits: North Cornwall (SX 15 97), Devon (SS 7010) and west Somerset (ST 06 17).
Parent Unit: Holsworthy Group (HOWY)
Previous Name(s): Eggesford-Type Culm (-47)
Middle Or Exeter-Type Culm (-3835)
Okehampton Sandstone Group (-3836)
Southerly Down Sandstone Group (-3931)
Welcome Measures (-3932)
Holmingham Beds (-2701)
Crackington Measures (-2086)
Instow Beds (-2087)
Kiddens Formation (-2088)
Whitchurch Down Greywacke Group (-2089)
Yahney Beds (-2090)
Cockington Beds (-3842)
Appledore Formation (-4541)
Black Down Sandstone Group (-1433)
Welcome Beds (-1435)
Welcome Formation (-785)
Limekiln Beds [Obsolete Name And Code: See DHLS And CKF] (LMK)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  Bonhat Road section, Exeter district. 
Partial Type Section  Sea cliffs between Rusey Cliff and Wanson Mouth (about 10km of coast), north Cornwall. Named after Crackington Haven, 3km northeast of Rusey Cliff. This section is pervasively folded and cut by low angle thrusts and faults. The lower two thirds of the Formation is exposed, comprising predominantly mudstones with subordinate thin- to medium-bedded sandstones. Neither the basal nor upper contacts are seen due to faulting. Freshney et al. (1972). 
Partial Type Section  Sea cliffs, at Crackington Haven, Cornwall. Freshney and Taylor (1972); McKeown et al, (1973). 
Reference Section  Pinhoe Brickpit, Exeter district. 
Reference Section  Railway cuttings of former Teign Valley Railway, near Dunsford. 
Reference(s):
Moore, E W J. 1929. The occurrence of Reticuloceras reticulatum in the Culm of north Devon. Gelogical Magazine, Vol 66, 356-358. 
Ussher, W A E. 1887. The Culm of Devonshire. Geological Magazine, Vol.24, 10-17. 
Webby, B D. and Thomas, J M. 1965. Whitsun field meeting: Devonian of west Somerset and the Carboniferous of north-east Devon. Proceedings of the Geologists Association, Vol. 76, 179-194. 
Prentice, J E. 1960. The Dinantian, Namurian and Lower Westphalian rocks of the region of southwest of Barnstaple, north Devon. Quarterly Journal of the Geological Society, London, Vol.115, 261-290. 
Ashwin, D P, 1958. The coastal outcrop of the Culm Measures of south-west England. Abstracts of the Proceedings of the Conference of Geologists and Geomorphologists in south-west England, Vol.2, 2-3. 
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). 
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. 
Dearman W R and Butcher, N E. 1959. The geology of the Devonian and Carboniferous rocks of the northwest border of the Dartmoor Granite, Devonshire. Proceedings of the Geologists' Association, Vol.70, 51-92. 
Lucas, M D. 1960. The Upper Palaeozoic geology of the Dulverton district. MSc thesis, University of Bristol. 
Macintosh, D M. 1964. The sedimentation of the Crackington Measures. Proceedings of the Ussher Society, Vol.1, 88-89. 
Money, N J. 1966. Carboniferous rocks of north Devon. Proceedings of the Ussher Society, Vol.1, 235-238. 
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. 
Edmonds, E A, McKeown, M C and Williams, M. 1969. British Regional Geology: Southwest England. Third edition [London: HMSO for Institute of Geological Sciences.] 
Ussher, W A E. 1882. The British Culm Measures. Proceedings of the Somerset Archeological and Natural History Society, Vol.38, 111-219. 
Chesher, J A, 1968. The succession and structure of the Middle Teign Valley. Proceedings of the Ussher Society, Vol.2, 15-17. 
Freshney, E C, McKeown, M C and Williams, M. 1972. Geology of the coast between Tintagel and Bude. Memoir of the Geological Survey of Great Britain, part of sheet 322 (England and Wales). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E307 E308 E309 E293 E310 E339 E323