The BGS Lexicon of Named Rock Units — Result Details

Teign Valley Group

Computer Code: TEVY Preferred Map Code: notEntered
Status Code: Full
Age range: Famennian Age (DA) — Marsdenian Substage (CZ)
Lithological Description: Predominantly comprises black commonly pyritic mudstones, siliceous mudstones, bedded cherts and some thin limestones. Formations dominated by limestone or sandstone turbidites are locally developed. Shallower water formations comprising greenish mudstones with thin limestones and black mudstones with quartzitic sandstones are confined to north Cornwall. Scattered formations and units of basaltic tuff and lava locally are present on the south crop of the group.
Definition of Lower Boundary: On the north crop in north Devon and west Somerset, the lower boundary is taken at the incoming of black, locally siliceous mudstones of the Teign Valley Group (Doddiscombe Mudstone Formation) above the soft mid to dark grey mudstones with scattered very thin fossiliferous limestones, siltstones and sandstones of the underlying Exmoor Group (Pilton Mudstone Formation). On the south crop in south Devon, the lower boundary is taken at the incoming of black, locally siliceous mudstones of the Teign Valley Group (Combe Mudstone Formation) above the olive green and pale grey mudstones of the Trusham Mudstone Formation. Elsewhere on the south crop the base is thrust out.
Definition of Upper Boundary: Due to thrusting along the south crop of the Group, the upper boundary is seen only between Okehampton (SX 59 54) and the Teign Valley (SX 84 84), east Dartmoor, Devon. The top is gradational and taken at the incoming of dark grey mudstones with abundant thin- to medium-bedded turbidite sandstones of the overlying Holsworthy Group (Crackington Formation) above the dark grey mudstones with sparse scattered thin turbidite sandstones of the Teign Valley Group (Dowhills Mudstone Formation). On the north crop of the Group, between the Bideford area (SS 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 top needs further investigation as the Dowhills Mudstone Formation (Limekiln Beds of Prentice, 1960), was included in the Crackington Formation by Edmonds et al. (1979). Between Bodmin and Dartmoor the group occurs in tectonic slices and no top is seen. The top here is thought to be an unconformity where the overlying Holsworthy Group (Bealsmill Formation) rests unconformably on rocks of the Teign Valley Group, which had already been deformed by D1.
Thickness: 100 to 720m
Geographical Limits: Between Bideford (SS 45 27), north Devon and Westleigh (ST 06 17), west Somerset. Between the north Cornish coast at Boscastle (SX10 91) and east Dartmoor (SX 84 87), south Devon, including the Launceston area (SX 33 84), the St Mellion area (SX 39 65), Okehampton area (SX 59 94), Teign Valley area (SX 84 84), Ilsington area (SX78 76) and south of Ilsington (SX 79 74).
Parent Unit: Culm Supergroup (CU)
Previous Name(s): Lower Culm Measures (-77)
Lower Culm Group [Obsolete Name And Code: See TEVY and BPLM] (LCU)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Middle Teign valley between Dunsford and Chudleigh, south Devon. The valley contains several sections through the Group, which is repeated by folding. Neither lower nor upper contacts are exposed. Selwood et al., 1984. 
Reference(s):
Waters, C N, Brown, M A E, Dean, M T and Powell, J H. In press. Lithostratigraphical framework for Carboniferous successions of Great Britain (Onshore). British Geological Survey Research Report, RR/05/06. 
Ussher, W A E. 1887. The Culm of Devonshire. Geological Magazine, Vol.24, 10-17. 
Ussher, W A E. 1882. The British Culm Measures. Proceedings of the Somerset Archeological and Natural History Society, Vol.38, 111-219. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable