The BGS Lexicon of Named Rock Units — Result Details

Teign Valley Group and Tintagel Group (Undifferentiated)

Computer Code: TEVYT Preferred Map Code: notEntered
Status Code: Full
Age range: Courceyan Substage (CF) — Arundian Substage (CJ)
Lithological Description: Tintagel Group comprises grey-black slates with siltstones and carbonate nodules, and limestone; coarse- and fine-grained tuffs also occur with agglomerate and lavas. Teign Valley Group predominantly comprises black commonly pyritic mudstones, siliceous mudstones, bedded cherts and some thin limestones. 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. In the west, around Tintagel, the base is taken at the base of grey-black slates of the Barras Nose Formation where they rest apparently conformably on grey slates of the Yeolmbridge Formation.
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. 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: Southwest England in Culm Basin from Tintagel to Barnstaple, south to Tavistock and east to Exeter; present at depth south and east of Taunton.
Parent Unit: none recorded or not applicable
Previous Name(s): none recorded or not applicable
Alternative Name(s): Teign Valley Group
Tintagel Group
Stratotypes:
none recorded or not applicable
Reference(s):
British Geological Survey. 2007. Bedrock Geology UK South. 1:625 000 scale (Keyworth, Nottingham: British Geological Survey). 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable