The BGS Lexicon of Named Rock Units — Result Details

Teign Chert Formation

Computer Code: TCH Preferred Map Code: TCh
Status Code: Full
Age range: Visean Stage (CV) — Visean Stage (CV)
Lithological Description: Thin to medium bedded cherts varying from white, grey and green to bluish black; rarely jasper red. Beds range from a few millimetres to 0.6m, generally separated by siliceous mudstone beds and partings. Radiolaria are locally abundant in the cherts and siliceous mudstones. Also present are bluish black mudstone and thin to medium-bedded argillaceous limestones. Some areas are dominated by mudstone, others by chert or limestone. Manganiferous beds are present in the Teign valley. Various volcanic rocks are present in the Teign valley and Ilsington area, south Devon and in the St Mellion area of east Cornwall. They include quartz-keratophyre tuffs, basic spilitic, and felsitic lavas. The spilitic lavas occur near the base of the formation in the Teign valley and are up to 12m thick; units of tuff up to 12m thick occur throughout. In the Teign valley the succession comprises interbedded cherts and subordinate mudstones and is capped by the 30 to 45m-thick Posidonia Beds, a distinctive unit of thin bedded cherts, limestones and shales with abundant pectinoid bivalves and spirally striated goniatites. South of Ilsington, south Devon, a 10m-thick unit (the "Ararat Slate") of black siliceous slatey mudstones, locally containing decalcified nodules, is present at the lowermost part of the formation. In the Okehampton area, north Dartmoor, the formation comprises interbedded cherts, limestones and mudstones. Locally limestones and mudstones are dominant and reported to occur in mappable lenticular units. This suggests that some cherts might be silicified limestones. The Posidonia Beds have not been mapped but their fauna occurs in the limestone lenses. In the St Mellion area of east Cornwall, the lower part of the formation comprises siliceous mudstone with or without scattered cherts beds and the upper part comprises cherts with thin mudstone interbeds. The Posidonia beds are present. In the Boscastle area, north Cornwall, the formation is dominated by mudstone; the Wilsey
Definition of Lower Boundary: In the Teign valley and Ilsington area the lower boundary is taken at the incoming of interbedded cherts and siliceous mudstones of the Teign Chert Formation above the bluish black and black mudstones of the underlying Combe Mudstone Formation. Boundary taken at the base of the first bedded chert. South of Ilsington, it is taken at the sharp boundary of the black mudstones of the "Ararat Slate" (Teign Chert Formation) or the buff weathering calcareous laminated siltstones at the top of the underlying "Rora Slate" (?Torpoint Formation). In the Okehampton area it is taken at the incoming of black argillaceous limestones with subordinate black cherts and siliceous mudstones and mudstones of the Teigh Chert Formation above the thin bedded mudstones and quartzites of the Meldon Shale and Quartzite Formation. Along the rest of the south crop of the Teign Valley Group, between Dartmoor and Bodmin moor and the north Cornish coast, the formation occurs in thrust slices and basal contacts are tectonic.
Definition of Upper Boundary: In the Teign valley the upper boundary is taken at the incoming of bluish grey micaceous mudstones with scattered thin sandstones and siltstones of the overlying Aston Mudstone Member of the Dowhills Mudstone Formation above the fossiliferous interbedded cherts, limestones and mudstones (Posidonia beds) of the Teign Chert Formation. In the Okehampton and Ilsington areas, the Dowhills Mudstone Formation has not been recognized, Crackington Formation being mapped overlying the Teign Chert Formation. Further work is needed in these areas at this level. Along the rest of the south crop of the Teign Valley Group, the formation occurs in thrust slices and the upper contact is tectonic.
Thickness: 150 to 230m in the Teign valley. South of Ilsington about 70m+ thick; in the Okehampton area it is 73m; a structural thickness of 100m has been calculated in the St Mellion area, east Cornwall.
Geographical Limits: Middle Teign Valley, Devon.
Parent Unit: Teign Valley Group (TEVY)
Previous Name(s): Barracados Cherts (BCDC)
Cannapark Limestone (CNPL)
Firebeacon Chert (-2477)
Chert Series (-4920)
Newton Chert Member (Of Brendon Formation) (-4921)
Newton Chert Formation [Obsolete Name And Code: Use COH] (NCH)
Pillaton Chert Formation (-4364)
Spara Bridge Formation (-1764)
Meldon Chert Formation [Obsolete Name And Code: Use COH] (MCF)
Lower Culm Measures (Pars) (-3354)
Chert Bodies In Crocaden Sandstone Formation (-1859)
Calcareous Group (Lower Culm Measures) (-3076)
Mount Ararat Chert (-3077)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Road section (B3193), west of Lower Ashton, Teign valley, south Devon. Most of the formation is exposed, from the junction with the underlying Combe Mudstone Formation up to the lowermost part of the Posidonia beds. The contact with the overlying Aston Mudstone Member of the Dowhills Mudstone Formation is not seen. The section mainly comprises cherts with subordinate siliceous shales. Two mapped tuff units are seen. Limestones are restricted to the Posidonia beds. 
Type Section  Supplementary section. Teign Lane, east of Crockham Quarry. 
Reference(s):
Ussher, W A E, 1892. The British Culm Measures. Proceedings of the Somerset Archeological and Natural History Society, Vol.38, 111-219. 
Selwood, E B, Edwards, R A, Simpson, S, Chesher, J A, Hamblin, R J O, Henson, M R, Riddolls, B W and Waters, R A. 1984. Geology of the countryside around Newton Abbot. Memoir of the British Geological Survey, Sheet 339 (England and Wales). 
Leveridge B E, Holder, M T, Goode, A J J, Scrivener R C, Jones N S and Merriman, R J. 2002. Geology of the Plymouth and south-east Cornwall area. Memoir of the British Geological Survey, Sheet 348 (England and Wales). 
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). 
Morton, R D. 1958. The stratigraphy, structure and igneous rocks of the Central Teign Valley. Unpublished Ph.D. Thesis, University of Nottingham. 
Dearman, W R. 1959. The structure of the Culm Measures at Meldon, near Okehampton, north Devon. Quarterly Journal of the Geological Society of London, Vol.115, 65-106. 
Ussher, W A E. 1887. The Culm of Devonshire. Geological Magazine, Vol.24, 10-17. 
Chesher, J A, 1968. The succession and structure of the Middle Teign Valley. Proceedings of the Ussher Society, Vol.2, 15-17. 
McKeown, M C, Edmonds, E A, Williams, M, Freshney, E C and Masson Smith, D J. 1973. Geology of the country around Boscastle and Holsworthy. Memoir of the British Geological Survey, sheets 322 and 323 (England and Wales). 
Selwood, E B. 1958. The Upper Devonian and Carboniferous geology of the Launceston area of Cornwall. Unpublished PhD thesis, University of Bristol. 
Whiteley, M J. 1983. The geology of the St Mellion outlier, Cornwall and its regional setting. Unpublished PhD thesis, University of Exeter. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E339