The BGS Lexicon of Named Rock Units — Result Details
Trevose Slate Formation [Obsolete: use SAH]
| Computer Code: | TVS | Preferred Map Code: | notEntered |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Eifelian Age (DI) — Frasnian Age (DR) | ||
| Lithological Description: | [Obsolete: use SAH] Regular lamination comprising alternating grey and darker grey laminae; paler laminae consist of fine grained siltstone, and the the darker laminae are silty mudstone. Paler laminae typically have sharper bases and gradational tops, so the sequence is comprised of two-laminae couplets (commonly less than 1 mm thick and laterally persistant). Beds of structureless siltstone are regularly interbedded with laminated facies; between 0.03 and 0.1 m thick, with planar bedding surfaces. In the upper part of the succession (but below the Marble Cliff Limestone), sporadic laminae and thin laminated beds of limestone up to 0.10 m thick, with a diverse and commonly pyritised fauna. Above the Marble Cliff Limestone Member, laminated beds are less common and the formation is dominated by more homogeneous beds and thin beds of limestone, lava, tuff and volcaniclastic rocks up to 1.5 m thick occur sporadically. Pale buff siltstone beds are developed towards the top of the Trevose Slate east of the Camel Estuary. | ||
| Definition of Lower Boundary: | The lower boundary of the Trevose Slate Formation is marked by a conformable junction with the underlying Bedruthan Formation in the cliffs to the south of Lower Butter Cove, at [SW 8487 6993], near Queen Bess Rock. The transition in the field would be noted by a change from the grey-buff fine-grained sandstones with siltstone at the top of the Bedruthan Formation to the dark grey homogeneous mudstones with well-spaced pale grey laminae and sporadic lenticular bands of black slate at the base of the Trevose Slate Formation. | ||
| Definition of Upper Boundary: | There is a conformable transition from the underlying Trevose Slate Formation to the overlying Harbour Cove Slate Formation. At the top of the Trevose Slate Formation, pale buff siltstones are present to the east of the Camel Estuary, associated with a 1 m thick debris flow deposit consisting of matrix-supported clasts of siltstones and shale up to 0.03 m in diameter. In the overlying Harbour Cove Slate Formation, the dominant lithologies at the base of the Formation are grey and green slates with variable colour banding and dark grey pyritous horizons with ammonoids. | ||
| Thickness: | Up to 3900 m. | ||
| Geographical Limits: | Exposure along the north Cornwall coast from near Queen Bess Rock [SW 8487 6993] to south of Gunver Head [SW 8940 7700]. Inland, a broad outcrop extending to the east through Wadebridge and to the margin of the Bodmin Granite. The Trevose Slate is not defined on the Bodmin sheet (sheet 347) to the south, so the exposure of Trevose Slate terminates at the southern boundary of the Trevose Head and Camelford sheet. | ||
| Parent Unit: | none recorded or not applicable | ||
| Previous Name(s): | Grey Slate [Obsolete Name and Code: Use TVS]
(-2505)
Porthcovan Slate Formation (PCOT) Treyarnon Slate Formation (TREY) Constantine Bay Slate Formation (CONBS) Treyarnon Slates (TREYS) Longcarrow Cove Tuff Beds (LCTB) Marble Cliff Beds (MCLFB) Light Grey Slate [Obsolete Name and Code: Use TVS] (-1245) C2 and C2-3 Slate [Obsolete Name and Code: Use TVS] (-3734) |
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| Alternative Name(s): | none recorded or not applicable | ||
| Stratotypes: | |||
| Type Section | Coastal section from the cliffs near to Queen Bess Island to the bay south of Gunver Head. Selwood et al., 1998. | ||
| Reference(s): | |||
| Ripley M J, 1964. The geology of the coastal area in the vicinity of Newquay, Cornwall. Unpublished PhD thesis, University of Birmingham. | |||
| Selwood, E B, Thomas, J M, Williams, B J, Clayton, R, Durning, B, Smith, O and Warr, L N, 1998. Geology of the country around Trevose Head and Camelford. Memoir of the British Geological Survey, sheets 335 and 336 (England and Wales). | |||
| House, M R, 1961. The Devonian successions of the Padstow area. Abstract of the Proceedings of the 3rd conference on the geology and geomorphology of south-west England, 4-5. | |||
| Beese, A P. 1982. The argillite facies of the Middle Devonian succession in north Cornwall. Proceedings of the Ussher Society. Vol. 5, 321-332. | |||
| Gauss, G A, 1966. Some aspects of slaty cleavage in the Padstow area of North Cornwall. Beese, A P, 1982. The Argillite facies of the middle Devonian succession in north Cornwall. Proceedings of the Ussher Society, Vol.5, 321-332. | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| none recorded or not applicable | |||