The BGS Lexicon of Named Rock Units — Result Details
Tredorn Slate Formation
| Computer Code: | TSL | Preferred Map Code: | TSl |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Famennian Age (DA) — Famennian Age (DA) | ||
| Lithological Description: | Greenish-grey slates. Local thin lenticular interbeds of bioclastic limestone and dolomite beds up to 0.15 m thick, with sandstone, siltstone and rare tuff beds. Slate and siltstone are interlaminated in places; within the aureole of the Bodmin granite this gave rise to distinctively banded slates ('Woolgarden Phyllites'). Rust-spotting generated by the oxidation of pyrite and spotting caused by low-grade metamorphism are extensively developed: orthoclase and chloritoid minerals are locally present and may be sufficiently large as to coarsen the texture of the slate. Towards the top of the Formation, a lenticular body of well cleaved, fine-grained bluish grey slate which is hard, close-textured and with a silky lustre: this is the Delabole Member within the Tredorn Slate Formation, which crops out in an arcuate outcrop around the Davidstow Anticline to the north of Camelford [SX 1060 8362]. | ||
| Definition of Lower Boundary: | Thrust boundary with the Trevose Slate or Jacket's Point Formation; normal faulted contact with the granite. Along the thrust contant between Trevose Slate and Tredorn Slate Formation, the contact would be recognised in the field by an abrupt change from the grey and dark grey laminae in silty mudstone with locally paler laminae (Trevose Slate Formation) to the greenish-grey slates with thin local limestone, sandstone, siltstone or tuff beds of the Tredorn Slate Formation (as described in lithology section above). Along the thrust boundary between the Jacket's Point Fornation and the Tredorn Slate Formation, the contact would be recognised in the field by an abrupt change from featureless mid-grey/green mudstone with local fine lamination and sheet or lenticular sandstone bodies (Jacket's Point Formation) to greenish-grey slates with thin local limestone, siltstone or tuff beds of the Tredorn Slate Formation. | ||
| Definition of Upper Boundary: | Faulted contact with either Barras Nose Formation or Tintagel Volcanic Formation. In both cases, an abrupt contact between the Tredorn Slate Formation (greenish grey slates with thin local limestone, siltstone or tuff beds) and the adjacent lithology would be identified. | ||
| Thickness: | Between 500 - 800 m structural thickness. | ||
| Geographical Limits: | North Cornwall. Exposed along the coast from [SX 0368 8345] as far north as [SX 0870 9085]. Inland, the Tredorne Slate Formation crops out to the east in a band that trends approximately east - west. | ||
| Parent Unit: | Not Applicable (-) | ||
| Previous Name(s): | Woolgarden Phyllite [Obsolete Name and Code: Use TVY, TSL]
(-2506)
Delabole Slates [Obsolete Name and Code: Use UDSL, TVY, DLBS, TSL] (-3996) Tredorn Beds [Obsolete Name and Code: Use TSL, TVY] (-4939) Tredorn Phyllites [Obsolete Name and Code: Use TSL, TVY] (-4940) |
||
| Alternative Name(s): | none recorded or not applicable | ||
| Stratotypes: | |||
| Reference Section | Coastal section between Saddle Rocks and California Quarry. Selwood, et al., 1998. | ||
| Type Area | Tredorn, south-east of Boscastle. | ||
| Reference(s): | |||
| 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). | |||
| 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). | |||
| Parkinson, J, 1903. On the geology of the Tintagel and Davidstow district, Quarterly Journal of the Geological Society,Vol.59,408-421. | |||
| Dewey, H, 1909. On Overthrusts at Tintagel (North Cornwall). Quarterly Journal of the Geological Society, Vol.65, 265-280. | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| E322 E336 E337 | |||