The BGS Lexicon of Named Rock Units — Result Details
Harbour Cove Slate Formation
| Computer Code: | HCSL | Preferred Map Code: | HCSl |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Frasnian Age (DR) — Famennian Age (DA) | ||
| Lithological Description: | Homogeneous green and grey-green slaty mudstones with thin bands of black, commonly pyritous mudstone and yellowish brown sideritic mudstone. At the base of the Formation, there are 40 m of grey and green slates with variable colour banding which contain dark grey pyritous horizons with ammonoids. The higher parts of the Formation contain purple mudstone with small-scale colour banding, local bands of coarse-grained tuffs (up to 100 m thick) and, towards the top, beds of ferruginous and calcareous nodules become increasingly common. Wide variation in slate colour is characteristic of Harbour Cove Slate Formation. Bioturbation is locally abundant; minor slumping is sparadic. The Gravel Carverns Conglomerate is a member in the highest part of the Harbour Cove Slate: it is comprised of a greenish-grey mudstone with thin black calcareous beds and beds of poorly sorted, matrix-supported conglomerate up to 1.6 m thick. The conglomeratic beds contain clasts of well-rounded siliceous mudstone, limestone, quartz-veined conglomerate and a variety of locally-derived igneous rocks. | ||
| Definition of Lower 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 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. | ||
| Definition of Upper Boundary: | The Harbour Cove Slate Formation has a conformable contact with the overlying Polzeath Slate Formation. The upper boundary of the Harbour Cove Slates is placed where the colour changes from dominantly grey-green to purple in a gradational mudstone sequence, with the purple, cleaved slates being attributed to the Polzeath Slate Formation. On the coast, the contact is affected by normal faulting; near Hawker's Cove [SW 9130 7765], amoung other locations, the Harbour Cove Slates are thrust over the Polzeath Slate. | ||
| Thickness: | In excess of 430 m. | ||
| Geographical Limits: | To the west and east of the Camel Estuary, North Cornwall. Most northerly outcrop is in Port Isaac Bay [SX 0168 8130] at Bound's Cliff. | ||
| Parent Unit: | Not Applicable (-) | ||
| Previous Name(s): | Upper Devonian Purple And Green Slate
(-954)
Harbour Cove Slates (HCSLS) Merope Island Beds (MRI) |
||
| Alternative Name(s): | none recorded or not applicable | ||
| Stratotypes: | |||
| Type Section | Cliffs sections between St George's Cove and Harbour Cove. Beese, 1984. | ||
| Reference(s): | |||
| 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. 1984. Stratigraphy of the Upper Devonian argillite succession in north Cornwall. Geological Magazine, Vol. 121, 61-69. | |||
| 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). | |||
| Gauss, G A and House, M R, 1972. The Devonian successions in the Padstow area, north Cornwall. Journal of the Geological Society of London, Vol.128, 151-172. | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| E336 | |||