The BGS Lexicon of Named Rock Units — Result Details
Porthleven Breccia Member
| Computer Code: | PBR | Preferred Map Code: | notEntered |
|---|---|---|---|
| Status Code: | Full, without ratification | ||
| Age range: | Famennian Age (DA) — Famennian Age (DA) | ||
| Lithological Description: | Mylor Slate Formation lithofacies (dark grey slate with pale siltstone laminae of variable spacing with some grading where the laminae are wider, thin beds of siltstone and fine sandstone and grey-green slate interbedded with siltstone or fine sandstone) as the matrix to a variably framework or matrix-supported breccia deposit. The clasts, of varying sizes up to 2 m in length, are of Portscatho Formation lithofacies - clasts include sandstone, siltstone, mudstone and trachyte. | ||
| Definition of Lower Boundary: | Thrust contact; Porthleven Breccia Member is in the footwall of the Carrick Thrust, with Portscatho Formation rocks structurally above. This contact would be identified in the field by an abrupt transition from the dark slate matrix and varied clasts of the Breccia Member (described above) to the sandstone turbidites with interbedded grey slate of the Portscatho Formation. | ||
| Definition of Upper Boundary: | Conformable and gradational with the lower part of the Mylor Slate Formation. Transitional relationship from dark grey, locally grey-green slates, interbedded thin bands and laminae of sandstone, basic lavas and sedimentary breccias to the overyling framework and matrix supported breccias with coarse sandstone clasts and grey slate matrix (as described above). | ||
| Thickness: | Less than 3300 m. | ||
| Geographical Limits: | South-west Cornwall; in the area around Falmouth. | ||
| Parent Unit: | Mylor Slate Formation (MRSL) | ||
| Previous Name(s): | Mylor Series
(MRSR)
Mylor Slates [Obsolete Name and Code: Use MRSL, PBR] (-407) |
||
| Alternative Name(s): | none recorded or not applicable | ||
| Stratotypes: | |||
| Reference Section | Foreshore sections at Flushing and Falmouth. Leveridge and Holder, 1985. | ||
| Reference(s): | |||
| Holder, M T and Leveridge, B E, 1986. A model for the tectonic evolution of south Cornwall. Journal of the Geological Society, London, Vol.143, 125-134. | |||
| Hill, J B and MacCallister, D A. 1906. The geology of Falmouth and Truro and of the mining district of Camborne and Redruth. Memoirs of the Geological Survey. Explanation of Sheet 352 (England and Wales). | |||
| Leveridge B E and Holder M T. 1985. Olisostromic Breccias at Mylor/Gramscatho boundary, south Cornwall. Proceedings of the Ussher Society, Vol. 6, 147-154. | |||
| Reid, C and Flett, J S. 1907. Geology of the Land's End district. Memoir of the Geological Survey of Great Britain. Sheets 351 and 358. | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| E352 | |||