The BGS Lexicon of Named Rock Units — Result Details

Gramscatho Group

Computer Code: GRO Preferred Map Code: notEntered
Status Code: Full, without ratification
Age range: Eifelian Age (DI) — Frasnian Age (DR)
Lithological Description: The Gramscatho Group is present in both an allochthonous and a parautochthonous sequence. In the allochtonous setting of the Veryan Nappe, bound at the base by the Veryan Thrust, the Group comprises (from base to top) Portscatho, Pendower, Carne and Roseland Breccia Formations, respectively. In the parautochthon, the Group is comprised of the Porthtowan Formation. Dark grey and grey-green slates are the major component of the interbedded greywacke sandstone and thin siltstone bands in the various formations within the Gramscatho Group; towards the top of the Group, the sandstones coarsen and their proportion increases. At the top of the Group, the Roseland Breccia is an olistostrome deposit, comprising a grey slaty mudstone with framework and matrix supported polymict conglomerate/breccia.
Definition of Lower Boundary: In the parautochthonous part of the Gramscatho Group, the base of the Group is at the base of the Porthtowan Formation, where it is underlain by the Grampound Formation. The precise nature of the boundary is not known, but the transition will be noted by the change from dark grey mudstone and mid-grey siltstone with thin lenticular beds of limestone (Grampound Formation) to grey and grey-green slaty mudstones with thin bedded medium- to fine-grained sandstone and siltstone turbidites (Porthowan Formation). The lower boundary of the allochthonous parts of the Group is the Carrick Thrust, which has a transgressive ramped contact with the parautochthon; the north of the Lizard it rests on the Porthleven Breccia Member and Mylor Slate Formation; further to the east it rests on Porthtowan Formation and Grampound Formation, respectively.
Definition of Upper Boundary: The top of the Gramscatho Group is placed at the top of the Roseland Breccia Formation; this is seen to the north west of Dodman Point [SW 9920 4042] where the contact is a thrust fault (the Dodman Thrust) with the allochthonous succession of the Gramscatho Group in the footwall. To the north of the contact (the Gramscatho Group) the dominant lithologies are those of the Roseland Breccia Formation (grey slaty mudstone with framework and matrix supported polymict conglomerate/breccia); to the south, meta-sandstone and siltstone turbidites with interbedded phyllitic mudstone.
Thickness: Up to 2500 m.
Geographical Limits: South-west Cornwall. The parautochthon is shown on existing maps to be exposed on the coast from near to Perranporth [SW 7552 5445] south to St Ives Bay [SW 5398 3821]; the allochthonous part of the Group crops out to the north of the Lizard Peninsula and to the north and west of Dodman Point.
Parent Unit: Not Applicable (-)
Previous Name(s): Gramscathos Beds [Obsolete Name and Code: Use GRO] (-3445)
Grampound and Probus Series [Obsolete Name and Code: Use GRO, PORO] (-4266)
Gramscatho Beds [Obsolete Name and Code: Use POAN, GRO] (-2375)
Veryan Limestone [Obsolete Name And Code: Use PWER] (VEL)
Portscatho Beds [Obsolete Name And Code: Use PORO] (PTB)
Falmouth Beds [Obsolete Name And Code: Use PORO] (FAB)
Falmouth Series [Obsolete Name and Code: Use GRO, PORO] (-465)
Manaccan Beds (MAB)
Hayle Sandstones (HLS)
Alternative Name(s): none recorded or not applicable
none recorded or not applicable
Leveridge, B E, Holder, M T and Goode, A J J, 1990. Geology of the country around Falmouth. Memoir of the British Geological Survey, Sheet 352 (England and Wales). 
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. 
Hendriks, E M L, 1931. The stratigraphy of South Cornwall Report of the British Association for 1930, 332 et seq. 
Hendriks, E M L, 1937. Rock Succession and structure in South Cornwall, a revision. With notes on the central European facies and Variscan folding there present. Quarterly Journal of the Geological Society, London, Vol.93, 322-360. 
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. 
British Geological Survey. 2000. Mevagissey. England and Wales Sheet 353. Solid and Drift Geology. 1:50 000. (Keyworth, Nottingham: British Geological Survey.) 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable