The BGS Lexicon of Named Rock Units — Result Details
Clwyd Limestone Group
| Computer Code: | CLWYD | Preferred Map Code: | notEntered |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Visean Age (CV) — Visean Age (CV) | ||
| Lithological Description: | The Visean succession of North Wales comprises a sequence of dominantly shallow marine ramp and platform carbonates defined as the Clwyd Limestone Group. The succession comprises the Foel Formation, overlain by the Llanarmon Limestone Formation. This is overlain by the Leete Limestone Formation and in turn by the Loggerheads Limestone Formation. The upper part of the group comprises the Cefn Mawr Limestone Formation. This is locally overlain by the Red Wharf Limestone Formation or Minera Formation. A diverse range of limestone facies with subordinate sandstone and mudstone units, and exhibiting local dolomitisation. Visean 'knoll reef' limestones are recognised in the Llandudno and Prestatyn areas (Warren et al., 1984) and channelised sandstone and conglomerate bodies are a feature of the succession exposed on Anglesey (Davies, J R, 1991). The group records the initiation and growth of a carbonate platform along the northern flank of the Wales-Brabant Massif. | ||
| Definition of Lower Boundary: | The group rests locally upon several formations formerly collectively known as 'Basement Beds', or elsewhere with marked unconformity on Silurian rocks. | ||
| Definition of Upper Boundary: | Conformable base of Pentre Chert Formation or Cefn-y-fedw Sandstone Formation in northeast Wales. Elsewhere marked by an unconformity with younger Carboniferous or Permo-Triassic rocks. | ||
| Thickness: | The group is up to 900 m thick. | ||
| Geographical Limits: | Crops out on Anglesey (SH 50 80) and on the North Wales mainland between Bangor (SH 54 70) and Oswestry (SJ 27 23), and on the Corwen Outlier (SJ 05 43). | ||
| Parent Unit: | Carboniferous Limestone Supergroup (CL) | ||
| Previous Name(s): | Mountain Limestone [Obsolete Name and Code: Use CLWYD]
(-2100)
Gronant Group [Obsolete Name and Code: Use CLWYD, CRAV] (GRN) Carboniferous Limestone Series [Obsolete Name and Code: Use CLWYD, PKLM] (-2703) Dyserth Limestone Group [Obsolete Name and Code: Use CLWYD] (DYL) |
||
| Alternative Name(s): | none recorded or not applicable | ||
| Stratotypes: | |||
| Type Area | The Dinantian outcrop between Prestatyn and Llandegla in North Wales (see also component formations for reference sections). | ||
| Reference(s): | |||
| Waters, C N, Smith, K, Hopson, P M, Wilson, D, Bridge, D M, Carney, J N, Cooper, A H, Crofts, R G, Ellison, R A, Mathers, S J, Moorlock, B S P, Scrivener, R C, McMillan, A A, Ambrose, K, Barclay, W J, and Barron, A J M. 2007. Stratigraphical Chart of the United Kingdom: Southern Britain. British Geological Survey, 1 poster. | |||
| Davies, J R, Riley, N J and Wilson, D, 1989. Distribution of Chadian and earliest Arundian strata in North Wales. Geological Journal, Vol 24/1, 31-47. | |||
| Earp, J R and Taylor, B J, 1986. Geology of the country around Chester and Winsford. Memoir of the British Geological Survey, Sheet 109 (England and Wales). | |||
| George, T N, Johnson, G A L, Mitchell, M, Prentice, J E, Ramsbottam, W H C, Sevastopulo, G D and Wilson, R B. 1976. A correlation of the Dinantian rocks of the British Isles. Special Report of the Geological Society of London, No 7. | |||
| Warren, P T, Price, D, Nutt, M J C and Smith, E G. 1984. Geology of the country around Rhyl and Denbigh. Memoir for 1:50 000 geological sheets 95 and 107 and parts of sheets 94 and 106. | |||
| Davies, J R. 1991. Karstification and pedogenesis on a late Dinantian carbonate platform, Anglesey, north Wales. Proceedings of the Yorkshire Geological Society, Vol. 48, 297-321. | |||
| Magraw, D, and Ramsbottom, W H C. 1956. A deep borehole for oil at Croxteth Park, near Liverpool. Liverpool and Manchester Geological Journal, Vol. 1, 512-535. | |||
| Somerville, I D, and Strank, A R E. 1984a. The recognition of the Asbian/Brigantian boundary fauna and marker horizons in the Dinantian of North Wales. Geological Journal, Vol. 19, 227-237. | |||
| Waters, C N, Waters, R A, Barclay, W J, and Davies, J R. 2009. Lithostratigraphical framework for Carboniferous successions of Southern Great Britain (Onshore). British Geological Survey Research Report, RR/09/01. 184pp. | |||
| Davies, J R, Wilson, D and Williamson, I T. 2004. Geology of the country around Flint. Memoir for 1:50 000 Geological Sheet 108. | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| E096 E093 E094 E095 E105 E106 E107 E108 E120 E121 E137 | |||