The BGS Lexicon of Named Rock Units — Result Details
Nant Ffrancon Subgroup
| Computer Code: | NFR | Preferred Map Code: | NFr |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Arenig Series (OR) — Caradoc Series (O4) | ||
| Lithological Description: | Mudstones, silty mudstones and sandstones; bioturbated, finely laminated, graptolitic: some volcanic units named as formations. Note that most of the Subgroup is not divided into formations. The named volcanic formations "float" within the undivided sedimentary Subgroup. | ||
| Definition of Lower Boundary: | Tremadoc-Arenig unconformity which lies at the top of the Dol-Cyn-Afon Formation (Mawddach Group) or top of Marchlyn Formation. Basal unit is Allt Lwyd Formation where present, otherwise undivided Nant Ffrancon Subgroup. | ||
| Definition of Upper Boundary: | Base of the Capel Curig Volcanic Formation (where present), otherwise the base of the Cwm Eigiau Formation. | ||
| Thickness: | >2km, variable | ||
| Geographical Limits: | Northern and central Snowdonia, east of the Harlech Dome. | ||
| Parent Unit: | Ogwen Group (OGW) | ||
| Previous Name(s): | Aberdaron Bay Group [Obsolete Name And Code: Use NFR]
(ABB)
Nant Ffrancon Group [Obsolete Name and Code: Use NFR] (-4217) Nant Ffrancon Formation [Obsolete Name and Code: Use NFR] (-1724) |
||
| Alternative Name(s): | none recorded or not applicable | ||
| Stratotypes: | |||
| Type Area | South side of Nant Ffrancon, Snowdonia. | ||
| Reference(s): | |||
| Young, T P, Gibbons, W and McCarroll, D. 2002. Geology of the country around Pwllheli. Memoir of the British Geological Survey, Sheet 134 (England and Wales). | |||
| Gibbons W and McCarroll D. 1993. Geology of the country around Aberdaron, including Bardsey Island. Memoir of the British Geological Survey, Sheet 133 (England and Wales). (London: H.M.S.O.). 88pp. | |||
| Howells, M F, Leveridge, B E, Addison, R and Reedman, A J. 1983. The lithostratigraphical division of the Ordovician underlying the Snowdon and Crafnant volcanic groups, North Wales. Report of the Institute of Geological Sciences, No. 83/1. | |||
| 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. | |||
| Howells, M F and Smith, M. 1997. Geology of the country around Snowdon. Memoir of the British Geological Survey, Sheet 119 (England and Wales). (London: The Stationery Office). 104pp. | |||
| Rushton, A W A and Howells, M F. 1998. Stratigraphical framework for the Ordovician of Snowdonia and the Lleyn Peninsula. British Geological Survey Research Report, RR/99/08. | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| E119 E106 E094 E133 E134 | |||