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: Not Applicable (-)
Previous Name(s): Nant Ffrancon Group (-4217)
Nant Ffrancon Formation (-1724)
Aberdaron Bay Group [Obsolete Name And Code: Use NFR] (ABB)
Alternative Name(s): Includes Aberdaron Bay Group
Aberdaron Bay Group
Stratotypes:
Type Area  South side of Nant Ffrancon, Snowdonia. 
Reference(s):
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. 
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. 
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. 
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). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E119 E106 E094 E133 E134