The BGS Lexicon of Named Rock Units — Result Details

Arfon Group

Computer Code: ARN Preferred Map Code: notEntered
Status Code: Pending Upgrade
Age range: Neoproterozoic Era (AZ) — Cambrian Period (E)
Lithological Description: Welded, felsic ash-flow tuff, volcanogenic conglomeratic sandstone, fine-grained interlaminated tuffite and tuffaceous sediment.
Definition of Lower Boundary: Drawn at the base of welded ash-flow tuffs of the Padarn Tuff Formation (base not exposed). (Correlative of Padarn Tuff Formation = Bwlch Gwyn Felsite, Anglesey).
Definition of Upper Boundary: Drawn at conformable upward passage from siltstone and sandstone of the Fachwen Formation (east of Aber-Dinlle Fault) or tuff and tuffite of the Bangor Formation (west of Aber-Dinlle Fault) to purple-red siltstone of the Llanberis Slates Formation.
Thickness: Almost 4000m - derived from geophysical evidence as base in not exposed.
Geographical Limits: Anglesey and Gwynedd, Wales.
Parent Unit: Not Applicable (-)
Previous Name(s): Arvonian [Obsolete Name And Code: Use ARN] (A)
Arvonian Formation (-4435)
Alternative Name(s): none recorded or not applicable
Type Area  The Arvon lowland between the Menai Straits and Snowdonia, Gwynedd, Wales. 
Hicks, H, 1878. On some Pre-Cambrian areas in Wales. Geological Magazine (2), Vol.5, 460-461. 
Greenly, E. 1930. A question of nomenclature. Geological Magazine, Vol.67, 287-288. 
Howells, M F, 1985. Geology of the country around Bangor. Memoir of the British Geological Survey, Sheet 106. (England and Wales). 
Reedman, A J, Leveridge, B E and Evans, R B. 1984. The Arfon Group (Arvonian?) of North Wales. Proceedings of the Geologists' Association, Vol.95, 313-321. 
Greenly, E. 1944. The Arvonian rocks of Arvon. Quarterly Journal of the Geological Society, London, 100, 269-287. 
1:50K maps on which the lithostratigraphical unit is found, and map code used: