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
Alternative Name(s): none recorded or not applicable
Type Area  The Arvon lowland between the Menai Straits and Snowdonia, Gwynedd, Wales. 
Greenly E. 1944. The Arvonian rocks of Arvon. Quarterly Journal of the Geological Society of London, Vol. 100, 269-284. 
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. 
Reedman A J, Leveridge B E and Evans R B. The Arfon Group ("Arvonian")] of North Wales. Proceedings of the Geologists' Association, Vol.95, 313-321. 
Howells, M F, 1985. Geology of the country around Bangor. Memoir of the British Geological Survey, Sheet 106. (England and Wales). 
1:50K maps on which the lithostratigraphical unit is found, and map code used: