The Arfon Group is a stratigraphically defined geological group of rocks that are present in the Cymru terrane around Bangor in North Wales, United Kingdom. The group contains volcanogenic conglomeratic sandstone, fine-grained tuffite and tuffaceous sediment.
Lithology and outcrop This Precambrian volcano-sedimentary group incorporates over 4000 m of deposits and were previously considered to be of Cambrian age. Uranium–lead (U–Pb) isotope ratio data suggests that the whole succession is indeed Precambrian Neoproterozoic age.A. J. Reedman, B. E. Leveridge, R. B. Evans, The Arfon Group (Arvonian) of North Wales., 1984, Proceedings of the Geologists Association, 95, 313-321 Exposure of the lower unit (Padarn Tuff) is exhibited on a ridge between Bangor and Caernarfon (both located in North Wales) and also on a ridge near Llyn Padarn.P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd Ed This is noted to be a thick sequence of acid ash flow tuffs and exhibits welding and are thought to be rapid deposition of thin air-fall tuffs and rhyolite flows placed in a bounded half-graben (or graben).
Zircon (U–Pb) radioisotopic dating Isotope data shows a zircon from the lower part of the succession as having a U–Pb isotope date of 614±2 MaR. D. Tucker & T. C. Pharaoh, U-Pb Zircon ages for late-Precambrian rocks in southern Britain., 1991, Journal of the Geological Society of London, 148, 435-433 and 604.7±1.6 MaW. Compston, A. E. Wright, P. Toghill, Dating the Late Precambrian volcanicity of England and Wales. 2002, Journal of the Geological Society of London. 159 323-339. and as such confirms a Neoproterozoic age.
References
External links The Geology of Wales - BGS The BGS Online Rock Lexicon