Arfon Group

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. [1]

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. [2] 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.[3] 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).[2]

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 Ma[4] and 604.7±1.6 Ma[5] and as such confirms a Neoproterozoic age.[3]

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References
  1. http://www.bgs.ac.uk/Lexicon/lexicon.cfm?pub=ARN
  2. 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
  3. P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd Ed
  4. R. 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
  5. W. 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.
  • The Geology of Wales - BGS
  • The BGS Online Rock Lexicon

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