The Arfon Group is a stratigraphically defined geological group of rocks that are present in the

Cymru Terrane The Cymru Terrane is one of five inferred fault bounded terranes that make up the basement rocks of the southern United Kingdom. The other notable geological terranes are the Charnwood Terrane, Fenland Terrane, Wrekin Terrane and the Monian Co ...

around Bangor in North

Wales Wales ( cy, Cymru ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by England to the Wales–England border, east, the Irish Sea to the north and west, the Celtic Sea to the south west and the ...

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and North ...

. The group contains volcanogenic conglomeratic sandstone, fine-grained tuffite and tuffaceous

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...

Lithology and outcrop

This

Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...

volcano-sedimentary group incorporates over 4000 m of deposits and were previously considered to be of

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

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 Padarn ( la, Paternus, Padarnus; cy, Padarn; br, Padern; ? – 550 AD) was an early 6th century British Christian abbot-bishop who founded St Padarn's Church in Ceredigion, Wales. He appears to be one and the same with the first bishop of Bra ...

) is exhibited on a ridge between Bangor and

Caernarfon Caernarfon (; ) is a royal town, community and port in Gwynedd, Wales, with a population of 9,852 (with Caeathro). It lies along the A487 road, on the eastern shore of the Menai Strait, opposite the Isle of Anglesey. The city of Bangor is ...

(both located in North Wales) and also on a ridge near

Llyn Padarn Llyn Padarn is a glacially formed lake in Snowdonia, Gwynedd, north Wales, and is an example of a moraine dammed lake. The lake is approximately long (about 240 acres) and at its deepest point is deep, and is one of the largest natural lakes i ...

.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 Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

and exhibits welding and are thought to be rapid deposition of thin air-fall tuffs and rhyolite flows placed in a bounded

half-graben A half-graben is a geological structure bounded by a fault along one side of its boundaries, unlike a full graben where a depressed block of land is bordered by parallel faults. Rift and fault structure A rift is a region where the lithosphere ...

(or

graben In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic contex ...

Zircon (U–Pb) radioisotopic dating

Isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

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 - BG

*The BGS Online Rock Lexico

{{Coord missing, Gwynedd Precambrian Europe Geology of Gwynedd Geological groups of the United Kingdom