The Arfon Group is a stratigraphically defined geological group of rocks that are present in the Cymru terrane around Bangor in North

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

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...

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

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

Lithology and outcrop

This

Precambrian The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, 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 t ...

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

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

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 Caernarfon (; ) is a List of place names with royal patronage in the United Kingdom, royal town, Community (Wales), community and port in Gwynedd, Wales. It has a population of 9,852 (with Caeathro). It lies along the A487 road, on the easter ...

(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 ) and at its deepest point is deep, and is one of the largest natural lakes in Wales. ...

.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 depression (geology), depressed block of the Crust (geology), crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German language, German, meaning 'ditch' or 't ...

Zircon (U–Pb) radioisotopic dating

Isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

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 The Neoproterozoic Era is the last of the three geologic eras of the Proterozoic geologic eon, eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era an ...

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