The Welsh Basin was a northeast-southwest aligned back-arc depositional basin during the

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 ...

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

and

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

periods during which a considerable thickness of marine

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 ...

s was laid down in the area. To the southeast lay the Midland Platform (a part of the micro-continent of Avalonia) and to the northwest, within the Iapetus Ocean, through what is now

Ireland Ireland (, ; ; Ulster Scots dialect, Ulster-Scots: ) is an island in the North Atlantic Ocean, in Northwestern Europe. Geopolitically, the island is divided between the Republic of Ireland (officially Names of the Irish state, named Irelan ...

and the

Lake District The Lake District, also known as ''the Lakes'' or ''Lakeland'', is a mountainous region and National parks of the United Kingdom, national park in Cumbria, North West England. It is famous for its landscape, including its lakes, coast, and mou ...

, was an

island arc Island arcs are long archipelago, chains of active volcanoes with intense earthquake, seismic activity found along convergent boundary, convergent plate tectonics, tectonic plate boundaries. Most island arcs originate on oceanic crust and have re ...

; a northeast-southwest aligned Irish Sea landmass which was associated with volcanic activity. Examination of the sediments and associated fossils allows the deeper centre of the basin to be distinguished from shallower 'platform' areas along its southeastern margins. From the middle Silurian onwards, collision of Avalonia with the more northerly continent of Laurentia occurred giving rise to the Caledonian Orogeny. The inversion of the basin occurred at that time i.e. its uplift and deformation. The southeastern limit of the Welsh Basin can be defined as the Welsh Borderland Fault Zone comprising the Pontesford-Linley Fault and Church Stretton Fault which stretch from

Pembrokeshire Pembrokeshire ( ; ) is a Principal areas of Wales, county in the South West Wales, south-west of Wales. It is bordered by Carmarthenshire to the east, Ceredigion to the northeast, and otherwise by the sea. Haverfordwest is the largest town and ...

Shropshire Shropshire (; abbreviated SalopAlso used officially as the name of the county from 1974–1980. The demonym for inhabitants of the county "Salopian" derives from this name.) is a Ceremonial counties of England, ceremonial county in the West M ...

and, in the northwest, the

Menai Strait The Menai Strait () is a strait which separates the island of Anglesey from Gwynedd, on the mainland of Wales. It is situated between Caernarfon Bay in the south-west and Conwy Bay in the north-east, which are both inlets of the Irish Sea. The s ...

Fault Zone. Its extent to the southwest and northeast is uncertain as these areas are cloaked by more recent rocks. Siliciclastic material was derived from the landmass to its southeast Pretannia, some being deposited in the shallower margins before subsequently being redeposited as

turbidite A turbidite is the geologic Deposition (geology), deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing ...

s as the sediment mass became unstable from time to time. In addition to the

sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...

s of the Welsh Basin, there are a number of volcanic formations within the overall sequence, particularly within the Ordovician system. Typical of the sediments of the Welsh Basin are the

Llandovery Llandovery (; ) is a market town and community (Wales), community in Carmarthenshire, Wales. It lies on the River Tywi and at the junction of the A40 road, A40 and A483 road, A483 roads, about north-east of Carmarthen, north of Swansea and w ...

age Aberystwyth Grits which outcrop along the middle coastal section of Cardigan Bay and comprise alternating mudstones and turbiditic

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

s.Toghill, P. 2000 ''The Geology of Britain: an introduction'' Airlife Publishing.

References

{{reflist Geology of Wales