The Anglo-Welsh Basin was one of the larger terrestrial sedimentary basins of Great Britain active during the Devonian period. The sediments which accumulated within it between around 419 and 362 million years ago are now found across a broad swathe of South Wales from Pembrokeshire in the west eastwards to the English border and throughout the southern Welsh Marches.
Traditional divisions
The succession has traditionally been divided into three informal parts; the Lower Old Red Sandstone, Middle Old Red Sandstone and Upper Old Red Sandstone. The Middle unit is largely missing within this basin though is represented by the Ridgeway Conglomerate Formation of southern Pembrokeshire. The Lower Old Red Sandstone was divided into three local stages Downtonian, Dittonian, Breconian and the Upper Old Red Sandstone assigned to the local Farlovian stage. These local stage names continue in informal use though individual formations are now assigned to one of the eight or so standard international stages.
Modern divisions
The Old Red Sandstone is defined as a supergroup. The standard international stages of the late/upper Silurian Pridoli and the lower Devonian Lochkovian, Pragian and Emsian more or less equate to the Lower Old Red Sandstone. The Eifelian, Givetian and Frasnian equate to the Middle ORS and the Fammenian equates to the Upper ORS. A number of fish biozones and fish ranges have been established for the system as have miospore assemblage biozones and thelodont assemblage biozones.
Lower Old Red Sandstone
The Lower Old Red Sandstone comprises a great thickness of sandstones, mudstones and siltstones with occasional calcretes, assigned to the latest Silurian and early Devonian Periods. Marine conditions alternated with sedimentation by braided and meandering rivers during the Pridoli epoch.
Tilestones Formation
The formation is assigned to the Pridoli epoch, the uppermost/latest of the Silurian period.
Temeside Mudstone Formation
The formation is assigned to the Pridoli epoch. It comprises olive green mudstones and mica rich sandstones.
Raglan Mudstone Formation
The formation is assigned to the Lochkovian epoch, the lowermost/earliest of the Devonian period and in earlier literature to the locally defined Downtonian. Consisting largely of mudstones, the formation also includes sandstone beds and calcretes. The latter represent occasional periods of uplift when soils began to develop in an arid climate. They range from discrete calciferous nodules to laterally persistent beds such as the Bishop's Frome Limestone Member, referred to in earlier literature as the Psammosteus Limestone and which is approximatley coincident with the boundary between the Devonian and Silurian periods though it is slightly diachronous.
St Maughans Formation
The formation is assigned to the Lochkovian and Pragian epochs and in earlier literature to the locally defined Dittonian.
Senni Formation
The formation is assigned to the Pragian and Emsian epochs and in earlier literature to the locally defined Breconian.
Brownstones Formation
The formation is assigned to the Pragian and Emsian epochs and in earlier literature to the locally defined Breconian.
Upper Old Red Sandstone
The Upper Old Red Sandstone comprises sandstones largely assigned to the Fammenian epoch of the Upper Devonian. This equates to the upper part of the locally defined Farlovian stage. In the east the succession is much thinner than that of the Lower Old Red Sandstone though it thickens to the west. It unconformably overlies the Lower Old Red Sandstone
Plateau Beds Formation
The Plateau Beds occur eastwards from Milford Haven through Carmarthenshire and into the Brecon Beacons. They comprise up to 58m of sandstones which contain subordinate mudstones but which are also conglomeratic in parts. Their origin is fluvial, aeolian and shallow marine, representing stages in a marine transgression during the Fammenian.
Grey Grits Formation
The Grey Grits comprise up to 20m of quartzitic sandstones which unconformably overlie the Plateau Beds in an outcrop eastwards from the Black Mountain through the Brecon Beacons
Pembrokeshire and Carmarthenshire
Uppermost Silurian strata (assigned to the Pridoli epoch) are present in a band stretching across southern Pembrokeshire from St Brides Bay and Skokholm via Milford Haven to Manorbier and Caldey Island. Similar mudstone strata with occasional sandstoness and conglomerates are present in a second band which extends eastwards from Minwear on the Eastern Cleddau, widening to the south of Narberth and on to southwestern Carmarthenshire. The outcrop continues eastwards from Pendine and Laugharne beneath the Tywi estuary eastwards to the Black Mountain of the Brecon Beacons National Park. [1]
Powys and Brecon Beacons National Park
The Old Red Sandstone achieves its greatest prominence within the Brecon Beacons National Park where it forms the Brecon Beacons themselves, the Black Mountains and the larger parts of Fforest Fawr and the Black Mountain.
To the north the outcrop extends across the Usk Valley and across Mynydd Epynt where the lithologies are predominantly the mudstones with subordinate sandstones of the St Maughans and Raglan Mudstone Formations. The thin Tilestones Formation and overlying Temeside Mudstones Formation lie at the base of the sequence, marking out the northern and western margins of Mynydd Bwlch-y-Groes and Mynydd Epynt. [2]
Gower
Both the upper ORS and the lower ORS are present in Gower. They can be seen at outcrop at Rhossili and along a faulted anticline extending from Llanmadoc Hill through Reynoldston eastwards to near Three Cliffs Bay. The present structure is a legacy of Variscan folding and thrusting. [3]
Gwent and the Vale of Glamorgan
The ORS is present at depth beneath the South Wales Coalfield and appears on its southern margin north of Cardiff and through the Ely valley and in inliers at Cowbridge where strata of the Upper ORS Quartz Conglomerate Group and Cwrt-yr-ala Formation and the Lower ORS Brownstones and Llanishen Conglomerate are faulted and folded. [4] It outcrops extensively to the east of the coalfield, around Newport and across much of Monmouthshire. [5]
References
- ^ British Geological Survey 1:50K map sheets 227 Milford Haven, 228 Haverfordwest & 244/245 Pembroke and Linney Head
- ^ British Geological Survey 1:50K map sheets 196 Builth Wells & 213 Brecon
- ^ British Geological Survey 1:50K map sheets 246 Worms Head & 247 Swansea
- ^ British Geological Survey 1:50K map sheet 262 part 261 Bridgend
- ^ British Geological Survey 1:50K map sheet 249 Newport & 250 Chepstow
| System |
Series (NW Europe) |
Stage (NW Europe) |
Series (ICS) |
Stage (ICS) |
Age ( Ma) |
|---|---|---|---|---|---|
| Carboniferous | younger | ||||
| Devonian | Silesian | Stephanian | Pennsylvanian | Gzhelian | 299–303,9 |
| Westphalian | Kasimovian | 303,9–306,5 | |||
| Moscovian | 306,5–311,7 | ||||
| Bashkirian | 311,7–318,1 | ||||
| Namurian | |||||
| Mississippian | Serpukhovian | 318,1–326,4 | |||
| Dinantian | Visean | Visean | 326,4–345,3 | ||
| Tournaisian | Tournaisian | 345,3–359,2 | |||
| Silurian | older | ||||
| Subdivisions of the Old Red Sandstone in South Wales compared with the official ICS-stages. | |||||