Devonian, introduction, Wales

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From: Howells, M F. 2007. British regional geology: Wales. Keyworth, Nottingham: British Geological Survey.

Distribution of continents in Late Proterozoic and early Palaeozoic time. Note: England and Wales are greatly exaggerated in size (adapted from Mitchell, 2004). P916144.
Vertical sections illustrating the Old Red Sandstone of Wales (adapted from Dineley, 1992). Thickness in metres. P916178.

The Caledonian Orogeny resulted in major changes to palaeogeography and sedimentation patterns across Wales and adjacent areas. The marine basin and adjacent platform that had influenced Lower Palaeozoic sedimentation were uplifted during the collision between Laurentia and East Avalonia. Together with Baltica, these two continents amalgamated to form Laurussia, a continent some distance to the north of Gondwana (P916144). The folded and uplifted sequence was a fragment of the extensive Caledonide mountain chain that developed along the length of the collision zone between Ireland and the Scottish Hebrides, and beyond.

The late Silurian and Devonian Old Red Sandstone rocks of Wales accumulated in the Anglo-Welsh Basin, south of the Caledonides, as the synorogenic to post-orogenic (‘molasse’) deposits of the Caledonian Orogeny. At the beginning of Devonian times, the shoreline lay close to south Devon and Cornwall, and later sedimentation in south-west England, the historical type area of the Devonian System, was dominantly marine in the evolving Rheic Ocean. Northwards from Devon, land was continuous into northern Scotland, and Wales and the adjacent areas became the site of extensive fluvial, alluvial and less common lacustrine sedimentation that characterises the Old Red Sandstone. South Wales lay in an external (extramontane) basin some distance south of the mountain chain, while Anglesey probably lay in an isolated basin at its edge. Drainage dispersal was mainly to the south and east on the northern margins of the Rheic Ocean. Palaeomagnetic evidence suggests that the region lay at subtropical latitudes of about 17°S; recent palaeogeographical reconstructions suggest a position about 30°S.

Detailed sedimentological studies have provided new insights into the environments and shown that contemporaneous fault activity was an important influence. The recognition that the limestones (‘concretionery cornstones’) are fossil carbonate soils (calcretes) has been particularly helpful in the interpretation of the depositional environment. The climate was seasonally wet, semi-arid and tropical. Most of the succession is characteristically red because of its iron oxide content.

Work on palynological (miospore) assemblages has refined stratigraphical correlation of successions in the Anglo-Welsh Basin, as has the recognition of widespread lithological markers such as air fall tuffs and emergent surfaces. However, problems of correlation remain between the terrestrial Old Red Sandstone succession and the standard marine Devonian stages, but progress is being made using miospores. The fossils that are present in the Old Red Sandstone illustrate the profound changes that were taking place in the evolutionary record during Devonian times, with the continued colonisation of terrestrial habitats by vascular plants and the expansion of vertebrates, including their eventual emergence on to land. The culmination of the Caledonian Orogeny in the Early Devonian (Emsian) Acadian Phase resulted in erosion and non-deposition throughout Mid Devonian times in most of Wales. In north Wales the Old Red Sandstone is exposed solely on Anglesey. In central Wales it is exposed in isolated outliers on Long Mountain and Clun Forest. Farther south, it crops out widely in the Black Mountains, Brecon Beacons, the Carmarthenshire Fans and on the limbs of several Variscan folds in southwest Pembrokeshire where it is spectacularly exposed in the sea cliffs (P916178).

Bibliography

ALLEN, J R L. 1974. The Devonian rocks of Wales and the Welsh Borderland. 47–84 in The Upper Palaeozoic and post-Palaeozoic rocks of Wales. OWEN, T R (editor). (Cardiff: University of Wales Press.)

ALLEN, J R L, THOMAS, R L, and WILLIAMS, B P J. 1982. The Old Red Sandstone north of Milford Haven. 123–149 in Geological excursions in Dyfed, south-west Wales. BASSETT, M G (editor). (Cardiff: Published for the Geologists’ Association by the National Museum of Wales.)

BARCLAY, W J, MCMILLAN, A A, PICKETT, E A, STONE, P, and WILBY, P R. 2005. The Old Red Sandstone of Great Britain. Geological Conservation Review Series, No. 31.

DAVIES, J R, MCNESTRY, A, and WATERS, R A. 1991. Palaeoenvironments and palynofacies of a pulsed transgression: the late Devonian and early Dinantian (Lower Carboniferous) rocks of southeast Wales. Geological Magazine, Vol. 128 (4), 355–380.