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Devonian

Chronostratigraphy The Devonian crops out principally in six areas in Great Britain, which reflect the original sedimentary basins in which they were deposited. These are 1) the Orcadian Basin in Orkney, Shetland and NE Scotland; 2) the Midland Valley of Scotland, 3) the Scottish Border Basin (Scottish Borders and Northumberland ), 4) the Mell Fell Trough of the southern Lake District 5) the Anglo-Welsh Basin in S. Wales, the Welsh Borderlands and Bristol 6) the basins of SW England.

The Devonian in Britain is divided into a predominantly marine successions in SW England and terrestrial red-bed successions (the Old Red Sandstone, ORS) in the remainder of England, Scotland, Wales and N. Ireland. The marine Devonian rocks of South-West England have been widely regarded as having a palaeogeographic location mirroring their current position, adjacent to the non-marine ORS rocks of south Wales, where fluvial and lacustrine Old Red Sandstone deposits pass southwards, inter-fingering with shallow-marine deposits, in south Devon and southern Cornwall. However, regional tectonic reconstructions suggest that the juxtaposition of SW England and S. Wales might not have occurred until the Late Carboniferous.

The Devonian System defined in fully marine, graptolite-bearing successions now places the basal parts of the Old Red Sandstone in the Silurian. The strata from the Ludlow Bone Bed (traditional base of Old Red Sandstone) up to the base of the Devonian, in the Old Red Sandstone, belong to the Pridoli Stage, the uppermost Stage of the Silurian. The stages of the Devonian, applied to the terrestrial Old Red Sandstone succession are applied with difficulty because of the absence of marine fossils.

In the marine Devonian, there are biostratigraphical zonal schemes for graptolites, ammonoids, brachiopods, fish, conodonts, microvertebrates and miospores. Lower Devonian Lochkovian and Pragian stages are defined in the deep-water, graptolite-bearing succession of the Prague Basin in the Czech Republic. The highest Lower Devonian stage is the Emsian, defined in Belgium. The Middle Devonian stages (Eifelian and Givetian) are defined in Germany, and Upper Devonian stages (Frasnian and Famennian) are named from the carbonate-bearing marine succession of southern Belgium.

The problems of correlation of the Old Red Sandstone of Great Britain are inherent in its terrestrial origins and the patchy preservation of its non-marine fossils. The principal macrofossils are fish fragments which have been used in erecting zonations for the Old Red Sandstone in the Anglo-Welsh Basin, but the occurrence of fish remains is patchy and of limited use in high-resolution correlation. Miospore classifications and microvertebrate classifications are also used, although problems of detailed correlation of the ORS successions with the Bohemian, German and Belgian marine stages, in which miospores are rare, is problematic. Correlations based on miospores (common to the Old Red Sandstone) and German marine succession, allows correlation of the Anglo-Welsh and Rhenish Gedinnian–Siegenian successions. A widespread volcanic ash deposit (the Townsend Tuff Bed) and a basin-wide calcrete (the Psammosteus Limestone) are valuable marker horizons in the Anglo-Welsh Basin, providing lithostratigraphical correlation of the successions.

Further Information
Barclay, W.J., Browne, M.A.E., 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, Joint Nature Conservation Committee, Peterborough, 393pp

Leveridge, B. E. & Hartley, A. J. (2006). The Variscan Orogeny: the development and deformation of Devonian/Carboniferous basins in SW England and South Wales. In: The Geology of England and Wales, 2nd Edn. (eds. P.J. Brenchley & P.F. Rawson), pp. 225-255, Geological Society, London.

Leveridge BE, Shail RK. (2011) Marine Devonian stratigraphy of Great Britain, Proceedings of the Geologists' Association. 122, 540-567.
(MWH)