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Permian

Chronostratigraphy
Permian rocks occur widely in the British Isles, in a number of largely separate depositional basins, from the Hebrides and west of Scotland, through Northern England, the English Midlands to Devon. An angular unconformity between folded and faulted older strata (Carboniferous and older) and a horizontal or shallow-dipping Permian succession is commonly seen throughout Britain (e.g. SW and NW England). Correlation and dating of the British Permian red beds has long been problematic, and historically the successions can be primarily divided into two main sub-divisions. The lower ‘continental’ division comprises red alluvial breccias, red fluvial and aeolian sandstones and basaltic volcanic lavas (and associated intrusives), which represent perhaps most of the Lower and Middle Permian. In most areas this lower ‘continental’ division is probably punctuated by major unconformities and disconformities. There are some locations (e.g. the English Midlands) where latest Carboniferous red-beds pass into assumed Cisuralian (Lower Permian) strata of similar lithology. The upper (‘marine’) division is represented by the Zechstein Group, which comprising marine carbonates, evaporites and mudstones, which perhaps represent probably only a part of the Lopingian (Upper Permian).

In the lower ‘continental division’ there is a general tendency in each UK basin for water-laid sands and breccias to be succeeded by aeolian sands (reflecting a decrease in rainfall and relief through time), these in turn are followed by water-laid sediments, indicating an increase in rainfall, suggesting synchronous long-time-scale climatic and landscape change expressed in the sediments. The age of the lower continental division has been primarily obtained through radiometric dating of associated volcanic and igneous units, particularly in SW England (the Exeter Group). The base of the Permian succession in the English Midlands is tentatively placed at the base of the Clent Formation (equivalent alluvial breccia units are found throughout the Midlands), where fossil amphibians and reptiles in the overlying Kenilworth Sandstone Formation constrain the younger age of the Clent Formation. In the Mauchline Basin in south-west Scotland, plant fossils and radiometric dating of associated lavas suggest that the lowest red beds are probably of earliest Permian (earliest Cisuralian) age, and this is broadly accepted for other basins that contain similar red beds.

Dating of the Zechstein Group has been primarily performed through well-log correlation to somewhat poorly dated successions (but better than UK) using conodonts, ammonoids and bivalves in successions from Germany and Poland. The primary problem with dating the Zechstein is the faunas are not particularly comparable to the biostratigraphy of the Permian marine stages, which are mostly based around low latitude fully marine Tethyan faunas.

Further Information

Benton, M., Cook, E. & Turner, P., (2002). Permian and Triassic Red Beds and the Penarth Group of Great Britain, Geological Conservation Review Series, No. 24, Joint Nature Conservation Committee, Peterborough, 337 pages.

Powell, J.H., Chisholm, J.I. Bridge, D. McC, Rees, J.G. Glover, B.W., Besley, B.M. (2000). Stratigraphical framework for Westphalian to early Permian red-bed successions of the Pennine basin . BGS research report, RR/00/01.

Ruffell, A.H., Holliday, D.W. & Smith, D.B. (2006) Permian: arid basins and hypersaline seas. In: The geology of England and Wales, 2nd Edn. (eds. P.J. Brenchley & P.F. Rawson), pp. 269-294, Geological Society, London.

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