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Triassic

Chronostratigraphy The British Triassic is dominated by red-coloured terrestrial sediments. These formed in a series of linked basins which often initially developed during the Permian and continued to develop during the Triassic. The seven stages of the Triassic Period cannot be adequately identified in the British Triassic, due to the rarity of fossils, hence the British onshore Triassic is broadly divided into three lithostratigrphic units, the Sherwood Sandstone Group, the Mercia Mudstone Group and the Penarth Group, these correspond approximately to the Induan and Olenekian, Anisian through to the Rhaetian, and the latest Rhaetian respectively. The means of defining the age of these units has been multi-fold. Palynology has been the primary tool providing age constraints often at stage level (to better dated Triassic successions in Europe), assisted by regional correlations using borehole sonic and gamma-ray log signatures in the subsurface, to better dated successions often in the southern North Sea. Fossil vertebrates locally provide age dating in some successions in southern England (Otter Sandstone Fm), the Midlands and NW England and in Scotland. Magnetostratigraphy has also been used locally providing direct correlation to well dated marine successions outside Britain. Magnetostratigraphic and biostratigraphic work in Germany accurately places the Permian- Triassic boundary there within the lowest part of the Lower Buntsandstein; which can be correlated using geophysical logs to a level near the base of the UK offshore Bunter Shale Formation- a level which can also be recognised in the East Irish Sea Basin also. In other areas major changes in lithology or well-log response, which mark the base of the Sherwood Sandstone Group, are often chosen as the base of the Triassic.

Fluvial sandstones dominate in the Sherwood Sandstone Group, showing northwards directed river systems sourced from the Variscan uplands of Brittany and Iberia which flowed northwards through the Wessex Basin, towards the East Irish Sea and North Sea basins. By the Middle Triassic (Anisian) this river system had diminished and locally sourced material was more important, with the development locally of lacustrine and marine playa systems in the major British Triassic Basins, often with substantial amounts of halite deposition during the Anisian and Carnian. This loss of far travelled sandy sediment continues through into the Upper Triassic, when most of Britain was dominated by deposition of red to green lacustrine, playa mudstones (Sidmouth and Branscombe Mudstone formations). Exceptions are the Carnian (Arden Sandstone Formation) when widespread grey lacustrine sandstones and mudstones were deposited in basinal areas in southern England. This trend was reversed in the Rhaetian (deposition of the Blue Anchor Formation and Penarth Group) when marine conditions (displayed by the Penarth Group) became widely established again for the first time in the British Mesozoic. The vertebrate and bivalve bearing fossiliferous strata of the Penarth Group has traditionally attracted most of the biostratigraphic work on the British Triassic.

Further Information

Hounslow, M.W. & Ruffell, A.H. (2006). Triassic: seasonal rivers, dusty deserts and saline lakes. In: The geology of England and Wales, 2nd Edn. (eds. P.J. Brenchley & P.F. Rawson), pp 295-324, Geological Society, London.

Howard, A., S. Warrington, G., Ambrose, K. & Rees, J.G
. (2005). A formational framework for the Mercia Mudstone Group of England and Wales. British Geological Survey Research Report.

McKie, T. & Williams, B.P.J. (2009). Triassic palaeogeography and fluvial dispersal across the northwest European Basins. Geological Journal 44, 711-741.
(MWH)