The BGS Lexicon of Named Rock Units — Result Details

Sidmouth Mudstone Formation

Computer Code: SIM Preferred Map Code: notEntered
Status Code: Full
Age range: Olenekian Age (TO) — Carnian Age (TC)
Lithological Description: The formation consists of dominantly mudstone and siltstone, red-brown with common grey-green reduction patches and spots. The mudstones are mostly structureless, with a blocky weathering habit, but intervals up to 15m thick of interlaminated mudstone and siltstone occur within parts of the formation, notably in the Bollin and Byley Mudstone members of the Cheshire Basin (Wilson, 1993) and in the Radcliffe Member of the East Midlands (Charsley et al., 1990). Heterolithic units consisting of several thin beds of grey-green dolomitic siltstone and very fine-grained sandstone, interbedded with mudstone, occur at intervals throughout the formation. These units range from a few centimetres up to 4m thick, and locally form mappable cuesta features. Named examples include the Cotgrave Sandstone Member and Clarborough Member of the East Midlands (Charsley et al., 1990; Smith et al., 1973; Warrington et al., 1980) and the Weatheroak Sandstone Member of the Redditch area (Old et al., 1991). Units of halite up to 400m thick are present at several stratigraphical levels in the thicker basinal sequences of the East Irish Sea, Cheshire, Staffordshire, Cleveland, Worcestershire, Somerset and Dorset, though are most prevalent towards the top. Breccias produced by contemporaneous dissolution of salt are common throughout the formation. In Cheshire, more recent salt dissolution (some induced artificially by brine pumping activities) gives rise to dissolution hollows and collapse breccias associated with the halite members. Gypsum/anhydrite also occurs throughout the formation as nodules and veins.
Definition of Lower Boundary: The boundary is both gradational and conformable, and is drawn arbitrarily within an upward-fining, interdigitating succession where mudstones of the Sidmouth Mudstone Formation predominate over the siltstones and fine-grained sandstones of the underlying Tarporley Siltstone Formation; the junction is diachronous. Where the underlying Tarporley Siltstone Formation is absent or not differentiated (including at the type section), the boundary is marked by a rapid upward transition from sandstone- to mudstone-dominated lithologies immediately above the Sherwood Sandstone Group. At outcrop, the boundary is associated with a change from the brown sandy soils of underlying formations to the clayey, distinctively reddish brown soils of the Sidmouth Mudstone Formation. A concave break of slope is associated with the boundary in some areas.
Definition of Upper Boundary: Placed at a rapid upward transition from red mudstones or siltstones to the dominantly grey or green, partly or largely arenaceous beds of the overlying Arden Sandstone Formation. In the Cheshire Basin and parts of the Wessex Basin, where the Arden Sandstone Formation cannot be differentiated within thick halite deposits, the boundary is drawn immediately above the highest halite beds of the Sidmouth Mudstone Formation, which is conformable in all areas. At outcrop, the boundary is typically marked by a change from the reddish-brown clayey soils of the Sidmouth Mudstone Formation to the grey, slightly sandy clay soils of the overlying Arden Sandstone Formation. Where the latter forms a marked cuesta feature, the boundary typically lies at a slight concave break in the scarp slope. In the Cheshire Basin, where the Arden Sandstone Formation is absent, the boundary is mapped at the approximate down-dip limit of subsidence hollows and collapse breccias, which are associated with near-surface solution of halites (Wilkesley Halite Member) at the top of the Sidmouth Mudstone (Taylor et al., 1963).
Thickness: 165m at the type section (Gallois, 2001). Elsewhere, it ranges from 120-130m in the East Midlands, to 1600m in the Cheshire Basin.
Geographical Limits: The surface outcrop extends from the South Devon coast east of Sidmouth northwards into Somerset, Avon, Worcestershire, Warwickshire, Leicestershire, Nottinghamshire and North Yorkshire; and westwards into Derbyshire, Staffordshire, Cheshire and Lancashire. The formation occurs in the subsurface below much of southern England, the south Midlands and eastern England. Equivalents occur below the southern North Sea and eastern Irish Sea.
Parent Unit: Mercia Mudstone Group (MMG)
Previous Name(s): Eldersfield Mudstone Formation [Obsolete Name And Code: Use SIM] (ELM)
Sub-Arden Keuper Marls (-3677)
Lower Keuper Marl(s) (-1822)
Mudstone I (-529)
Lower Keuper Marl (-1702)
Alternative Name(s): Dowsing Formation And Dudgeon Formation
Leyland Fm Plus Preesall Halite Fm, Dowbridge Mudstone Fm And Warton Halite Fm
Type Section  The south Devon Coast between Sidmouth and Weston Mouth. The full sequence, younging gradually eastwards, is well-exposed in a series of cliff sections extending for 3.5km, with only minor disturbance by faulting. Gallois, 2001; Jeans, 1978. 
Reference Section  Eldersfield Borehole (SO73SE/6), near Tewkesbury, from the start of coring at 4.77m (c. 2m below the top of the formation) to the boundary with the underlying Bromsgrove Sandstone Formation at 363.53m depth. Curated core held at the National Geological Records Centre, BGS Keyworth. Barclay et al. (1997), Worssam et al. (1989). 
Wills, L J. 1970. The Triassic succession in the central Midlands in its regional setting. Quarterly Journal of the Geological Society of London, Vol.126, 225-285. 
Gallois, R W. 2001. The lithostratigraphy of the Mercia Mudstone Group (mid to late Triassic) of the south Devon coast. Geoscience in south-west England, Vol.10, 195-204 
Brodie, P B. 1870. On the geology of Warwickshire. 34th Annual Report of the Warwickshire Natural History and Archaeological Society, 10-34. 
Charsley, T J, Rathbone, P A and Lowe, D J, 1990. Nottingham: A geological background for planning and development. British Geological Survey Technical Report WA/90/1. 
Jackson, D I, Johnson, H and Smith, N J P. 1997. Stratigraphical relationships and a revised lithostratigraphical nomenclature for the Carboniferous, Permian and Triassic rocks of the offshore East Irish Sea Basin. 11-32 in Petroleum Geology of the Irish Sea and Adjacent Areas. Meadows, N S, Trueblood, S P, Hardman, M and Cowan, G. (editors). Geological Society Special Publication, No.124. 
Jeans, C V. 1978. The origin of the Triassic clay assemblages of Europe with special reference to the Keuper Marl and Rhaetic of parts of England. Philosophical Transactions of the Royal Society of London, Series A, Vol.289, 549-639. 
Taylor, B J, Price, R H and Trotter, F M. 1963. Geology of the country around Stockport and Knutsford. Memoir of the Geological Survey of Great Britain. 
Wills, L J. 1976. The Trias of Worcestershire and Warwickshire. Report of the Institute of Geological Sciences, No.76/2. 
Wilson, A A. 1993. The Mercia Mudstone Group (Trias) of the Cheshire Basin. Proceedings of the Yorkshire Geological Society, Vol.49, 171-188. 
Barclay, W J, Ambrose K, Chadwick, R A and Pharaoh, T C. 1997. Geology of the country around Worcester. Memoir of the British Geological Survey, Sheet 199 (England and Wales). 
Harrison, W J. 1876. On the occurrence of the Rhaetic Beds in Leicestershire. Quarterly Journal of the Geological Society of London, Vol.12, 212-218. 
Johnson, H, Warrington, G and Stoker, S J. 1994. 6. Permian and Triassic of the Southern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. 
Matley, C A. 1912. The Upper Keuper (or Arden) Sandstone Group and associated rocks of Warwickshire. Quarterly Journal of the Geological Society, London, Vol.68, 252-280. 
Smith, E G, Rhys, G H and Goossens, R F. 1973. Geology of the country around East Retford. Memoir of the Geological Survey of Great Britain, Sheet 101 (England and Wales). 
Howard, A S, Warrington, G, Ambrose, K, and Rees, J G. 2008. A formational framework for the Mercia Mudstone Group (Triassic) of England and Wales. British Geological Survey Research Report, RR/08/04. 
Old R A, Hamblin, R J O, Ambrose, K, and Warrington G. 1991. Geology of the country around Redditch. Memoir of the British Geological Survey, Sheet 183 (England and Wales). 
Worssam, B C, Ellison, R A and Moorlock, B S P. 1989. Geology of the country around Tewkesbury. Memoir of the British Geological Survey, Sheet 216 (England and Wales). 
Warrington, G, Audley-Charles, M G, Elliott, R E, Evans, W B, Ivimey-Cook, H C, Kent, P E, Robinson, P L, Shotton, F W and Taylor, F M. 1980. A correlation of the Triassic rocks in the British Isles. Special Report of the Geological Society of London, No.13. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E326 E340