The BGS Lexicon of Named Rock Units — Result Details

Sidmouth Mudstone Formation

Computer Code: SIM Preferred Map Code: notEntered
Status Code: Full
Age range: Anisian Age (TA) — Carnian Age (TC)
Lithological Description: Dominantly red-brown mudstone and siltstone with common grey-green reduction patches and spots. The mudstones are mostly structureless, with a blocky weathering habit, but units up to 15 m thick of interlaminated mudstone and siltstone occur in 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; Howard et al., in press). 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 4 m thick, and locally form mappable cuesta features. Named examples include the Cotgrave Sandstone Member and Clarborough Member of the East Midlands (Smith et al., 1973; Warrington et al., 1980; Charsley et al., 1990) and the Weatheroak Sandstone of the Redditch area (Old et al., 1991). The Redcliffe Sandstone Member, a distinctive, deep red calcareous and ferruginous sandstone, is a marginal facies of the formation developed locally in the Bristol area (Kellaway and Welch, 1993). Units of halite up to 400 m thick are present at several stratigraphical levels in the thicker basinal sequences in west Lancashire and south Cumbria, Cheshire, Staffordshire, north-east Yorkshire, Worcestershire, Somerset and Dorset. Breccias produced by contemporaneous solution of salt are common throughout the formation. In the Cheshire Basin, more recent salt solution (some induced artificially by brine pumping activities) gives rise to solution hollows and collapse breccias associated with the halite members. Gypsum/anhydrite also occurs throughout the formation as nodules and veins. Mid Triassic (Anisian) to Late Triassic (Carnian).
Definition of Lower Boundary: Gradational and conformable, and is drawn arbitrarily within an upward-fining succession at the level where mudstones predominate over the siltstones and sandstones of the underlying Tarporley Siltstone Formation; the junction is diachronous. Where the underlying Tarporley Siltstone Formation is 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 (Jones, 1993; Gallois, 2001, 2004). Where not exposed, the boundary is associated with a change from the brown sandy clay soils of underlying formations to the more 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 the 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. At the type section, this boundary coincides with the base of a prominent breccia bed (see sections in Jeans, 1978) at the base of the Arden Sandstone Formation. In the Cheshire Basin and parts of the Wessex Basin, where the Arden Sandstone Formation cannot be recognised in the presence of thick halite deposits, the boundary is drawn immediately above the highest halite beds. The boundary 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: Over 200 m thick in South Devon, and 165 m at its type section (Gallois, 2001). Elsewhere, it ranges from 120 to 130 m in the East Midlands to 370 m in the Worcester Basin and 1600 m in the Cheshire Basin.
Geographical Limits: The surface outcrop extends from the south Devon coast east of Sidmouth northwards into Somerset, Gloucestershire, Worcestershire, Warwickshire, Leicestershire, Nottinghamshire, east and north Yorkshire, and hence westwards into Derbyshire, Staffordshire, north Shropshire, Cheshire and Lancashire. Separate outcrops occur in south Cumbria and the Carlisle Basin. 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. The type area is the South Devon coast.
Parent Unit: Mercia Mudstone Group (MMG)
Previous Name(s): Sub-Arden Keuper Marls [Obsolete Name and Code: Use SIM] (-3677)
Mudstone I [Obsolete Name and Code: Use SIM] (-529)
Eldersfield Mudstone Formation [Obsolete Name and Code: Use SIM] (ELM)
Keuper Marl, Lower [Obsolete Name and Code: Use MMG, SIM] (LKM)
Alternative Name(s): Dowsing Formation and Dudgeon Formation
Leyland Fm plus Preesall Halite Fm, Dowbridge Mudstone Fm and Warton Halite Fm
Stratotypes:
Reference Section  Cropwell Bridge Borehole (SK63NE/28) [SK 6773 3547], Cropwell Bishop, Nottinghamshire: from 42.03 m to 170.70 m depth (Howard et al., 2009). Curated core held at the National Geosciences Records Centre, BGS, Keyworth. 
Reference Section  Fulbeck F/B1 Borehole (SK85SE/25) [SK 8889 5053], Fulbeck, Lincolnshire: from 158.8 to 301.58 m depth (Berridge et al., 1999). Curated core held at the National Geosciences Records Centre, BGS, Keyworth. 
Reference Section  Worcester Borehole (SO85NE/23) [SO 8624 5762], Worcestershire: from surface (core from 30 m) to 293.51 m (Barclay et al., 1997). Curated core held at the National Geosciences Records Centre, BGS, Keyworth. 
Reference Section  Asfordby Hydrogeological Borehole (SK72SW/71) [SK 7252 2061], Asfordby, Leicestershire: from 211.37 to 327.87 m depth (Carney et al., 2004). Curated core held at the National Geosciences Records Centre, BGS, Keyworth. 
Type Section  South Devon coast between Sidmouth [SY 129 873] and Weston Mouth [SY 163 879] (Jeans, 1978; Warrington and Scrivener, 1980; Gallois 2001, 2004). The full sequence, younging gradually eastwards, is well-exposed in a series of cliff sections extending for 3.5 km, with only minor disturbance by faulting (Gallois, 2001; Jeans, 1978). 
Reference Section  Eldersfield Borehole (SO73SE/6) [SO 7891 3221], Eldersfield, near Tewkesbury, Gloucestershire: from the start of coring to 348.47 m depth (Barclay et al., 1997; Worssam et al., 1989). Curated core held at the National Geosciences Records Centre, BGS, Keyworth. 
Reference Section  Wilkesley Borehole (SJ64SW/7) [SJ 6286 4144], Cheshire: from 347.8 to 1527.4 m depth (Poole and Whiteman, 1966; Wilson, 1993). 
Reference Section  Home Farm Borehole (SP 47SW/72) [SP 4317 7309], near Rugby, Warwickshire: from 76.67 to 201.22 m depth (Old et al., 1987). Curated core held at the National Geosciences Records Centre, BGS, Keyworth. 
Reference Section  Twycross Borehole (SK30NW/13) [SK 3387 0564], Twycross, Leicestershire (Worssam and Old, 1988): from base of Superficial Deposits at 17.40 to 127.97 m depth. Curated core held at the National Geosciences Records Centre, BGS, Keyworth. 
Reference Section  Leicester Forest East Borehole (SK50SW/71) [SK 5245 0283], Leicestershire: from base of Superficial Deposits at 4.0 m to 119.51 m depth (Worssam and Old, 1988). Curated core held at the National Geosciences Records Centre, BGS, Keyworth. 
Reference Section  Knights Lane Borehole (SP25SW/1) [SP 2242 5497], Stratford-upon-Avon, Warwickshire: from base of Superficial Deposits at 6.1 m to 151.23 m depth (Williams and Whittaker, 1974). Curated core held at the National Geosciences Records Centre, BGS, Keyworth. 
Reference Section  Kirkham Borehole (SD43SW/6) [SD 4324 3247], Lancashire: from base of Superficial Deposits at 36.58 m to 366.19 m depth (Wilson, 1990; Wilson and Evans, 1990). Curated core held at the National Geosciences Records Centre, BGS, Keyworth. 
Reference(s):
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). 
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. 
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. 
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. 
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). 
Waters, C N, Smith, K, Hopson, P M, Wilson, D, Bridge, D M, Carney, J N, Cooper, A H, Crofts, R G, Ellison, R A, Mathers, S J, Moorlock, B S P, Scrivener, R C, McMillan, A A, Ambrose, K, Barclay, W J, and Barron, A J M. 2007. Stratigraphical Chart of the United Kingdom: Southern Britain. British Geological Survey, 1 poster. 
Berridge, N G, Pattison, J, Samuel, M D A, Brandon, A, Howard, A S, Pharaoh, T C and Riley, N J. 1999. Geology of the Grantham district. Memoir of the British Geological Survey, sheet 127 (England and Wales). 
Carney, J N, Ambrose, K, Brandon, A, Royles, C P, Lewis, M A, and Sheppard, H. 2004. Geology of the country around Melton Mowbray. Sheet Description of the British Geological Survey, 1:50 000 Series Sheet 142 Melton Mowbray (England and Wales). 
Hounslow, M W, and McIntosh, G. 2003. Magnetostratigraphy of the Sherwood Sandstone Group (Lower and Middle Triassic), south Devon, UK: detailed correlation of the marine and nonmarine Anisian. Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 193, 325–348. 
Howard, A S, Warrington, G, Carney, J N, Ambrose, K, Young, S R and Pharaoh, T C. 2009. Geology of the country around Nottingham. Memoir of the British Geological Survey, Sheet 126 (England and Wales). 
Kellaway, G A and Welch, F B A. 1993. Geology of the Bristol district. Memoir of the British Geological Survey Special Sheet [England and Wales]. 
Old, R A, Sumbler, M G and Ambrose, K, 1987. Geology of the country around Warwick. Memoir of the British Geological Survey, Sheet 184 (England and Wales). 
Poole, E G, and Whiteman, A J. 1966. Geology of the country around Nantwich and Whitchurch. Memoir of the Geological Survey of Great Britain, Sheet 122 (England and Wales). 
Warrington, G, and Scrivener, R C. 1980. The Lyme Regis (1901) Borehole succession and its relationship to the Triassic sequence of the east Devon coast. Proceedings of the Ussher Society, Vol. 5, 24-32. 
Williams, B J and Whittaker, A. 1974. Geology of the country around Stratford-upon-Avon and Evesham. Memoir of the Geological Survey of Great Britain Sheet 200 (England and Wales). 
Wilson, A A. 1990. The Mercia Mudstone Group (Trias) of the East Irish Sea Basin. Proceedings of the Yorkshire Geological Society, Vol.48, 1-22. 
Wilson, A A and Evans, W B. 1990 Geology of the country around Blackpool. Memoir of the British Geological Survey, Sheet 66. (England and Wales). 
Worssam, B C and Old, R A, 1988. Geology of the country around Coalville. Memoir of the British Geological Survey, Sheet 155 (England and Wales) 
Gallois, R W. 2004. The type section of the junction of the Otter Sandstone Formation and the Mercia Mudstone Group (Mid Triassic) at Pennington Point, Sidmouth. Geoscience in southwest England, Vol. 11, 51-58. 
Jones, N S. 1993. Sedimentology of selected Triassic and Cretaceous successions from the Sidmouth area, Devon. British Geological Survey Technical Report, WH/93/61R. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E326 E340