The BGS Lexicon of Named Rock Units — Result Details

Branscombe Mudstone Formation

Computer Code: BCMU Preferred Map Code: notEntered
Status Code: Full
Age range: Norian Age (TN) — Rhaetian Age (TR)
Lithological Description: Mudstone and siltstone, red-brown with common grey-green reduction patches and spots. The mudstones are mostly structureless, with a blocky weathering habit. Gypsum/anhydrite, locally of economic importance, is common throughout in beds, nodules and veins. Many of these sulphate beds have been named (e.g. Tutbury Gypsum and Newark Gypsum of the East Midlands) and form distinct markers on geophysical logs. Sporadic thin beds of argillaceous sandstone and silty dolomite occur in the lower part of the formation. Beds of thinly interlaminated, dark grey-green mudstone and dolomitic siltstone occur locally towards the top of the formation (formerly separated in the East Midlands as the Glen Parva Formation, Warrington et al., 1980). Halite is recorded towards the base of the formation in Nettlecombe Borehole, Dorset, SY59NW/1, [SY50520 95440], but is absent elsewhere; halite pseudomorphs are rare. Late Triassic, Norian to ?Rhaetian.
Definition of Lower Boundary: Conformable in all areas. Placed at the base of predominantly red, structureless mudstones where they rest on the interbedded, laminated, dark green-grey siltstones and pale grey fine-grained sandstones of the underlying Arden Sandstone Formation; the boundary is abrupt or a rapid, interbedded transition. Where the Arden Sandstone Formation cannot be recognised with confidence due to the presence of halite (Cheshire Basin and parts of the Wessex Basin), the boundary is placed immediately above the highest halite beds of the Sidmouth Mudstone Formation (Wilkesley Halite Member, Somerset Halite Member, Dorset Halite Member). At the type section, the base of the formation is drawn at a lower stratigraphical level than originally defined by Gallois (2001) in order to encompass approximately 11 m of reddish-brown mudstone with subordinate greenish grey interbeds that was included by Gallois in the Dunscombe Mudstone Formation. Where not exposed, the boundary is marked typically by a change in soil characteristics from grey, slightly sandy clay soils of the Arden Sandstone Formation to the reddish brown clayey soils of the Branscombe Mudstone Formation. Where the Arden Sandstone forms a marked cuesta feature, the boundary lies at the down-dip limit of the dip slope. In the Cheshire Basin, where the Arden Sandstone Formation is not recognisable, the boundary is mapped at the approximate down-dip limit of subsidence hollows and collapse breccias, which are associated with near-surface solution of halite (Wilkesley Halite Member) at the top of the Sidmouth Mudstone (Taylor et al., 1963).
Definition of Upper Boundary: Placed at an abrupt or rapidly gradational upward transition from the red-brown, silty, mudstones of the Branscombe Mudstone Formation to green or grey-green, dolomitic mudstones of the Blue Anchor Formation. Locally in southern England and Wales, a more gradational transition occurs above interbedded red and green lithologies (e.g. on the west Somerset and south Devon coasts); in these areas the boundary is drawn above the highest significant red mudstone bed. In south Devon, this coincides with a prominent bed of dolomitic limestone (Gallois, 2001). The boundary may be locally erosional around the margins of the London-Brabant Massif (Horton et al., 1987; Old et al., 1987). At outcrop, the boundary is marked by a change in soil colour from the reddish brown clayey soils of the Branscombe Mudstone Formation to grey clayey soils of the overlying Blue Anchor Formation. Typically, this boundary lies near the base of a scarp slope formed by the Blue Anchor Formation and capped by the Penarth Group.
Thickness: Up to 240 m in the Wessex Basin, 170 to 190 m in the Worcester Basin, 25 to 60 m in the East Midlands and up to 160 m in the Cheshire Basin.
Geographical Limits: The surface outcrop extends from south Devon northwards into Somerset, Gloucesterhire, south Glamorgan and Worcestershire. The formation crops out below parts of the Midlands, Cheshire and the Carlisle Basin, and in a strip extending northwards from Nottingham into North Yorkshire. The formation occurs in the subsurface below much of the south Midlands and eastern England. It has equivalents in the Southern North Sea and East Irish Sea Basins. The type area is the south Devon coast.
Parent Unit: Mercia Mudstone Group (MMG)
Previous Name(s): Trent Formation plus the lower part of the Parva Formation [Obsolete Name and Code: Use BCMU] (-2586)
Mudstone III [Obsolete Name and Code: Use BCMU] (-677)
Brooks Mill Mudstone Formation [Obsolete Name and Code: Use BCMU] (BMI)
Dunscombe Mudstone Formation [Obsolete Name and Code: Use AS, BCMU] (DUM)
Cropwell Bishop Formation [Obsolete Name and Code: Use BCMU] (CBP)
Supra-Arden Keuper Marls [Obsolete Name and Code: Use BCMU] (-1972)
Twyning Mudstone Formation [Obsolete Name and Code: Use BCMU] (TWM)
Upper Keuper Marl [Obsolete Name and Code: Use BCMU] (UKM)
Trent Formation Plus Glen Parva Formation [Obsolete Name and Code: Use BCMU] (-44)
Alternative Name(s): Triton Formation
Elswick Mudstone Formation
Stratotypes:
Reference Section  Aust Cliff, south Gloucestershire [ST 5645 8920]: uppermost 35 m is exposed in cliffs on the east side of the River Severn below the Blue Anchor Formation (Hamilton, 1977; Kellaway and Welch, 1993). 
Reference Section  St Audrie's Bay, north Somerset [ST 120 437 to 106 431]: uppermost 68 m exposed in the cliffs and foreshore (Hounslow et al., 2004). 
Reference Section  Asfordby Hydrogeological Borehole (SK72SW/71) [SK 7252 2061], Asfordby, Leicestershire: from 170.08 to 211.37 m depth (Carney et al., 2004). Curated core held at the National Geosciences Records Centre, BGS, Keyworth. 
Reference Section  Cropwell Bridge Borehole (SK63NE/28) [SK 6773 3547], Cropwell Bishop, Nottinghamshire: from surface to 37.50 m depth (uppermost beds not proved) (Howard et al., 2009). Curated core held at the NGRC Core Store, BGS, Keyworth. 
Reference Section  Fulbeck F/B1 Borehole (SK85SE/25) [SK 8889 5053], Fulbeck, Lincolnshire: from 117.52 to 156.10 m depth (Berridge et al., 1999). Curated core held at the National Geosciences Records Centre, BGS, Keyworth. 
Reference Section  Wilkesley Borehole (Reg. No. SJ64SW/7) [SJ 6286 4144]: from 186.5 to 347.8 m depth (Poole and Whiteman, 1966; Wilson, 1993). 
Reference Section  Twyning Borehole (SO83NE/5) [SO 8943 3664], Twyning, near Tewkesbury, Gloucestershire: from 141.5 to 310.51 m depth (Barclay et al., 1997; Worssam et al., 1989). 
Type Section  South Devon coast between Weston Cliff and Branscombe Mouth [SY 171 879 to 207 881] (lower two thirds of formation) and in Haven Cliff, east of Seaton [SY 256 898 to 260 897] (upper third of formation) (Gallois, 2001). 
Reference(s):
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). 
Horton, A, Poole, E G, Williams, B J, Illing, V C and Hobson, G D. 1987. Geology of the country around Chipping Norton. Memoir of the British Geological Survey, Sheet 218 (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). 
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. 
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). 
Hamilton, D. 1977. Aust Cliff. 111-118 in Geological excursions in the Bristol district. Savage, R J G (editor). (Bristol: University of Bristol.) 
Hounslow, M W, Posen, P E, and Warrington, G. 2004. Magnetostratigraphy and biostratigraphy of the Upper Triassic and lowermost Jurassic succession, St Audrie's Bay, UK. Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 213, 331-358. 
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 
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). 
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. 
Elliott, R E 1961. The stratigraphy of the Keuper Series in southern Notinghamshire. Proceedings of the Yorkshire Geological Society, Vol.33, 197-231. 
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. 
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). 
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. 
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]. 
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. 
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). 
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. 
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. 
Rees, J G and Wilson, A A. 1998. Geology of the country around Stoke-on-Trent. Memoir of the British Geological Survey, Sheet 123 (England and Wales). 
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. 
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. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E326 E340