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 structurless, 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.
Definition of Lower Boundary: Conformable in all areas. Placed at the base of predominantly red, structureless mudstones where they rest on the interbedded 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 is absent (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). At the type section, the base of the formation is drawn at a lower stratigraphical level than originally defined by Gallois (2001), to encompass approximately 15m of predominantly reddish-brown mudstone that was included by Gallois in the Dunscombe Mudstone Formation. At outcrop, the boundary is typically marked by a change from the 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 absent, the boundary is mapped at the approximate down-dip limit of subsidence hollows and collapse breccias, which are associated with near-surface dissolution of halites (Wilkesley Halite Member) at the top of the Sidmouth Mudstone (Taylor et al., 1963).
Definition of Upper Boundary: Placed at an abrupt or rapid upward change from the red-brown, silty mudstones of the Branscombe Mudstone Formation to the green or grey-green, dolomitic mudstones of the Blue Anchor Formation. In parts of southern England and Wales the transition occurs above a unit of interbedded red and green strata (e.g. on the west Somerset and south Devon coasts). The boundary may be locally unconformable, as suggested in the Warwickshire area by 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 240m in the Wessex Basin, 170m in the Worcester Basin; 25 to 60m in the East Midlands, up to 160m in the Cheshire Basin
Geographical Limits: The surface outcrop extends from the South Devon coast northwards into Somerset, Avon and Worcestershire. The formation crops out below parts of the English Central Midlands and Cheshire, 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.
Parent Unit: Mercia Mudstone Group (MMG)
Type Section  South Devon coast between Western Cliff and Branscombe Mouth (lower two thirds of Formation) and in Seaton Bay and Haven Cliff (upper third of Formation, although 40m of strata are omitted - see Comments). The succession youngs eastwards in a discontinuous series of cliff exposures. 
Reference Section  Aust Cliff, Avon, exposes uppermost 35m of the Formation and the boundary with the overlying Blue Anchor Formation. 
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. 
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). 
Berridge, N G and Pattison, J, 1994. Geology of the country around Grimsby and Patrington. Memoir of the British Geological Survey, Sheets 90, 91, 81 and 82 (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. In press. 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. 
Johnson, H, Warrington, G and Stoker, S J. 1994. Permian and Triassic of the Southern North Sea. In Knox, R W O'B and Cordey, W G (editors), Lithostratigraphic nomenclature of the UK North Sea. [Nottingham: British Geological Survey.] 
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). 
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). 
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. 
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. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E326 E340