The BGS Lexicon of Named Rock Units — Result Details

Mercia Mudstone Group

Computer Code: MMG Preferred Map Code: MMG
Status Code: Full
Age range: Anisian Age (TA) — Rhaetian Age (TR)
Lithological Description: Dominantly red, less commonly green-grey, mudstones and subordinate siltstones with thick halite-bearing units in some basinal areas. Thin beds of gypsum/anhydrite are widespread; thin sandstones are also present. Mid Triassic (Anisian) to latest Triassic (Rhaetian).
Definition of Lower Boundary: The boundary with the underlying Sherwood Sandstone Group is taken at the upward transition from sandstone to mudstone- or siltstone-dominated lithologies and the incoming of pseudomorphs after halite; the boundary is usually gradational, conformable and markedly diachronous, younging towards the south. In the East Midlands, the boundary with the underlying Sherwood Sandstone Group is sharp and unconformable (Warrington 1974, fig. 40; Howard et al., 2008). The base of the group onlaps onto Carboniferous and older rocks at basin margins.
Definition of Upper Boundary: The boundary with the overlying Penarth Group is typically a slight unconformity marked by an abrupt upward change from the green or grey-green, dolomitic mudstones of the Blue Anchor Formation to black, fossiliferous shales of the Westbury Formation (Penarth Group). The unconformity surface may be slightly irregular and small clasts of Blue Anchor Formation lithologies commonly occur at the base of the Westbury Formation. The Blue Anchor Formation may be burrowed or have animal borings to a depth of several centimetres below the unconformity surface.
Thickness: Thickness variation is considerable, ranging up to 1350 m in the Cheshire Basin.
Geographical Limits: Onshore UK and contiguous offshore areas (equivalent strata in the Southern North Sea Basin are assigned to the Haisborough Group (HAI)).
Parent Unit: New Red Sandstone Supergroup (NRS)
Previous Name(s): Red Marl [Obsolete Name and Code: Use MMG, RDM] (-4292)
Keuper Marl, Middle [Obsolete Name And Code: Use MMG] (MKM)
Keuper Series, Lower [Obsolete Name And Code: Use SSG or MMG] (LKSR)
Skerry Band [Obsolete Name And Code Use: MMG] (SKE)
Red Marls (Saliferous) [Obsolete Name And Code Use: MMG] (SRM)
Stanwix Shales Formation [Obsolete Name And Code: Use MMG] (STSH)
Keuper Marl, Lower [Obsolete Name and Code: Use MMG, SIM] (LKM)
Keuper Marl [Obsolete Name And Code: See MMG] (KM)
Keuper Marl And Sandstone [Obsolete Name And Code: Use MMG And SSG] (KMS)
Keuper "Series" [Obsolete Name And Code: Use SSG And MMG] (KSR)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  South Devon Coast between Sidmouth [SY 129 873] and Haven Cliff [SY 256 898] to [SY 273 894] (Jeans, 1978; Warrington and Scrivener, 1980; Gallois, 2001, 2004). 
Reference Section  Fulbeck F/B1 Borehole, Fulbeck, Lincolnshire, from 111.52 to 375.24 m depth (Berridge et al., 1999). Curated core held at the National Geosciences Records Centre, BGS, Keyworth. 
Type Area  "Mercia" i.e. the English Midlands; the area south and east of Nottingham is the most suitable part for the type area (Elliott, 1961; Charsley et al., 1990). 
Reference(s):
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. 
Waters, C N, Gillespie, M R, Smith, K, Auton, C A, Floyd, J D, Leslie, A G, Millward, D, Mitchell, W I, McMillan, A A, Stone, P, Barron, A J M, Dean, M T, Hopson, P M, Krabbendam, M, Browne, M A E, Stephenson, D, Akhurst, M C, and Barnes, R P. 2007. Stratigraphical Chart of the United Kingdom: Northern Britain. (British Geological Survey.) 
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. 
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. 
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. 
Elliott, R E 1961. The stratigraphy of the Keuper Series in southern Notinghamshire. Proceedings of the Yorkshire Geological Society, Vol.33, 197-231. 
Benton, M J, and three others. 1994. A review of the British Middle Triassic tetrapod assemblages. 131-160 in In the shadow of the dinosaurs. Fraser, N C, and Sues, H-D (editors). (Cambridge: University Press, Cambridge.) 
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). 
Hull, E, 1869. The Triassic and Permian rocks of the Midland Counties of England. Memoir of the Geological Survey of Great Britain. 
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 
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. 
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. 
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, and four others. 1999. Stratigraphy and sedimentology. 10-40 in The Cheshire Basin: basin evolution, fluid movement and mineral resources in a Permo-Triassic rift setting. Plant, J A, Jones, D G, and Haslam, H W (editors). (Keyworth, Nottingham, British Geological Survey.) 
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. 
Warrington, G. 1974. Trias. 145-160 in The geology and mineral resources of Yorkshire. Rayner, D H, and Hemingway, J E. Yorkshire Geological Society Special Publication, No. 2. (Leeds: Yorkshire Geological Society.) 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E084 E075 E062 E124 E140 E216 E183 E184 E043 E263 E262 E234 E047 E037 E199 E139 E154 N35 E017 E279 E102 E055 E065 E064 E072 E080 E081 E089 E090 E091 E058 E018 E023 E027 E103 E114 E115 E127 E128 E158 E172 E187 E201 E202 E217 E142 E048 E235 E236 E237 E250 E253 E264 E265 E141 E280 E326 E340 E281 E341 E342 E343 E400 N28