The BGS Lexicon of Named Rock Units — Result Details

Blue Anchor Formation

Computer Code: BAN Preferred Map Code: BAn
Status Code: Full
Age range: Norian Age (TN) — Rhaetian Age (TR)
Lithological Description: Typically comprises pale green-grey, dolomitic silty mudstones and siltstones with thin argillaceous or arenaceous laminae and lenses and a few thin, commonly discontinuous beds of hard, dolomitic, pale-cream to buff, porcellanous mudstone and siltstone ('Tea Green Marl' of Etheridge, 1865). In southern England and Wales only, the 'Tea Green Marl' is overlain by the 'Grey Marls' (Richardson, 1906). This unit (equivalent to the upper part of the Rydon Member and the whole of the Williton Member of Mayall, 1981) comprises grey, black, green and, rarely, red-brown dolomitic mudstones with, in the higher beds, yellowish-grey dolostones; also present are laminated siltstone beds with mudcracks, scarce pseudomorphs after halite, and locally abundant gypsum; miospores occur sporadically, and bivalve fossils and bioturbation occur locally in the upper beds. Late Triassic, ?Norian to Rhaetian.
Definition of Lower 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 over a greater thickness of beds or 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, as suggested in the Warwickshire area around the margins of the London-Brabant Massif (Horton et al., 1987; Old et al., 1987). Where not exposed, the boundary is marked by a change in soil colour from the reddish brown clayey soils of the underlying Branscombe Mudstone Formation to the grey clayey soils of the Blue Anchor Formation. Typically, this boundary lies near the base of a scarp slope formed by the Blue Anchor Formation and capped by limestones of the Penarth Group and lowermost Lias Group.
Definition of Upper Boundary: Slightly unconformable or disconformable, marked by an abrupt upward transition from green or grey-green, dolomitic mudstones of the Blue Anchor Formation to black, fossiliferous shales of the Westbury Formation (Penarth Group). The unconformity surface is commonly slightly irregular and small clasts of Blue Anchor Formation lithologies may occur at the base of the Westbury Formation. The Blue Anchor Formation is both burrowed and bored to a depth of several centimetres below the boundary. In parts of southern England and Wales (e.g. on the west Somerset coast) the upper part of the formation (Williton Member; Mayall, 1981) includes lithologies similar to those of the overlying Westbury Formation, giving rise to an apparently gradational and interdigitating upper boundary. Where not exposed, the boundary is marked by a change in soil type from the generally paler grey clays associated with the Blue Anchor Formation to the darker grey clays of the overlying Westbury Formation of the Penarth Group. Typically, this boundary lies about half way up a scarp slope capped by limestones of the Penarth Group and lowermost Lias Group.
Thickness: Type section: 36.54m (Warrington and Whittaker, 1984); primary reference section: 31.18m (Warrington and Ivimey-Cook, 1995). Maximum recorded thickness: 67.20m, Winterborne Kingston Borehole (BGS Borehole reference number SY89NW/1), Dorset (Rhys et al., 1982), inferred from geophysical logs. Elsewhere the Formation typically ranges from 5 to 20m; it is generally thinner (7 to 10m) in the more northerly basins.
Geographical Limits: The formation is the highest unit of the Mercia Mudstone Group in the British Isles. The outcrop extends northwards from the south Devon coast, through Somerset, Avon, Gloucestershire, Worcestershire, Warwickshire, Leicestershire, Nottinghamshire, Lincolnshire, Humberside and Yorkshire and into Cleveland. Other outcrops occur in Gwent and Glamorgan, South Wales, and in the Cheshire and Carlisle basins. Outcrops in south Devon and Cleveland are in spatial continuity with occurrences of equivalent units offshore in the English Channel and Southern North Sea basins respectively. The type area is the west Somerset coastline.
Parent Unit: Mercia Mudstone Group (MMG)
Previous Name(s): Grey Marls [Obsolete Name and Code: Use BAN] (-3191)
Parva Formation (upper part) [Obsolete Name and Code: Use BAN] (-3192)
Sully Beds (lower part) [Obsolete Name and Code: Use BAN] (-1966)
Pale Green and Grey Marls [Obsolete Name and Code: Use BAN] (-1328)
Tea-Green Marl [Obsolete Name and Code: Use BAN] (TGM)
Tea Green and Grey Marls [Obsolete Name and Code: Use BAN] (-2573)
Grey and Tea Green Marls and Sully Beds [Obsolete Name and Code: Use BAN] (-667)
Alternative Name(s): Triton Formation
Stratotypes:
Reference Section  St Audrie's Bay [ST 1045 4310], west Somerset coast (see Whittaker and Green, 1983; Warrington and Ivimey-Cook, 1995). 
Reference Section  Coastal section at Lavernock Point [ST 188 682], South Glamorgan, exposing the full thickness (31.18m) of the Formation (Waters and Lawrence, 1987; Warrington and Ivimey-Cook, 1995). 
Reference Section  South Devon coast east of Seaton, between Sparrowbush Ledge [SY 260 897] and Culverhole Point [SY 273 894] (Gallois, 2001). 
Reference Section  Fulbeck F/B1 Borehole (SK85SE/25) [SK 8889 5053], Fulbeck, Lincolnshire: from 111.52 to 117.52 m depth (Berridge et al., 1999). Curated core held at the National Geosciences Records Centre, BGS, Keyworth. 
Reference Section  Staithes No.20 Borehole (NZ71NE/14) [NZ 76034 18000], Boulby mine site, North Yorkshire: from 391.36 to 397.00 m depth (Woods, 1973). Curated core held at the National Geosciences Records Centre, BGS, Keyworth. 
Type Section  Coastal cliffs at Blue Anchor, Somerset [ST 0385 4368], exposing the full thickness (36.54 m) of the formation (Warrington and Whittaker, 1984). 
Reference(s):
Richardson, L. 1906. On the Rhaetic and contiguous deposits of Devon and Dorset. Proceedings of the Geologists' Association, Vol.19, 401-409. 
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). 
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 
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). 
Whittaker A and Green G W. 1983. Geology of the country around Weston-super-Mare. Memoir of the Geological Survey, England and Wales, Sheet 279 with parts of sheets 263 and 295. 
Woods, P J E. 1973. Potash exploration in Yorkshire: Boulby Mine pilot borehole. Transactions of the Institution of Mining and Metallurgy, Vol.B82, 99-106. 
Ivimey-Cook, H C. 1974. The Permian and Triassic deposits of Wales. 295-321 in The Upper Palaeozoic and post-palaeozoic rocks of Wales. Owen, T R (editor). (Cardiff: University of Wales Press.) 
Hamilton, D and Whittaker, A. 1977. Coastal exposures near Blue Anchor, Watchet and St Audrie's Bay, north Somerset, 101-109 "in" Savage, R J G (Editor). Geological Excursions in the Bristol District. University of Bristol, 101-109. 
Warrington, G and Whittaker, A. 1984. The Blue Anchor Formation (late Triassic) in Somerset. Proceedings of the Ussher Society, Vol.6, 100-107. 
Bristow, H W and Etheridge, R. 1873. Vertical sections of the Lower Lias and Rhaetic or Penarth Beds of Glamorgan, Somerset and Gloucester-shires. No. 47. Geological Survey of England and Wales. 
Elliott, R E 1961. The stratigraphy of the Keuper Series in southern Notinghamshire. Proceedings of the Yorkshire Geological Society, Vol.33, 197-231. 
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. 
Mayall, M J. 1981. The late Triassic Blue Anchor Formation and the initial Rhaetian transgression in south-west Britain. Geological Magazine, Vol.118, 377-384. 
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). 
Warrington, G and Ivimey-Cook, H C. 1995. The Late Triassic and Early Jurassic of coastal sections in west Somerset and South and Mid-Glamorgan. Field Geology of the British Jurassic, 9-30. Geological Society, London. 
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. 
Bristow, W H, Etheridge, R and Woodward, H W. 1873. Vertical sections of the Lower Lias and Rhaetic or Penarth Beds of Somerset and Gloucester-shires, No.46. Geological Survey of England and Wales. 
Etheridge, R. 1865. On the Rhaetic or Avicula contorta Beds at Garden Cliff, Westbury-upon-Severn, Gloucestershire. Proceedings of the Cotteswold Naturalists Field Club, Vol.3, 218-235. 
Waters, R A and Lawrence, D J D. 1987. Geology of the South Wales Coalfield - Part III: the country around Cardiff. Memoir of the British Geological Survey, Sheet 263. (England and Wales). 
Rhys, G H, Lott, G K, and Calver, M A. 1982. The Winterborne Kingston borehole, Dorset, England. Report of the Institute of Geological Sciences, No.81/3. 
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.) 
Richardson, L. 1911. The Rhaetic and contiguous deposits of west, mid, and part of east Somerset. Quarterly Journal of the Geological Society of London, Vol. 67, 1-74 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E216 E199 E183 E262 E263 E279 E127 E142 E141 E280 E264 E326 E122 E123 E126 E138 E140 E169 E182 E184 E200 E201 E234 E250 E251 E261 E278 E294 E295 E296 E340