| Lithological Description: |
| The formation typically comprises pale green-grey, dolomitic silty mudstones and siltstones with thin arenaceous lenses and a few thin, commonly discontinuous beds of hard, dolomitic, pale yellowish-grey, porcellanous mudstone and silltstone ("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 throughout and bivalve fossils and bioturbation become increasingly common upwards. |
| Definition of Lower 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 over a greater thickness of beds or 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 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, which is 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 of Mayall, 1981) includes lithologies similar to that of the overlying Westbury Formation, giving rise to an apparently gradational and interdigitating upper boundary. At outcrop, the boundary is marked by a change in soil type from the generally paler grey clay of the Blue Anchor Formation, to the darker grey clay 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. |
| Reference(s): |
| 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. |
| 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.] |
| 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. |
| Rhys, G H, Lott, G K and Calver, M A (editors), 1982. The Winterborne Kingston borehole, Dorset, England. Report of the Institute of Geological Sciences, No.81/3. |
| 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). |
| 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. |
| 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. |
| 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. |
| 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). |
| 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. |
| Warrington, G and Whittaker, A. 1984. The Blue Anchor Formation (late Triassic) in Somerset. Proceedings of the Ussher Society, Vol.6, 100-107. |
| 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. |
| Richardson, L. 1906. On the Rhaetic and contiguous deposits of Devon and Dorset. Proceedings of the Geologists' Association, Vol.19, 401-409. |
| 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. |
| 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. |
