Blue Anchor Formation

From Earthwise
Jump to navigation Jump to search

Blue Anchor Formation (BAN)[edit]

Previous nomenclature[edit]

Tea Green Marl (Etheridge, 1865)

Tea Green and Grey Marls (Richardson, 1906; Hamilton and Whittaker, 1977)

Grey and Tea Green Marls, and Sully Beds (Richardson, 1911)

Pale Green and Grey Marls (Bristow and Etheridge, 1873; Bristow et al., 1873)

Parva Formation (upper part) (Elliott, 1961)

Sully Beds (lower part) (Warrington et al., 1980, fig.1).

Approximately equivalent to:

Borehole geophysical log unit ‘F’ (Lott et al., 1982).

Parent unit[edit]

Mercia Mudstone Group

Derivation of name[edit]

From the type section, Blue Anchor, west Somerset Coast (Warrington et al., 1980; Warrington and Whittaker, 1984).

Type area[edit]

West Somerset coastline.

Type section[edit]

Blue Anchor cliff (ST 0385 4368), Somerset (Warrington and Whittaker, 1984).

Primary reference section[edit]

St Audrie’s Bay (ST 1045 4310), west Somerset coast (see: Whittaker and Green, 1983; Warrington and Ivimey-Cook, 1995).

Other reference sections[edit]

Lavernock Point (ST 188 682), south Glamorgan (Waters and Lawrence, 1987; Warrington and Ivimey-Cook, 1995).

South Devon coast east of Seaton, between Sparrowbush Ledge (SY 260 897) and Culverhole Point (SY 273 894) (Gallois, 2001).

Fulbeck F/B1 Borehole (SK85SE/25) (SK 8889 5053), Fulbeck, Lincolnshire, from 111.52 to 117.52m depth (Berridge et al., 1999). Curated core held at the National Geosciences Records Centre, BGS, Keyworth.

Staithes No.20 Borehole (NZ71NE/14) (NZ 76034 18000), Boulby mine site, North Yorkshire; from 391.36 to 397.00m depth (Woods, 1973). Curated core held at the National Geosciences Records Centre, BGS, Keyworth.

Extant exposures/sections[edit]

Good sections occur at several sites on the west Somerset coast (see: Whittaker & Green, 1983; Warrington and Whittaker, 1984; Edmonds and Williams, 1985; Edwards, 1999), and in south Glamorgan (Richardson, 1905; Ivimey-Cook, 1974; Waters and Lawrence, 1987). Others occur on the south Devon coast (SY 270 894), east of Seaton (Richardson, 1906; Gallois, 2001), and adjacent to the Bristol Channel or lower River Severn, e.g. in cliffs at Aust (ST 5645 8920) (Reynolds, 1946; Hamilton, 1977), Garden Cliff, Westbury-on-Severn (SO 719 126) (Etheridge, 1865; Benton et al., 2002) and Wainlode (SO 847 259) (Worssam et al., 1989).

Elsewhere the formation is poorly exposed and is known largely from boreholes and temporary excavations. A good section is currently (2005) seen in the British Gypsum Bantycock opencast mine (SK 8123 4949) near Newark, Nottinghamshire. Information on inland outcrop or borehole sections elsewhere in England and south Wales is mostly contained, or cited, in memoirs or other publications of the Geological Survey. An occurrence in the Carlisle Basin has been documented by Ivimey-Cook et al. (1995)

Lithology[edit]

The formation 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.

Lower boundary[edit]

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).

Where not exposed, the boundary is marked by a change in soil colour from the reddish brown clayey soils of the Branscombe Mudstone Formation to the 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.

Upper boundary[edit]

Slighty 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 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 outcrop 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[edit]

Type section: c.36.54m (Warrington and Whittaker, 1984); primary reference section: 31.18m Warrington and Ivimey-Cook, 1995).

Maximum recorded thickness: 67.20m, Winterborne Kingston Borehole, Dorset (Rhys et al., 1982), inferred from geophysical logs.

Elsewhere, the formation typically ranges from 5-20m; it is generally thinner (7-10m) in the more northerly basins.

Age[edit]

Late Triassic; late Norian to early Rhaetian on palynological evidence

Equivalent units[edit]

Upper part of the Triton Formation, Southern North Sea Basin (Johnson et al., 1994)

Geographical extent[edit]

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, Gloucestershire, Worcestershire, Warwickshire, Leicestershire, Nottinghamshire and Lincolnshire, into Yorkshire. Other outcrops occur in Gwent and Glamorgan, south Wales, and in the Needwood, Cheshire and Carlisle basins. Outcrops in south Devon and Lincolnshire and Yorkshire are in spatial continuity with occurrences of equivalent units offshore in the English Channel and Southern North Sea basins.

Subdivisions[edit]

Rydon Member (Mayall, 1981)

References[edit]