Mercia Mudstone Group

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Mercia Mudstone Group (MMG)[edit]

Previous nomenclature[edit]

Approximately equivalent to:

Upper Red Marls and Gypsum (Sedgwick, 1829)

Red Marl, New Red Marl or Keuper Marl (Hull, 1869)

Keuper Marls (Lamplugh et al., 1908)

Nottingham Group (Balchin and Ridd, 1970)

Stanwix Marls or Stanwix Shales (Holmes, 1881, 1899)

Derivation of name[edit]

From Mercia, the Anglo-Saxon kingdom that occupied much of central and southern England.

Primary reference section[edit]

South Devon Coast between Sidmouth (SY 129 873) and Haven Cliff (SY 256 898 to 273 894) (Jeans, 1978; Warrington and Scrivener, 1980; Gallois, 2001, 2004).

Reference sections[edit]

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.

Base of group: surface section, former railway cutting, Colwick Road, Sneinton, Nottingham (SK5924 3968 to 5920 3980). Exposes lower boundary and lowermost 20 m of group (Charsley, 1989; Benton et al., 2002; Howard, 2003; Howard et al., in press).

Top of group: St Audrie’s Bay (ST 1045 4310), west Somerset coast (Whittaker and Green, 1983; Warrington and Ivimey-Cook, 1995; Benton et al., 2002; Hounslow et al., 2004).

Extant exposures/sections[edit]

See above, and entries for constituent formations. All formations within the group are poorly exposed and known largely from boreholes and temporary excavations. Information on inland outcrop or boreholes is mostly contained, or cited, in memoirs or other publications of the British Geological Survey.

Lithology[edit]

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.

Lower boundary[edit]

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., in press). The base of the group onlaps onto Carboniferous and older rocks at basin margins.

Upper boundary[edit]

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[edit]

Thickness variation is considerable, ranging up to 1350 m in the Cheshire Basin.

Age[edit]

Mid Triassic (Anisian) to latest Triassic (Rhaetian). The base of the group is diachronous, younging to the south. Age assignments in Warrington et al. (1980) have been revised (Benton et al., 1994; Rees and Wilson, 1998; Warrington et al., 1999).

Equivalent units[edit]

Haisborough Group, Southern North Sea Basin (Rhys, 1974; Cameron et al., 1992; Johnson et al., 1994)

Geographical extent[edit]

Onshore UK and contiguous offshore areas (equivalent strata in the Southern North Sea Basin are assigned to the Haisborough Group (see above).

Subdivisions[edit]

Tarporley Siltstone Formation

Sidmouth Mudstone Formation

Arden Sandstone Formation

Branscombe Mudstone Formation

Blue Anchor Formation

References[edit]