Wilmslow Sandstone Formation

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Wilmslow Sandstone Formation (WLSF)[edit]

Previous nomenclature[edit]

  • Upper Mottled Sandstone (Hull, 1860a)
  • Upper Red and Mottled Sandstone (Hull, 1860b)
  • Moulding Sand Group (Wills, 1948)
  • Bunter Upper Mottled Sandstone (Thompson, 1970a)
  • ‘Upper’ St Bees Sandstone (Jackson et al., 1987)
  • Upper Wildmoor Beds (Wills, 1970)
  • Lower Wildmoor Beds (Wills, 1970)
  • Wildmoor Beds (Wills, 1976)

Parent unit[edit]

Sherwood Sandstone Group

Derivation of name[edit]

From the town of Wilmslow, in north-east Cheshire.

Type section[edit]

Wilmslow–Alderley Edge area, north-east Cheshire (Taylor et al., 1963, p.61–62; Thompson, 1970a; Warrington and Thompson, 1971).

Reference sections[edit]

  • SO 949 762: Wildmoor (Wildmoor Sandstone Member), north of Bromsgrove, Worcestershire (Warrington et al., 1980).
  • NY 04268 03080: Bottom part, Sellafield Borehole 10B, Cumbria. BGS Registered No. NY00SW/40, 22.22 to 190.35 m.
  • NY 04506 00184: Sellafield Borehole 13B, Cumbria. BGS Registered No. NY00SW/42, 176.42 to 295.70 m.

Extant exposures/sections[edit]

The formation is exposed over parts of the West Midlands (mainly Worcestershire and Shropshire) (Whitehead and Pocock, 1947; Mitchell et al., 1962). There are a few exposures in Cheshire, mainly around the Alderley Edge–Wilmslow area. Extensive unweathered sections are accessible in disused mine workings at the Alderley Edge Geological SSSI in north-east Cheshire (Warrington, 2010). In Cumbria there are discontinuous exposures in the River Calder valley (Jones and Ambrose, 1994).

Lithology[edit]

In the Cheshire Basin and northwards into Cumbria, the formation comprises red-brown to brick-red, fine-to medium-grained, generally pebble-free, cross-bedded sandstones, with sporadic siltstones. The sand grains are well rounded and indicate an aeolian origin; in Cumbria, they are coarse-to very coarse-grained. There are thin units of fluvial sandstones that have subangular to subrounded sand grains and more common beds of mudstone. Southwards, these pass into bright orange-red to dark brick-red, generally silty or argillaceous, fine-to medium-grained, micaceous sandstones with subordinate siltstone and mudstone beds. The sand grains are subangular to subrounded and pebbles are rare.

Lower boundary[edit]

The lower boundary, at the top of a sequence dominated by pebbly sandstones and conglomerates of the Chester Formation, is transitional. It may be taken at the highest occurrence of pebbles, either as a conglomerate or a pebbly sandstone. In Cumbria it is sharp and is taken at the upward change to typical aeolian sandstones from typical fluvial sandstones, which are finer grained and better cemented.

Upper boundary[edit]

It is overlain unconformably by the Helsby Sandstone Formation. In Cumbria, it is placed at the top of a thick (40 to 50 m) fluvial sandstone unit that is well-cemented and overlain by coarser, less well-cemented aeolian sandstones of the Helsby Sandstone Formation. It is not a formational boundary with characteristic features on borehole geophysical logs.

Thickness[edit]

Thicknesses are variable across the country. In Cumbria the Sellafield boreholes proved 469 to 529 m. In the West Midlands and the Worcester Basin it is up to 284 m thick. In the Cheshire Basin, in the Knutsford Borehole, pebbles indicative of the Chester Formation were encountered at 1870 to 1878 m depth, giving a thickness of about 920 m for the Wilmslow Sandstone; this includes 325 m of strata below the ‘Top Silicified Zone’ which, in the EISB and regions 8 and 9, has been used to mark the base of the Wilmslow Sandstone Formation (Jackson et al., 1987).

Age[edit]

No direct evidence; probably Olenekian (late Early Triassic)

Equivalent units[edit]

  • Calder Sandstone Member, St Bees Sandstone Formation, East Irish Sea Basin (Jackson and Johnson, 1996; Chadwick et al., 2001).
  • Smith Bank Formation (pars.), Heron Group, Central North Sea (Cameron, 1993).
  • Bunter Sandstone Formation, Bacton Group, Southern North Sea Basin (Cameron et al., 1992; Johnson et al., 1994; Hounslow and Ruffell, 2006; Bachmann et al., 2010).

Geographical extent[edit]

The Wilmslow Sandstone is exposed from just south of Stourport, across the West Midlands to Stoke-on-Trent and Lichfield, from Macclesfield to Manchester and Liverpool, and as far north as Ormskirk. In Lancashire, north of Ormskirk, it cannot be distinguished from the Helsby Sandstone and Chester formations. In Cumbria it is a distinctive unit of aeolian facies with thin fluvial interbeds. It is present in the subsurface in the Worcester and Needwood basins but absent in the Wessex and Somerset basins. There is some uncertainty about its presence between the West Midlands and the Needwood Basin. It is absent to the east of the Worcester Basin and up the eastern side of England to Doncaster. North of Doncaster, the SSG is undifferentiated and the Wilmslow Sandstone Formation cannot be distinguished.

Subdivision[edit]

Wildmoor Sandstone Member

References[edit]