The BGS Lexicon of Named Rock Units — Result Details

Wilmslow Sandstone Formation

Computer Code: WLSF Preferred Map Code: WlS
Status Code: Full
Age range: Olenekian Age (TO) — Olenekian Age (TO)
Lithological Description: 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.
Definition of Lower Boundary: 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.
Definition of Upper Boundary: 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: 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).
Geographical Limits: 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.
Parent Unit: Sherwood Sandstone Group (SSG)
Previous Name(s): MOTTLED SANDSTONE, UPPER (UMS)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Wilmslow-Alderley Edge area, north Cheshire (Taylor and others, 1963, p.61-62; Thompson, 1970; Warrington and Thompson, 1971). 
Reference Section  Stockport Corporation boreholes (the 'Keuper Sandstone' of Alty, 1926); "in" Taylor and others, 1963, pp61, 155. Best in BH No.3. (SJ88SW/7). 
Reference Section  Tiltston stream section. 10km northwest of Whitchurch, Cheshire. 
Reference(s):
Taylor, B J, Price, R H and Trotter, F M, 1963. Geology of the country around Stockport and Knutsford. Memoir of the Geological Survey of the United Kingdom. p.183. 
Thompson, D B, 1970. Itineries X and XI, Alderley Edge, "in" Broadhurst, F M and others. The area around Manchester. Geologists' Association Guide No.7 (Revised edition). Benhan, Colchester. p.39-51. 
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. 
Warrington, G and Thompson, D B, 1971. The Triassic rocks of Alderley Edge, Cheshire. Mercian Geologist, Vol.4, p.69-72. 
Alty, S. 1926. The petrographic features of Keuper rocks from a boring at Wilmslow, near Stockport, Cheshire. Proceedings of the Liverpool Geological Society, Vol.14, 278-283. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E123 E151 E137 E096