Helsby Sandstone Formation

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Helsby Sandstone Formation[edit]

Previous nomenclature[edit]

  • Lower Keuper Sandstone (Hull, 1860a, b)
  • Waterstones* (Binney, 1846)
  • Building Stones (Hull, 1869)
  • Basement Beds (Hull, 1869)
  • Upper Sandstone(s) (Ussher, 1902, 1913)
  • Grinshill Sandstone (Pocock and Wray, 1925)
  • Ruyton Sandstone (Pocock and Wray, 1925)
  • Otter Sandstones (Laming, 1968)
  • Otter Sandstone (Henson, 1970)
  • Waterstones Group (Wills, 1970)
  • Keuper Sandstone Formation (Thompson, 1970a)
  • Burcot Breccia (Wills, 1976)
  • Bulkeley Hill Sandstone Formation (Warrington et al., 1980)

* The beds formerly termed ‘Waterstones’ have, over most of the country, now been assigned to the MMG although, in the past, they were included in the ‘Lower Keuper Sandstone’ (various workers) and the Bromsgrove Sandstone (Worssam and Old, 1988). In the Bromsgrove area, this part of the sequence comprises more than 80 per cent sandstone and has been included in the Helsby Sandstone Formation as the Sugarbrook Member (4.6).

Parent unit[edit]

Sherwood Sandstone Group

Derivation of name[edit]

From the town of Helsby, north Cheshire

Type section[edit]

SJ 495 755: Helsby Hill, Cheshire (Earp and Taylor, 1986, p.26) exposing about 43m of the formation.

Reference sections[edit]

  • SK 059 388 to SK 072 422: Between Hollington and Alton, Staffordshire.
  • SO 96 70: Bromsgrove, north Worcestershire (Warrington, et al., 1980); basal Burcot Breccia exposures (Wills, 1976, p 37); other localities (Old, et al., 1991, pp. 18–30).
  • NY 4000 7240: Small exposures in river banks north of Longtown, Cumbria (Dixon et al, 1926).
  • NY 04506 00184: Sellafield Borehole 13B, Cumbria. BGS Registered No. NY00SW/42, 35.97 to 176.42 m (Barnes et al., 1994).
  • SY 060 816 to SY 130 873: Sea cliffs between Budleigh Salterton and Sidmouth, south Devon (Hounslow and McIntosh, 2003).

Extant exposures/sections[edit]

The formation is very well exposed on the coast between Budleigh Salterton and Sidmouth, south Devon (Barton et al., 2011); a few exposures occur inland in east Devon and Somerset. It is well-exposed in the Warwick (Old et al., 1987), Bromsgrove (Old et al., 1991), Stourport and Kidderminster areas (Mitchell et al., 1962), and across the West Midlands into Staffordshire (e.g. Bridge and Hough, 2002). In north Cheshire, south Lancashire and Merseyside there are numerous exposures, including natural cliff sections at Helsby Hill and sections in disused quarries on Overton Hill, near Frodsham (Woodcock, 2002), and in disused quarries and road cuttings on the Wirral peninsula (Benton et al., 2002). It is also well-exposed at Frogsmouth Quarry, Runcorn (Mountney and Thompson, 2002). The Frodsham Sandstone Member is well-exposed at Frodsham, notably in the railway cutting (Thompson, 1969; Benton et al., 2002). Extensive unweathered sections are accessible in disused mine workings at the Alderley Edge Geological SSSI, north-east Cheshire (Warrington, 2010).

Lithology[edit]

Fine-to medium-grained, locally micaceous, cross-bedded and flat-bedded sandstones, weathering to sand near surface. Sandstones are of fluvial (subangular to subrounded grains) and aeolian (well-rounded grains) facies. Pebbles may be common, particularly near the base of the formation, and thin units of hard intraformational conglomerate occur in the south-west. Thin lenticular beds of reddish brown siltstone and mudstone occur and may be common in fining-upward sequences; calcretes and rhizocretions occur at some horizons in the south-west (Purvis and Wright, 1991; Hounslow and McIntosh, 2003; Newell, 2006).

Lower boundary[edit]

Rests disconformably on the Wilmslow Sandstone Formation and unconformably on the Chester Formation in different areas of the country. In the West Midlands and Lancashire it is placed at the base of the lowest pebbly bed or conglomerate. In Cumbria it is placed where aeolian sandstones overlie a thick fluvial sandstone sequence assigned to the Wilmslow Sandstone Formation. It is not a formational boundary with characteristic features on borehole geophysical logs.

Upper boundary[edit]

Transitional into the overlying Tarporley Siltstone Formation or, where that is absent, the Sidmouth Mudstone Formation of the MMG.

Thickness[edit]

It is locally absent in areas that were contemporary high ground such as Charnwood Forest, Breedon Hill and Mountsorrel, and possibly the Lickey Hills. The formation is thinnest in the Warwickshire, Derbyshire and north Staffordshire areas. The thickest known development (up to 500 m) is in the Worcester Basin. It thins northwards to around 250 m in the Cheshire–Lancashire area and is 90 to over 176 m thick in Cumbria. The maximum known thickness there is 176.42 m in the Sellafield 13B borehole where the top was not proved.

Age[edit]

Anisian (early Mid Triassic), from bio-and magnetostratigraphy

Equivalent units[edit]

  • Smith Bank Formation (pars.), Heron Group, Central North Sea (Cameron, 1993).
  • Dowsing Dolomitic Formation (basal part), Haisborough Group, Southern North Sea Basin (Cameron et al., 1992; Johnson et al., 1994; Bachmann et al., 2010).
  • Ormskirk Sandstone Formation, East Irish Sea Basin (Jackson and Johnson, 1996; Chadwick et al., 2001).

Geographical extent[edit]

The formation crops out almost continuously from the south Devon coast, northwards, to near Taunton, Somerset. There is one outcrop in north-west Gloucestershire, around Newent, and extensive outcrops in north Worcestershire and in Staffordshire, up to near Stoke-on-Trent. There is continuous outcrop from Warwick to Nuneaton with outcrops elsewhere in Warwickshire and extensive but broken outcrops in south Derbyshire and north-west Leicestershire, including the north side of Charnwood Forest. Further fragmentary outcrops occur in Cheshire and south-west of Manchester, with more continuous outcrops around Merseyside and in southern Lancashire. The formation crops out very locally on the Cumbrian coast and more extensively in the Eden valley.

Subdivisions[edit]

  • Burcot Member (Old et al., 1991)
  • Delamere Sandstone Member (Thompson, 1970a)
  • Finstall Member (Old et al., 1991)
  • Frodsham Sandstone Member (Thompson, 1969)
  • Kibblestone Member, downgraded from Kibblestone Formation (Rees and Wilson, 1998)
  • Pennington Point Member (Gallois, 2004)
  • Shepshed Sandstone Member (Carney et al., 2002)
  • Sugarbrook Member (Old et al., 1991)
  • Thurstaston Sandstone Member (Thompson, 1970a)
  • Thurstaston Hard Sandstone Bed (Thompson, 1970b)

References[edit]