Arden Sandstone Formation

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Arden Sandstone Formation (AS)[edit]

Previous nomenclature[edit]

Arden Sandstone Member (Warrington et al., 1980)

Arden Sandstone Group (Matley, 1912)

Arden Sandstone Horizon (Wills, 1970; 1976)

Dane Hills Sandstone Member (Warrington et al., 1980)

Dane Hills Sandstone Group (Horwood, 1913)

Hollygate Skerry (Elliott, 1961)

Hollygate Sandstone Member (Charsley et al., 1990; Howard et al., in press)

Keuper sandstone (Murchison and Strickland, 1840; Symonds, 1855)

North Curry Sandstone Member (Warrington et al., 1980)

Upper Keuper Sandstone (Phillips, 1848; Brodie, 1856; Plant, 1856; Ussher, 1908; Wills and Campbell Smith, 1913)

Approximately equivalent to:

Sandstone Group of the Weston Cycle (Jeans, 1978)

Weston Mouth Sandstone Member (Warrington et al., 1980)

Dunscombe Mudstone Formation (lowest 24m only) (Gallois, 2001)

Parent unit[edit]

Mercia Mudstone Group

Derivation of name[edit]

From the Forest of Arden, Warwickshire[edit]

Type area[edit]

Knowle Basin, Warwickshire

Type section[edit]

Canal cutting (SP 2118 6744), Shrewley, Warwickshire (Old et al., 1991, Plate 10; Benton et al., 2002)

Primary reference section[edit]

Canal cutting (SP 2010 6905), Rowington, Warwickshire (Old et al., 1991)

Other reference sections[edit]

Twyning Borehole (SO83NE/5) (SO 8943 3664), Twyning, near Tewkesbury, Gloucestershire; from 310.51 to 315.00m (Barclay et al., 1997). Curated core held at the National Geosciences Records Centre, BGS, Keyworth.

Cropwell Bridge Borehole (SK63NE/28) (SK 6773 3547) Cropwell Bishop, Nottinghamshire; from 37.50m to 42.03m depth (Howard et al., in press). Curated core held at the National Geosciences Records Centre, BGS, Keyworth.

Higher Dunscombe and Weston Cliffs (SY 152 877 to SY171 879), south Devon coast (Jeans, 1978; Warrington and Scrivener, 1980; Gallois, 2001).

Newnham river cliff, River Severn, Gloucestershire (SO 6923 1168).

Extant exposures/sections[edit]

The formation usually gives rise to a mappable cuesta-type feature (e.g. Old et al., 1991, Plate 1) but is not well exposed. Most sections are in former quarries (e.g. at Inkberrow (SP 009 569), Warwickshire, and Longdon (SO 835 354), Worcestershire), or in road cuttings (e.g. at Inkberrow (SP 004 563, SP 011 577) and roadside exposures (e.g. at Henley-in-Arden (SP 156 654), Warwickshire). Similar exposures of local representatives of the formation occur around Taunton and Bridgwater, Somerset (Buckland, 1837; Warrington and Williams, 1984; Ruffell and Warrington, 1988; Ruffell, 1990), and on the south Devon coast (Jeans, 1978; Warrington and Scrivener, 1980; Gallois, 2001).

Lithology[edit]

The Arden Sandstone consists of grey, green and purple mudstone interbedded with paler grey-green to buff-coloured siltstone and fine to medium-grained, varicoloured (green, brown, buff, mauve) sandstone; beds of conglomerate occur locally. Breccias occur at the base and top of the formation on the south Devon coast (Jeans, 1978), and may have been formed by penecontemporaneous solution of halite. Laminated and thinly interbedded sediments are commonly extensively bioturbated and show structures indicative of thixotropic deformation. Invertebrate and vertebrate macrofossils are present, locally in abundance, and miospores and burrows are common. The siltstones and finer sandstones show small-scale ripple-drift cross-bedding; thicker sandstone beds show trough and planar cross-bedding. The proportion of fine to coarse clastics varies laterally within the formation. The thicker sandstone units, composed of several individual beds, have a lenticular geometry and occupy the inferred former courses of distributary channels in a deltaic or estuarine environment. Less arenaceous, mudstone- and siltstone-dominated successions represent intervening interdistributary areas.

The formation is differentiated from the reddish brown, blocky weathering mudstones of adjacent formations by its predominantly greenish grey colour, the presence of a significant (though often subordinate) proportion of sandstone, the predominance of finely laminated lithologies throughout, and its comparatively fossiliferous nature. The formation is characterised by an exotic clay mineral assemblage, which is rich in mixed-layer clays and distinct from the illite-dominated units below and above (Jeans 1978, Bloodworth and Prior, 1993; Carney et al., 2004; Jeans et al., 2005).

Lower boundary[edit]

The lower boundary of the formation is placed at an abrupt upward change from the red mudstone or siltstone of the Sidmouth Mudstone Formation to the dominantly grey or green, partly or largely arenaceous beds of the Arden Sandstone Formation. On the south Devon coast, this boundary coincides with the base of a prominent breccia bed (see sections in Jeans, 1978), and is conformable in all areas.

Where not exposed, the boundary is typically marked by a change from the reddish brown clayey soils of the Sidmouth Mudstone Formation to the grey, slightly sandy clay soils of the overlying Arden Sandstone Formation. Where the latter forms a marked cuesta feature, the boundary typically lies at a slight concave break in the scarp slope.

Upper boundary[edit]

Placed at the base of red, structureless mudstone where it rests on the interbedded dark grey-green siltstone and pale grey sandstone of the underlying Arden Sandstone Formation, the boundary is abrupt or is a rapid interbedded transition. On the south Devon coast, the upper boundary coincides with the top of a prominent breccia bed (see sections in Jeans, 1978). This boundary is conformable in all areas.

Where not exposed, the boundary is typically marked by a change from the grey, slightly sandy clay soils of the Arden Sandstone Formation to the reddish-brown clayey soils of the Branscombe Mudstone Formation. Where the Arden Sandstone forms a marked cuesta feature, the boundary lies at the down-dip limit of the dip slope.

Thickness[edit]

Typically 7 to 8m, rising to 20m in the Stowell Park Borehole (SP01SE/1, (SP 0835 1176) (Green and Melville, 1956) towards the south of the Worcester Basin, and 24 m on the south Devon coast (cf. Jeans, 1978). Locally thins to 2-3m in parts of the Worcester Basin, Knowle Basin and East Midlands areas.

Age[edit]

Late Triassic, late Carnian (Tuvalian), on palynological evidence.

Equivalent units[edit]

Sandstone groups in Dunscombe and Weston cycles (Jeans, 1978), which represent a single unit (Warrington and Scrivener, 1980).

Geographical extent[edit]

The outcrop of the formation is traceable nearly continuously from Gloucestershire northward through Worcestershire and eastwards though Warwickshire into Leicestershire and Nottinghamshire. Less continuous outcrop is mapped in Somerset and east Devon. In the subcrop, the formation is proved in several cored and geophysically logged boreholes in the subsurface in the Central Midlands (Worcester and Knowle basins), East Midlands, and in southern England (Wessex Basin). It is not recognised in the Cheshire Basin and other basins farther north-west. At depth in Dorset and Somerset, the partly equiavalent Dunscombe Mudstone Formation has been interpreted to pass laterally into halite (Gallois, 2001, 2003).

Subdivisions[edit]

None

References[edit]