Sidmouth Mudstone Formation
Sidmouth Mudstone Formation
Previous nomenclature
Radcliffe Formation plus Carlton Formation, Harlequin Formation and Edwalton Formation (excluding Hollygate Skerry) (Elliott, 1961)
Radcliffe Formation plus Gunthorpe Formation and Edwalton Formation (excluding Hollygate Sandstone Member) (Charsley et al., 1990)
Hambleton Mudstone Formation plus Singleton Mudstone Formation, Kirkham Mudstone Formation, and Breckells Mudstone Formation (Wilson and Evans, 1990)
Bollin Mudstone Formation plus Northwich Halite Formation, Byley Mudstone Formation, Wych Mudstone Formation and Wilkesley Halite Formation (Wilson, 1993).
Eldersfield Mudstone Formation (Barclay et al., 1997)
Approximately equivalent to: Lower Keuper Marl (s) (Brodie, 1870; Matley, 1912)
Mudstone I (Jeans, 1978)
Sub-Arden Keuper Marls (Wills, 1970; 1976)
Parent unit
Derivation of name
From the town of Sidmouth in south Devon, located immediately to the west of the type section.
Type area
South Devon coast
Type section
South Devon coast between Sidmouth (SY 129 873) and Weston Mouth (SY 163 879) (Jeans, 1978; Warrington and Scrivener, 1980; Gallois 2001; 2004).
Primary reference sections
Eldersfield Borehole (SO73SE/6) (SO 7891 3221); Eldersfield, near Tewkesbury, Gloucestershire; from the start of coring to 348.47 m depth. (Barclay et al., 1997; Worssam et al., 1989). Curated core held at the National Geosciences Records Centre, BGS, Keyworth.
Other reference sections
Worcester Borehole (SO85NE/23) (SO 8624 5762), Worcestershire; from surface (core from 30 m) to 293.51 m (Barclay et al., 1997). Curated core held at the National Geosciences Records Centre, BGS, Keyworth.
Asfordby Hydrogeological Borehole (SK72SW/71) (SK 7252 2061), Asfordby, Leicestershire; from 211.37 to 327.87 m depth (Carney et al., 2004). Curated core held at the National Geosciences Records Centre, BGS, Keyworth.
Cropwell Bridge Borehole (SK63NE/28) (SK 6773 3547); Cropwell Bishop, Nottinghamshire; from 42.03m to 170.70m depth (Howard et al., in press). Curated core held at the National Geosciences Records Centre, BGS, Keyworth.
Fulbeck F/B1 Borehole (SK85SE/25) (SK 8889 5053), Fulbeck, Lincolnshire; from 158.8 to 301.58 m depth (Berridge et al., 1999). Curated core held at the National Geosciences Records Centre, BGS, Keyworth.
Wilkesley Borehole (SJ64SW/7) (SJ 6286 4144), Cheshire, from 347.8m to 1527.4 m to depth (Poole and Whiteman, 1966; Wilson, 1993).
Home Farm Borehole (SP47SW/72) (SP 4317 7309), near Rugby, Warwickshire; from 76.67 to 201.22 m depth (Old et al., 1987). Curated core held at the National Geosciences Records Centre, BGS, Keyworth.
Twycross Borehole (SK30NW/13) (SK 3387 0564), Twycross, Leicestershire (Worssam and Old, 1988); from base of Superficial Deposits at 17.40 m to 127.97 m depth. Curated core held at the National Geosciences Records Centre, BGS, Keyworth.
Leicester Forest East Borehole (SK50SW/71) (SK 5245 0283), Leicestershire; from base of Superficial Deposits at 4.0 m to 119.51 m depth (Worssam and Old, 1988). Curated core held at the National Geosciences Records Centre, BGS, Keyworth.
Knights Lane Borehole (SP25SW/1) (SP 2242 5497), Stratford-upon-Avon, Warwickshire; from base of Superficial Deposits at 6.1 m to 151.23 m depth (Williams and Whittaker, 1974). Curated core held at the National Geosciences Records Centre, BGS, Keyworth.
Kirkham Borehole (SD43SW/6) (SD 4324 3247), Lancashire; from base of Superficial Deposits at 36.58 m to 366.19 m depth (Wilson, 1990; Wilson and Evans, 1990). Curated core held at the National Geosciences Records Centre, BGS, Keyworth.
Extant exposures/sections
The formation is exposed on the south Devon coast in a series of cliff sections extending for 3.5 km between Sidmouth (SY 129 873) and Weston Mouth (SY 163 879) (Jeans, 1978; Warrington and Scrivener, 1980, Gallois, 2001; 2004). Access to the uppermost part of the succession is difficult, although landslipped material can be examined at beach level.
Elsewhere, the formation is generally very poorly exposed at surface. Sections of the lower and middle parts of the formation (Radcliffe and Gunthorpe members) occur alongside the River Trent at Radcliffe-on-Trent, near Nottingham (SK 6461 3987 and SK 6507 4064) (Howard, 2003), and the middle part of the formation (Gunthorpe Member) is currently exposed in Dorket Head Brick Pit (SK 598 347), near Nottingham (Rathbone, 1989; Howard et al., in press).
Lithology
The formation consists dominantly of red-brown mudstone and siltstone with common grey-green reduction patches and spots. The mudstones are mostly structureless, with a blocky weathering habit, but units up to 15m thick of interlaminated mudstone and siltstone occur in parts of the formation, notably in the Bollin and Byley Mudstone members of the Cheshire Basin (Wilson, 1993) and in the Radcliffe Member of the East Midlands (Charsley et al., 1990; Howard et al., in press). Heterolithic units consisting of several thin beds of grey-green dolomitic siltstone and very fine-grained sandstone, interbedded with mudstone, occur at intervals throughout the formation. These units range from a few centimetres up to 4m thick, and locally form mappable cuesta features. Named examples include the Cotgrave Sandstone Member and Clarborough Member of the East Midlands (Smith et al., 1973; Warrington et al., 1980; Charsley et al., 1990) and the Weatheroak Sandstone of the Redditch area (Old et al., 1991). The Redcliffe Sandstone Member, a distinctive, deep red calcareous and ferruginous sandstone, is a marginal facies of the formation developed locally in the Bristol area (Kellaway and Welch, 1993). Units of halite up to 400 m thick are present at several stratigraphical levels in the thicker basinal sequences in west Lancashire and south Cumbria, Cheshire, Staffordshire, north-east Yorkshire, Worcestershire, Somerset and Dorset. Breccias produced by contemporaneous solution of salt are common throughout the formation. In the Cheshire Basin, more recent salt solution (some induced artificially by brine pumping activities) gives rise to solution hollows and collapse breccias associated with the halite members. Gypsum/anhydrite also occurs throughout the formation as nodules and veins.
Lower boundary
The boundary is both gradational and conformable, and is drawn arbitrarily within an upward-fining succession at the level where mudstones predominate over the siltstones and sandstones of the underlying Tarporley Siltstone Formation; the junction is diachronous. Where the underlying Tarporley Siltstone Formation is not differentiated (including at the type section), the boundary is marked by a rapid upward transition from sandstone to mudstone-dominated lithologies immediately above the Sherwood Sandstone Group (Jones, 1993; Gallois, 2001, 2004).
Where not exposed, the boundary is associated with a change from the brown sandy clay soils of underlying formations to the more clayey, distinctively reddish brown soils of the Sidmouth Mudstone Formation. A concave break of slope is associated with the boundary in some areas.
Upper boundary
This boundary is placed at the rapid upward transition from red mudstones or siltstones to the dominantly grey or green, partly or largely arenaceous beds of the overlying Arden Sandstone Formation. At the type section, this boundary coincides with the base of a prominent breccia bed (see sections in Jeans, 1978) at the base of the Arden Sandstone Formation. In the Cheshire Basin and parts of the Wessex Basin, where the Arden Sandstone Formation cannot be recognised in the presence of thick halite deposits, the boundary is drawn immediately above the highest halite beds. The boundary is conformable in all areas.
At outcrop, 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. In the Cheshire Basin, where the Arden Sandstone Formation is absent, the boundary is mapped at the approximate down-dip limit of subsidence hollows and collapse breccias, which are associated with near-surface solution of halites (Wilkesley Halite Member) at the top of the Sidmouth Mudstone (Taylor et al., 1963).
Thickness
The formation is over 200 m thick in south Devon (Figure 3). Elsewhere, it ranges from 120-130 m in the East Midlands to 370 m in the Worcester Basin and 1600 m in the Cheshire Basin.
Age
Mid Triassic (Anisian) to Late Triassic (Carnian). Miospore assemblages ranging in age from Anisian to Carnian have been recovered from this formation throughout England. These dates are further constrained by Anisian ages from the underlying Tarporley Siltstone and late Carnian dates from the Arden Sandstone. Magnetostratigraphical correlation (Hounslow and McIntosh, 2003) indicates that the base of the Ladinian lies close to the base of the formation on the south Devon coast.
Equivalent units
Dowsing Formation and Dudgeon Formation (Southern North Sea Basin) (Johnson et al., 1994).
Leyland Formation plus Preesall Halite Formation, Dowbridge Mudstone Formation and Warton Halite Formation (East Irish Sea Basin) (Jackson et al., 1997).
Geographical extent
The surface outcrop extends from the south Devon coast east of Sidmouth northwards into Somerset, Gloucestershire, Worcestershire, Warwickshire, Leicestershire, Nottinghamshire, east and north Yorkshire; and westwards into Derbyshire, Staffordshire, north Shropshire, Cheshire and Lancashire. Separate outcrops occur in south Cumbria and the Carlisle Basin. The formation occurs in the subsurface below much of southern England, the south Midlands and eastern England. Equivalents occur below the southern North Sea and eastern Irish Sea.
Subdivisions
* indicates units that were previously defined as formations
Sid Mudstone Member, Salcombe Hill Mudstone Member, Salcombe Mouth Member, Hook Ebb Mudstone Member, Little Weston Mudstone Member (Wessex Basin) (Gallois, 2001)
*Radcliffe Member, *Gunthorpe Member, Cotgrave Sandstone Member, *Edwalton Member, *Clarborough Member, *Seaton Carew Member, *Esk Evaporite Member (East Midlands Shelf North) (Smith, 1980; Warrington et al., 1980, Charsley et al., 1990)
*Bollin Mudstone Member, *Northwich Halite Member, *Byley Mudstone Member, *Wych Mudstone Member, *Wilkesley Halite Member (Cheshire Basin) (Wilson, 1993)
*Hambleton Mudstone Member, Rossall Halite Member, *Singleton Mudstone Member, Mythop Halite Member, Thornton Mudstone Member, Coat Walls Mudstone Member, *Breckells Mudstone Member (west Lancashire) (Wilson and Evans, 1990)
*Stafford Halite Member (Stafford Basin) (Warrington et al., 1980)
*Droitwich Halite Member, *Weatheroak Sandstone Member (Worcester and Knowle Basins) (Warrington et al., 1980; Old et al., 1991; Barclay et al., 1997)
*Redcliffe Sandstone Member (Bristol area) (Kellaway and Welch, 1993)
*Somerset Halite Member, Dorset Halite Member (Wessex Basin) (Warrington et al., 1980; Harvey and Stewart, 1998)