Branscombe Mudstone Formation

Branscombe Mudstone Formation (BCMU)

Previous nomenclature

Brooks Mill Mudstone Formation (Wilson, 1993: Rees and Wilson, 1998)

Cropwell Bishop Formation (Charsley et al., 1990, Berridge et al., 1999)

Twyning Mudstone Formation (Barclay et al., 1997)

Trent Formation plus the lower part of the Parva Formation (Elliott, 1961)

Trent Formation plus Glen Parva Formation (Warrington et al., 1980).

Approximately equivalent to:

Mudstone III (Jeans, 1978)

Supra-Arden Keuper Marls (Wills, 1970; 1976)

Upper Keuper Marl(s) of Harrison (1876), Matley (1912)

Parent unit

Mercia Mudstone Group

Derivation of name

From the village of Branscombe, south Devon.

Type area

South Devon coast.

Type section

South Devon coast between Weston Cliff and Branscombe Mouth (SY 171 879 to SY 207 881) (lower two thirds of formation) and in Haven Cliff, east of Seaton (SY 256 898 to SY 260 897) (upper third of formation) (Gallois, 2001).

Primary reference section

Twyning Borehole (SO83NE/5) (SO 8943 3664), Twyning, near Tewkesbury, Gloucestershire; from 141.5-310.51m depth. (Barclay et al., 1997; Worssam et al., 1989).

Other reference sections

At Audrie’s Bay, north Somerset (ST 120 437 to ST 106 431); uppermost c.68 m exposed in the cliffs and foreshore (Talbot et al., 1974; Hounslow et al., 2004).

Aust Cliff, south Gloucestershire (ST 5645 8920); uppermost 35m is exposed in cliffs on the east side of the River Severn below the Blue Anchor Formation (Hamilton, 1977; Kellaway and Welch, 1993).

Asfordby Hydrogeological Borehole (SK72SW/71) (SK 7252 2061), Asfordby, Leicestershire; from 170.08m to 211.37m 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 surface to 37.50m depth (uppermost beds not proved) (Howard et al., in press). Curated core held at the NGRC Core Store, BGS, Keyworth.

Fulbeck F/B1 Borehole (SK85SE/25) (SK 8889 5053), Fulbeck, Lincolnshire, from 117.52 m to 156.10 m depth (Berridge et al., 1999). Curated core held at the National Geosciences Records Centre, BGS, Keyworth.

Wilkesley Borehole (Reg. No. SJ64SW/7) (SJ 6286 4144), from 186.5 m to 347.8 m depth (Poole and Whiteman, 1966; Wilson, 1993).

Extant exposures/sections

The formation is exposed on the south Devon coast between Weston Cliff and Branscombe Mouth (SY 171 879 to 207 881) and intermittently between Beer and Haven Cliff, Seaton (SY 235 896 to SY 260 897), (Jeans, 1978; Gallois, 2001). The succession generally youngs eastwards in a discontinuous series of cliff exposures; approximately 40m of strata are obscured towards the top of the formation in Seaton Bay.

Most of the formation is very poorly exposed at surface. There are, however, several good coastal or river cliff sections of the uppermost part of the formation adjacent to the Bristol Channel and River Severn in west Somerset, Gloucestershire and south Glamorgan. These include the sections at Aust Cliff and St Audrie’s Bay (see above), and also north of Lavernock Point, south Glamorgan (ST 187 682 to ST 186 690) (Waters and Lawrence, 1987; Warrington and Ivimey-Cook, 1995), Garden Cliff, Westbury-on-Severn (SO 719 126) (Etheridge, 1865; Benton et al., 2002) and Wainlode, near Gloucester (SO 847 259) (Worssam et al., 1989).

Lithology

Mudstone and siltstone, red-brown, with common grey-green reduction patches and spots. The mudstones are mostly structureless, with a blocky weathering habit. Gypsum/anhydrite, locally of economic importance, is common throughout in beds, nodules and veins. Many of these sulphate beds have been named (see Subdivisions, below) and form distinct markers on geophysical logs. Sporadic thin beds of argillaceous sandstone and silty dolostone occur in the lower part of the formation. In south Devon, Somerset and Gloucestershire, the highest 10-20 metres of the formation include common beds of greenish grey mudstone, giving rise to markedly colour-banded sections where exposed in coastal or river cliffs. A similar ‘colour-banded’ unit in the lowest 11m of the formation was included in the Dunscombe Mudstone Formation by Gallois (2001). In the East Midlands, beds of thinly interlaminated, dark grey-green mudstone and dolomitic siltstone occur locally towards the top of the formation (formerly separated as the Glen Parva Formation, Elliott, 1961, Warrington et al., 1980). Halite is interpreted to occur towards the base of the formation in Dorset (Nettlecombe Borehole, SY59NW/1 (SY 5052 9544); Figure 3) but is absent elsewhere; pseudomorphs after halite are rare.

Lower boundary

The lwer boundary is placed at the base of predominantly red, structureless mudstones where they rest on the interbedded, laminated, dark grey-green siltstones and pale grey fine-grained sandstones of the underlying Arden Sandstone Formation. The boundary is abrupt or a rapid, interbedded transition. Where the Arden Sandstone Formation cannot be recognised with confidence due to the presence of halite (Cheshire Basin and parts of the Wessex Basin), the boundary is placed immediately above the highest halite beds of the Sidmouth Mudstone Formation (Wilkesley Halite Member, Somerset Halite Member, Dorset Halite Member). At the type section, the base of the formation is drawn at a lower stratigraphical level than originally defined by Gallois (2001) in order to encompass some 11 m of reddish brown mudstone with subordinate greenish grey interbeds that Gallois included in the Dunscombe Mudstone Formation. The 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. In the Cheshire Basin, where the Arden Sandstone Formation is not recognisable, 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).

Upper boundary

Placed at an abrupt or rapidly gradational upward transition from the red-brown, silty, mudstones of the Branscombe Mudstone Formation to green or grey-green, dolomitic mudstones of the Blue Anchor Formation. Locally in southern England and Wales, a more gradational transition occurs above interbedded red and green lithologies (e.g. on the west Somerset and south Devon coasts); in these areas the boundary is drawn above the highest significant red mudstone bed. In south Devon, this coincides with a prominent bed of dolomitic limestone (Gallois, 2001). The boundary may be locally erosional around the margins of the London-Brabant Massif (Horton et al., 1987; Old et al., 1987)

Where not exposed, the boundary is marked by a change in soil colour from the reddish brown clayey soils of the Branscombe Mudstone Formation to the grey clayey soils of the overlying Blue Anchor Formation. Typically, this boundary lies near the base of a scarp slope formed by the Blue Anchor Formation and capped by the Penarth Group.

Thickness

Up to 240 m in the Wessex Basin, 170 to 190 m in the Worcester Basin; 25 to 60 m in the East Midlands, up to 160 m in the Cheshire Basin.

Age

Late Triassic, Norian to ?Rhaetian. Not independently dated; age is inferred from late Carnian dating of the underlying Arden Sandstone Formation (see above) and late Norian - Rhaetian dating of the overlying Blue Anchor Formation (see below)

Equivalent units

Lower and middle part of Triton Formation, Southern North Sea Basin (Johnson et al., 1994) Elswick Mudstone Formation of the East Irish Sea Basin (Jackson et al., 1997).

Geographical extent

The surface outcrop extends from south Devon northwards into Somerset, Gloucesterhire, south Glamorgan and Worcestershire. The formation crops out below parts of the English Central Midlands, Cheshire and the Carlisle Basin, and in a strip extending northwards from Nottingham into North Yorkshire. The formation occurs in the subsurface below much of the south Midlands and eastern England. It has equivalents in the Southern North Sea and East Irish Sea Basins.

Subdivisions

• Littlecombe Shoot Mudstone Member (Wessex Basin) (Gallois, 2001)
• Red Rock Gypsum Member (Wessex Basin) (Gallois, 2001)
• Seaton Mudstone Member (Wessex Basin) (Gallois, 2001)
• Haven Cliff Mudstone Member (Wessex Basin) (Gallois, 2001)

Units of undefined status include:

• Newark Gypsum (East Midlands Shelf South) (Sherlock and Hollingworth, 1938)
• Tutbury Gypsum or Tutbury Sulphate Bed (East Midlands Shelf South) (Sherlock and Hollingworth, 1938; Taylor, 1983)
• Windmill Skerries (East Midlands Shelf South) (Elliott, 1961)
• Woodford Hill Sandstone (Bristol area) (Kellaway and Welch, 1993)