Tarporley Siltstone Formation

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Tarporley Siltstone Formation (TPSF)[edit]

Previous nomenclature[edit]

Denstone Formation (Charsley, 1982)

Holling Member (Barclay et al., 1997)

Maer Formation (Rees and Wilson, 1998)

Retford Formation plus Colwick Formation (Warrington et al., 1980)

Sneinton Formation (Charsley et al., 1990; Berridge et al., 1999)

Woodthorpe Formation plus Colwick Formation (Warrington et al., 1980)

Woodthorpe Formation plus Waterstones Formation (Elliott, 1961)

Approximately equivalent to: Keuper Waterstones (Lamplugh et al. 1908; Smith, 1912; Swinnerton, 1918; Poole and Whiteman, 1966; Evans et al., 1968)

Waterstones (Hull, 1869; Strahan, 1882; Gibson, 1925)

Waterstones Group (Wills, 1970, 1976)

Parent unit[edit]

Mercia Mudstone Group

Derivation of name[edit]

From the village of Tarporley, Cheshire

Type area[edit]

Several sections (mostly small exposures) around the village of Tarporley, Cheshire (Warrington et al., 1980)

Primary reference sections[edit]

Windyharbour Borehole, Staffordshire (SK04SE/16) (SK 0854 4100) between 51.57 and 103.16 m. (Charsley, 1982). Curated core held at the National Geosciences Records Centre, BGS, Keyworth.

Surface section, former railway cutting, Colwick Road, Sneinton, Nottingham (SK5924 3968 to 5920 3980). Exposes lower boundary and lowermost 20m of formation (Charsley, 1989; Benton et al., 2002; Howard, 2003; Howard et al, in press).

Other reference sections[edit]

Cropwell Bridge Borehole (SK63NE/28) (SK 6773 3547), Nottinghamshire between 170.70m and 214.33m (Charsley et al., 1990; Howard et al., in press). Curated core held at the National Geosciences Records Centre, BGS, Keyworth.

Fulbeck F/B1 Borehole, Lincolnshire (SK85SE/25) (SK8889 5053) between 301.58 and 375.24m (Berridge et al., 1999). Curated core held at the National Geosciences Records Centre, BGS, Keyworth.

Extant exposures/sections[edit]

Cheshire Basin: exposures in the River Dane, north-east of Congleton (SJ 900 656 to 892 657) (Wilson, 1993) and The Dungeon, Wirral, Merseyside (SJ 252 832) (Benton et al., 2002)

Stafford Basin: former railway cutting north of Maer (SJ 7909 3803), and a stream section (SJ 921 353) east of The Hayes, near Oulton (Rees and Wilson, 1998).

Needwood Basin, sections in streams west of Upper Tean (SK 0069 3946 to SK 0050 3931) and Broadgatehall Drumble, near Hollington (SK 0392 3848 to SK 0446 3755) (Charsley, 1982).

East Midlands Shelf South: sections in a railway cutting at Sneinton (see above) and in a road cutting at Redhill (SK 5837 4713 to 5835 4689) (Rathbone, 1989).

Lithology[edit]

Heterolithic, comprising interlaminated and interbedded siltstone, mudstone and sandstone in variable but approximately equal proportions. The siltstones are micaceous and laminated, or interlaminated with mudstones or sandstones; the mudstones usually appear structureless, with a blocky habit. The sandstones are mostly very fine- to fine-grained, well-sorted, and micaceous. They are typically cemented by ferroan calcite or dolomite. Sandstone units are commonly less than 0.5m thick though composite units, consisting of several individual sandstone beds, may reach over 5m thick. Intraformational mudclast conglomerates are common, with mudclasts concentrated at the bases of sandstone beds. Conglomerates with extraformational pebbles are rare, and tend to be less than 1m thick; the ‘Waterstones Conglomerate’ of Nottinghamshire is a notable example (Elliott, 1961). Most mudstone and siltstone beds are reddish brown, though green-grey mottles and laminae are common. Gypsum occurs sporadically in the mudstone as small nodules. The sandstones are grey-brown in colour and conspicuously paler than the siltstones and mudstones. Bedding is generally planar. Most beds are tabular and laterally extensive, though lenticular channel sandstone bodies occur and typically have incised, erosional bases. The upper surfaces of sandstones and coarse siltstones are commonly wave or current rippled; in section the sandstones are usually planar or ripple-laminated, and exhibit sporadic rootlets. Fossils include miospores (e.g. Fisher, 1972; Charsley, 1982; Earp and Taylor, 1986; Rees and Wilson, 1998; Warrington et al., 1999), invertebrate trace fossils (Ireland et al., 1978; Pollard, 1981), vertebrate tracks (Sarjeant, 1974) and the brachiopod Lingula (Rose and Kent, 1955).

Lower boundary[edit]

The boundary is diachronous, and in most regions there is interdigitation with the underlying Sherwood Sandstone Group. Within individual sections, the boundary is drawn arbitrarily within an interbedded, generally upward-fining gradation at the level where the micaceous mudstones, siltstones and fine sandstones of the Tarporley Siltstone predominate over the coarser grained, cross-stratified sandstone of the underlying Sherwood Sandstone Group (Warrington et al., 1980). Locally, for example above the Kibblestone Formation of the Stafford basin (Rees and Wilson, 1998), the lowest part of the formation is dominated by sandstones. In these circumstances, the boundary is placed at the base of micaceous, flaggy and fine-grained sandstones, typical of the Tarporley Siltstone Formation, but differing markedly from the coarser, cross-stratified sandstones in the underlying Sherwood Sandstone Group. In the Cheshire Basin the junction between the Tarporley Siltstone Formation and the underlying Helsby Sandstone Formation is complex, being both transitional and diachronous, and locally there is a lateral passage between the two formations. For example, in the south-east of the basin, in the Stoke-on-Trent district (Rees and Wilson, 1998), siltstones, mudstones and thin sandstones (like those of the Tarporley Siltstone) alternate with thicker sandstones of Helsby Sandstone type; here the finer grained lithologies are included within the latter formation. This succession passes laterally into one where mudstones are dominant, assigned to the Tarporley Siltstone Formation. In the Nottingham area, the base of the unit is unconformable and is marked by a patchily distributed basal conglomerate up to 1m thick with a strong calcareous cement (Howard, 2003; Howard et al., in press).

Where not exposed, the boundary is typically marked by a change in soil type from the sandy soils of the Sherwood Sandstone Group to the heavier, brown sandy clay soils (commonly containing fragments of fine-grained micaceous sandstone) associated with the Tarporley Siltstone Formation. A concave break of slope may mark the boundary in some areas.

Upper boundary[edit]

The boundary is diachronous, and interdigitates with the overlying Sidmouth Mudstone Formation. Within individual sections, the boundary is drawn arbitrarily within an upward-fining, interbedded succession at the level where mudstones of the overlying Sidmouth Mudstone Formation predominate over the siltstones and sandstones of the Tarporley Siltstone Formation.

Where not exposed, the boundary is typically marked by a change in soil type from the brown, sandy clay soils of the Tarporley Siltstone Formation to the more clayey, markedly reddish brown soils of the Sidmouth Mudstone Formation. A concave break of slope is associated with the boundary in some areas.

Thickness[edit]

Varies from 20-60 m in the East Midlands, and 50-75 m in Staffordshire, up to a maximum of 220 m in the Cheshire basin. The 273 m thick succession logged in the Ashley Borehole, Cheshire (Taylor et al., 1963) probably includes a substantial thickness of the overlying Sidmouth Mudstone Formation.

Age[edit]

On palynological evidence, the formation is Mid Triassic (Anisian) in age. Both the basal and upper boundaries are diachronous, younging generally towards the south (Warrington, 1970; Warrington et al., 1980; with revisions in Benton et al., 1994; Rees and Wilson, 1998; Warrington et al., 1999).

Equivalent units[edit]

None. Coeval strata in contiguous offshore areas are developed in a finer-grained lithofacies resembling the overlying Sidmouth Mudstone Formation.

Geographical extent[edit]

The outcrop extends northwards from Warwickshire through Staffordshire, Cheshire, Leicestershire, and Nottinghamshire; the formation is also proved by numerous boreholes in the adjacent subcrop. To the south of a line between Wolverhampton and Coventry, the formation passes laterally by interdigitation into the uppermost part of the Sherwood Sandstone Group. Where, south of that line, correlatives of the formation have been recognised, for example the Sugarbrook Member of the Redditch area (Old et al., 1991), they are typically included in the upper part of the Sherwood Sandstone Group and have not been mapped separately. An exception is the Holling Member of the Worcester area, included in the Eldersfield (now Sidmouth) Mudstone Formation by Barclay et al. (1997) and here re-assigned to the Tarporley Siltstone Formation. To the north of the outcrop, the formation passes laterally into mudstone-dominated facies. It appears to be absent in north and west Lancashire and the central part of the East Irish Sea Basin (Jackson et al. 1987; 1995), where the Sherwood Sandstone Group is directly overlain by a silt and sand-poor sequence typical of the bulk of the Mercia Mudstone Group farther south. However, in the Solway and Carlisle Basins, a ‘Waterstones Lithology’ has been tentatively identified (Jackson et al. 1995). On the East Midlands Shelf, although the formation is identified in boreholes in the Grantham and Grimsby districts (Berridge et al. 1999; Berridge and Pattison, 1994), it cannot be confidently identified farther north in the Kingston upon Hull district (Gaunt et al. 1992). The lateral transition into mudstone-dominated facies in Eastern England has been described by Smith and Warrington (1971). Coeval strata in the Southern North Sea Basin are in mudstone and halite-dominated facies (Cameron et al., 1992; Johnson et al., 1994).

Subdivisions[edit]

Malpas Sandstone Member (Cheshire Basin) (Stephens, 1961; Poole and Whiteman, 1966; Wilson, 1993).

Woodthorpe Member (East Midlands Shelf South) (formerly Woodthorpe Formation, Elliott, 1961)

Retford Member (East Midlands Shelf South) (formerly Retford Formation, Warrington et al., 1980)

Colwick Member (East Midlands Shelf South) (formerly Colwick Formation, Warrington et al., 1980)

References[edit]