Sherwood Sandstone Group of England, Wales and south-west Scotland - Lithostratigraphy

From Earthwise
Jump to navigation Jump to search

Sherwood Sandstone Group of England, Wales and south-west Scotland - Lithostratigraphy[edit]

From: Ambrose, K, Hough, E, Smith, N J P, and Warrington, G. 2014. Lithostratigraphy of the Sherwood Sandstone Group of England, Wales and south-west Scotland. British Geological Survey Research Report, RR/14/01.

Introduction[edit]

In England, the Sherwood Sandstone Group (SSG; Warrington et al., 1980) is present continuously at the surface and at subcrop from the south Devon coast northwards to Teesside on the north-east coast, Lancashire and Cumbria on the north-west coast, and in the Vale of Eden (Figures 1 and 2). Outcrops occur mainly in east Devon, Somerset, the central and south Midlands (Gloucestershire, Worcestershire, Warwickshire, Leicestershire, Staffordshire, south Derbyshire and north Shropshire), Cheshire, Lancashire, Cumbria, Nottinghamshire, Yorkshire and Durham. The SSG also occurs in south-west Scotland, in Dumfries and Galloway, and on the Isle of Arran. Rocks currently assigned to the Group also crop out in Clwyd, North Wales (Figures 1 and 2). Deposition occurred principally in a number of actively subsiding¸ fault-bounded continental basins (Figure 1); thinner sequences formed in the intervening areas. The current nomenclature for the constituent formations of the SSG reflects distinct depositional areas, with many local names currently in use on BGS maps and in literature. Lithological similarity and borehole geophysical logs have been used to demonstrate continuity of formations between basins and this, combined with the need for a more unified stratigraphical approach for the BGS DiGMapGB and 3D modelling studies, has necessitated this review of the SSG nomenclature. The aim of this discussion document is to determine the scope for rationalising the formation names used in BGS publications and thereby achieve a more unified lithostratigraphical scheme for the Group.

The SSG of England, Wales and south-west Scotland is composed mainly of brown, red, buff, greenish grey and yellow sandstones, commonly with colour mottling. Extra- and intraformational clasts are common in the lower part and are scattered throughout some of the higher formations. Extraformational clasts become less common northwards into Cheshire, south Lancashire, Nottinghamshire and South Yorkshire. Much of the succession is of fluvial origin, deposited from fast-flowing braided rivers that were superseded by meandering river systems in which fining-upward cycles, with subordinate beds of mudstone, were deposited (Warrington, 1970a; Old et al., 1991; Warrington and Ivimey-Cook, 1992; Powell et al., 2000). Aeolian units occur in east Devon, Cheshire, Lancashire, Cumbria, south-west Scotland, the Isle of Arran and offshore, below the East Irish Sea. They may be present but unrecognised in other areas. Palaeontological evidence of temporary marine influence occurs in the upper part of the SSG (Finstall and Sugarbrook members of the Bromsgrove Sandstone Formation) in Oxfordshire, north Worcestershire and south Warwickshire (Warrington, 1967, 1970a, p.21 in Williams and Whittaker, 1974; Ivimey-Cook, p.13 in Poole, 1978; Old et al., 1991; Benton et al., 1994).

Breccias of Permian and Triassic age occur at the base of the arenaceous sequence but of these, only the Huntley Formation (Ashbourne district, sheet 124; Chisholm et al., 1988) is currently included within the SSG. The Moira Formation in Leicestershire and Derbyshire is demonstrably diachronous, interdigitating with at least two SSG formations in the Coalville (Sheet 155; Worssam and Old, 1988) and Loughborough (Sheet 141; Carney et al., 2001) districts; in the Derby district (Sheet 125; Frost and Smart, 1979) it underlies the Tarporley Siltstone Formation of the Mercia Mudstone Group (MMG). The age range of these breccias is uncertain and they may span the Permian–Triassic boundary. They always occur in isolation from underlying rocks of known Permian age (Appleby, Cumbrian Coast and Zechstein groups), so their relationship to those is not known.

The lower parts of some continuous arenaceous sequences currently included in the SSG are possibly partly of Permian age. These include the Lenton Sandstone in the East Midlands and the Kinnerton Sandstone in the Cheshire Basin (Warrington et al., 1980).

The base of the SSG is currently defined in the BGS Lexicon of Named Rock Units as ‘Gradational on the Roxby Formation (formerly the Permian Upper Marls) in the type area of Sherwood Forest, Nottinghamshire, and similar relationship elsewhere, but sharp and unconformable on Carboniferous or older rocks locally’. However, there is a disconformable relationship with the Lenton Sandstone Formation in the eponymous area of Sherwood Forest, Nottinghamshire, and in the West Midlands the SSG overlies the aeolian Permian Bridgnorth Sandstone Formation; in east Devon, Lancashire and Cumbria it overlies other beds of Permian age. A revised definition is needed to take these variations into account.

Figure 1 Outcrop of the SSG (onshore only). Figure 2 Map showing the 11 regions of the SSG review.

The top of the SSG, and the base of the MMG, is placed at the ‘upward transition from sandstone to mudstone- or siltstone-dominated lithologies and the incoming of pseudomorphs after halite; the boundary is usually gradational, conformable and diachronous, younging towards the south’ (Howard et al., 2008; see also Warrington, 1970b; Warrington and Ivimey-Cook, 1992; Herries and Cowan, 1997; Thompson and Meadows, 1997; Meadows, 2006, fig.8). In the East Midlands, the boundary is sharp and unconformable (Warrington, 1974, fig. 40). In Devon, the boundary is conformable (Gallois, 2004).

The total thickness of the SSG is highly variable. The Knutsford Borehole (Warrington et al. 1999), in Cheshire, proved an incomplete sequence of 1330 m. The thickest known complete sequence is 945.2 m in the Kempsey Borehole (Barclay et al., 1997) Worcestershire, from where substantial thinning occurs towards the south-west, and to the east and south-east where the SSG is overlapped by the MMG around the London–Brabant Massif (Figure 2; Holloway, 1985).

The SSG ranges in age from latest Permian (?) and Early Triassic (Induan to Olenekian) to early Mid Triassic (Anisian). It overlies rocks that range in age from Precambrian to Permian and possibly earliest Triassic.

Tectonic and depositional setting[edit]

During Permian and Triassic times, Britain lay in the interior of the Pangaea supercontinent, to the north of the Variscan mountain chain. Both Pangaea and the Variscan mountains were the product of continental collision in the late Carboniferous but, by the early Permian, Pangaea was showing signs of breaking apart. In what is now southern, central and north-west England, crustal tension led to the formation of a series of fault-bounded basins (e.g. McLean, 1978; Chadwick and Evans, 1995; Chadwick, 1997). In contrast, eastern England lay at the western margin of a much larger depocentre (the ‘Southern Permian Basin’ area; Bachmann et al., 2010) that extended eastwards to Poland. These basins influenced deposition throughout the Permian and Triassic periods. In the Early Triassic, rain falling on the Variscan mountains in northern France fed a major river system that flowed northwards across southern Britain and deposited the thick sequences of sands and gravels that form the SSG (e.g. Wills, 1956, 1970; Audley-Charles, 1970; Warrington, 1970a; Warrington and Ivimey-Cook, 1992; Tyrell et al., 2012). Deposition of the lower formations of the SSG was dominantly fluviatile and largely restricted to the basins, although sediments were transported across the intervening highs; a subordinate part of the succession is likely to be aeolian. However, the upper formations of the SSG, which include both aeolian and fluvial sandstones, overlapped progressively onto the adjacent highs. Temporary marine influence is evident locally in the upper SSG in the Midlands (see 1.2), and marine environments, represented initially by intertidal deposits in the basal (Tarporley Siltstone) formation of the MMG (e.g. Ireland et al., 1978), gradually encroached on areas of previously mainly fluvial deposition. There is palaeontological and geochemical evidence of marine influence during deposition of the MMG until the end of Carnian times (Warrington, 1970b, 1981; Warrington and Ivimey-Cook, 1992).

Stratigraphical overview[edit]

The lithostratigraphical classification of Triassic rocks in England and Wales currently adopted by BGS is based on a review carried out by the Triassic Working Group of the Geological Society of London between 1967 and 1979 (Warrington et al., 1980). The terms ‘Bunter’ and ‘Keuper’, based on an inferred time correlation with the German Triassic sequence, were abandoned in favour of a rigorous lithostratigraphical approach using the gross lithological characteristics of the various rock units. Parts of the former ‘Lower Mottled Sandstone’, and all of the former ‘Bunter Sandstone’, ‘Upper Mottled Sandstone’ and ‘Lower Keuper Sandstone’ were combined into the SSG, with the MMG corresponding closely, but not exactly, with the former ‘Keuper Marl’.

In some regions, strata of known or probable Permian age have, in the absence of preserved stratigraphical marker beds, been included within the SSG (e.g. the Kinnerton Sandstone of the Cheshire Basin). In other areas, strata of Permian age are included in the older Exeter and Aylesbeare (Devon), Appleby and Cumbrian Coast (north-west England), and Rotliegendes and Zechstein (East Midlands and north-east England) groups (see 3.2). The current situation is confusing, with sandstones of similar lithology and age being included in different groups. A resolution of this inconsistency is proposed, whereby the SSG does not include units of known or inferred Permian age, and with an upwards change from aeolian to dominantly fluvial sedimentation being a main criterion for defining the base of the SSG.

Chronostratigraphical subdivision and correlation of Triassic rocks in England, Wales and Scotland is hampered by a scarcity of age-diagnostic fossils. The few fossils known from the lower formations of the SSG comprise mostly vertebrate footprints and other trace fossils; a few crustaceans (Euestheria) and a fragment of a fish have also been recorded (Wills, 1970; Warrington and Thompson, 1971; Warrington, 1976; Old et al., 1991; Bridge and Hough, 2002). However, in Devon, Gloucestershire, Oxfordshire, Warwickshire, Worcestershire, Staffordshire and Shropshire, the youngest formation has yielded more diverse fossil assemblages. In east Devon, the Otter Sandstone Formation has yielded remains of fish, amphibians and reptiles, in addition to some invertebrates (Benton et al., 1994, 2002). Miospores (plant spores and pollen) and plant debris occur in the Bromsgrove Sandstone Formation in Gloucestershire (Green and Melville, 1956; Warrington, 1970a) and, with Euestheria, a brachiopod (Lingula), a bivalve, and ostracods, in that formation in Oxfordshire (Worssam, 1963; Poole, 1969; Ivimey-Cook, p.13 in Poole, 1978; Warrington, p.22 in Poole, 1978). In north Worcestershire, the same formation has yielded trace fossils, marine microplankton (acritarchs), miospores, plant macrofossils, annelids, molluscs, Euestheria, arachnids and remains of fish, amphibians and reptiles; miospores, acritarchs and amphibian and reptilian remains also occur in this formation in south Warwickshire (Warrington, 1967, 1970a, p.21 in Williams and Whittaker, 1974, 1976; Old et al., 1987, 1991; Warrington and Ivimey-Cook, 1992; Benton et al., 1994, 2002; Barclay et al., 1997; Powell et al., 2000). The Euestheria indicate fresh- to brackish-water conditions; other fossils (e.g. acritarchs and Lingula) are indicative of marine environments. In Shropshire (Benton et al., 1994) and Staffordshire (Chisholm et al., 1988) vertebrate remains have been recovered from the Grinshill Sandstone and Hollington formations respectively. In Cheshire the Helsby Sandstone Formation has yielded trace fossils, particularly vertebrate tracks known notably from the Wirral and Runcorn areas (Tresise and Sarjeant, 1997); miospores, macrofloral debris, and Euestheria also occur (Warrington et al., 1999).

A review of the borehole geophysical log signatures of the component units of the SSG in different parts of the UK forms part of this report (Figures 3, 4 and 5), with one log illustrating the signatures for each of regions 1 to 7, two for each of regions 8 and 9, but none for regions 10 and 11 (Figure 2), where such data is unavailable.

Many local names have been applied to the constituent formations of the SSG. These names, and their usage on BGS maps, reflect the following factors:

  • original depositional restriction of units to individual basins;
  • postdepositional, spatial isolation of formations by faulting or erosion;
  • ‘campaign boundaries’ within the BGS mapping programme, resulting in different nomenclatures being introduced for different projects, often many years apart.

With a few notable exceptions, formation names within the SSG are unique to individual basins and in some cases to areas within basins (see Table 1).

Current lithostratigraphical framework[edit]

Spatial distribution of the SSG[edit]

The outcrop (Figure 1) of the SSG extends from the south Devon coast, northwards into Somerset. There are scattered isolated outcrops along the east side of the Malvern Hills, in Gloucestershire and Worcestershire. A continuous outcrop commences in north Worcestershire, near Droitwich, extends northwards into Staffordshire and north Shropshire, and encircles the South Staffordshire Coalfield and the Stafford Basin. A narrow outcrop on the south-eastern side of the Cheshire Basin expands around the southern and western sides of that basin and branches onto the Wirral and eastwards from Merseyside to Greater Manchester. From Merseyside it extends northwards, through west Lancashire, to the south side of Morecambe Bay. The outcrop is discontinuous on the north side of Morecambe Bay; it skirts the Cumbrian coast to Whitehaven, then reappears at Maryport from where it extends eastwards, on the south side of the Solway Firth and divides near Carlisle, southwards into the Vale of Eden and northwards to the north side of the Solway Firth in Scotland. The SSG also crops out on the Isle of Arran. Eastwards from Worcestershire the outcrop extends through Warwickshire, on the eastern side of the Warwickshire Coalfield, into Leicestershire. Farther north it runs nearly continuously eastwards from Staffordshire, through south Derbyshire and into Nottinghamshire. It then forms a broad outcrop running northwards through Nottinghamshire, South, West and North Yorkshire and into Durham, reaching the coast on Teesside. Isolated outcrops occur in North Wales, in the Vale of Clwyd and around Rhyl. Two outliers occur in Staffordshire, in the Leek–Congleton area (Evans et al., 1968; Aitkenhead et al., 1985; Chisholm et al., 1988). Strata equivalent to the SSG also occur on the Isle of Man (Chadwick et al., 2001) and in Northern Ireland (Mitchell, 2004) but these are beyond the scope of this report.

Eastwards from outcrops in central and eastern England, west- and southwards from those in Lancashire and on Arran respectively, and in the centres of the Cheshire and Carlisle basins, the SSG is present in the subsurface beneath the MMG and younger Mesozoic deposits. It thins towards and pinches out at subsurface around the London–Brabant Massif (Figure 2), an ancient cratonic area composed of Lower Palaeozoic rocks (see figures in Hull, 1860a, Holloway, 1985, Warrington and Ivimey-Cook, 1992 and Horton et al., 1995). The SSG successions onshore in England, Wales and south-west Scotland, including Arran, are in continuity with those in the adjacent offshore areas (Evans, 1990; Cameron et al., 1992; Hamblin et al., 1992; Fyfe et al., 1993; Johnson et al., 1994; Tappin et al., 1994; Jackson et al., 1995; Jackson and Johnson, 1996; Chadwick et al., 2001; Bachmann et al., 2010). Occurrences of the SSG are here divided into 11 spatially distinct regions (Figure 2), some of which represent isolated basins, whereas others represent outcrop areas that are isolated at surface but not in the subsurface. There is subsurface continuity between Regions 1 to 8. Regions 8 to 10, although not in continuity onshore, are in continuity offshore in Liverpool Bay and the East Irish Sea; Region 11 is isolated.

Current lithostratigraphical nomenclature[edit]

The current names adopted in BGS publications for the constituent formations of the SSG and the associated underlying groups of Permian age are shown in Table 1. Further explanation and information sources are given below. Lexicon entries for most of these units can be viewed on the BGS Website:

http://www.bgs.ac.uk/bgs/w3/free/lexicon/lexicon.html http://www.bgs.ac.uk/bgs/w3/free/lexicon/lexicon.html

Region 1 – south-west England, Somerset and Wessex basins[edit]

Lithostratigraphy based on Warrington et al. (1980), Whittaker and Green (1983), Selwood et al. (1984), Edmonds and Williams (1985), Edwards and Freshney (1987), Bristow et al. (1991, 1995), Edwards (1999), Edwards and Scrivener (1999), Edwards and Gallois (2004), Gallois (2004), Chadwick and Evans (2005) and Barton et al. (2011). The succession comprises the Budleigh Salterton Pebble Beds Formation (up to 48 m thick) and the overlying Otter Sandstone Formation, a mixed aeolian–fluvial sequence up to 210 m thick. At outcrop the contact between these formations appears disconformable but at subcrop an angular unconformity occurs and an intervening (unnamed) argillaceous unit is present locally (Butler, 1998; Warrington, 2005; Barton et al., 2011). Table 1 The current stratigraphical nomenclature for the SSG review.

Borehole geophysical log characteristics of the SSG in the region were illustrated by Rhys et al. (1982), Penn (1987), Holloway et al. (1989), Bristow et al. (1991) and Butler (1998). The succession is in subsurface continuity with those in Region 2 and offshore, below the Bristol Channel (Tappin et al., 1994) and the English Channel (Evans, 1990; Hamblin et al., 1992).

Region 2 – Worcester and Knowle basins[edit]

Lithostratigraphy based on Green and Melville (1956), Warrington et al. (1980), Horton et al. (1987, 1995), Old et al. (1987, 1991), Worssam et al. (1989), Barclay et al. (1997), Powell et al. (2000), Sumbler et al. (2000), Barron et al. (2002) and Sumbler (2002). The succession is based on mapping, particularly in the northern part of this region, on the Kempsey Borehole (Whittaker, 1980) which proved the most complete SSG succession in the Worcester Basin, on boreholes in the Bromsgrove area which proved the Bromsgrove Sandstone Formation (Old et al., 1991), and on seismic data (Chadwick, 1985; Chadwick and Smith, 1988; Chadwick and Evans, 2005). In southern and eastern parts of this region (Gloucestershire and Oxfordshire) the succession has an extensive subcrop beneath younger Mesozoic deposits.

The succession comprises the Hopwas Breccia, of uncertain age, and the Triassic Kidderminster, Wildmoor Sandstone and Bromsgrove Sandstone formations. In the Bromsgrove area a conglomerate unit at the base of, and interdigitating with, the Kidderminster Formation was mapped separately and named the ‘Quartzite Breccia’ (Old et al., 1991). However, in the Birmingham area this term was not used, ‘Hopwas Breccia’ being preferred (Powell et al., 2000). In the Bromsgove area, the Bromsgrove Sandstone Formation has been subdivided into the Burcot, Finstall and Sugarbrook members. Borehole geophysical log characteristics of the SSG in the region were illustrated by Green and Melville (1956), Bullerwell (1963), Bennett (1969), Poole (1977, 1978), Horton et al. (1987), Penn (1987), Barclay et al. (1997), Powell et al. (2000) and Sumbler et al. (2000). The succession is in subsurface continuity with those in regions 1 and 3 and in spatial continuity with those in regions 5 and 6, although not all constituent formations are physically continuous between those areas, the Wildmoor Sandstone being identified only in regions 2 and 6.

Region 3 – Hinckley Basin, with parts of south Derbyshire, north-west Leicestershire and Warwickshire[edit]

Lithostratigraphy based on Warrington et al. (1980), Old et al. (1987), Worssam and Old (1988), Bridge et al. (1998), and Carney et al. (2001, 2002, 2009). The formations are largely defined from boreholes and surface mapping in the Warwick, Coventry, Coalville, Loughborough and Leicester areas. The succession includes the Moira, Polesworth and Bromsgrove Sandstone formations. Borehole geophysical log characteristics of the SSG in the region were illustrated by Worssam and Old (1988), Bridge et al. (1998) and Carney et al. (2001). The succession is in subsurface continuity with those in regions 2, 4 and 5, although not all constituent formations are in physical continuity between those areas; the Wildmoor Sandstone is only present in Region 2 and the Bromsgrove Sandstone is largely absent from Region 4.

Region 4 – East Midlands Shelf[edit]

Lithostratigraphy based on Elliott (1961), Smith and Warrington (1971), Smith et al. (1973), Warrington et al. (1980), Berridge and Pattison (1994), Gaunt (1994), Berridge et al. (1999), Carney et al. (2004), Hough et al. (2007) and Howard et al. (2009). A clear division into the Lenton Sandstone and Nottingham Castle Sandstone formations can be made in the southern part of this region and as far north as Doncaster. However, northwards from there, to the coast around Hartlepool, pebbles are scarce and the SSG is undivided (Gaunt, 1994; Aitkenhead et al., 2002). The cover of superficial deposits becomes very extensive north of Retford, but considerable information is available from boreholes; in this area the SSG is up to about 400 m thick. Borehole geophysical log characteristics of the SSG in the region were illustrated by Berridge and Pattison (1994), Chadwick et al. (1995), Berridge et al. (1999), Carney et al. (2004) and Howard et al. (2009), The succession is in spatial continuity with that in Region 3; it is in subsurface continuity with the Bacton Group succession in the Southern North Sea Basin (Cameron et al., 1992; Johnson et al., 1994).

Region 5 – Needwood Basin[edit]

Lithostratigraphy based on Stevenson and Mitchell (1955), Warrington et al. (1980), Charsley (1982) and Chisholm et al. (1988). The succession includes the Huntley, Hawksmoor and Hollington formations. The succession is in subsurface continuity with that in Region 3 and in spatial continuity with those in regions 2 and 6. The Wildmoor Sandstone Formation is absent in regions 3 and 5.

Region 6 – Stafford Basin[edit]

Lithostratigraphy based on Warrington et al. (1980), Rees and Wilson (1998) and Bridge and Hough (2002). The SSG comprises three main formations, together with the Permian Bridgnorth and Kinnerton Sandstone formations. The Hopwas Breccia is also present in this region. The Kidderminster Formation is up to 200 m thick. It is succeeded conformably by the Wildmoor Sandstone Formation which is up to 284 m thick; the junction is gradational and poorly defined. The overlying Bromsgrove Sandstone Formation is up to 500 m thick and rests on a major disconformity; near Stone, an aeolian facies equivalent to this formation has been mapped as the Kibblestone Formation (Rees and Wilson, 1998). Warrington et al. (1980) introduced the ‘Cannock Chase Formation’ in the eponymous area, with a lower conglomeratic unit and an upper ‘pebble free’ unit that were considered equivalents of the Kidderminster and Polesworth formations respectively. Borehole geophysical log characteristics of the SSG in the region were illustrated by Bridge and Hough (2002). The succession is in spatial continuity with those in regions 2, 5 and 7; the Wildmoor Sandstone Formation is absent in regions 5 and 7.

Region 7 – Cheshire Basin[edit]

Lithostratigraphy based on Thompson (1970a), Warrington et al. (1980, 1999), Earp and Taylor (1986), Evans et al. (1993), Rees and Wilson (1998), Aitkenhead et al. (2002) and Howard et al. (2007). The Cheshire Basin is fault-bounded to the south-east and east. To the north and west it is largely bounded by older rocks on which the basin-fill deposits rest unconformably. The Llŷn–Rossendale ridge, which separates the basin from the East Irish Sea and west Lancashire areas, defines its north-western limit.

The Kinnerton Sandstone Formation, generally regarded as dominantly Permian in age, was included within the SSG by Warrington et al. (1980). This is underlain in parts of the region by the Manchester Marls Formation. Where that formation is not present, the Kinnerton Sandstone includes beds equivalent to the Manchester Marls and underlying Collyhurst Sandstone formations. This part of the group is characterised by fine-grained sandstones deposited as aeolian dunes in a desert environment. The remainder of the SSG comprises three formations, based predominantly on sandstone sedimentology and lithology.

The Chester Pebble Beds Formation comprises red-brown sandstone with sporadic mudstone interbeds; pebbles become rare northwards within the basin (Thompson, 1970a). It is succeeded by the Wilmslow Sandstone Formation, composed of red and orange, fine-grained, poorly cemented, friable and predominantly aeolian sandstones. The Thurstaston Sandstone Member was included in the Wilmslow Sandstone Formation by Howard et al. (2007), rather than in the Helsby Sandstone as originally defined by Thompson (1970a). It comprises a dominantly aeolian sequence with a thin bed (the Thurstaston Hard Sandstone Bed; Thompson, 1970b) of fluvial sandstone at the base of the member, and is similar to the Wilmslow Sandstone. However, borehole geophysical log correlation (Figure 3, Formby Borehole) suggests that this member is better placed in the Helsby Sandstone Formation.

Figure 3 Formby Borehole geophysical log.

The Helsby Sandstone Formation overlies the Wilmslow Sandstone Formation and typically comprises up to 250 m of brown and red-brown, rarely cream coloured, sandstones and pebbly sandstones. Many local names have been used for units or members in this formation, including the Grinshill and Ruyton sandstones of the Wem district (Pocock and Wray, 1925; Thompson, 1993) in the south of the basin, the Bulkeley Hill Formation (Warrington et al., 1980), formerly the ‘Keuper Sandstone Passage Beds’ of Poole and Whiteman (1966), in the west of the basin, and the ‘Engine Vein Conglomerates’, ‘Beacon Lodge Soft Sandstones’, ‘Wood Mine Conglomerates’, ‘West Mine Soft Sandstones’ and ‘Nether Alderley Red Sandstone’ members at Alderley Edge (Thompson, 1970b), in the north-eastern part of the basin. In the north-western part of the basin the formation comprises, in ascending order, the ‘Thurstaston Soft Sandstone’ (with the ‘Thurstaston Hard Sandstone Bed’), the ‘Delamere Pebbly Sandstone’ and the ‘Frodsham Soft Sandstone’ members. The ‘Delamere Pebbly Sandstone Member’ is fluvial in origin and the ‘Frodsham Soft Sandstone Member’ is largely aeolian (Thompson, 1969, 1970a, 1970b; Mountney and Thompson, 2002).

Borehole geophysical log characteristics of the SSG in the region were illustrated by Colter (1978), Penn (1987), Rees and Wilson (1998), Warrington et al. (1999) and Bloomfield et al. (2006). The succession is in spatial continuity with those in regions 6 and 8, and in subsurface continuity with that offshore, below the East Irish Sea.

Region 8 – West Lancashire[edit]

Lithostratigraphy after Warrington et al. (1980), Wilson and Evans (1990) and Jackson et al. (1995). This region is the onshore part of the East Irish Sea Basin (EISB). Outcrops in the Formby and Fylde areas are covered by thick superficial deposits and the formations are defined largely from borehole core. In the southern part of this region recent mapping (E Hough, pers. com) has allowed separation of the Wilmslow Sandstone and overlying Ormskirk Sandstone formations; this division is recognisable as far north as Burscough (SD 43 10). The Chester Pebble Beds Formation is present at depth but does not crop out. Borehole geophysical log characteristics are illustrated in de Pater and Baisch (2011) (available on the internet at:

(http://www.cuadrillaresources.com/wp-content/uploads/2012/02/Final_Report_Bowland_Seismicity_02-11-11.pdf http://www.cuadrillaresources.com/wp-content/uploads/2012/02/Final_Report_Bowland_Seismicity_02-11-11.pdf)

Figure 3 illustrates the three-fold subdivision of the Helsby Sandstone Formation in the Formby No. 5 borehole from this region, based on that of Thompson (1970a), seen in Region 7 (p.10). The uppermost unit of the SSG comprises a lower fluvial and an upper aeolian unit. Farther north the SSG is poorly exposed, lacks lithological variation, and is undivided.

The succession is in spatial continuity with that in Region 7 and in subsurface continuity with those below the East Irish Sea and in Region 9.

Region 9 – Cumbria, Carlisle Basin and Vale of Eden[edit]

Lithostratigraphy after Dixon et al. (1926), Eastwood (1930), Day (1970), Warrington et al. (1980), Barnes et al. (1994), Jackson et al. (1995), Akhurst et al. (1997), Johnson et al. (2001) and Holliday et al. (2004). The SSG crops out on the west coast of the Lake District and eastwards, to Carlisle, then southwards into the Vale of Eden and northwards into Scotland. In the Barrow-in-Furness area only the St Bees Sandstone is recognised (Rose and Dunham, 1977) whereas drilling to the north, around Sellafield, allowed the recognition of three units; in ascending order, the St Bees Sandstone, Calder Sandstone and Ormskirk Sandstone formations (Barnes et al., 1994; Akhurst et al., 1997). These formations can be recognised from Millom to Whitehaven, with thicknesses of 250 to 600 m, 650 m and 150 to 250 m respectively (Akhurst et al., 1997). Borehole geophysical log characteristics of the SSG in the region were illustrated by Barnes et al. (1994), Jones and Ambrose (1994), Chadwick et al. (1995), Akhurst et al. (1997) and Holliday et al. (2001, 2004).

Offshore, below the East Irish Sea, only two formations, the St Bees Sandstone and the Ormskirk Sandstone, are recognised (Jackson and Johnson, 1996; Chadwick et al., 2001), with the Calder Sandstone reduced to member status and forming the upper part of the St Bees Sandstone, the lower part of which is termed the Rottington Sandstone Member.

The lithostratigraphy of the SSG in the Carlisle Basin and Vale of Eden, from Maryport to Carlisle and into south-west Scotland, is uncertain due to a thick cover of superficial deposits, poor exposure, and a scarcity of borehole information (Holliday et al., 2006, 2008). Here two formations have previously been recognised, namely the St Bees Sandstone and Kirklinton Sandstone formations (Dixon et al., 1926; Eastwood, 1930; Day, 1970; Holliday et al., 2004). The SSG is 502 m thick in the Silloth 1A Borehole whereas to the north, in the main part of the Carlisle Basin, and in the Vale of Eden the St Bees Sandstone Formation ranges from 150 to 430 m and the Kirklinton Sandstone Formation is thought to be about 100 m thick (Dixon et al., 1926; Day, 1970). Holliday et al. (2006, 2008) proposed adopting the offshore EISB nomenclature for the Carlisle Basin and Vale of Eden successions, and discarding the term ‘Kirklinton Sandstone’.

In the main part of the Vale of Eden, Goodchild (1893) recognised two divisions of the SSG, equivalent to the St Bees Sandstone and the Kirklinton Sandstone formations. Arthurton and Wadge (1981) considered it impractical to map both formations and only the St Bees Sandstone Formation is recognised on recent BGS maps in this area. However, from published descriptions (Goodchild, 1893; Arthurton and Wadge, 1981) it would seem that the Ormskirk Sandstone Formation is present in the Vale of Eden. In the Vale of Eden the St Bees Sandstone Formation varies in thickness from 125 to 600 m (Burgess and Holliday, 1979; Arthurton and Wadge, 1981).

The SSG is in spatial continuity with that in Region 8 and in subsurface continuity with that below the East Irish Sea.

Region 10 – Vale of Clwyd, North Wales[edit]

Lithostratigraphy after Ivimey-Cook (1974), Warrington et al. (1980), Warren et al. (1984) and Davies et al. (2004). The Vale of Clwyd is an asymmetric half graben occupied by a sandstone succession up to 800 m thick that comprises aeolian and fluvial red beds and is in spatial continuity with that offshore below the East Irish Sea.

The outcrop, which is largely obscured by superficial deposits, is mapped as Kinnerton Sandstone which was included in the SSG by Warrington et al. (1980) and was therefore considered in this study. However, the Kinnerton Sandstone is here assigned to Framework Unit A (see 3.1, 3.2) which is not included in the SSG and the region is not, therefore, considered further.

Region 11 – Isle of Arran, Firth of Clyde, south-west Scotland[edit]

Warrington et al. (1980) defined two formations as part of the SSG, the Lamlash Sandstone and the Glen Dubh Sandstone formations. The former comprises sandstones with interbedded conglomerates and mudstones and the latter is a calcareous aeolian sandstone. The Lamlash Sandstone Formation shows similarities to the Kidderminster Formation of England but the pebbles are thought to be largely locally sourced (MacGregor et al., 1983). Because this outcrop is detached and remote from those in England and Wales it is recommended that the local lithostratigraphical nomenclature is retained.

References[edit]