Mercia Mudstone Group (Triassic) of England and Wales – a review 2 – revised lithostratigraphical framework

Mercia Mudstone Group (Triassic) of England and Wales – a review 2 – revised lithostratigraphical framework

From: Howard, A S, Warrington, G, Ambrose, K, and Rees, J G. 2008. A formational framework for the Mercia Mudstone Group (Triassic) of England and Wales. British Geological Survey Research Report, RR/08/04.

This article summarises the revised formation nomenclature for the Mercia Mudstone Group. An introduction to the Mercia Mudstone Group and the reasons for the revised nomenclature can be found in the article: Mercia Mudstone Group (Triassic) of England and Wales – a review 1 – introduction and stratigraphical framework

Approach to the review

The following objectives have guided this review and revision of the lithostratigraphy of the Mercia Mudstone Group in England and Wales:

• the requirement for a logical and robust scheme that adequately describes the range of lithological and stratigraphical variability within the Mercia Mudstone Group
• the need for stability in nomenclature to avoid confusing the users, especially those without specialist understanding of stratigraphical concepts
• the requirement for a rationalised, mappable stratigraphy that is applicable on geological maps throughout England and Wales, eliminating, as far as practicable, changes in nomenclature that currently occur across the boundaries between geological maps of different vintage
• the need for a nomenclature that supports the compilation, by the BGS, of a ‘seamless’ digital geological map and 3D model of the geology of the UK to formation level

These objectives are potentially conflicting and therefore required appropriate weighting for a review to be effective. Options for rationalising the Mercia Mudstone Group lithostratigraphy of England and Wales were identified using three procedures.

1. Identification, irrespective of current nomenclature, of units within the Mercia Mudstone Group that were likely to have been in stratigraphical continuity prior to any subsequent tectonism or erosion, and which therefore correspond to the definition of a formation (North American Commission on Stratigraphic Nomenclature, 1983, 2005; Whittaker et al., 1991; Rawson et al., 2002). Such units may be diachronous.
2. Determination of the present surface and subsurface continuity of these units within the Mercia Mudstone Group.
3. Identification of the practical needs for rationalising local names to ensure that geological maps and digital 3D subsurface models can be linked without involving arbitrary changes in nomenclature across gaps in outcrop or across faults.

Revised formation nomenclature

The following section summarises the revised formation nomenclature for the Mercia Mudstone Group. In choosing the most appropriate name for each of the framework units, we have preferred names that are based on outcrop sections rather than boreholes, with type sections that are readily and permanently accessible to facilitate study. Formal descriptions and diagnoses of each formation are given in section 4.

Unit A

The name Tarporley Siltstone Formation, introduced by Warrington et al. (1980), is applied to this unit. Usage of this name has previously been confined to the Cheshire Basin. The terms Denstone, Maer and Sneinton formations, applied to this unit in other regions, are junior synonyms and are abandoned.

Unit B

The name Sidmouth Mudstone Formation has been applied to this unit at outcrop on the south Devon coast (Gallois, 2001). This name is adopted here in preference to the equivalent Eldersfield Mudstone Formation of the Worcester district (Barclay et al., 1997), which was defined based on borehole cores and associated geophysical logs. Existing formations named within this unit in the East Midlands Shelf South, Cheshire Basin and west Lancashire regions are downgraded to members; most subdivisions presently of member status in those areas will retain that status, though with Sidmouth Mudstone as their new ‘parent’ formation.

Unit C

The name Arden Sandstone Formation is preferred for this unit due to its familiarity as a widespread and mappable stratigraphical marker throughout much of the Worcester and Knowle basins, and its use on published geological maps of those regions. Its stratigraphical significance was recognised by Matley (1912), who introduced the name ‘Arden Sandstone Group’. The name ‘Arden Sandstone Member’ was formalised by Warrington et al. (1980), and various sections in the type area, including the stratotype, were described by Old et al. (1991). The name was elevated to formation status by Barclay et al. (1997). Numerous synonyms applied in other regions (Figure 2; see also Warrington et al., 1980) are abandoned.

Unit D

The name Branscombe Mudstone Formation has been applied to this unit at outcrop on the south Devon coast (Gallois, 2001), where it occurs in discontinuous, faulted sections that are partly obscured by landslides. Both the base and top of the formation are exposed. Predominantly structureless, red-brown and subordinate grey-green mudstones that were included (see section 4) in the upper part of the Dunscombe Mudstone Formation by Gallois (2001) are more typical of the Branscombe Mudstone and are here included in that unit.

This name is preferred to the equivalent Twyning Mudstone Formation of the Worcester district (Barclay et al., 1997), which was based on borehole cores and associated geophysical logs, and to the Cropwell Bishop Formation of the Nottingham area for which no published formal definition exists. The equivalent Brooks Mill Mudstone Formation, introduced by Wilson (1993) in the Cheshire Basin, has been applied to a succession that is atypical, in that the base overlies the Wilkesley Halite Member of the Sidmouth Mudstone Formation, and the Arden Sandstone Formation is not developed in that region.

Unit E

A single name, Blue Anchor Formation (Warrington et al., 1980), is already applied to this unit in all regions. No modification is necessary.

Marginal facies these are mainly dolomitic, arenaceous or conglomeratic deposits of local provenance and distribution; it is considered inappropriate to treat them as formations (cf. Whittaker et al., 1991). Defining them as members would also be unsuitable, because many of these deposits can be shown to be lateral facies equivalents of two or more vertically adjacent formations; some may also be laterally equivalent to parts of the overlying Penarth Group. The use of informal and descriptive local names remains the most practical option for these deposits. Published geological maps in the Bristol area indicate that the Redcliffe Sandstone is a marginal facies equivalent of the Sidmouth Mudstone Formation (Kellaway and Welch, 1993); it is therefore re-defined here as a member of that formation.

Regional lithostratigraphy and subdivisions

Region 1 Wessex Basin

Former lithostratigraphy after Warrington et al. (1980) and Gallois (2001, 2003) The outcrop in this area (Figure 1) is isolated from Region 2 by an outcrop of the overlying Penarth and Lias Groups that extends from Glastonbury to Bridgwater Bay. There is, however, continuity in the subsurface with Region 2 and also with the succession proved offshore below the English Channel (Hamblin et al., 1992).

Region 2 – Bristol and South Wales

Former lithostratigraphy based on Green and Welch (1965), Ivimey-Cook (1974), Warrington et al. (1980), Waters and Lawrence (1987), Wilson et al. (1990) and Whittaker and Green (1983) The outcrop pattern of the Mercia Mudstone Group in this region is complicated by numerous inliers, outliers and tongues of older and younger rocks. It is separated from Region 3 by an outcrop of Devonian rocks that crosses the River Severn at Sharpness, Gloucestershire, but there is continuity in the subsurface beneath the Cotswolds to the east.

Region 3 Worcester and Knowle Basins

Former lithostratigraphy based on Green and Melville (1956), Barclay et al. (1997), Worssam et al. (1989), Powell et al. (2000), Warrington et al. (1980), Old et al. (1987), Old et al. (1991), and Williams and Whittaker (1974) The entire succession is in subsurface continuity with that of Region 5 (East Midlands Shelf South), but only the lower part is in continuity with Region 4 (Needwood Basin). The cover of superficial deposits (drift) becomes extensive in east Warwickshire, hampering correlation with succession at outcrop to the north-east in the East Midlands Shelf South.

Region 4 Needwood Basin

Former lithostratigraphy based on Warrington et al. (1980), Charsley (1982), Chisholm et al. (1988) and Stevenson and Mitchell (1955) Only the lower part of the succession is in spatial continuity with Region 3 to the south (Worcester and Knowle basins) and Region 5 (East Midlands Shelf South). A narrow horst of upfaulted Sherwood Sandstone Group separates this region from the Stafford Basin (Region 7) to the west.

Region 5 East Midlands Shelf South

Former lithostratigraphy based on Elliott (1961), Worssam and Old (1988), Warrington et al. (1980), Charsley et al. (1990), and Howard et al. (in press) The formations were largely defined from boreholes and surface mapping in south Nottinghamshire and north Leicestershire. The outcrop in Leicestershire and Warwickshire is covered extensively with superficial deposits. There is full spatial continuity in the subsurface with Region 6 (East Midlands Shelf North) and with the equivalent succession (Haisborough Group, Rhys, 1974) in the Southern North Sea Basin. The formations mapped at surface in this region have not been traced north of Newark.

Region 6 East Midlands Shelf North

Former lithostratigraphy based on Smith and Warrington (1971), Smith et al. (1973), Smith (1980), Riddler (1981) and Warrington et al. (1980) Superficial deposits cover is extensive north of Retford in north Nottinghamshire. There is full continuity with the succession offshore in the Southern North Sea Basin (Johnson et al., 1994), correlation with which is illustrated in Figure 11 (see also Cameron et al., 1992; Warrington, 1974).

Region 7 Stafford Basin

Former lithostratigraphy based on Warrington et al. (1980) and Rees and Wilson (1998) The Mercia Mudstone Group succession in the basin is isolated from the adjacent regions by faulting and erosion.

Region 8 Cheshire Basin

Former lithostratigraphy based on Wilson (1993) and Rees and Wilson (1998) The Mercia Mudstone Group succession in the Cheshire Basin is spatially isolated from the Stafford Basin by faulting and from west Lancashire (Region 9) by an outcrop of the Sherwood Sandstone Group extending from the Wirral towards Preston (Llyn–Rossendale Ridge of Jackson et al., 1997). The outcrop is extensively masked by superficial deposits and the existing formations have been mapped mainly using borehole evidence. The halite deposits were used by Pugh (1960) and subsequently Wilson (1993) as dividers for defining a formation-level stratigraphy, with the exception of the Byley and Wych Mudstone formations (Wilson, 1993), which are not separated by a halite and are differentiated on lithological characteristics recognised mainly in borehole core.

Region 9 - West Lancashire

Former lithostratigraphy after Warrington et al. (1980), Wilson and Evans (1990) and Jackson et al. (1995; 1997) The region is the onshore extension of the East Irish Sea Basin, the nomenclature of which (Figure 11) has been defined by Jackson et al. (1997). Onshore, superficial deposits conceal much of the outcrop in the Wirral, Formby, Fylde and Walney–Barrow-in-Furness areas, and the existing formations have been defined and mapped largely using data from cored boreholes (Wilson, 1990; Wilson and Evans, 1990).

Region 10 Carlisle Basin

Lithostratigraphy after Warrington et al. 1980, Barnes et al. (1994), Ivimey-Cook et al. (1995), Jackson et al. (1995; 1997) and Holliday et al. (2004) The lower part of the Mercia Mudstone Group in this region is partly in continuity with that offshore in the Solway Firth Basin, but both basins are isolated from the East Irish Sea Basin by a sea floor outcrop of the Sherwood Sandstone along the Ramsey–Whitehaven ridge (Jackson et al., 1995). The outcrop in the Carlisle Basin is masked by superficial deposits. The name Stanwix Marls or Stanwix Shales (Holmes, 1881; 1899) has been applied to the Mercia Mudstone Group in the Carlisle Basin but is abandoned in this review. Borehole evidence in this basin provides insufficient detail to differentiate formations within the Group, other than the Blue Anchor Formation (Ivimey-Cook et al., 1995).