Mercia Mudstone Group (Triassic) of England and Wales – a review 1 – introduction and stratigraphical framework
- 1 Mercia Mudstone Group (Triassic) of England and Wales – a review 1 – introduction and stratigraphical framework
- 2 Review of existing nomenclature
- 2.1 Distribution of the Mercia Mudstone Group
- 2.2 Stratigraphical framework
- 2.3 Marginal facies
- 2.4 Recommendations for rationalising Mercia Mudstone Group lithostratigraphy
- 3 References
Mercia Mudstone Group (Triassic) of England and Wales – a review 1 – introduction and stratigraphical 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 presents a review of the lithostratigraphical nomenclature for the Mercia Mudstone Group (Triassic) of England and Wales. Irrespective of existing nomenclature, five units that were in stratigraphical continuity prior to any subsequent tectonism or erosion are identified as constituting the group. These units are formations as defined by international and UK codes of stratigraphical practice (North American Commission on Stratigraphic Nomenclature, 1983; 2005; Whittaker et al., 1991; Rawson et al., 2002). Where present, the five units are consistently mappable at outcrop using conventional geological surveying techniques, and are recognisable in the subsurface in both borehole core and geophysical logs. The various local lithostratigraphical names formerly applied to these mappable subdivisions are replaced by a single formation name for each unit. These are, in upward succession, the Tarporley Siltstone Formation, Sidmouth Mudstone Formation, Arden Sandstone Formation, Branscombe Mudstone Formation and Blue Anchor Formation.
Local synonyms of these newly defined formations are abandoned in this revised scheme. In some regions, these newly defined units may encompass two or more subdivisions that were formerly defined as formations. In such cases, the former ‘formations’ are downgraded to member status, with the newly defined formation as their parent stratigraphical unit. Informal nomenclature is retained for some highly discontinuous, locally distributed units at basin margins, where correlation with adjacent formations is uncertain.
The report includes formal definitions of the Mercia Mudstone Group and its five component formations. Definitions of members, beds and informal subdivisions within these formations are given in the BGS Lexicon of Named Rock Units, which can be consulted on the BGS website (http://www.bgs.ac.uk/lexicon/lexicon_intro.html).
The Mercia Mudstone Group (Mercia Mudstone Group) of England and Wales is composed mainly of red and, less commonly, green and grey mudstone and siltstone. Substantial deposits of halite occur in the thicker, basinal successions of Dorset, Somerset, Worcestershire, Staffordshire, Cheshire, west Lancashire and south Cumbria, and east and north Yorkshire. Sulphate deposits (gypsum and anhydrite) and sandstone beds are common at some stratigraphical levels but are a minor constituent throughout most of the group. Fossils occur mostly in green and grey-coloured units; miospores are the most widespread stratigraphically. The Mercia Mudstone Group ranges in age from Mid Triassic (Anisian) to latest Triassic (Rhaetian). In most areas it overlies sandstones of the Sherwood Sandstone Group (Sherwood Sandstone Group), but locally overlaps these to rest on Permian, Carboniferous or older rocks. The Mercia Mudstone Group is overlain by the grey to black, fossiliferous mudstone of the Westbury Formation of the Penarth Group.
Tectonic and depositional setting
During Permian and Triassic times, England and Wales lay in the interior of the supercontinent of Pangaea, to the north of the Variscan mountain chain. Both Pangaea and the Variscan mountains were the product of continental collision during the late Carboniferous but, by the early Permian, Pangaea was already showing the first signs of breaking apart. In what is now southern, central and north-west England, the resulting crustal tension led to the formation of a series of fault-bounded basins (Warrington and Ivimey-Cook, 1992; Chadwick and Evans, 1995). Eastern England lay on the margin of a much larger subsiding depocentre, the Southern North Sea Basin, which covered much of north-west Europe. The interplay of sea-level change, extension-driven basinal subsidence and climatic fluctuations profoundly influenced deposition throughout the Permian and Triassic (Ruffell and Shelton, 1999). In the early Triassic, monsoonal rains falling on the Variscan mountains fed a major river system that flowed northwards across southern Britain (the ‘Budleighensis River’ of Wills, 1956, 1970) and deposited thick sequences of pebbly sands, now preserved in the lower part of the Sherwood Sandstone Group. Deposition of the lower formations within the Sherwood Sandstone Group was largely restricted to the fault-bounded basins, although sediments were transported across the intervening highs. However, the upper formations of the Sherwood Sandstone Group, which include both aeolian and fluvial sandstones, overlapped progressively onto the adjacent highs. During Anisian times, fluvial environments of the upper Sherwood Sandstone Group retreated southwards and were replaced diachronously by the sub-aqueous hypersaline and evaporitic mudflat environments of the Mercia Mudstone Group (Warrington and Ivimey-Cook, 1992). Four main depositional processes prevailed on these mudflats (Arthurton, 1980; Warrington and Ivimey-Cook, 1992; Talbot et al., 1994):
- settling-out of mud and silt in brackish or hypersaline water bodies
- rapid deposition of sheets of silt and fine sand transported by flash floods
- accumulation of wind-blown dust on wet mudflat surfaces
- chemical precipitation of salts, principally halite and gypsum, from marine-sourced hypersaline water bodies and from contemporary groundwaters
The thickest Mercia Mudstone Group sequences accumulated within fault-bounded basins, but it overlapped the Sherwood Sandstone Group and Permian strata and overstepped onto Carboniferous and older rocks that formed adjacent highs, so that the basin infills coalesced. Deposition of the Mercia Mudstone Group ended in latest Triassic (Rhaetian) times when rising sea level flooded the mudflats and laid down the widespread, dark grey to black marine mud that formed the Westbury Formation at the base of the Penarth Group (Warrington and Ivimey-Cook, 1992).
The current lithostratigraphical nomenclature of Triassic rocks in England and Wales is based on an extensive review carried out by the Triassic Working Group of the Geological Society of London, published in 1980 (Warrington et al., 1980). Prior to this, the terms ‘Bunter’ and ‘Keuper’ were applied to these rocks, based on lithological similarity with units of those names in the German Triassic. The Triassic Working Group abandoned those terms because of their implied time connotation, and adopted a rigorous lithostratigraphical approach. The former ‘Bunter’ and ‘Lower Keuper Sandstone’ units were combined to form the Sherwood Sandstone Group. The overlying Mercia Mudstone Group was constituted from strata between the Sherwood Sandstone Group and the Penarth Group (formerly the ‘Rhaetic’), corresponding broadly with the former ‘Keuper Marl’.
Chronostratigraphical subdivision and correlation of Triassic rocks in England and Wales is hampered by a scarcity of fossils. Chronostratigraphical determinations of the formations in the Mercia Mudstone Group are effected mainly by the use of spore and pollen assemblages (Warrington et al., 1980). Recovery of these fossils varies stratigraphically and geographically, however, and the recognition of ‘time lines’ within the Mercia Mudstone Group succession is generally approximate. Geophysical log markers and clay mineral stratigraphy have in some cases been used to infer approximate chronostratigraphical correlations where spores and pollen are absent. Magnetostratigraphy is available for the upper part of the Mercia Mudstone Group (Hounslow et al., 2004) and has potential for dating and correlation at other levels.
A plethora of local names has been applied to formations and members within the Mercia Mudstone Group; many of these have been introduced since the Geological Society of London published the special report on the Correlation of Triassic rocks in the British Isles (Warrington et al., 1980). These local names, and their usage on British Geological Survey (BGS) maps, reflect the following factors:
- original depositional restriction of units to individual basins
- postdepositional, geographic isolation of outcrops by faulting or erosion
- differences in lithostratigraphical approach across boundaries between separate BGS mapping projects, especially where the surveying of adjacent 1:50 000 map sheets was separated by many years and new knowledge became available over the interim period.
With the exception of the Blue Anchor Formation, nomenclature of constituent formations of the Mercia Mudstone Group is unique to individual basins and in some cases to geographical regions within basins. Additionally, some formations were originally defined on lithological or geophysical characteristics recognisable only in borehole core and their mappability at surface has not been tested. This applies, for example, to some of the formations defined in west Lancashire and Cheshire, where exposures are scarce and most of the outcrop lies below thick Quaternary cover. This review presents a rationalisation of existing nomenclature, with the objective of a more unified, lithostratigraphical scheme for the Mercia Mudstone Group in England and Wales.
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.
- 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.
- Determination of the present surface and subsurface continuity of these units within the Mercia Mudstone Group.
- 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.
Review of existing nomenclature
Distribution of the Mercia Mudstone Group
The Mercia Mudstone Group crops out (Figure 1) in east Devon, Somerset and in the Bristol region, extending westwards into Glamorgan and Gwent, south Wales. The outcrop continues northwards through the Gloucester and Worcester regions before broadening out to underlie much of the central Midlands in Warwickshire, Staffordshire and Leicestershire. Farther north, the outcrop bifurcates around the Pennines. To the east, it extends northwards through Nottinghamshire and Yorkshire and reaches the North Sea coast near Hartlepool. To the west it underlies northern Shropshire, Cheshire and Merseyside and much of the Formby and Fylde peninsulas, passing offshore below the Irish Sea before re-appearing onshore to the south-west of the Lake District at Walney and Barrow-in-Furness, and farther north near Carlisle. In Cheshire, Warwickshire, Yorkshire and the Carlisle area, large parts of the outcrop are masked by thick Quaternary deposits (mainly glacial till), with more patchy cover of superficial deposits elsewhere.
From the outcrop, the Mercia Mudstone Group extends eastwards below younger Mesozoic rocks and pinches out in the subcrop around the London-Brabant Massif (Horton et al., 1987; Sumbler et al., 2000), a cratonic area composed of Palaeozoic rocks (Figure 1; see also Warrington and Ivimey-Cook, 1992). Red beds proved beneath the Jurassic by boreholes in Surrey, Sussex and Kent have been tentatively assigned to the Mercia Mudstone Group (Warrington et al., 1980).
The Mercia Mudstone Group crops out in a number of distinct regions (Figure 1); these are named after their areas of outcrop but their extent includes adjacent subcrops. Some of these contain successions that are now geographically isolated within structural basins, but were formerly connected during deposition of the group. Other regions simply represent outcrop areas that are isolated at surface but connected in the subsurface. The entire group is in continuity in the subsurface between regions 1 to 3 and 5 to 6. The lower part of the Mercia Mudstone Group in Region 4 (Needwood Basin) is also connected in the subsurface with the successions in regions 3 (Knowle Basin) and 5 (East Midlands Shelf South). The successions in regions 7 to 10 are isolated either by faulting or by intervening outcrops of older rocks.
As a basis for the rationalisation of Mercia Mudstone Group lithostratigraphy, we have identified a framework of five lithostratigraphical units (A to E, described below) that either possess, or can reasonably be inferred to have once possessed, a high degree of continuity. These units are mappable both at surface and in the subsurface on a regional rather than local basis, and thus comply with the definition of a formation (sensu North American Commission on Stratigraphic Nomenclature, 1983, 2005; Whittaker et al., 1991; Rawson et al., 2002). Each has distinctive lithological characteristics and geophysical log profiles. The lithostratigraphical nomenclature currently applied to these units is shown in Figure 2 and summarised below. A revised nomenclature is considered in section 3.
This heterolithic unit consists of brown mudstone and siltstone interbedded with a variable but approximately equal proportion of paler, grey-brown sandstone. Stratification is generally planar or subplanar, and most sandstone beds are less than 0.5 m thick, with intervening mudstone and siltstone partings of similar thickness. The sandstones are typically very fine to fine grained, less commonly medium-grained, highly micaceous, and weakly cemented by ferroan calcite or dolomite. Beds of fine- to medium-grained sandstone up to 5 m thick are present locally; these have a lenticular geometry with internal cross-stratification. Sulphates (gypsum and anhydrite) are present as small veins and nodules but are not as abundant as in the higher units. Fossils are generally restricted to miospores (e.g. Fisher, 1972; Charsley, 1982; Earp and Taylor, 1986; Rees and Wilson, 1998; Warrington et al., 1999), but invertebrate trace fossils (Ireland et al., 1978; Pollard, 1981), vertebrate tracks (Sarjeant, 1974) and the brachiopod Lingula (Rose and Kent, 1955) have been recorded locally.
The unit is typically a few tens of metres thick, but reaches a maximum of 220 m in the Cheshire Basin.
Unit A was formerly known in many areas as the ‘Waterstones’, the name originating from its association with springs (Hull, 1869, p.67) or from an oft-quoted resemblance of the micaceous bedding planes to ‘watered silk’. Of the five Mercia Mudstone Group units, this is the most problematical in terms of stratigraphical treatment, and it has not been differentiated in all regions. Where recognised, both the base and top of the unit are gradational, and interdigitate with the upper beds of the Sherwood Sandstone Group and the lower beds, Unit B, of the Mercia Mudstone Group, respectively. In southern England and south Wales (regions 1 and 2, Figure 1), the unit has not been differentiated, and Mercia Mudstone Group Unit B rests directly on the Sherwood Sandstone Group. In the Worcester and Knowle basins (Region 3, Figure 1), Unit A has not been mapped as a formation in its own right and, where the facies has been recognised, it has usually been included within the Bromsgrove Sandstone Formation (or equivalent) in the uppermost part of the Sherwood Sandstone Group. From the Central Midlands northwards (regions 4-8, Figure 1), Unit A has been mapped as a distinct formation and has been assigned a different name in each basin (e.g. Tarporley Siltstone Formation, Maer Formation, Denstone Formation, Sneinton Formation, see Figure 2).
The base of Unit A (or where the unit is not recognised, the base of the Mercia Mudstone Group) is diachronous, becoming generally younger southwards. An exception is Eastern England (regions 5 and 6), where the base of the Mercia Mudstone Group is unconformable and Unit A onlaps westwards onto the weathered surface of the Sherwood Sandstone Group. This is overlain locally by a thin conglomerate containing re-worked, wind-facetted pebbles, and has long been considered to represent the Hardegsen Disconformity of north-west Europe (Warrington, 1970; Wills, 1970). Geological mapping and geophysical log correlation suggests that the lower part of Unit A in the Nottingham area was deposited in isolation from coeval strata in the Needwood Basin (Region 4, Figure 1) to the west, but that the upper part is in subsurface continuity between these areas (Howard et al., in press).
Unit A is Mid Triassic (Anisian) in age (Figure 2).
Unit B consists mainly of red and, less commonly, green and grey dolomitic mudstones and siltstones. These range from finely laminated to almost totally structureless; in many cases, the primary depositional lamination has been deformed penecontemporaneously by frequent wetting and drying of the substrate and growth and solution of salts. Thin beds of coarse siltstone and very fine-grained sandstone occur at intervals throughout the unit. Individual sandstone beds are typically 0.02 to 0.06 m thick, greenish grey in colour, planar or current ripple laminated and have strong, intergranular dolomite cement. Less commonly, gypsum cement occurs; this is usually dissolved by meteoric waters in the near surface to leave weakly cemented or uncemented sand at outcrop. Sandstones are usually grouped into composite units of three or more beds, with greenish grey mudstone intercalations of equal thickness. These composite units vary in thickness from 0.15 to 1 m and many are sufficiently resistant to form low, cuesta-like landforms; these resistant beds are locally termed ‘skerries’, the more persistent of which have been named and mapped in some basins. Sulphates (gypsum and anhydrite) are present throughout the unit, but occur as thin veins, small nodules and intergranular cements that are of no economic importance. Plant spores and pollen are virtually the only fossils in this unit.
Deposits of halite, some of substantial thickness and considerable economic importance, occur within the lower to middle part of this unit in the thicker basinal successions of Cheshire, west Lancashire, Carlisle and the East Midlands Shelf North (Figure 2). Higher halite units in the Cheshire, Stafford, Needwood, Worcester and Wessex basins are included in this unit, but may in part be coeval with Unit C (Figures 2; 3). Due to dissolution, the halite beds do not crop out at surface, but their projected surface position is commonly marked by subsidence hollows and collapse breccias formed in overlying strata. These features are formed not only by natural dissolution but also as a result of salt extraction by uncontrolled brine pumping.
Geophysical log markers within the unit can usually be correlated within basins, for example the Regional Gamma Ray Marker in Eastern England (Balchin and Ridd, 1970) and the geophysical log units recognised in the Wessex Basin by Lott et al. (1982). However, correlation of the markers within Unit B between basins is untenable, mainly due to the local presence of thick halite deposits with low gamma/high sonic signatures that break up and mask any distinctive patterns in the logs.
Unit B is typically 150 to 300 m thick, though with substantial variation between basins. Up to 1600 m occurs in the Cheshire Basin, where two halite units with an aggregate thickness exceeding 600 m are developed.
In the Worcester Basin, Unit B was named the Eldersfield Mudstone, based on a cored and geophysically logged borehole section at Eldersfield (Barclay et al., 1997). Gallois (2001) introduced the name Sidmouth Mudstone Formation for this unit, based on the cliff sections east of Sidmouth, south Devon. In the Cheshire Basin, west Lancashire and the East Midlands Shelf South, Unit B encompasses numerous locally defined formations based on various lithological criteria. In the Cheshire Basin and west Lancashire, the succession was originally subdivided using the major halite units (Pugh, 1960), followed by further subdivision of the mudstones based on lithological character (Wilson, 1990, 1993). In the East Midlands Shelf South, formations were based partly on subtle lithological characteristics evident in borehole core, and partly on the use of ‘skerries’ as mappable marker beds at surface (Elliott, 1961; Charsley et al., 1990). The unit has not been named elsewhere, though some component units bear names (e.g. Somerset Halite Formation, Droitwich Halite Formation; Figure 2).
Taking the unit as a whole, at least partial depositional continuity was likely to have existed between all basins. However, depositional continuity of the subdivisions currently recognised within this unit, especially the halites, is more questionable. The halites of west Lancashire and the Cheshire Basin have been correlated on the basis of approximate time-equivalence (Wilson, 1993; Jackson et al., 1995), but may not have been in depositional continuity across the high that separates the two basins. Most of the currently recognised subdivisions cannot be correlated between basins either by geological mapping or geophysical log interpretation and their stratigraphical status is re-evaluated accordingly in this report.
Unit B ranges from Anisian to Carnian in age.
Unit C is distinguished from those below and above on the basis of contrasting colour and a unique combination of lithological, mineralogical, sedimentological and palaeontological features. It is a heterolithic unit that consists predominantly of grey and green mudstone interbedded with paler grey-green to buff-coloured siltstone and fine- to medium-grained, varicoloured (green, brown, buff, mauve) sandstone; fine conglomerates occur locally. Invertebrate and vertebrate macrofossils are present, locally in abundance, and miospores and trace fossils are common. The unit is characterised by an exotic clay mineral assemblage, which is rich in mixed-layer clays and distinct from that of the units below and above (Jeans 1978, Bloodworth and Prior, 1993; Carney et al., 2004; Jeans et al., 2005). Two lithofacies can be distinguished. A mudstone-dominated lithofacies consists of dark greenish grey laminated mudstone and siltstone with subordinate purplish grey mudstone and a few thin beds of very fine- to fine-grained dolomitic sandstone. A sandstone-dominated lithofacies consists mainly of thin- to medium-bedded sandstone that forms discontinuous and lenticular bodies that are enclosed within the mudstone-dominated lithofacies. The sandstone is moderately to strongly calcareous or dolomitic, and the thicker, most resistant beds have been widely used for building stone. In the Wessex Basin and more generally in the English Midlands, the unit has been interpreted as comprising sand-filled distributary channels separated by mudflats within a deltaic or estuarine environment (Ruffell, 1991; Warrington and Ivimey-Cook, 1992). Brecciated units have been recorded at the base and top of the unit (Jeans, 1978; Gallois, 2001) on the south Devon coast.
The thickness of Unit C is typically less than 10 m thick, but locally ranges up to 21 m in the Worcester Basin and 24 m in the Wessex Basin.
Mapping by the Geological Survey prior to 1980 emphasised the discontinuous sandstone lithofacies, which forms pronounced but localised topographic features. The outcrop was consequentially shown as discontinuous on these older geological maps (e.g. Sheet 295 Taunton). More recent surveys in the Worcester and Knowle basins, where the outcrop is most extensive and continuous, have included the sandstones and the associated distinctive green mudstones and siltstones as a single unit—Arden Sandstone Formation (Old et al., 1991; Barclay et al., 1997).
Occurrences in the Leicester and Nottingham areas of the East Midlands Shelf South have been termed the Dane Hills Sandstone and Hollygate Sandstone members, respectively (Warrington et al., 1980; Howard et al., in press). The unit has not been mapped in the Needwood, Stafford or Cheshire Basins, or north of Newark in Nottinghamshire. In south-west England the unit encompasses the North Curry Sandstones of Somerset and possibly the Butcombe Sandstone and Stoke Park Rock Bed of the Mendips and Bristol areas, respectively (Green and Welch, 1965; Kellaway and Welch, 1993). On the south Devon coast, it includes the Weston Mouth Sandstone Member of Warrington et al. (1980). The Dunscombe Mudstone Formation of Gallois (2001) is a rather thicker unit (35 m) that is partly equivalent to Unit C but includes, in its upper part, a succession of red-brown and subordinate grey-green mudstones that are more typical of the overlying Unit D (see section 4).
Where borehole geophysical logs can be related to core, Unit C is seen to have a distinctive log signature that enables wider correlation in the subsurface within and between basins. However, in the Cheshire basin and parts of the Wessex Basin, where thick halites are present, the log signature cannot be distinguished, and the unit may, in part, be coeval with the halite (Gallois, 2003). The lack of cored borehole data in the Wessex Basin and adjacent offshore areas has previously led to the miscorrelation of the unit with evaporites at a higher level in the succession (e.g. Lott et al., 1982, Hamblin et al., 1992). The recent availability of core from Wiscombe Park 2 Borehole† in south Devon has enabled the unit to be correctly identified (Warrington, 1999) and correlated within the Wessex Basin (Figure 3). The unit can now be recognised with confidence in boreholes from Devon to the East Midlands (Regions 1, 2, 3 and 5, Figures 1 and 4) and is likely to be continuous in the subsurface between these areas. The unit is late Carnian (Tuvalian) in age, based on palynological evidence (Warrington and Ivimey-Cook, 1992).
† BGS borehole registration numbers and grid references are given in the Appendix
Unit D superficially resembles Unit B, but resistant dolomitic sandstone-rich units (‘skerries’) are less common, and structureless red-brown, dolomitic mudstones dominate. Geophysical log interpretation of boreholes in the Wessex Basin (Figure 3; Harvey and Stewart, 1998) suggests that a thin halite may be present locally towards the base of the unit, but the mineral is absent elsewhere (though pseudomorphs after halite occur sporadically). Gypsum and anhydrite are abundant, either as thick beds or as large nodular masses and veins. Locally, gypsum forms deposits of economic importance, for example near Burton on Trent, Nottingham and Newark (regions 4 and 5, Figure 1). Gypsum is usually absent in the near surface due to dissolution by meteoric water. The unit has a remarkably consistent geophysical log profile, and several log markers, including those recognised by Lott et al. (1982), can be correlated between onshore basins (Figures 3; 4) and with offshore equivalents in the Southern North Sea Basin. The unit is typically devoid of fossils, though plant spores and pollen occur within the higher beds in Somerset and Devon (Region 1, Figure 1).
The unit is represented by the Twyning Mudstone in the Worcester Basin (Barclay et al., 1997), the Brooks Mill Mudstone in the Cheshire Basin (Wilson, 1993), and the Cropwell Bishop Formation in the East Midlands Shelf South (Charsley et al., 1990). In Devon the unit includes the Branscombe Mudstone Formation together with the upper part of the Dunscombe Mudstone Formation of Gallois (2001). Elsewhere it is un-named. As with Unit B (though not as markedly), the unit thickens into the depocentres, with the thickest sequences (up to 240 m) developed in the Wessex Basin and the thinnest (25 m) in the East Midlands.
Depositional continuity can be reasonably inferred for this unit between all the basins (Figure 2). Continuity exists in the subsurface between south Devon and North Yorkshire (regions 1, 2, 3, 5 and 6, Figure 1), but the successions in the Needwood, Stafford and Cheshire basins, and in west Lancashire and the Carlisle basin (regions 4, 7, 8, 9 and 10, Figure 1) are isolated by faulting and/or subsequent erosion.
There is no direct evidence of age, but a Norian age is inferred because of palynological evidence—a Carnian and Norian (?) age is indicated for the underlying Unit C and a Rhaetian age for the overlying Unit E.
This thin but widespread unit is the uppermost formation of the Mercia Mudstone Group and is recognised in all the regions (Figure 2) except west Lancashire. A single name, the Blue Anchor Formation, has been applied to this unit throughout England and Wales (Warrington et al., 1980). In south-west England and south Wales (regions 1 and 2, Figure 1) the unit consists of interbedded greenish grey, dark grey and green dolomitic mudstones and dolostones with common gypsum. Elsewhere, it is more homogeneous in lithology and consists of the apparently structureless, pale greenish grey dolomitic mudstones and siltstones known formerly as the ‘Tea Green Marl’.
The unit is typically around 30 m thick in south-west England (Region 1, Figure 1), reaching a maximum of 67 m; elsewhere it is generally less than 10 m thick. It was probably deposited in a coastal sabkha environment with periodic marine influence, presaging the widespread marine transgression that deposited dark grey to black mud that formed the lower part of the overlying Penarth Group (Westbury Formation) (Warrington and Ivimey-Cook, 1992). The base of the unit is generally regarded as conformable, though Horton et al. (1987) and Old et al. (1987) refer to significant erosion below this horizon around the margins of the London-Brabant Massif. The base of the overlying Penarth Group is more widely regarded as a non-sequence; it rests on the Blue Anchor Formation that typically bears evidence of animal borings and shrinkage cracks on the top surface.
A late Norian(?) to Rhaetian age is indicated by palynological evidence (Hounslow et al., 2004).
Towards the margins of depositional basins and on the flanks of contemporaneous landmasses such as the Mendips and Charnwood Forest, the Mercia Mudstone Group contains laterally impersistent beds of conglomerate and breccia, commonly cemented strongly by dolomite. These conglomerates were deposited as alluvial fan gravels or screes, and contain abundant, large, commonly angular pebbles of local provenance. They are especially common towards the base of the Mercia Mudstone Group where it onlaps onto Carboniferous or older rocks. In south Glamorgan, the marginal facies consists of fenestral and algal carbonates interdigitating with conglomerates and breccias, interpreted as shoreline deposits (Tucker, 1977; Waters and Lawrence, 1987, Benton et al., 2002). Localised pocket deposits filling palaeokarstic cavities in the underlying Carboniferous Limestone occur locally on the Welsh (Wilson et al., 1990), and English flanks of the Bristol Channel (Robinson, 1957; Savage, 1993; Fraser, 1994; Wall and Jenkyns, 2004) and also in Leicestershire (Carney et al., 2001). Sandstone beds also occur locally towards the basin margins in some other areas; the Redcliffe Sandstone Formation of the Bristol area (Kellaway and Welch, 1993), which is up to 50 m thick, is a notable example.
Recommendations for rationalising Mercia Mudstone Group lithostratigraphy
The recognition of framework units A to E enables consideration of three options for the rationalisation of Mercia Mudstone Group lithostratigraphy and analysis of their relative advantages and disadvantages.
Status quo option
This option would largely retain the existing provincial nomenclature illustrated in Figure 2, with some minor rationalisation of nomenclature within some regions to remove obvious synonyms.
- Nomenclature used on recently published BGS maps, memoirs and sheet explanations is maintained.
- Assigns ‘formation’ status to units that are mappable only on a local scale.
- Retains inconsistencies in unit status between regions with, for example, halite units being assigned member status in west Lancashire but formation status elsewhere.
- Arbitrary ‘boundaries’ of little or no stratigraphical significance would need to be created to separate these local formations on ‘seamless’ digital maps or 3D subsurface models.
Partial rationalisation option
This option rationalises formation names for each of the five ‘framework units’ where they are in continuity, but retains the existing local nomenclature in
- Production of ‘seamless’ digital geological maps and 3D subsurface models would not be hindered, because all changes in nomenclature would take place across structural or erosional discontinuities between outcrops, removing the need for arbitrary boundaries.
- Retains inconsistencies in unit status between regions with, for example, halite units being assigned formation status in the Cheshire Basin but member status elsewhere.
- The scheme locally modifies the lithostratigraphy published on recent BGS maps (e.g. Nottingham Sheet 126) and may thus confuse the users of those maps until new editions are published, although the number of sheets affected would be less than for the full rationalisation option (see below).
Full rationalisation option
This option applies a formation name to each of the five ‘framework units’ across all regions. Existing formations that represent local subdivisions of the new ‘framework’ formations are downgraded to member status. Names that represent junior synonyms are abandoned.
- Potentially, it presents a ‘once and for all’ solution that, because of its simplicity, should become readily accepted and applied by the user community in the medium to long term.
- The formations defined in this scheme are closest in concept to the definitions of the North American Commission on Stratigraphic Nomenclature (1983, 2005), Whittaker et al. (1991) and Rawson et al. (2002).
- Lithostratigraphical divisions maintain a consistency of status between regions; for example equivalent halite units have member status in all occurrences.
- It is the most favourable option for construction of ‘seamless’ digital maps and 3D subsurface models to formation level.
- The scheme updates the lithostratigraphy published on recent BGS maps, and may thus confuse the users of those maps until new editions are published.
In consideration of these alternatives, the conclusion of the BGS Stratigraphy Committee is to adopt full rationalisation as the preferred option, on the grounds that it most closely satisfies the objectives stated in section 1.3, and offers the nearest approach to the formation definitions favoured by Whittaker et al. (1991), Rawson et al. (2002), and the North American Commission on Stratigraphic Nomenclature (1983, 2005). The revised lithostratigraphy arising from this rationalisation is described in section 3; formal definitions of the five formations are given in section 4 and in the BGS Lexicon of Named Rock Units, accessible via the BGS website (http://www.bgs.ac.uk/lexicon/lexicon_intro.html).