Engineering geology of British rocks and soils Mudstones of the ...

cates around the Pennine Anticline, with the eastern limb
running through Nottinghamshire into the Vale of York,
before eventually reaching the North Sea coast at Teesside.
The western limb underlies northern Shropshire, Cheshire
and Merseyside and much of the Formby and Fylde peninsulas,
passing offshore below the Irish Sea before
extending onshore again on the northern side of the Lake
District near Carlisle. In Cheshire, Warwickshire, the Vale
of York 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.
Thick sequences of the group dip below younger Mesozoic
rocks in Dorset, Hampshire, north-east England and the
southern North Sea. In the south-east of England, the group
pinches out in the subsurface around the margins of the
London Brabant Massif, an ancient cratonic area composed
of Lower Palaeozoic rocks. The group is absent in the subsurface
below London and the Home Counties.
2.3 LITHOSTRATIGRAPHICAL CLASSIFICATION
The mainly arid, continental depositional environment has
resulted in few fossils being preserved and this, coupled with
similar material being deposited over long periods of time
within, but not necessarily uniformly, over the area has
resulted in extreme difficulty in correlating deposits from
one area to another. The current lithostratigraphical classification
of Triassic rocks in England and Wales is based on an
extensive review carried out by the Geological Society of
London (Warrington et al., 1980). The review correlates
stratigraphical sequences from 28 areas of Britain. The terms
Bunter and Keuper, based on supposed time correlation with
the German Triassic sequence, were discontinued in favour
of a more rigorous lithostratigraphical approach using the
gross lithological characteristics of the various rock units.
The former Bunter and Lower Keuper Sandstone units are
now combined into the Sherwood Sandstone Group, with the
Mercia Mudstone Group corresponding closely to the former
Keuper Marl division.
Many local names have been applied to formations
within the Mercia Mudstone Group as shown in the correlation
charts of Warrington et al. (1980). The profusion of
names reflects either the original depositional restriction of
a unit to an individual basin or the subsequent geographical
isolation of a formation at outcrop due to post-Triassic
erosion. Thus, many of the lithostratigraphical subdivisions
that have been recognised in the Mercia Mudstone are
unique to individual basins. However, despite this localised
nomenclature, five broad subdivisions, labelled A to E in
Table 2.1, are recognisable within the group in most basins
and are used in the following description (Howard et al.,
1998). The relationship of current stratigraphical divisions
to former terminology has been addressed by Powell
(1998) and includes terms relating to the Mercia Mudstone
Group.
2.4 LITHOLOGICAL CHARACTERISTICS
The lithological characteristics of the subdivisions A to E of
Howard et al. (1998) are as follows:
Unit A
This is a transitional lithological unit between the Sherwood
Sandstone and Mercia Mudstone groups, and is characterised
4
by interbedding of brown mudstones and siltstones with paler
grey-brown sandstones in approximately equal proportions.
Bedding is generally planar or sub-planar, and most sandstone
beds are less than 0.5 m thick with intervening mudstone and
siltstone partings of similar thickness. The sandstone is
typically very fine to fine-grained, less commonly mediumgrained,
highly micaceous, and moderately 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 and
probably represent sand-filled fluvial, distributary channels.
Sulphates (gypsum and anhydrite) are present as small veins
and nodules but are not as abundant as in higher units. The
unit is typically a few tens of metres thick, but reaches a
maximum of 270 m in the Cheshire Basin.
This unit was formerly known in many areas as the
'Waterstones' due to the supposed resemblance of the highly
micaceous bedding planes to ‘watered silk’. This is the most
difficult unit of the Mercia Mudstone Group to define because
in most basins, both the base and top are gradational, and
lateral facies transition into the upper beds of the Sherwood
Sandstone and the lower beds of Unit B of the Mercia
Mudstone can be demonstrated. In many areas it has not been
distinguished and Unit B lies directly on the Sherwood
Sandstone. Thus, south of Birmingham, Unit A tends not to be
mappable as a formation and, where recognised, is usually
included in the top of the Sherwood Sandstone Group.
Elsewhere, the unit forms the basal formation of the Mercia
Mudstone Group and has been assigned a different formational
name (e.g. Tarporley Siltstone, Sneinton Formation) in
each basin. The base of the unit is both conformable and gradational
but also diachronous, becoming generally younger
southwards from the East Irish Sea area to Worcestershire.
The Eastern England Shelf is a notable exception; there the
base of the unit is unconformable and is marked by a patchily
distributed basal conglomerate up to 1 m thick with a strong,
calcareous cement. Geophysical log correlation indicates that
the lower part of Unit A in the Nottingham area is stratigraphically
isolated from strata of the same age in the Needwood
Basin to the west, but the upper part is in spatial continuity in
the subsurface between these zones.
Unit B
This unit consists mainly of red and, less commonly, green
and grey dolomitic mudstones and siltstones. These show a
variety of fabrics ranging from finely laminated to almost
totally structureless. In many cases the primary depositional
fabric has been deformed by frequent wetting and
drying of the substrate following deposition and the consequent
growth and solution of salts. Thin beds of coarse
siltstone and very fine sandstone occur at intervals throughout
the unit. Individual sandstone beds are typically 20 –
60 mm thick, greenish grey in colour, planar and or
current-ripple laminated and have strong, intergranular
dolomite cements. Less commonly, gypsum cements occur;
these may be dissolved by meteoric waters in the nearsurface
zone to leave a weakly-cemented or uncemented
sand at outcrop. Sandstone beds are usually grouped into
composite units of three or more beds, with greenish grey
mudstone interbeds of equal thickness. These composite
units vary from 0.15 to 1 m thick 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 in some basins.
Substantial deposits of halite, some of considerable
economic importance, occur within this unit in the thicker,