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

asinal successions of Dorset, Somerset, Worcestershire,
Staffordshire, Cheshire, west Lancashire and the East
Midlands (Table 2.1). The halite beds do not crop out at
surface, but their projected surface position is often marked
by subsidence hollows and collapse breccias formed in
overlying strata. These features are formed not only by
natural dissolution but also by the effects of salt extraction
by brine pumping. Sulphates (gypsum and anhydrite) are
abundant throughout the unit as veins but are not of
economic importance.
The unit is typically 150 to 300 m thick, though with
substantial variation between basins. Up to 1200 m occurs
in the Cheshire Basin, which includes two thick halite units
with a combined thickness of over 600 m.
In the Worcester Basin, the unit is assigned to a single
formation, the Eldersfield Mudstone. In the Cheshire Basin
and west Lancashire, major halite units have been used to
subdivide the succession, with some further division of the
mudstone based on lithological character. In the East
Midlands, the formations are based partly on fairly subtle
lithological characteristics and partly by using skerries as
mappable marker beds. The unit is not named in other
basins, though some of its components are (e.g. Somerset
Halite, Droitwich Halite).
Taking the unit as a whole, original depositional continuity
is likely, at least in part, between all the basins.
However, correlation of individual formations between the
basins is highly uncertain and most should be considered as
being restricted to individual basins or even parts of basins.
Unit C
This is a thin but widespread unit that has been mapped at
surface, albeit discontinuously, from the Dorset coast into
Nottinghamshire. The unit has not been formally identified
in the Needwood or other basins to the west and north-west,
although it may be represented by a series of un-named
sandstone beds lying just above the Wilkesley Halite in the
Cheshire Basin (Wilson, 1993). In Worcestershire and
Warwickshire, the unit is represented by the Arden
Sandstone. Equivalents in south-west England are the
Butcombe, North Curry and Weston Mouth sandstones. The
Dane Hills Sandstone of Leicestershire and the Hollygate
Sandstone of the Nottingham district represent this unit in
the East Midlands.
In central and south-west England the unit typically
consists of up to 12 m of thickly bedded, medium to coarsegrained,
cross-stratified sandstone. The sandstone is moderately
to strongly calcareous or dolomitic and in some places
cemented by quartz. The most resistant beds have been
quarried locally for building stone. However, these beds of
sandstone are discontinuous and lenticular in geometry and
probably represent the fills of fluvial distributary channels.
Where thick sandstone beds are absent the unit is represented
by dark greenish grey siltstone and mudstone with a
few thin beds of dolomitic, very fine to fine-grained
sandstone. In the East Midlands, the Hollygate Sandstone
consists of up to 8 m of fine to medium-grained pale grey
sandstone interbedded with predominantly red-brown
mudstone; the sandstone beds are cemented mainly by
gypsum; stronger cementation by intergranular dolomite or
quartz overgrowths occurs only in small patches and thin
beds. The sandstone weathers to a very poorly cemented or
uncemented sand in the near-surface zone but is more
competent below a few metres depth from the surface.
The unit forms a distinct marker in geophysical logs that
can be traced in boreholes from Dorset to North Yorkshire;
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it is probably continuous in the subsurface between these
areas. Generally, BGS maps surveyed since 1980 show the
unit as a continuous, though locally very thin formation
except in areas thickly covered by superficial deposits. Older
maps, including those covering Somerset and Dorset, show
the unit as a series of isolated outcrops. Therefore, the
apparent discontinuity of surface outcrop may be partly due
to differences of approach between 'old' and 'new' mapping.
Unit D
This unit resembles Unit B, but resistant dolomitic
sandstone units ('skerries') are less common and structureless
red-brown, dolomitic mudstone dominates. Halite is
absent (although pseudomorphs after halite occur sporadically)
but beds, nodules and veins of gypsum are abundant
either as thick beds and veins or as nodular masses.
Locally, gypsum forms deposits of economic importance,
for example near Burton-on-Trent, Nottingham and
Newark. Gypsum is absent in the near-surface zone due to
dissolution by meteoric water, weakening the fabric of the
rock and locally resulting in a general lowering of the land
surface by up to 3 m.
The unit is represented by the Twyning Mudstone in the
Worcester Basin, the Brooks Mill Mudstone in the
Cheshire Basin and the Cropwell Bishop Formation in the
East Midlands. The unit is unnamed elsewhere. As with
Unit B, (though not as markedly) the unit thickens substantially
into the more rapidly subsiding depositional basins,
with the thickest sequence (140 m) developed in the
Cheshire Basin and the thinnest (30 m) in the East
Midlands.
As a whole, the original depositional continuity can be
inferred between all the basins in Table 2.1. Spatial continuity
is preserved in the subsurface between Dorset and
Yorkshire, but the successions in the Needwood, Stafford,
Cheshire, west Lancashire and Carlisle basins are spatially
isolated.
Unit E
This thin but widespread unit is the uppermost within the
Mercia Mudstone Group and is represented in all basins
except the west Lancashire area. A single name, the Blue
Anchor Formation, has been applied to this unit throughout
England and Wales since 1980 (Warrington et al., 1980). In
south-west England and South Wales, the unit consists of
interbedded greenish-grey, dark grey and green dolomitic
mudstones and dolostones with common gypsum.
Elsewhere, the unit is more homogeneous in lithology and
consists of apparently structureless, pale greenish grey
dolomitic mudstones and siltstones known formerly as the
Tea Green Marl.
The unit is up to 40 m thick in south-west England but is
generally less than 15 m thick elsewhere. It was probably
deposited in a coastal sabkha environment with periodic
marine influence, presaging the widespread marine transgression
that deposited the dark grey to black mudstones of
the lower part of the overlying Penarth Group (Westbury
Formation). The base of the Penarth Group is a nonsequence,
typically resting on a shrinkage-cracked and
bored top surface of the Blue Anchor Formation.
2.5 MARGINAL CONGLOMERATES
Towards the margins of depositional basins and on the
flanks of contemporaneous landmasses such as the