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

changed to a ‘detrital’ assemblage of illite and chlorite with
a minor component of illite-smectite and trace quantities of
palygorskite at some levels (Mayall, 1981; Leslie, 1989).
At Branscombe the transition from less than 85% to greater
than 85% illite and chlorite took place between 45 and 65
m below the base of the Blue Anchor Formation. Nodular
gypsum was abundant in laterally impersistent beds in the
lower part of the Blue Anchor Formation and the upper 300
mm of red mudstone.
A similar transition in clay mineral content, was identified
between 50 and 70 m below the Blue Anchor
Formation at St. Audrie’s Bay in Somerset. The authigenic
clay minerals were mainly mixed layer chlorite-smectite
with smectite and in some beds minor sepiolite or palygorskite
(Leslie et al., 1993). Above the transitional zone
and in the Blue Anchor Formation more than 90% of the
clay mineral assemblage comprised illite and chlorite with
minor chlorite-smectite, intermittent smectite, and palygorskite.
The Blue Anchor Formation may contain between
60% and 80% carbonate in beds of 0.2 to 0.9 m thick.
In the Upper Mercia Mudstone Group at the Penarth Cliffs
in South Wales, the clay mineral assemblage was illite,
chlorite and minor mixed-layer chlorite-smectite clay.
Smectite, palygorskite and sepiolite have not been identified.
3.4.2 West Midlands
The Eldersfield Mudstone Formation forms the lower part
of the Mercia Mudstone and contains between 2% and 33%
dolomite. The lower parts of this formation usually contain
less than 10% calcite and it may be absent, for example in
the Stowell Borehole in Gloucestershire. The dolomite
content of the Arden Sandstone Formation is usually
between 10% and 30%. The dolomite content of the upper
red mudstone, the Twyning Mudstone Formation, is
normally between 10 and 20% and its calcite content is
usually similar to the rest of the succession. The Blue
Anchor Formation contains between 10 and 30% dolomite
(Jeans, 1978).
Illite and chlorite are present at the base of the Mercia
Mudstone Group. The clay mineral assemblage in most of
the middle and upper Eldersfield Mudstone Formation,
Arden Sandstone Formation and the lower part of the
Twyning Mudstone Formation comprises a mixture of
illite, chlorite, and mixed layer chlorite-smectite clay (often
present as corrensite). Smectite is present, sometimes with
sepiolite in, and adjacent to, the Arden Sandstone
Formation. The dominant clay minerals in the upper part of
the Twyning Mudstone Formation and the Blue Anchor
Formation are illite with subordinate or minor chlorite.
Skerries in the Warwickshire and Worcestershire area
are usually dolomitic sandstones or siltstones with angular
to sub angular quartz grains which are often cemented with
dolomite. Quartz, gypsum and barycelestite cements are
rare (Old et al., 1987). The Weatheroak Sandstone, near
Redditch (Old et al., 1991) is a pale, flaggy sandstone,
typically cavernous, calcareous or felspathic with green
mudstone interbeds. Copper mineralisation is present at its
base. Secondary malachite predominates, mostly as grain
coatings. In particularly heavily mineralised specimens
cuprite and native copper inclusions, tenorite overgrowths,
chalcocite and hydrous copper silicate are also present. A
vuggy, fine-grained, felspathic sandstone above the
Weatheroak Sandstone is composed of angular to subrounded
quartz with some feldspar and is grain supported
with a well developed secondary silica cement. In this area
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the Arden Sandstone Formation is usually a dolomitic,
sometimes calcareous, quartz sandstone or siltstone. It may
be close-packed and contain some grain to grain pressure
welding and minor secondary quartz growth. Phosphate
bioclasts may also be present (Strong, 1983).
Skerries in the Blue Anchor Formation may be
composed of angular or sub-angular quartz in a micrite
matrix; or composed of silt-size, and less commonly, sandsize
quartz grains scattered in a uniform microspar matrix.
Some skerries are composed of bioclastic fragments up to
several mm in diameter in a micrite or microspar matrix
(Strong, 1979; 1983).
3.4.3 Cheshire Basin
The eight formations recognised within the Mercia
Mudstone Group in the Cheshire Basin (Evans et al., 1993;
Wilson, 1993) are the Tarporley Siltstone Formation,
Bollin Formation, Northwich Halite Formation, Byley
Mudstone Formation, Wych Mudstone Formation,
Wilkesley Halite Formation, Brooks Mill Mudstone
Formation and the Blue Anchor Formation. They were
described by Wilson (1993) and the mineralogy, petrology
and diagenesis were studied by Milodowski et al., (1994).
TARPORLEY SILTSTONE FORMATION
Tarporley Siltstone Formation (formerly known as the
Keuper Waterstones) is the basal formation of the Mercia
Mudstone Group. The Helsby Sandstone Formation, the
highest formation of the Sherwood Sandstone passes gradationally
into, and in part is laterally equivalent to, the
Tarporley Siltstone. This represents a transition from the
dominantly aeolian and fluvial quartz sandstones of the
Sherwood Sandstone to inter-tidal deposits at the base of
the Mercia Mudstone Group. Typically the Tarporley
Siltstone Formation comprises interbedded siltstone,
reddish brown and greenish grey mudstone and thin, fineto
medium-grained sandstone. In some areas the coarser
sandstone and siltstone facies are more typical of the
Helsby Sandstone Formation. In the Malpas area a major
part of the Tarporley Siltstone passes into a dominantly
sandstone facies, the Malpas Sandstone. The rocks
comprise largely detrital, angular, quartz silt and clay
minerals with minor amounts of detrital K-feldspar, albite,
mica (muscovite and biotite), chert fragments, and
titanium-iron oxides. Minor amounts of sub-micron-sized
iron oxide are present in the red-brown facies but are
largely absent in the grey or green facies. Detrital heavy
minerals are present mainly at the base of siltstone
laminae and interbeds, and include ilmenite, zircon, rutile,
magnetite, tourmaline, apatite, monazite, xenotime and
chromite. The clay minerals are dominated by illite with
variable proportions of smectite and chlorite. However,
corrensite was dominant with minor illite in a sample from
the Bridge Quarry, Grinshill (NGR SJ 523 238)
(Milodowski et al., 1994). The petrography of the
Tarporley Siltstone Formation shows that many of these
sediments comprise well defined, small, complex, fining
upwards cycles rather than discrete siltstone and mudstone
laminae.
The Malpas Sandstone contains well-rounded low grade,
metamorphic silty mudstone grains composed of illite,
quartz and chlorite. This assemblage is typical of the
Lower Palaeozoic basement rocks from the Midlands or
Wales.