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

chlorite-smectite is present as corrensite or more smectiterich
mixed clays. This contrasts with the Sneinton
Formation that contains no mixed layer clay minerals
(Bloodworth and Prior, 1993).
Near East Retford the arenaceous members of the
Sneinton Formation comprise well sorted, closely packed,
coarse silt- to fine-sand sized, sub-angular to angular
clastic grains (Smith et al., 1973). The quartz has
secondary overgrowths. Feldspars are also present and
show alteration with secondary overgrowths. The overgrowths
do not appear to form a coherent cement. Where
the rock is red, haematite form secondary coatings along
joints. The detrital heavy minerals include zircon, apatite,
tourmaline, rutile, garnet, leucoxene and anatase. The clay
mineral composition is typical of the Sneinton Formation
and comprises mainly illite with chlorite.
Between the Sneinton Formation and the Clarborough
Beds the rocks comprise red mudstones with bands of silty
micaceous mudstone that contain illite and minor chlorite.
The skerries are laminated, usually dolomitic and are
sometimes argillaceous and micaceous (Smith et al., 1973).
The Clarborough Beds consist of about 3 to 13 m of
mudstone and silty mudstones with much gypsum and
skerries. These beds are probably equivalent to the East
Bridgford Gypsum at the base of the Gunthorpe Formation.
The gypsum occurs as irregular bands and veins. The
skerries are variable in composition. Some are very porous,
greyish green, argillaceous, dolomitic siltstone with small
cavities and consist of well-sorted, angular, fine sand size,
quartz grains with plagioclase, orthoclase, mica and
dolomite of coarse and very fine silt size. However, they
can also be dense with fine sand-sized, roughly bedded
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aggregates of evenly sorted clastic quartz and feldspar
scattered in a coarsely crystalline, sometimes interlocking
gypsum matrix, with laminated fine-grained dolomite.
Other skerries contain silt-sized quartz with scattered
feldspars and mica with patches of intergranular fibrous
gypsum and streaks of dolomite (Smith et al., 1973).
Gypsum has been quarried from the Clarborough Beds in
the East Retford area.
The Gunthorpe Formation above the Clarborough Beds,
at Gringley-on-the-Hill, north Nottinghamshire, contains
angular to sub-rounded, equant, irregular silt- to sand-sized
quartz, slightly eroded potassium feldspar and calcite. Mica
flakes up to 0.1 mm across are common. The porosity
varies significantly from the silty areas to the more clayey
areas that enclose them. In the siltier areas, the clay matrix
and dolomite cement reduce the intergranular porosity.
Dolomite and calcite commonly occur as granular to
massive cements. Dolomite occasionally occurs as
euhedral rhombs up to 0.01 mm across and shows no
evidence of dissolution in the near-surface zone. The clay
matrix is dominated by illite with minor amounts of
chlorite and corrensite. Chlorite occasionally forms
discrete but irregular patches up to several tenths of a millimetre
across (Pearce et al., 1996).
The upper part of the red beds of the Mercia Mudstone
Group comprises red mudstones with green bands and
some skerries. Gypsum is common as bands and stringers.
The Blue Anchor Formation consists of light green, grey
or greyish green mudstone or silty mudstone and is calcareous
in some places. Grey, fine-grained bands of sandstone
are also present. Pale greyish green dolomitic limestone
has also been recorded.