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

3.4 REGIONAL MINERALOGICAL VARIATION
This section is limited to those areas for which mineralogically
information was available and does not include the
East Irish Sea Basin and north Cumbria.
3.4.1 South-west England and South Wales
MARGINAL DEPOSITS
The marginal deposits of the sedimentary basin in the southwest
comprise breccias, conglomerates and sandstones with
intercalations of finer material. They were largely derived
from local sources as scree and flash flood deposits and were
formerly named ‘dolomitic conglomerate’. The most highly
dolomitised breccias and conglomerates are buff, yellow or
orange-brown in colour, whereas the less dolomitised are red
and green or grey green. In areas where they lie on
Carboniferous Limestone the basal deposits may fill palaeocaves
in the limestone.
The marginal deposits in the Cardiff area of South Wales
comprise up to 35 m of coarse clasts with finer interbeds
and have been subdivided into continental and lacustrine
shore subfacies (Waters and Lawrence, 1988). The continental
subfacies comprises conglomerate, much of which is
derived from the Carboniferous Limestone, and sandstone
with local fine intercalations. These deposits interdigitate
with red mudstone and grade into the typical red mudstone
of the Mercia Mudstone Group. The distribution of the
deposits was controlled by the basement topography at the
time of their deposition.
The lacustrine shore deposits, to the west and south of
Cardiff, are of clastic or carbonate types. The clastic
sediments are reworked deposits of the lake shore and the
carbonate deposits are of limestone and associated evaporitic
beds. They can be seen at Sully Island and at Dinas
Powys. At the base of the Sully Island succession is a thin,
clastic shore zone deposit above which is a residual ferricrete
or perilittoral dolomite. Above this there is an evaporitic
dolomite comprising an array of zoned rhombic
crystals, up to 300 mm in length, and partially dolomitised
calcite spar. The rhombic dolomite may be ferroan and
haematite cement is common. Some parts of the bed have
suffered dissolution and fracturing and the cavities are
filled by ferroan calcite spar. The rhombic dolomite may be
replaced by haematite stringers or irregular laminae with a
residue of quartz grains.
The Evaporitic Dolomite Unit (Leslie, 1989) consists of
6 – 9.2 m of red dolomite, detrital clays (mainly illite) and
quartz with a haematite-rich cement. This unit comprises
four sections. Lower and upper sections are of laminated
and rippled dolomite. The dolomite contains between 52
and 53% calcium carbonate. Most of the sedimentary structures
have been lost due to recrystallisation. Above the
lower section there are about 3 m of dolomite containing
replaced evaporite nodules. Some nodules, originally
composed of calcite spar with abundant anhydrite inclusions,
have been partly dolomitised. Other nodules contain
either carbonate or quartz-replacement sulphate nodules.
Underlying the upper unit is about 2 m of carbonate that
precipitated at or just below the water table.
BASIN DEPOSITS
In the main part of the basin in south-west Britain red or
green, dolomitic or calcareous, mudstones, siltstones and
sandstones, typical of the Mercia Mudstone Group, were
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deposited. Evaporite deposits of gypsum and anhydrite
occur as nodules, beds or net-like deposits and, in the
centre of the basins, thick halite deposits such as the
Somerset Halite Formation were formed. Sandstones are
siliceous and/or dolomitic. In Gloucestershire and
Somerset the evaporites contain strontium sulphate
(celestite) and barium sulphate (barite). Very small
amounts of galena and zinc blende are present disseminated
in the evaporites or in small cavities.
Jeans (1978) described three mudstone facies separated
by two cycles of carbonate-sandstone-carbonate, the
‘Dunscombe Cycle’ and the ‘Weston Cycle’ between
Sidmouth and Branscombe and between Seaton and
Charton, in south Devon. However, these two cycles are
the same and probably equivalent to the Arden Sandstone
Member (Warrington et al., 1980). In the lowest mudstone,
the dolomite content ranged from 1% to 20% and was
dominant over calcite that was in the range 0% to 5%.
Above this mudstone is a cycle of carbonate-sandstonecarbonate.
Within the carbonate facies the relative importance
of the two carbonates varied, sometimes dominated
by calcite but most of the ‘cycle’ was dolomite-rich (up to
50% dolomite). The sandstone group also contained more
dolomite than calcite. The upper mudstone, usually,
contained more calcite than dolomite particularly near the
top. Dolomite was the dominant carbonate of the Blue
Anchor Formation, of which it formed up to 50% in some
beds.
Illite and chlorite dominated the clay mineralogy of the
lower mudstone. Higher up the succession there was a
change to a greater proportion of the mixed layer illitesmectite
in south Devon and to chlorite-smectite in other
areas. Higher up and into the carbonate-sandstonecarbonate
cycle the clay mineral assemblage consisted of
illite, chlorite, mixed layer illite-smectite, smectite and
sepiolite. The more complex assemblage was also present
in the lower part of upper mudstone. However, higher up
and into the Blue Anchor Formation the clay assemblage
Present usually in minor quantities but sporadically distributed
Minor constituent: Clay minerals ~30% of clay particles
Major or minor constituent
Mineral currently exploited
Key to Figure 3.3 (opposite) The distribution of clay
minerals in the Mercia Mudstone Group.