Lithostratigraphical framework for Carboniferous successions of ...

5.6 MILLSTONE GRIT GROUP
The millstone Grit Group (Millstone Grit facies), is limited
within the Northern england province to the askrigg Block
(Figure 6).
The group is about 500 m thick, significantly thinner on
the askrigg Block than the equivalent strata present within
the Central pennine Basin to the south.
The base of the millstone Grit Group is marked by a
slightly angular intra-pendleian (e 1c ) unconformity. The
presence of a mid-Carboniferous unconformity is indicated
by the absence of alportian strata across the askrigg Block
(Ramsbottom, 1977).
The group is typified by cyclic successions of quartzfeldspathic
sandstone, grey mudstone, thin coal and
prominent seatearths. ammonoid-bearing marine mudstones
are not common. marine bands of mostly Kinderscoutian
age comprise near-shore calcareous sandstones with a
benthic fauna of brachiopods, crinoids and bryozoans, but
lacking ammonoids.
The sandstones are typically fluviatile, with distributary
channel palaeoflows to the south or south-west. some
seatearths may result from prolonged periods of
emergence.
The group ranges from pendleian to Yeadonian age.
5.7 PENNINE COAL MEASURES GROUP
The pennine Coal measures Group (Coal Measures facies)
extends from the wales–Brabant High northwards to the
southern uplands. The group has been given the epithet of
‘pennine’ to distinguish these coal measures from those
present in the midland valley of scotland, and from those
south of the wales–Brabant High.
The group is subdivided into pennine Lower, middle and
upper Coal measures using the same marker marine bands
used for the remainder of the pennine Basin. The main
description of the group is provided in the section for Central
england and wales, where this group is most extensively
developed. variations relevant to northern england are
discussed here.
The pennine Coal measures Group in the main area of
the Northumberland–durham coalfield is up to 900 m thick.
in west Cumbria, the group is between 300 and 400 m thick,
thickening offshore to the north-west into the solway Basin.
in the Canonbie Coalfield, the group is about 1000 m thick,
with a westward thinning and overstep of the Coal measures
onto a structural high. about 800 m of this succession
belongs to the pennine upper Coal measures.
The base of the group generally rests conformably upon
the Yoredale Group. However, in the Canonbie Coalfield
there is an unconformity at the base of the group. in the
north-west of this coalfield, the pennine upper Coal
measures also rest unconformably upon the pennine middle
Coal measures. Over much of the west Cumbria Coalfield,
strata from the upper Similis-pulcra Bivalve Zone appear to
be cut out below the unconformable base of the whitehaven
sandstone.
marine bands are dominated by foraminifera, Lingula
sp., fish remains and shallow marine benthic productid
fauna. in the Northumberland–durham Coalfield, the
Millstone Grit facies with sheet-like coarse-grained
sandstones (including the ‘Third Grit’), common marine
bands and thin coals persists into the Langsettian. The
pennine upper Coal measures of the Canonbie Coalfield is
typically reddened secondarily.
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The pennine Coal measures Group of the Northern
england province typically range from Langsettian to
Bolsovian. The pennine upper Coal measures of the
Canonbie Coalfield include fauna indicative of the
Anthraconauta phillipsii Zone, Bolsovian in age.
5.8 WARWICKSHIRE GROUP
The warwickshire Group, of Red-bed subfacies, of the
southern and central pennine Basin (powell et al., 2000) is
here considered to extend to the northern parts of the basin.
a single whitehaven sandstone Formation is recognised in
north and west Cumbria. in the Canonbie Coalfield, a
broadly upward-coarsening, primary red-bed succession is
now considered to represent the warwickshire Group and
three new formations have been proposed, in ascending
order: eskbank wood, Canonbie, Bridge sandstone and
Beckless sandstone formations.
The whitehaven sandstone Formation is at least 300 m
thick, whereas in the Canonbie Coalfield the group is up to
500 m thick.
The whitehaven sandstone Formation unconformably
overlies the pennine Coal measures Group and comprises a
red-bed succession. The lower 100 m consists of red to deep
purple or purplish brown, cross-bedded, micaceous, medium-
to coarse-grained sandstone, known as the whitehaven
sandstone (akhurst et al., 1997). There are interbeds of pink
to red or grey mudstone and siltstone and thin palaeosols are
present locally. The sandstones are overlain by a heterogeneous,
dominantly red succession of mudstone, sandstone and marl
with thin coals and limestones with Spirorbis sp.
The lower part of the whitehaven sandstone Formation is
interpreted as deposits from a major braided river system that
flowed from the north-east. The upper part of the formation
represents deposition in interdistributary bay or lacustrine
environments with minor river channels (akhurst et al., 1997).
The reddening was either primary or early diagenetic.
plant remains and the presence of the zonal nonmarine
bivalve Anthraconauta phillipsii (eastwood et al., 1931)
indicate a late Bolsovian to early westphalian d age for the
whitehaven sandstone Formation. To the north-east, similar
strata have a westphalian d A.tenuis Zone fauna (eastwood
et al., 1968), with the red beds of the Canonbie Coalfield of
comparable age.
5.9 CORRELATION WITH THE NORTHERN
IRISH SEA
The northern irish sea includes the peel–solway Firth Trough
(the offshore continuation of the Northumberland Trough)
and the manx massif and Ramsey–whitehaven Ridge (the
offshore continuation of the Lake district High). Carboniferous
strata subcrop beneath Quaternary deposits in the western
part of the region and beneath a thick permo-Triassic cover in
the east. The offshore information is detailed by Jackson et al.
(1995), and the lithostratigraphical scheme of Jackson and
Johnson (1996) identifies three groups for the Carboniferous:
the Garwood, Bisat and Kidston. These groups, broadly
equate with the chronostratigraphical subdivisions of
dinantian, Namurian and westphalian–stephanian.
5.9.1 Garwood Group
The Garwood Group is present in the east irish sea as two
distinct facies. a Platform and ramp carbonate facies