A lithostratigraphical framework for the Carboniferous successions ...

Roosecote Borehole, Barrow-in-Furness (SD26NW/19)
[SD 2304 6866], includes an entire thickness of the formation
from 158.13 to 613.31 m depth (Rose and Dunham,
1977; Johnson et al., 2001).
Shellag Point Borehole, north Isle of Man [NX 4565
9965] cored from 100.60 m to 128.15 m, with neither the
base or top proved (Chadwick et al., 2001).
Formal subdivisions
The Ravensholme Limestone and Park Style Limestone
members are present at or near the base of the formation.
Sandstone-dominated turbidites are locally common,
including the Pendleside Sandstone Member. In the south
of the Isle of Man, the Scarlett Point and Scarlett Volcanic
members occur at the base and top of the formation, respectively
(described fully by Dean et al., in press).
Lithology
Mainly dark grey fissile and blocky mudstone, weakly
calcareous with subordinate sequences of interbedded limestone
and sandstone, fossiliferous in more-or-less discrete
bands.
In the Furness and Settle areas the formation comprises
thick-bedded, blocky to sub-fissile, dark grey and black,
organic-rich mudstone, with subordinate beds of dark grey
siltstone, sandstone and pale brown dolomitic limestone.
Marine bands are also present. The formation shows an
upwards decrease in carbonate turbidites and a concomitant
increase in siliciclastic sandstone turbidites (see Rose
and Dunham, 1977; Johnson et al., 2001; Arthurton et al.,
1988).
In the south Isle of Man, the Bowland Shale Formation
includes black claystone with localised deposition of carbonate
turbidites, debris flows, olistoliths, volcaniclastic
deposits and lavas (see Chadwick et al., 2001; Dickson et
al., 1987). At the base of the formation, the Scarlett Point
Member comprises cherty and pyritous tabular beds of pale
wackestone and lime mudstone (dolomitised in places),
which display gradational boundaries with interbedded
black, fissile, blocky claystone. The limestone is burrowed
and has inadunate crinoidal lags and scattered ammonoids.
At the top of the formation, the Scarlett Volcanic Member
is dominated by a series of volcaniclastic debris flows and
gravity slides. Claystone rafts and megaclasts are entrained
within the volcaniclastic rocks, and carbonate olistoliths
and pillow lavas also occur. Between the members, where
the Bowland Shale Formation oversteps the Balladoole
Formation (Great Scar Limestone Group), coarse-grained
detrital carbonates and debris beds are common. These
include erosively based, graded packstone beds, conglomerate,
megaclasts and large olistoliths (with reef limestone
and spectacularly preserved ammonoid faunas) derived
from the Balladoole Formation. The middle part of the
exposed part of the formation comprises black, calcareous,
platy claystone with subordinate beds (up to 2 m thick) of
dark wackestone, and dark detrital packstone debris.
In the north Isle of Man, the Shellag Point Borehole (see
below) cored through 27.55 m of siltstone, claystone and
ironstone. These included two marine bands.
Lower and upper boundaries
The conformable base upon the Pendleside Limestone
Formation in the Craven Basin, on the Widmerpool
Formation in the East Midlands, and the Pentre Chert and
Cefn-y-Fedw Sandstone Formation in North Wales, on the
Hodderense Limestone Formation in the south of the Isle
of Man, is taken at the first appearance of black mudstone
above variegated mudstones or fine-grained limestones.
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In south Cumbria the lower boundary of the formation is
taken at the base of the Cravenoceras leion Marine Band,
where the dark grey to black marine mudstone rests upon
thinly interbedded limestones, mudstones and subordinate
sandstones of the Alston Formation. The Bowland Shale
Formation onlaps onto, and eventually by Pendleian times
extends over, the carbonates of the Central Lancashire High
(Trawden Limestone Group). In the Craven Reef Belt, in
the Malham Cove–Gordale Scar area, the fissile mudstones
with nodules and thin beds of ironstone and limestone of
the Bowland Shale Formation, rest unconformably and
diachronously on the Malham Formation (Great Scar
Limestone Group), or Yoredale Group.
The top of the formation is taken at the base of the
Millstone Grit Group over most of the Pennine Basin and
at the base of the Morridge Formation in Staffordshire and
the East Midlands. The formation shows complex intertongueing
with the Morridge and Cefn-y-Fedw Sandstone
formations. It is seen as a conformable boundary defined
by the base of the lowermost thick feldspathic sandstone
of the Millstone Grit Group, or quartzitic sandstone of the
Morridge and Cefn-y-Fedw formations, above thick dark
grey mudstone of the Bowland Shale Formation.
Thickness
Generally between 120 m and 620 m thick. The formation
thickens north-eastwards along the axis of the Central
Lancashire High, from about 22 m in the Roddlesworth
Borehole (SD62SE/274), 68 m thick in the Holme Chapel
Borehole (SD82NE/69), and 102 m in the Boulsworth
Borehole (SD93SW/14) (Figure 11). The underlying
Trawden Limestone Group shows a thinning in the same
direction (Evans and Kirby, 1999), suggesting the thickening
of the Bowland Shale Formation reflects available
accommodation space.
In the Craven Reef Belt the Bowland Shale Formation
is perhaps 30 m to 200 m thick (see Arthurton et al., 1988,
figure 22). In south Cumbria, the Roosecote Borehole
(SD26NW/19) (see above) proved the formation to be
130 m thick (see Johnson et al., 2001; Rose and Dunham,
1977). On the Isle of Man, the Bowland Shale Formation
may be at least 186 m thick. In the north Isle of Man, the
Shellag Point Borehole [NX 4565 9965] proved only a part
of the formation 27.55 m thick.
Distribution
Widespread in the Craven Basin (the type area), including
Lancaster, Garstang, Settle, Clitheroe, Harrogate districts,
south Cumbria and the Isle of Man, but also in North Wales,
Staffordshire and the East Midlands.
Regional correlation
The upper part of the Bowland Shale Formation passes
northward into the Millstone Grit Group and to the south
into the Morridge Formation (Figure 10).
Genetic interpretation
The mudstones accumulated as hemipelagic deposits, predominantly
from suspension in moderately deep water
largely below the storm wave base. For much of the time the
water was brackish or fresh and occurred in the photic zone
(Collinson, 1988). Marine bands developed during periods
of higher salinity when connections with the open ocean
were established. The thin limestones and sandstones were
introduced by storms and/or as turbidites; the limestones
sourced from active carbonate shelves, the siliciclastic
sediments from active deltas accumulating on the margins
of the Central Pennine Basin. In the south of the Isle of
British Geological Survey
Research Report RR/09/01