Lithostratigraphical framework for Carboniferous successions of ...

7 south wales–Bristol
The area south of the wales–Brabant High developed a
succession similar to that seen north of the high. The earliest
Carboniferous strata are the avon Group, a mudstonedominated
shelf succession. This is overlain by a carbonate
succession, the pembroke Limestone Group, which shows
an evolution from ramp to platform facies during the
Tournaisian–visean. The platform carbonates are overlain
by latest visean to early Namurian, dominantly basinal
shales and cherts. The marros Group, of Namurian age,
represents incursions of mainly quartzose siliciclastic
sediments derived from the wales–Brabant High to the
north. The overlying south wales Coal measures Group, of
westphalian age, comprises fluviodeltaic facies comparable
to pennine Coal measures present to the north of the wales–
Brabant High. southerly derived sandstones of the pennant
sandstone Formation, part of the Bolsovian to stephanian
warwickshire Group, overlie the south wales Coal measures
Group in the south wales Basin.
7.1 AVON GROUP
The avon Group is a new name proposed to replace the
obsolete terms Lower Limestone shale Group and Cefn
Bryn shales. The avon Group is named from the type area
of the avon Gorge, south of Bristol, but also extends across
south wales. The mudstone-dominated succession is
considered to be sufficiently distinct from the overlying
platform and ramp carbonates of the pembroke Limestone
Group to be recognised as a separate group.
existing mapping allows only an undifferentiated group to
be identified over large areas of the south wales and Bristol
region, for example in pembrokeshire, Gower, monmouth
and Chepstow. However, recent mapping has allowed the
group to be subdivided in south wales into the Tongwynlais,
Castell Coch Limestone and Cwmyniscoy mudstone
formations (Figure 8) (waters and Lawrence, 1987; wilson et
al., 1990; davies et al., 1991). in the Bristol district, the avon
Group comprises a lower shirehampton Formation overlain
by undivided avon Group (formerly maesbury mudstone
Formation of Barton et al., 2002), although the extension of
this nomenclature southwards towards the mendip Hills is
uncertain and in need of further work.
The avon Group in south wales is up to 135 m thick. The
Tongwynlais Formation thins northward and is absent in the
north crop of the Carboniferous strata of south wales (Barclay
et al., 1989). in the Bristol region, the group is up to 150 m
thick within its type area. The shirehampton Formation is
about 30 to 50 m thick in the Bristol area, but thins southward
and passes into undivided avon Group in the mendip Hills.
The base of the avon Group is commonly sharp, but
conformable. in the north crop of the Carboniferous strata of
south wales, the Tongwynlais Formation is absent, with the
Castell Coch Limestone formation resting disconformably
upon the devonian upper Old Red sandstone Group.
The avon Group comprises dark grey or black mudstone
with interbedded crinoidal bioclastic or ooidal limestone.
The Tongwynlais Formation consists of interbedded thin
skeletal packstone limestone and grey mudstone, with
subordinate calcareous sandstone, oolite and hematitic
31
skeletal grainstones (Gayer et al., 1973; waters and
Lawrence, 1987). The Castell Coch Limestone Formation
comprises thick to well-bedded, commonly cross-bedded,
skeletal and ooidal grainstones, rich in crinoidal debris
(waters and Lawrence, 1987; wilson et al., 1990). The
formation shows secondary reddening, commonly associated
with dolomitisation. The Cwmyniscoy mudstone Formation
comprises dark grey, silty mudstone, sparsely fossiliferous,
with subordinate thin bioclastic limestone and calcareous
siltstone beds (waters and Lawrence, 1987; Barclay et al.,
1989; wilson et al., 1990). The mudstone beds are thinner
toward the gradational boundary with the overlying
pembroke Limestone Group.
The shirehampton Formation comprises coarse bioclastic
and ooidal limestone with subordinate calcareous sandstone
and mudstone that has a brackish water fauna with Lingula.
some interbeds of sandstone and red and green mudstone of
Old Red Sandstone facies occur locally. a reddened crinoidal
limestone, the ‘Bryozoa Bed’ occurs at the top of the
formation. The overlying undivided avon Group consists of
greenish grey mudstone with interbedded black crinoidal
limestone with a rich shelly fauna.
The group is interpreted as a mudstone-dominated
succession deposited within a shelf setting during a major
phase of northward-directed marine transgression. The
Castell Coch Limestone represents the first regional
shallowing event to affect the Tournaisian shelf, with
regional progradation of high-energy ooidal shoals.
The avon Group is entirely Tournaisian in age (davies et
al., 1991).
7.2 PEMBROKE LIMESTONE GROUP
The pembroke Limestone Group is a new name proposed
for the Platform and ramp carbonate facies of the south
wales and Bristol region. in the pembroke area, the group is
well exposed and shows a wide range of lithological
facies.
The Tournaisian to ?early Chadian succession of mid to
inner ramp/shelf bioclastics present across the Bristol region
and the south crop of the south wales Coalfield are
distinguished as the Black Rock Limestone subgroup,
formerly referred to as a group. within the north crop of
south wales a separate subgroup, the Clydach valley
subgroup, was deposited coevally in peritidal and oolitic
shoal environments with numerous palaeosols. in
pembrokeshire, the reef facies of the Berry slade Formation
is considered to be separate from both the subgroups.
during the Chadian to arundian no regional subgroup
has been established, although the Burrington Oolite
subgroup has been defined as restricted to the mendips.
during the latest arundian to Holkerian the Hunts Bay
Oolite subgroup, which has received local usage in the past,
is here extended across south wales (Figure 8).
The asbian to Brigantian succession has to date no
subgroup classification. However, the establishment of
marked cyclicity distinguishes this interval of strata from
underlying successions and may warrant establishment of a
subgroup.