Lithostratigraphical framework for Carboniferous successions of ...

There are similarities between a number of formations
from the south wales and Bristol regions, which may,
following further investigation, be recognised as single
laterally extensive formations.
7.2.1 Black Rock Limestone Subgroup
Formations within the Black Rock Limestone subgroup
have been identified in areas of recent mapping in the vale of
Glamorgan; namely the Barry Harbour Limestone, Brofiscin
Oolite and Friars point Limestone formations (waters and
Lawrence, 1987; wilson et al., 1990) (Figure 8).
in the Bristol region the subgroup was formerly subdivided
into Black Rock Limestone and Black Rock dolomite
formations. The dolostone generally occurs within the upper
part of the subgroup over the Bristol–mendips area, but is
present throughout the succession at Chepstow and the
Forest of dean. These formation names are now considered
obsolete as they represent non-dolomitised and dolomitised
limestone, respectively, rather than distinguished by primary
lithological variations. Further studies are necessary to
determine if component formations of the subgroup can be
identified in the Bristol region.
in pembrokeshire, the Black Rock Limestone subgroup
is overlain by the laterally impersistent Berry slade
Formation, which is of reef facies and is excluded from the
subgroup. in the Gower, the subgroup replaces the obsolete
penmaen Burrows Limestone Group, which comprised a
lower shipway Limestone, Tears point Limestone and upper
Langland dolomite (George et al., 1976). Neither area has
been mapped recently, and the relationship of the stratigraphy
with that defined for the Black Rock Limestone subgroup of
the vale of Glamorgan is in need of further research.
The subgroup shows a prominent southwards thickening,
to a maximum of 500 m in the vale of Glamorgan (waters
and Lawrence, 1987; wilson et al., 1990), and more than
250 m in the mendip Hills. at Chepstow and the Forest of
dean the subgroup is 90 to 120 m thick. Component
formations also thicken southward, with the exception of the
Brofiscin Oolite, which thins southwards and is absent in the
southern part of the Cardiff area (waters and Lawrence,
1987; wilson et al., 1990).
The base of the pembroke Limestone Group in the vale
of Glamorgan is sharp and conformable upon the mudstone
of the avon Group. in the Bristol region the base of the
subgroup is gradational and is marked by upward increase in
the proportion of limestone.
The Black Rock Limestone subgroup comprises dark
grey to black, poorly sorted crinoidal limestone, locally with
chert bands and nodules. The subgroup includes the middle
Hope volcanic member, a succession of up to 37 m of tuffs,
and pillow basalts present in the weston area of Bristol
(whittaker and Green, 1983; waters, 2003).
The Barry Harbour Limestone Formation comprises thinbedded,
dark grey, fine- to coarse-grained, crinoidal, skeletal
packstone with thin beds of shaly, calcareous mudstone and
common replacive chert lenses (waters and Lawrence, 1987;
wilson et al., 1990). intercalated beds of ooidal grainstone
occur locally towards the top of the formation. The Brofiscin
Oolite Formation comprises pale to dark grey, massive to
thick-bedded, well-sorted, predominantly ooidal and in part
skeletal grainstone, locally extensively dolomitised. The
Friars point Limestone Formation comprises thickly bedded,
dark grey to black, fine-grained, skeletal packstones with
subordinate thin interbeds of shaly, argillaceous, skeletal
packstone. The limestones are richly fossiliferous,
particularly crinoidal. Bioturbation and pressure solution
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gives the limestone an irregular nodular appearance (wilson
et al., 1990). The upper part of the formation is everywhere
dolomitised.
The Barry Harbour Limestone represents high-energy
storm deposits. The Brofiscin Oolite Formation formed as
oolite shoals in relatively shallow marine waters. The Friars
point Limestone Formation records renewed transgression
and re-establishment of deeper water shelf conditions
(wilson et al., 1990).
The subgroup contains a rich fauna of conodonts, corals
and foraminifera, and ranges from Tournaisian to Chadian
age. The Tournaisian–visean boundary is believed to occur
within the Friars point Limestone (waters and Lawrence,
1987).
7.2.2 Clydach Valley Subgroup
The Clydach valley subgroup of the north crop of the south
wales Coalfield is of equivalent age to the Black Rock
Limestone subgroup, but is of distinctive lithofacies and is
recognised as a distinct subgroup. The transition between
the two subgroups, present within the east crop of south
wales, is obscured by extensive dolomitisation and the
Namurian overstep. The interdigitating oolites and
dolostones of the Clydach valley subgroup have been
identified as informal formations, described by Barclay et
al. (1989). in upward succession, these are: the sychnant
dolomite, pwll-y-Cwm Oolite, pantydarren, Blaen Onnen
Oolite, Coed Ffyddlwn and Gilwern Oolite formations.
Towards the east of abergavenny the succession becomes
entirely dolomitic. To the west, in the merthyr Tydfil district,
the succession is entirely oolitic, forming the abercriban
Oolite subgroup.
The Clydach valley subgroup rests conformably upon
strata of the avon Group. about 45 m thickness is recorded
in the abergavenny area (Barclay et al., 1989). The subgroup
was deposited within peritidal and oolitic shoal environments
with numerous palaeosols.
The Blaen Onnen Oolite Formation contains a conodont
assemblage indicative of the Pseudopolygnathus
multistriatus Conodont Zone, of Tournaisian age (Barclay et
al., 1989). The Gilwern Oolite Formation has in the past
been attributed to Chadian and arundian age, though a
solely Chadian age is now generally accepted.
7.2.3 Chadian and Arundian succession
The Burrington Oolite subgroup is used in the mendip Hills,
where a monotonous ooidal limestone-dominated succession
could not be subdivided into component formations. in the
central and eastern parts of the mendip Hills, the subgroup
is underlain by, and passes into, a distinctively crinoidal,
non-oolitic facies, mapped as the vallis Limestone
Formation.
in the weston-super-mare area the Chadian and arundian
succession has been subdivided (Figure 8) into the Gully
Oolite, Caswell Bay mudstone, High Tor (formerly
Birnbeck) Limestone and Goblin Combe Oolite formations
(whittaker and Green, 1983). The relationship between the
Caswell Bay mudstone Formation and the Clifton down
mudstone Formation of the Bristol district is in need of
further research. it is probable that they are laterally
equivalent, though the latter is thicker and with a greater
chronostratigraphical range (late Chadian to arundian). The
High Tor Limestone and Goblin Combe Oolite formations
are absent north of the Bristol area, where they pass laterally
into the Clifton down mudstone Formation.