A lithostratigraphical framework for the Carboniferous successions ...

distibution of facies suggests a ramp-like setting, with
darker offshore limestones deposited in the north and east
(of the Clwydian Range) and shallower, inner ramp facies
to the west (Vale of Clwyd). Subsequently, during the late
Arundian, these high energy, inner ramp limestones prograded
northwards and eastwards to establish a low-gradient
carbonate platform. During the early Asbian, these
shoal facies withdrew to the outer edge of the platform to
form a protective barrier to the peritidal facies of the Leete
Limestone Formation.
Biostratigraphical characterisation
A commonly rolled coral assemblage is indicative of an
Arundian age (Somerville and Strank, 1984b). The basal few
metres of the formation locally contain foraminiferal assemblages
indicative of the Cf4�2 Subzone (Davies et al., 1989).
The upper levels of the thicker northern sequence are considerably
younger, containing Cf5 Biozone and Cf6� Subzone
assemblages, which confirm the presence of Holkerian and
early Asbian strata, respectively (Davies et al., 2004).
Age
Holkerian to Early Asbian
4.6.2.1 llWyN-y-frâN saNDstoNe memBer (lyfs)
Name
The member was defined by Davies et al. (1989).
Type section
Alyn Valley Borehole (SJ15NE/8) [SJ 1889 5741] and
Pistyll Gwyn Quarry: base of member proved at 16.72 m
depth in the borehole; top of member located c.14 m above
the base of the quarry section (Davies et al., 2004).
Lithology
Calcareous sandstones and sandy limestones (grainstones),
locally cross-bedded and dolomitised.
Lower and upper boundaries
The base of the member is marked by the entry of calcareous
sandstone 13 m above the base of the Llanarmon
Limestone Formation in the Alyn Valley Borehole.
The top of the member is marked by the entry of dark
brown-grey, less sandy, crinoidal limestones (packstones)
of overlying parts of the Llanarmon Limestone Formation.
Thickness
Between 20 and 30 m
Distribution
Llanarmon-yn-Ial area [SJ 19 58], east of the Clwydian
Range, north-east Wales.
Genetic interpretation
Records high energy, near shore bar deposition during the
early stages of the Arundian transgression of North Wales,
at a time when siliciclastic detritus was still actively being
supplied to the developing carbonate ramp margin (Davies
et al., 2004).
Age
Arundian
4.6.2.2 NaNt-y-gamar limestoNe memBer (Nygl)
Name
The unit, described by (Nichols, 1965) and (Bancroft et al.,
1988) is here defined formally as a member.
49
Type section
Quarry at Nant-y-Gamar, Llandudno [SH 8012 8165]: section
in upper 35 m of the bryozoan-rich limestones of the
member (Bancroft et al., 1988).
Lithology
Massive, bryozoan-rich reef limestone (bafflestone/framestone).
Lower and upper boundaries
The base of the member is not seen, but presumed to be a
contact between massive, bryozoan-rich, reef limestones
and underlying well bedded, extensively dolomitised limestones
(peloidal and skeletal grainstones) of the Llanarmon
Limestone Formation.
The top of the member is at the boundary between massive
‘reef’ limestones and well bedded, extensively dolomitised
limestones (peloidal and skeletal grainstones) of the
Llanarmon Limestone Formation.
Thickness
At least 35 m
Distribution
Nant-y-Gamar, Llandudno [SH 80 81]
Genetic interpretation
Early Asbian build-up of trepostome bryozoan-rich limestones
at the margin of North Wales Visean carbonate platform
(Bancroft et al., 1988).
Age
Early Asbian
4.6.3 Leete Limestone Formation (LEEL)
Name
Formerly known as the Lower Brown Limestone, the
Careg-Onen Limestone and Tandinas Limestone formations
in Anglesey (Davies, 1983), Dulas Limestone of Rhyll
(Warren et al., 1984), Ty-Nant Limestone of Llangollen
(Somerville, 1977) and Sychtyn Member of Oswestry
(Grey, 1981). The formation, named after Leete Country
Park [1985 6280] at Loggerheads, was first defined by
Somerville (Somerville, 1977, 1979), who included in it the
whole of the limestone sequence below the Loggerheads
Limestone. Following the recognition of earlier Arundian
formations, the base has subsequently been redefined by
Somerville and Strank (1984b) and again by Davies et
al. (1989). The unit was formally defined by Davies et al.
(2004).
Type section
Scarp east of River Alyn in Leete Country Park at
Loggerheads [SJ 1978 6284]: section through upper 65 m
of the formation (Davies et al., 2004).
Lithology
Lithologically varied, comprising diagnostic porcellanous
and fenestral limestones (wackestones and calcite mudstones)
interbedded with dark, foetid, argillaceous limestones
(packstones), and pale skeletal peloidal limestones
(packstones and grainstones) including beds rich in oncoids.
Thin grey and green mudstone beds and rare coals are
present locally. Lithologies are arranged in shoalingupwards
rhythms each capped by a porcellaneous limestone.
British Geological Survey
Research Report RR/09/01