Lithostratigraphical framework for Carboniferous successions of ...

2 principal lithofacies
Nine major lithofacies have been identified for the
Carboniferous of Great Britain (Figure 3), and these names
are shown in italics in the text that follows. The main
lithologies, environment of deposition, distribution and age
range are described below.
2.1 CONTINENTAL AND PERITIDAL FACIES
This facies occurs as two commonly interdigitating subfacies.
Continental fluvial clastic (‘cornstone’) subfacies
Lithology purple-red conglomerate, sandstone and red
mudstone, with characteristic nodules and thin beds of
concretionary carbonate (calcrete).
depositional environment alluvial fan, fluvial channel
and floodplain overbank, deposited in a semi-arid climate.
Peritidal marine and evaporite (‘cementstone’) subfacies
Lithology Grey mudstone, siltstone and sandstone,
characterised by the presence of nodules and beds of ferroan
dolostone (‘cementstone’) and evaporites (mainly gypsum
and anhydrite).
depositional environment alluvial plain and
marginal marine flats subject to periodic desiccation and
fluctuating salinity, in a semi-arid climate.
distribution of both subfacies widespread across
northern Britain from the midland valley of scotland to
central england (Figure 2a), deposited from Late devonian
to visean times. The two subfacies are commonly found to
interdigitate. The continental fluvial clastic subfacies
commonly forms the first basin infill and extends onto horst
and tilt-block highs. The peritidal marine and evaporite
subfacies is generally limited to troughs associated with
graben and half-graben, for example the midland valley of
scotland, Northumberland Trough and stainmore Trough.
2.2 HETEROLITHIC CLASTIC AND NON-
MARINE CARBONATE FACIES
Lithology interbedded grey sandstone, siltstone,
mudstone and locally oil shale. Thin, subordinate beds of
lacustrine limestone and dolostone, seatearth, coal and
sideritic ironstone are present variably in different
formations.
depositional environment Fluviatile, deltaic and
lacustrine environments with relatively sparse marine
mudstone deposits commonly alternating, in thin cycles.
distribution principally present in the eastern part of
the midland valley of scotland, of visean age (Figure 2b),
passing laterally into dominantly volcanic rocks to the west.
also present as early visean strata in parts of the
Northumberland Trough.
2.3 OPEN MARINE, PLATFORM AND RAMP
CARBONATES
This facies occurs as two subfacies.
6
Shallow shelf platform carbonate subfacies
Lithology Blanket bioclastic carbonate with crinoid
banks, shelly or coral biostromes and algal (Girvanella)
bands. The facies includes karstic bedding surfaces, overlain
by thin bentonites, interpreted as emersion surfaces.
depositional environment Tropical shallow marine
environment.
Ramp carbonate subfacies
Lithology Calcareous mudstone with common dark
bituminous and bioclastic limestones, and locally the
presence of carbonate breccias. ‘Knoll reefs’ or ‘waulsortian
reefs’, now commonly known as mud-mounds form a
distinctive biogenically precipitated bioherm.
depositional environment Ramps generally develop
during the early stage of platform carbonate evolution. They
represent deposits upon gently inclined shallow marine
slopes that may be marginal to platforms.
distribution of both subfacies upon platforms and
ramps developed on horst-blocks and half-graben tilt-blocks
during the visean over the alston and askrigg blocks, fringing
the manx-Lake district High and on the north and south
margins of the wales–Brabant High (Figure 2b, c). isolated
platform carbonates also developed on the derbyshire High,
Holme High and Central Lancashire High within a broadly
basinal environment. The above strata are collectively named
the Carboniferous Limestone supergroup.
2.4 HEMIPELAGIC FACIES
Lithology dark grey to black mudstone, in part
calcareous with calcareous nodules (‘bullions’). Thin
sandstone and limestone beds are locally common; breccia
may be present.
depositional environment Quiet and relatively deep
basinal environment with minor influx of sand-rich turbidites
within prodelta environments, and carbonate-rich turbidites
on carbonate slopes. Breccias may represent proximal
turbidites or slump deposits.
distribution Basinal (‘gulf’) areas of central england and
north wales developed during the visean and Namurian (Figure
2b, c, d). also, this facies was present within the Culm Basin
during the Late devonian to Namurian (Figure 2a, b, c, d).
2.5 MIXED SHELF CARBONATE AND DELTAIC
(YOREDALE) FACIES
Lithology Typically upward-coarsening cycles of
limestone, mudstone, sandstone, seatearth or ganister and
coal.
depositional environment The limestone, mudstone
and some sandstone were deposited in marine environments.
The majority of the sandstones and coals (the upper part of
the cycles) were deposited as progradational lobate deltas.
distribution widespread across the midland valley of
scotland and northern england, in strata of visean to
Namurian age, as far south as the Craven Fault system
(Figure 2c).