Lithostratigraphical framework for Carboniferous successions of ...

1 introduction
The Carboniferous strata of the uK comprise a wide range
of facies and depositional environments. This, in part,
represents the northward drift of the uK across the equator
during the Carboniferous (scotese and mcKerrow, 1990).
Both the beginning and end of the Carboniferous period are
marked by a climate that was arid, at least seasonally. This
led to widespread development of red continental alluvial
clastic-dominated facies during the Tournaisian and late
westphalian to stephanian times. The intervening period
was dominated by an equatorial climate.
The diverse lithofacies that developed throughout the
Carboniferous were also the consequence of tectonic
processes. a phase of Late devonian to early Carboniferous
rifting produced a marked palaeorelief with numerous basins
occupying subsiding graben and half-graben and emergent
highs associated with horsts and tilt-block highs (Leeder,
1982). Cessation of most rifting processes throughout large
parts of Great Britain in the visean was followed by a period
of regional subsidence and the resulting basins were infilled
by widespread deposits.
1.1 TECTONIC SETTING
1.1.1 Tournaisian–Visean
southern Britain is postulated by Leeder (1982) to have
been located on the northern margin of a Rheno-Hercynian
back-arc basin during devonian time. in the Late devonian
a phase of north–south rifting started to affect all of central
and northern Britain, initiating development of a series of
graben and half-graben, separated by platforms and tiltblock
highs (Leeder, 1982, 1988). From north to south these
Carboniferous blocks and basins include the midland valley
of scotland, Northumberland Trough, alston Block,
stainmore Trough, askrigg Block and Craven Basin
(Figure 1). The block and basin margins commonly reflected
reactivation of pre-existing basement lineaments. in southwest
england four rift basins formed sequentially northwards
during the devonian, with the youngest and northernmost,
the Culm Basin, initiating during the late Famennian.
The midland valley of scotland is an east-north-easttrending
graben, about 700 km long and up to 100 km wide,
which was initiated during the Late devonian by the
reactivation of pre-existing east-north-east-trending
Caledonian structures. The graben was flanked to the northwest
by the eroded remains of the Caledonian mountains,
north of the Highland Boundary Fault, and to the south-east
by the southern uplands. The origin of the graben remains
controversial with interpretations that include:
• pure shear during east–west tension (Haszeldine, 1984)
• pure shear during Tournaisian to early Visean north–
south tension followed by thermal subsidence in the
late visean to westphalian (Leeder, 1982)
• Early Devonian crustal rifting followed by Late
devonian and Carboniferous thermal subsidence with
superimposed dextral strike-slip (dewey, 1982)
1
Recent developments suggest that the main influence was
sinistral fault movement during the Late devonian to early
Carboniferous and largely dextral thereafter (Browne et al.,
2003). depocentres within the graben subsided at different
rates over time, and their location and trend also changed
(Browne and monro, 1989). superimposed upon this are
marked thickness variations resulting from synsedimentary
movement on north-east- and east-trending faults in a strikeslip
regime throughout the Carboniferous. associated with
this were minor phases of compression, most notable during
the mid Carboniferous (Read, 1988).
The southern uplands separated the Northumberland
Trough, including the Tweed and solway Firth basins, from the
midland valley of scotland. However, this was breached by
narrow basins of north-north-west trend. The Northumberland
Trough was bounded to the north by the North solway Fault
and to the south by the stublick–Ninety Fathom Fault, both
active during deposition (Chadwick et al., 1995).
The alston Block lies to the south of the Northumberland
Trough. This horst is bounded to the south by the Closehouse–
Lunedale–swindale Beck faults, active during the
Tournaisian and visean. The stainmore Trough, a halfgraben
basin, lies farther to the south, with the southern
margin defined by the stockdale monocline. This structure
also marks a transition in sedimentation between the basin
and the tilt-block of the askrigg Block to the south. The
manx–Lake district High occurs to the west of the alston
and askrigg blocks, and separated by a broadly north–southtrending
strike-slip pennine–dent Fault system. The block
areas are associated with development of platform carbonates
during the Tournaisian and/or visean.
The Craven Fault system defines the southern margin of the
askrigg Block. To the south of this fault system the block and
basin structures persist, though generally the high subsidence
rates created a province dominated by basinal facies. The
Craven sub-basin of Lancashire and the Harrogate sub-basin of
Yorkshire represent the northern examples of a series of linked
narrow embayments. in the south of the basin, the Gainsborough
Trough and widmerpool Gulf are separated by platform
carbonate shelves such as the linked east midlands shelf and
derbyshire High, and the west midlands shelf and Central
pennine High. These shelf areas represent the northern margin
of the wales–Brabant High, which formed a persistent
topographical feature throughout the Carboniferous.
along the southern margin of the wales–Brabant High
there is a gradual change southward from shelf/ramp
carbonates into a deep marine succession of the North devon
Basin of north devon and Culm Basin of north Cornwall
and southern and central devon. The Culm Basin evolved as
a central graben flanked by half-graben sub-basins to the
north and south. By the Late devonian, continental collision
resulted in the commencement of Hercynian compressional
deformation. The deformation commenced with closure of
the most southerly Gramscatho Basin (Leveridge et al.,
1990); the deformation front migrated northwards during
the Tournaisian and visean (selwood and Thomas, 1988),
with progressive closure, inversion and deformation of the
Looe, south devon and Tavy basins.