Page 42 - An Atlas of Carboniferous Basin Evolution in Northern England

Carboniferous basin development 23

Evans & Kirby 1999). These facies and thickness variations indicate a southern
fault-bounded basin margin situated at depth below the inversion-related
Pendle Monocline (Fig. 18), with carbonate platform margins developed in the
Chadian, Arundian and Asbian/Brigantian (Evans & Kirby 1999). The
northern margin of the basin is marked by facies and thickness changes on
to the Bowland High (Gawthorpe 1987a; Lawrence et al 1987; Arthurton et al.
1988) and the Askrigg Block (Tiddeman 1889; Hudson 1930).
Within the basin, basement has not been penetrated by any borehole to date,
the oldest proven sediments being of Courceyan age (EClb). These were
penetrated by the Swinden borehole and comprise sub-wavebase, argillaceous
packstones deposited on the distal portion of a carbonate ramp (Gawthorpe et
al. 1989). This general style of sedimentation continued into the Chadian (EC2;
Chatburn and Thornton Limestones). Thickness and facies variations indicate
some degree of structural control which was inherited from earlier late
Devonian-Courceyan (ECl) rifting. Waulsortian carbonate buildups are often
associated with the flanks of intra-basinal fault blocks (Miller & Grayson 1982;
Lees & Miller 1985) that formed the sites of major Variscan inversion
structures (e.g. Clitheroe, Hetton-Eshton and Slaidburn anticlines; Arthurton
1984; Gawthorpe 1987a; Arthurton et al 1988).
The late Chadian-Holkerian (EC3) was marked by the rapid development of
sea-floor topography, indicated by thickness and facies differentiation across
the basin (Gawthorpe 1987a). This topography was associated with reactiva-
tion of extensional faults and the development of local footwall unconformities
within the basin. The succession in the basin became dominated by mudstones
(Worston Shales) with local developments of siliciclastic turbidites and
sedimentary slides (Gawthorpe & Clemmey 1985). In addition Pb-Zn
mineralization is associated with these events (Gawthorpe et al. 1989). The
succeeding Holkerian to early Asbian (EC4) shows a progressive increase in
carbonate sedimentation, culminating in the development of carbonate-
rimmed shelves along the northern margin of the basin.
A further phase of tectonism occurred during the late Asbian to early
Brigantian (EC5) (Gawthorpe 1986). This phase, which comprised several
events, is characterized by major units of resedimented carbonate conglom-
erate, the common occurrence of soft sediment deformation features and major
facies and thickness variations within the basin and across the northern basin
margin. Background sedimentation became dominated by deep marine
mudstone (Bowland Shales) with influxes of coarse grained siliciclastic
turbidites (mid-Brigantian Pendleside Sandstone). Facies distributions and
thickness variations indicate southerly downthrow on the Middle Craven
Fault. However, slip reversal along this fault in the Brigantian may be
indicated by uplift and erosion of carbonate buildups in the hanging-wall
(Mundy 1980).
Namurian and Westphalian strata are some 2 km thick in the syncline that
separates the Pendle Monocline from the Rossendale High (Lancashire
Coalfield), and about 1500 m in the Holme Chapel and Boulsworth boreholes
(Fig. 18). There is no direct evidence for original thicknesses over the central
part of the basin, but they are likely to have been in excess of the preserved
2km. Pendleian facies are typical Millstone Grits and Coal Measures,
pervasive over this part of the central Pennine Basin. However, the initial
clastic infill of delta-front turbidites (the Pendleian aged Pendle Grit; LC1 a) is
much older than in the East Midlands (e.g. Widmerpool Gulf).

Fig. 21. Composite 2D seismic line and interpreted geological cross-section across the
Stainmore Trough (see Fig. 10 for location of the seismic).