Warwickshire Group, Carboniferous of the Tweed and Northumberland–Solway basins
|Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
In the Canonbie Coalfield, the Pennine Coal Measures Group is overlain by a red-bed succession that, together with other, similar upper Carboniferous sequences elsewhere in northern and central England, make up the Warwickshire Group (P912387). A key characteristic is reddening that occurred soon after sediment deposition, but which is not uniformly developed throughout; as a result, red beds alternate with unreddened, grey-green intervals. This phenomenon introduces local uncertainty as to the exact age of the Warwickshire Group strata and their stratigraphical relationship to apparently coeval, but unreddened rocks in the Pennine Coal Measures Group.
The Canonbie outcrop of the Warwickshire Group consists of interbedded mudstones, siltstones and sandstone similar to those of the underlying Pennine Coal Measures but mostly reddened; coal is rare. The group is late Bolsovian to Asturian in age. Its base at Canonbie is conformable, grading up from the grey mudstones of the Pennine Upper Coal Measures Formation into the overlying red beds. The sharp, unconformable base of the Permian succession defines the top of the group. At outcrop, about 290 m of Warwickshire Group red beds are exposed along the banks of the River Esk, but the maximum proved thickness is the 530 m seen in the Becklees Borehole, close to the central axis of the Solway Syncline. Seismic reflection data indicate that elsewhere in the centre of the syncline the group could be up to about 700 m thick.
Three formations have been recognised within the group, each with distinctive geophysical log signatures that allow them to be readily correlated in the subsurface. The lowermost, Eskbank Wood Formation (late Bolsovian to Asturian), ranges in thickness from 145 m to 175 m. Its base is gradational (and probably diachronous) across alternations of grey and primary red-bed strata but can conveniently be taken at the first appearance of pedogenic carbonate nodules just below the High Coal, which forms a prominent marker horizon (P912356). The formation comprises red mudstone and some sandstone, calcrete palaeosols, thin beds of Spirorbis limestone and Estheria-bearing mudstone. In the lower part there are a few thin coals and beds of grey mudstone, some of which contain nonmarine bivalves. The overlying Canonbie Bridge Sandstone Formation (Asturian) ranges in thickness from 131 m to 154 m. The base of the formation is sharp, marked by the incoming of thick units of medium- and coarse-grained, cross-bedded channel sandstone. A noticeable feature of these sandstones is their greenish grey colour, which can be related to the presence of abundant lithic grains. There are sporadic interbeds of mudstone and calcrete. The Becklees Sandstone Formation (Asturian) is the highest unit recognised from the Warwickshire Group of Canonbie and is overlain unconformably by Permian strata. Its full thickness is not known, but about 200 m is proved in the Becklees Borehole. Fine-grained sandstone with a distinct orange-brown colour is the dominant lithology, with some thin beds of mudstone and calcrete. Borehole cuttings suggest limestones and thin coals may also be present, though rare.
Warwickshire Group sedimentation in the Canonbie area largely took place on an alluvial plain drained by braided river systems, and was characterised by an early, primary oxidation of the strata. Palaeocurrent data from channels in the Canonbie Bridge Sandstone show that the rivers flowed towards the north. Overbank and floodplain mud was deposited between the channels, where soils were able to form during intervals of low aggradation. The ‘Spirorbis’ limestones of the Eskbank Wood Formation are most probably of lacustrine origin.
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