Canonbie Bridge Sandstone Formation
Canonbie Bridge Sandstone Formation is part of the Warwickshire Group
Interbedded fine- to coarse-grained reddish brown to greenish grey, moderately- to poorly sorted sandstones (50–70 per cent) and reddish brown mudstones and reddish brown, well-drained palaeosols including calcretes. Sandstones can be micaceous and are typically thick (10–30 m) forming sharp to erosively based channel sandstones in multistorey successions, cross-bedded and some show upwards-fining. Intraformational mudstone conglomerates occur scattered throughout the sandstones but are more common at channel bases. In hand specimen the sandstones contain a noticeable component of greenish grey grains, which, in thin section, can be seen to be lithic clasts. This high lithic component gives the sandstones a characteristic gamma ray log signature, with GR values ranging from 56–120 API (mean 97.3, median 99.53 API).
Deposition generally on a well-drained alluvial plain with large fluvial systems and rare ephemeral floodplain lakes. Sporadic periods of poorer drainage.
The formation outcrops at its type locality along the River Esk for 250 m south of Canonbie Bridge, for example at (NY 39551 76501), and in discontinuous exposures for about 600 m to the north of the bridge, for example at (NY 39502 76735) and (NY 39317 76792). A reference section is the Forge Diamond Bore (BGS Registration Number NY37NE/7) (NY 39456 76720), which cored the lower part of the formation from about 2.2 to 72.8 m depth.
Lower and upper boundaries
The base of the formation is taken at the base of the lowermost sandstone bed in a 20–30 m thick multistorey sandstone complex resting on the interbedded mainly mudstone and sandstone of the Eskbank Wood Formation (Figure 8, Column 10). The abrupt junction can be examined at outcrop at (NY 39194 76953) and has also been proven in numerous boreholes, including those at Becklees (BGS Registration Number NY37SE/3) (NY 35166 71578) (cored depth: 653 m; geophysical log depth: 656 m), Broadmeadows (BGS Registration Number NY37NE/15) (NY 37646 76265) (geophysical log depth: 176.6 m), and Glenzierfoot (BGS Registration Number NY37SE/2) (NY 36514 74275) (geophysical log depth: 368.2 m). In the Forge Diamond Bore (see above) the base is taken at a 0.6 m-thick conglomerate bed at 72.8 m depth.
At outcrop (NY 39360 76310) and (NY 39812 75760) the upper boundary is taken at the abrupt junction between the fine- to coarse-grained reddish brown lithic sandstones of the Canonbie Bridge Sandstone Formation and the overlying softer, orange brown, fine- to medium-grained sandstones that lack appreciable lithic grains (Becklees Sandstone Formation) (Figure 8, Column 10). In uncored boreholes the boundary is marked by a shift in the gamma ray log to lower values, reflecting the change in sandstone composition from lithic rich (higher GR values) to lithic poor, ‘cleaner’ sandstones with lower GR values. This can be seen for example at 485.3 m and 214 m depth in the Becklees and Glenzierfoot boreholes (see above) respectively.
The formation varies in thickness. It is 131 m and 154 m thick in the Broadmeadows and Glenzierfoot boreholes (see above) respectively. It has a maximum thickness of 168 m in the Becklees Borehole (see above).
Distribution and regional correlation
The formation is restricted to part of the Canonbie Coalfield between the Evertown (NY 3639 7594) and Rowanburn (NY 41140 7687) areas. It is also known in its subsurface south-western extension, where it has been proven for at least a distance of approximately 6 km.
Asturian (Westphalian D).
- Jones, N S, and Holliday, D W.2006.The stratigraphy and sedimentology of Upper Carboniferous Warwickshire Group red-bed facies in the Canonbie area of S.W. Scotland.British Geological Survey Internal Report, IR/06/043.
- Jones, N S, Holliday, D W, and McKervey, J A.In press.Warwickshire Group (Pennsylvanian) red-beds of the Canonbie coalfield, England–Scotland border, and their regional palaeogeographical implications.Geological Magazine.