Eskbank Wood Formation
Eskbank Wood Formation is part of the Warwickshire Group
Interbedded red mudstone (claystones and siltstones), fine- to medium-grained sandstones, calcrete palaeosols, thin beds of ‘Spirorbis’ limestone and Estheria-bearing mudstones. Sparse thin coals and grey mudstones are present in the lower part of the formation; some of these coals have been oxidised and altered to limestone (Jones and Holliday, 2006; Jones et al., in press; see also Mykura, 1960). Mudstones form 60–70 per cent of the formation, with sandstones forming most of the rest.
Deposition on an alluvial plain with lakes. The flood plain alternating from poorly to well drained.
The type area is Hush Pool, River Esk (NY 39149 77055 to 39194 76953). Approximately 75 m of nearly continuous outcrops of the formation are present along the eastern bank of the River Esk. Reference sections include the Forge Diamond Bore (BGS Registration Number NY37NE/7) (NY.39456 76720) from 72.8 to 235.5 m depth (the full thickness being cored), and the Becklees Borehole (BGS Registration Number NY37SE/3) (NY 35166 71578) from 653 to 816.8.m depth (the formation being cored from 744.41.m depth to its base).
Lower and upper boundaries
The base of the formation was not identified at outcrop and is defined in boreholes. The boundary is taken at the first major red-bed strata overlying the grey mudstone-dominated Pennine Upper Coal Measures Formation (Figure 8, Column 10). It is a conformable, gradational boundary. In the Becklees Borehole (see above) the base of the formation (at 816.8.m depth) is defined by the last significant downhole occurrence of primary red-bed lithologies. In this instance it comprises 1.3.m of mottled reddish brown and greenish grey silty mudstone with abundant carbonate nodules. These nodules are interpreted as pedogenic calcrete glaebules and represent the first good evidence for the development of primary red-bed conditions. A limited number of red lithologies are known below 816.8 m depth, but it is not known whether these represent primary red-beds or could be linked to a later secondary reddening event. The gradational boundary to the formation is well demonstrated by the reversion higher in the borehole to grey lithologies. This includes a prominent coal (the ‘High’ Coal), which is about 1 m in thickness and must represent a significant amount of time in which reducing conditions prevailed. In the Forge Diamond Bore (see above) the base is taken at the change from pale greenish mudstones to red and green mudstones at 235.5 m depth. The lower boundary is difficult to pick in uncored boreholes, but analysis of cored boreholes shows that close to the base of the formation the distinctive ‘High’ Coal is typically present, and can be identified from suitable geophysical logs (e.g. sonic, density). It is suggested that the position of this coal be taken as the base of the formation where core data are absent.
The top of the formation is taken at the base of the lowermost sandstone bed of the Canonbie Bridge Sandstone Formation (Figure 8, Column 10). This is typically a thick (20–30.m) multistorey sandstone complex. The abrupt junction can be examined at outcrop at (NY 39194 76953) and has also been proven in numerous boreholes, including the Becklees Borehole (see above) (cored depth: 653 m; geophysical log depth: 656 m), and those at 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, Becklees and Glenzierfoot boreholes (see above) the formation is about 163 m, 164 m and 146 m thick respectively. In the Rowanburnfoot Bore (BGS Registration Number NY47NW/27) (NY 41031 75743) it is 175 m thick.
Distribution and regional correlation
The formation is restricted to part of the Canonbie Coalfield between the Evertown (NY 3639 7594) and the 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.
Age and biostratigraphical characterisation
Late Bolsovian (Westphalian C) to Asturian (Westphalian D). The section along the eastern bank of the River Esk north of Canonbie Bridge comprises some mudstones that contain a nonmarine fauna, particularly bivalves, at various points in the section. These have been identified as Anthraconauta phillipsii, A. aff. phillipsii, A. cf. tenuis, A. cf. wrightii, possibly Anthracomya pruvosti and also ostracods (A. E. Trueman in Barrett and Richey 1945, p. 39). Trueman suggested that this indicates the A. tenuis or even the A. prolifera Nonmarine Bivalve Zone of Trueman and Weir (1946), which are indicative of an Asturian (Westphalian D) age.
- 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.
- Mykura, W.1960.The replacement of coal by limestone and the reddening of Coal Measures in the Ayrshire coalfield.Bulletin of the Geological Survey of Great Britain, No. 16, 69–109.
- Barrett, B H, and Richey, J E.1945.Economic geology of Canonbie Coalfield (Dumfriesshire and Cumberland).Wartime Pamphlet of the Geological Survey of Great Britain, No. 42.
- Trueman, A E, and Weir, J.1946.A monograph of British Carboniferous nonmarine lamellibranchia.Palaeontographical Society, London, monograph.