TWO CONTRASTING LACUSTRINE DELTA DEPOSITS AND THEIR EFFECTS ON COAL MINING IN A SURFACE MINE IN DERBYSHIRE, ENGLAND

Lodge House surface mine in the East Midlands Coalfield, England, exposed five coal seams of Upper Carboniferous (Pennsylvanian) age. At this time the site lay near the equator, north of the Wales-Brabant High, on the southern margin of the Pennine Basin, within a narrow, elongate seaway, which linked to the ocean to the east. Deposition of the section exposed in the mine occurred in an overall waterlogged fluvio-lacustrine environment but specific depositional environments for inter-seam intervals differ; two intervals are interpreted as lake deltas. The lower interval consists of a coarsening-up sequence of carbonaceous claystones, laminated siltstones, and interbedded sandstones and siltstones suggesting progradation of a small lake delta. The top part of the lower interval is cut by small channels with varied fills, interpreted as lake delta feeder channels. Early compaction of feeder channel fills caused the overlying seam to be locally thickened, forming depressions termed ‘swilleys’. Crevasse splay deposits above the swilleys also thicken, indicating compaction influenced sedimentation in the interval above. The upper lake delta interval coarsens up, from dark claystones and laminated siltstones to thin sandstones interbedded with siltstones indicating delta progradation. The coarser grained beds contain lenticular, wavy and flaser bedding, and straight crested ripples suggesting the lake was strongly influenced by wave activity. Interbedded sandstones pass laterally into feeder channels of carbonate-cemented, massive sandstones; both show soft sediment deformation. Soft sediment deformation and the massive nature of the channel fills indicates rapid sedimentation as hyperconcentrated sediment flows. The characteristics of both intervals have implications for mining. Changes in thickness and gradient associated with swilleys in the lower interval can cause problems when working coal underground by longwall faces, thus knowledge of their presence prior to mining is desirable. The hard, massive sandstone masses in the upper interval can be excavated in surface mines without blasting, but their high compressive strength, massive nature and high abrasivity are likely to cause difficulties when excavating such rock bodies underground.