Lacustrine delta deposits and their effects on coal mining in a surface mine in Derbyshire, England

Five late Carboniferous (Pennsylvanian) coal seams and associated inter-seam strata have been examined at Lodge House surface mine in the East Midlands Coalfield of England. Deposition of these coal seams and their accompanying inter-seam intervals occurred in an equatorial fluvio-lacustrine environment in an area approximately halfway between the depocentre and the southern margins of the Pennine Basin. Controls on the onset of inter-seam sedimentation and cessation of peat formation appear to have operated at two scales. Fourth order glacio-eustatic rises exerted the dominant control over formation of peats in laterally extensive mires. However, fifth order and smaller scale autocyclic mechanisms (specifically avulsion) are believed to have accompanied splitting of other seams close to, but beyond the confines of the mine.A study of contrasting lacustrine delta deposits occupying two of the inter-seam intervals has enabled proximal, distal, wave-dominated and delta distributary channel sub-facies to be recognised. The lacustrine deltas prograded rapidly into shallow, but extensive, freshwater lakes and both delta lobe switching and avulsion of delta distributary channels were important mechanisms. The lower lacustrine delta sequence is characterised by mud-filled delta distributary channels near the top of the interval. The upper sequence is dominated by wave-formed structures and massive sandstone delta distributary channels, which formed during a period of episodic floods and strong winds.Lacustrine delta deposits generally create good mining conditions, but delta distributary channels can present difficulties in underground mines. Mud-filled delta distributary channels subject to early compaction can give rise to ‘swilleys’ (elongate depressions) resulting in thickness changes and undulation of the overlying seam. When filled with strong, well-cemented, abrasive sandstone, delta distributary channel deposits can be hard to excavate. At Lodge House, the narrow width of the delta distributary channels meant that they were not detected by drilling prior to mining. Thus outcrop studies should be used to complement subsurface data as they can significantly enhance our understanding of small-scale features.

► Describes Pennsylvanian lacustrine deltas in East Midlands Coalfield, England ► Describes lacustrine delta sub-facies from strata between coal seams ► Discusses allocyclic versus autocyclic controls on coal seams and inter-seam strata ► Discusses effects of ‘swilleys’ and hard sandstone masses on underground mining