|
Exploration and Evaluation Techniques |
1 Geology and Cartography Division, Oxford Brookes University, Oxford OX3 0BP, UK
2 British Coal, Technical Services and Research Executive, Stanhope Bretby, Burton-upon-Trent, DE15 0QD, UK
3 Golder Associates (UK) Ltd, Landmere Lane, Edwalton, Nottingham NG12 4DG, UK
4 British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK
Important coal deposits are present in Britain north of the Wales-Brabant High in the Pennine Basin, which was initiated by late Devonian to early Carboniferous extension. The formation of coals of sufficient thickness, quality and continuity to be commercially exploited was favoured during the latest Westphalian A (Langsettian) and much of the Westphalian B (Duckmantian), when deposition took place in an environment similar to an upper delta plain, with limited marine influence. During the early part of the Westphalian A, marine influence was important, with the consequence that the coals were thinner, less persistent and of inferior quality. By the late Westphalian, alluvial deposition became important, and coals of commercial quality are rare.
Accurate determination of facies, lithological attributes and geometry is an important tool in exploration and mine planning. Sedimentary facies have thus been rationalized into those that may be consistently recognizable in boreholes and mine workings. However, facies may be further subdivided when conditions allow, and intermediate facies exist, reflecting the complexities of depositional environments. The characteristics of the following facies are described here: mire, palaeosol, marine, lacustrine, lacustrine delta, major channel, minor channel, overbank and crevasse splay. Channel deposits have a number of adverse effects on mining and the range of channel-fill deposits reflects the operation of many different processes.
A model of sedimentation is presented which emphasizes that local sedimentary and autocyclic processes, local tectonics, subsidence and compaction were important in controlling the distribution of facies at a mine scale.
