Basin evolution and tectonics of the Leicestershire coalfield.

The Leicestershire coalfield constitutes a NW-SE orientated area of approximately 125 km2, centred on the town of Coalville in the East Midlands. The area is underlain by Carboniferous, coal-bearing strata of Westphalian A and B age. In the south the Carboniferous rocks are concealed below Permo-Triassic sediments. The investigation was restricted to the parts of the Leicestershire coalfield worked by British Coal deep mines. A synoptic view of the entire coalfield was taken, using exploration borehole data and underground quality control samples (where available). The study describes aspects of these data as they relate to the geology and depositional setting of the Leicestershire coalfield. The observed geometry and distribution of the coal seams and interseam sediments are related to the causative depositional processes. Computer modelling of the data examined the lateral and vertical lithological relationships within the Westphalian A and B succession of the Leicestershire coalfield in three representative parts of the stratigraphic interval, namely the middle section of the Westphalian A, the Westphalian A-B contact, and the mid Westphalian B. The lateral and vertical lithological relationships were defined using isopach and structural contour maps as well as correlation (scatter) diagrams. This study shows that there is probably not one single control on sedimentation, but a complex interaction of factors that have led to the deposition of the coal and coal-bearing strata in the Westphalian of the Leicestershire coalfield. The most important factor appears to be base level change, and resultant changes in water table level. Base level changes in turn may be controlled by tectonics, eustacy or doming of the peat during growth. There is a significant change in sedimentary style between the mid Westphalian A and the mid Westphalian B. Controls on sedimentation changed during the Westphalian, from predominantly structural factors in the mid Westphalian A to factors inherent in the depositional system in Westphalian B. Subsidence in the Westphalian A was in response to regional crustal extension and took place along the faults bounding the block and basin topography of the basement. Subsidence was rapid and basin fill sequences were thick, but coal seams were thin. With relaxation of extensional tectonics isostatic readjustment and thermally induced subsidence took place in the Westphalian B, with sedimentation keeping pace with the subsidence so that deposition occurred in shallow water which meant that the sediment and coal accumulated as a result of compactional, topographical and depositional factors.