The association between tectonic deformation, inorganic composition and coal rank in the bituminous coals from the South Wales coalfield, United Kingdom

Abstract

Analysis of vertical distribution of coal rank in three collieries and one opencast mine in the SE of the South Wales coalfield has revealed deviations from Hilt's law in both proximate analyses and in vitrinite reflectance. The variations in rank parameters studied are closely correlated with variations in the intensity of tectonic deformation, measured using a numerical deformation index (NDI). Three seams in the sequence were also analysed for major and trace element geochemistry and Rock Eval Pyrolysis. Variations in these parameters are related to thrust inclusion of floor and/or roof mudrocks into the seams. A detailed conceptual model of a major thrust and associated in-seam thrust detachments and lag faults is developed to explain the vertical rank profile by post-coalification faulting. It is concluded that, although the model can explain the vertical pattern of rank, the complexity of the required faults render the model improbable. The alternative mechanisms for enhancement of the rank parameters associated with tectonic deformation, including shear stress control, frictional heating and localised fluid flow are discussed and are thought more likely to be responsible for the deviations from Hilt's law in the vertical rank profile. Rank suppression shown by three seams, two of which are overlain by marine bands, is thought to be related to biochemical activity in the peat environment resulting from the influx of saline waters.