Cyclic Sedimentation of the Barakar Formation, Singrauli Coalfield, India: Statistical Assessment from Borehole Logs

摘要

The succession of lithofacies of a part of the Barakar Formation of the Singrauli coalfield has been studied by statistical techniques. The lithologies have been grouped under five facies states viz. coarse-, medium-, and fine-grained sandstones, shale and coal for statistical analyses. Markov chain analysis indicates the arrangement of Barakar lithofacies in the form of fining-upward cycles. A complete cycle consists of conglomerate or coarse-grained sandstone at the base sequentially succeeded by medium- and fine-grained sandstones, shales and coal seam at the top. The entropy analysis puts the Barakar cycles in A-4 type of cyclicity, which consists of different proportions of lower-, side- and upper-truncated cycles of lithologic states. Regression analysis indicates a sympathetic relationship between total thickness of strata (net subsidence) and number of coal cycles and an antipathic relationship between average thickness and number of coal cycles. The cyclic sedimentation of the Barakar Formation was controlled by autocyclic process, which occurred due to the lateral migration of streams triggered by intrabasinal differential subsidence. In many instances, the clastic sediments issued from the laterally migrating rivers interrupted the sedimentation resulting in thinner cycles in areas where the numbers of cycles are more. Principal component and multivariate regression analyses suggest that the net subsidence of the basin is mostly controlled by number and thickness of sandstone beds and coal seams.