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Sedimentary rocks of Early Pennsylvanian age unconformably overlie Mississippian rocks of Chester age in the western Kentucky coal field. Regional truncation of the formations of Chester age and the existence of a series of southwestward-trending channels, commonly incised 200-300 ft into the formations of Chester age, are the main evidence of the unconformity. The main channels are filled with sandstone and shale; the smaller, shorter channels mainly are filled with shale. In the study area, well cuttings containing sparse microfossils, fragments of macrofossils, sandy and oolitic limestone, and glauconite indicate a marine environment during late phases of the filling of the channels. Both the sediments filling the Pennsylvanian channels and the remnant hills of Mississ ppian rocks between the channels are overlain by deposits of a former river and delta system that presently form a deep freshwater aquifer in the study area. The geometry of this Pennsylvanian river and delta system indicates the distribution of offshore barrier bars, lagoons, tidal channels, delta-front distributaries, and a bar that was probably formed by long-shore currents. Production and shows of oil and gas from laterally equivalent Pennsylvanian sandstone are peripheral to the barrier bars on their former seaward side.
Because the marine transgression probably extended across much of western Kentucky and part of Illinois, other examples of the depositional model suggested here may be present in a much larger area. An understanding of the complex details of early Pennsylvanian deposition may be obtained by applying the principles of the model to adjacent areas.
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