Temporal Changes in Coal-bearing Depositional Sequences (Lower and Middle Pennsylvanian) of the Central Appalachian Basin, U.S.A.

Abstract

Middle Pennsylvanian coal-measure sequences of the eastern Kentucky coal field, central Appalachian basin, occur in ordered groupings of five to six fourth-order coal-clastic cycles, between third-order marine flooding surfaces. Lower Pennsylvanian coal measures also are present, but are laterally truncated by at least four, 60–80-km (37–50-mi)-wide belts of quartz-pebble-bearing quartzarenites that were deposited in a longitudinal drainage system. Successive quartzarenite belts are truncated updip by the next youngest belt. Each belt consists of at least a pair of vertically stacked, composite sandstones separated by a coal bed and estuarine or marine shale facies. Although less marine than their middle Pennsylvanian counterparts, the base of these lower Pennsylvanian midformation shales also represents marine flooding surfaces, or the updip equivalents of flooding surfaces. Therefore, lower Pennsylvanian third-order genetic sequences can be defined that include both marginward quartzarenites and basinward coal-measure facies.Changes in foreland-basin subsidence, sedimentation patterns, climate, and marine influences affected depositional sequences from the early to middle Pennsylvanian. The westward shift of the longitudinal drainage belt was accompanied by a westward shift in basinward coal measures, resulting in increasingly more extensive coals with time. Increasing expanse and uniformity of coal measures was accompanied by decreasing foreland accommodation. In each third-order sequence, the greatest accommodation appears to occur in the regressive parts of the brackish to marine shales that bound each sequence. The greatest spatial changes in sequence thickness occur across the northern hinge line of the basin and along the basinward limit of successive quartzarenite belts. Foreland-basin subsidence influenced the stacking of successive lower Pennsylvanian quartzarenites, the westward overlap of successive quartzarenite belts, basinward increases in the number of coal beds, development of coal zones in third-order sequences, and basinward increases in the thickness of coal beds.