SPATIAL AND TEMPORAL TRENDS OF LOWER AND MIDDLE PENNSYLVANIAN COALS, CENTRAL APPALACHIAN BASIN, U.S.A

Mineable coal beds of the central Appalachian Basin exhibit both spatial and temporal variation in thickness and quality. Patterns of variability are a function of changes in tectonic accommodation, eustasy, sediment supply, and paleoclimate. Lower and Middle Pennsylvanian strata can be divided into 8 to 12 genetic sequences, each consisting of five to nine coal-clastic cycles, analogous in scale to cyclothems. Coal-clastic cycles increase in thickness and number from the northwest to the southeast, toward the basin axis. Some coal beds preserved on the northwest basin margin develop from single beds to multiple beds in coal zones toward the southeast. Architectural analysis of mined coal seams indicates that individual benches of coals within beds, and beds within zones, may exhibit distinct quality and thickness patterns. The combination of these bench and bed attributes can greatly affect the attributes of the seam mined at any one location.

Lateral thickness changes of sequences show strong influence of tectonic accommodation in the foreland basin in the Early Pennsylvanian, which then decreased into the late Middle Pennsylvanian when the basin axis shifted northward. Increased thickness and frequency of coal-clastic cycles, coal beds within the cycles, and the development of coal zones toward the basin axis all reflect foreland tectonic controls. Superimposed on these effects are temporal changes in sediment supply, marine influences and paleoclimate. Coal quality parameters show temporal changes within successive genetic sequences, which may be related to marine influences, as well as broader scale temporal changes related to changing Carboniferous paleoclimate. Ever-wet paleoclimates in the Early and early Middle Pennsylvanian allowed for the development of domed paleomires, and resultant low-sulfur, low-ash coals, although these generally are only represented in a portion of the coal bed. Sediment supply shifted from a northern quartzose source to a southeastern source throughout the Early Pennsylvanian, with quartzose sands occurring at the expense of coals toward the basin margin. Middle Pennsylvanian coals are more extensive than their lower Pennsylvanian counterparts because they are not truncated by lateral quartz arenites; this is a function of tectonics and sediment supply.