COAL-BENCH SPLITS VS. MERGES--A COMPLICATION TO PREDICTING TRENDS IN MULTIPLE-BENCH COAL SEAMS

Coal beds of the central Appalachian Basin are commonly separated by clastic partings and inertinite-rich layers into benches. Multiple coal benches characterize many thick, widespread mined seams. Bench-scale analyses of coal seams, based on thickness, quality, or compositional groups (petrography, palynology, sulfur content, ash yield), indicate that individual benches of coal commonly have distinct vertical and lateral trends. Meaningful resource assessments need to recognize that mined seams may represent different combinations of coal benches in different parts of the basin.

In some multiple-benched coal beds, partings grade laterally into thicker clastic intervals called splits. Seam splits most commonly are interpreted as syndepositional influx of sediment from a lateral source, such as a channel. The coal below the split is interpreted to have been briefly interrupted by a flood, and then the mire reoccupied the sedimentary deposit as a continuation of the preceding or lateral mire. Recent research in the central Appalachian Basin suggests that some seam splits represent merges of completely different mires. The difference is subtle, but in merges, a new mire, unconnected to the preceding mire, is interpreted to have draped the intervening clastic wedge. Upper or rider benches above the clastic wedges have compositional groupings suggestive of a new pioneering mire, rather than a simple continuation or a drowning phase of the previously existing mire. In other cases, coal benches drape scours or apparent paleotopographic lows and come near to the top of, or truncate, an underlying coal bed. These types of truncation merges occur in low-accommodation areas in the northern part of the basin and locally within coal zones of closely spaced coal beds. Because the upper benches of these merged coal beds accumulated as completely different, temporally separate mires from the coals they merge with or cut through, trends in their composition should be treated separately from underlying coal benches or beds, when possible. Where rider benches merge with, rather than split from, a seam, or lower coal benches thicken into scours and merge with leader benches, anomalously thick coal may occur. The increased thickness, however, is often at the expense of quality.