POTENTIAL PALEOTOPOGRAPHIC INFLUENCES ON SULFUR DISTRIBUTION IN THE ELM LICK COAL ZONE, TRADEWATER FORMATION (MIDDLE PENNSYLVANIAN) WESTERN KENTUCKY

Although the Illinois Basin is generally considered a high-sulfur coal basin, several Middle Pennsylvanian coal beds are locally low in sulfur content. High sulfur contents in Illinois Basin coals have largely been attributed to authigenic marine influences in transgressive facies in the immediate roof rocks above mined coal beds. Areas of low sulfur in other coal beds in the basin have been attributed to sediment wedges between the marine roof rocks and the coal, and to fresh-water tidal facies in the immediate roof. Other possible mechanisms for low sulfur contents in coal include original ombrogenous peat accumulation, fluid migration along faults, and late-stage water movement through permeable sandstones cutting into the coal.

The Elm Lick coal zone (Middle Pennsylvanian, Tradewater Formation) of western Kentucky, was locally mined by surface methods. The coal is overlain by dark gray shales with lingulid brachiopods suggestive of at least marginal marine conditions. In these mines, the sulfur content of the coal ranged from 5% to nearly 1% across relatively short distances. In one mine, sulfur increased toward a pinch out into gray shale and in another mine, sulfur gradually decreased across the pit with no apparent change in coal thickness or immediate roof geology. Although the coal is locally truncated by paleochannels, sulfur content does not increase or decrease toward the channels. Petrographic and palynological analyses of the coal support rheotrophic rather than ombrogenous peat accumulation in all of the mines sampled.

Changing sulfur content towards a pinchout of the coal suggests fluid migration and trapping of sulfates along the pinchout. However, rather than fluids from the immediate roof sediments, or some diagenetic hydrothermal event, evidence of paleovalley incisement 6 to 10 meters above the coal offers the possibility that patchy sulfur distribution in the coal may be related to paleogroundwater flow. Groundwater flow lines beneath an incised paleotopographic surface, or a succession of stacked paleotopographic surfaces could yield complex, patchy sulfur distributions in coals that would not follow distinct trends in the immediate roof geology, or be readily discernable without extensive highwalls and close data spacing to determine three-dimensional paleotopographic relief.