Coal beds mined in Kentucky often are not laterally continuous in thickness, quality, or roof condition. Regional and local variation is common. Because thickness, quality, and roof conditions are the result of geologic processes that were active when the coal was deposited as a peat swamp, a better understanding of the relationships between geology and major coal resources can aid in identifying geologic trends, which can be extrapolated beyond areas of present mining. The focus of this study is on the Fire Clay (Hazard No. 4) coal, one of the leading producers in the Eastern Kentucky Coal Field with 20 million short tons of annual production. More than 3,800 thickness measurements, highwall and outcrop descriptions, borehole and geophysical-log descriptions, and proximate analyses from 97 localities were used in conjunction with previous palynologic and petrographic studies to investigate the geology of the Fire Clay coal in a 15-quadrangle area of the Eastern Kentucky Coal Field.
The Fire Clay coal is commonly separated into two distinct layers or benches by a flint-clay and shale parting called the “jackrock parting” by miners. Maps of coal benches above and below the parting show that the lower bench is limited in extent and variable in thickness. In contrast, the coal above the jackrock parting occurs across most of the study area and is characterized by rectangular patterns of coal thickness.
Multiple coal benches resulted from the accumulation of multiple peat deposits, each with different characteristics. The lower bench of the coal was deposited when a peat accumulated above an irregular topographic surface. Because the peat was being deposited at or below the water table, it was often flooded by sediment from lateral sources, resulting in moderate to locally high ash yields. This peat was drowned and then covered by volcanic ash, which formed the flint clay in the jackrock parting. The upper coal bench accumulated above the ash deposit, after irregularities in the topography had been filled. The relatively flat surface allowed the swamps to spread outward and dome upward above the water table in some areas. Doming of the peat resulted in areas of coal with generally low ash yields and sulfur contents. Sharp, angular changes in the upper coal bench are inferred to represent subtle fault influence on upper peat accumulation.
The upper peat was buried by a series of river channels, which were bounded by levees, flood plains, and elongate bays. Several of the rivers eroded through the Fire Clay peats, forming cutouts in the coal. These cutouts often follow orientations similar to the angular trends of coal thinning, suggesting a relationship that can be extended beyond the present limits of mining. Also, additional peat swamps accumulated above the levees and flood plains bounding the channels. Along the thinning margins of these deposits, the peats came near or merged with the top of the Fire Clay coal, resulting in local areas of increased coal thickness.
Rider coal benches exhibit high to moderate sulfur contents and ash yields, so that although they may increase coal thickness, total coal quality generally decreases where riders combine with the Fire Clay coal.
Report of Investigations 2
Digital Object Identifier (DOI)
Some of the data used in the study were collected under a grant from the U.S. Geological Survey, National Coal Resource Data System.
Greb, Stephen F.; Hiett, John K.; Weisenfluh, Gerald A.; Andrews, Robert E.; and Sergeant, Richard E., "Geology of the Fire Clay Coal in Part of the Eastern Kentucky Coal Field" (1999). Kentucky Geological Survey Report of Investigations. 5.