Depositional history of the Fire Clay coal bed (Late Duckmantian), EasternKentucky, USA

More than 3800 coal thickness measurements, proximate analyses from 97 loca lities, and stratigraphic and sedimentological analyses from more than 300 outcrops and cores were used in conjunction with previously reported palyno logical and petrographic studies to map individual benches of the coal and document bench-scale variability in the Fin Clay (Hazard No. 4) coal bed ac ross a 1860 km(2) area of the Eastern Kentucky Coal Field. The bench archit ecture of the Fire Clay coal bed consists of uncommon leader benches, a per sistent but variable lower bench, a widespread, and generally thick upper b ench, and local, variable rider benches. Rheotrophic conditions are inferre d for the leader benches and lower bench based on sedimentological associat ions, mixed palynomorph assemblages, locally common cannel coal layers, and generally high ash yields. The lower bench consistently exhibits vertical variability in petrography and palynology that reflects changing trophic co nditions as topographic depressions infilled. Infilling also led to unconfi ned flooding and ultimately the drowning of the lower bench mire. The drown ed mire was covered by an air-fall volcanic-ash deposit, which produced the characteristic flint clay parting. The extent and uniform thickness of the parting suggests that the ash layer was deposited in water on a relatively flat surface without a thick canopy or extensive standing vegetation acros s most of the study area. Ash deposits led to regional pending and establis hment of a second planar mire. Because the topography had become a broadly uniform, nutrient-rich surface, upper-bench peats became widespread with la rge areas of the min distant to elastic sources. Vertical sections of thick (> 70 cm), low-ash yield, upper coal bench show a common palynomorph chang e from arborescent lycopod dominance upward to fern and densospore-producin g, small lycopod dominance, inferred as a shift from planar to ombrotrophic mire phases. Domed mires appear to have been surrounded by wide areas of p lanar mires, where the coal was thinner (< 70 cm), higher in ash yield, and dominated by arborescent lycopods. Rectangular thickness trends suggest th at syndepositional faulting influenced peat accumulation and possibly the p osition of thr domed mire phase, Faulting also influenced post-deposition c lastic environments of deposition, resulting in sandstone channels with ang ular changes in orientation. Channnels and lateral facies were locally drap ed by high-ash-yield rider coal benches, which sometimes merged with the up per coal bench. These arborescent-lycopod dominant rider coal benches were profoundly controlled by paleotopography, much like the leader coal benches . Each of the benches of coal documented here represent distinctly differen t mires that came together to form the Fire Clay coal bed, rather than a si ngle mire periodically split by elastic influx, This is significant as each bench of the coal has its own characteristics, which contribute to the tot al coal characteristics. The large data set allows interpretation of both v ertical and lateral limits to postulated domed phases in the upper coal ben ch and to the delineation of subtle tec tunic structures that allow for mea ningful thickness projections beyond the limits of present mining. (C) 1999 Elsevier Science B.V. All rights reserved.