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

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Abstract

More than 3800 coal thickness measurements, proximate analyses from 97 localities, and stratigraphic and sedimentological analyses from more than 300 outcrops and cores were used in conjunction with previously reported palynological and petrographic studies to map individual benches of the coal and document bench-scale variability in the Fire Clay (Hazard No. 4) coal bed across a 1860 km2 area of the Eastern Kentucky Coal Field. The bench architecture of the Fire Clay coal bed consists of uncommon leader benches, a persistent but variable lower bench, a widespread, and generally thick upper bench, and local, variable rider benches. Rheotrophic conditions are inferred for the leader benches and lower bench based on sedimentological associations, 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 conditions as topographic depressions infilled. Infilling also led to unconfined flooding and ultimately the drowning of the lower bench mire. The drowned 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 across most of the study area. Ash deposits led to regional ponding and establishment of a second planar mire. Because the topography had become a broadly uniform, nutrient-rich surface, upper-bench peats became widespread with large areas of the mire distant to clastic sources. Vertical sections of thick (>70 cm), low-ash yield, upper coal bench show a common palynomorph change from arborescent lycopod dominance upward to fern and densospore-producing, small lycopod dominance, inferred as a shift from planar to ombrotrophic mire phases. Domed mires appear to have been surrounded by wide areas of planar mires, where the coal was thinner (<70 cm), higher in ash yield, and dominated by arborescent lycopods. Rectangular thickness trends suggest that syndepositional faulting influenced peat accumulation, and possibly the position of the domed mire phase. Faulting also influenced post-depositional clastic environments of deposition, resulting in sandstone channels with angular changes in orientation. Channnels and lateral facies were locally draped by high-ash-yield rider coal benches, which sometimes merged with the upper 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 different mires that came together to form the Fire Clay coal bed, rather than a single mire periodically split by clastic influx. This is significant as each bench of the coal has its own characteristics, which contribute to the total coal characteristics. The large data set allows interpretation of both vertical and lateral limits to postulated domed phases in the upper coal bench, and to the delineation of subtle tectonic structures that allow for meaningful thickness projections beyond the limits of present mining.

Introduction

The Fire Clay (Hazard No. 4, Jackrock) is a Middle Pennsylvanian (Late Duckmantian) coal bed of the Hyden Formation in the Breathitt Group (Fig. 1A), previously referred to as the Breathitt Formation (Chesnut, 1992). The Fire Clay coal is one of the top producers in eastern Kentucky with annual production of more than 20 Mt. Where mined in the study area, the coal is a compliance resource, with a mean total sulfur content of 1.0 wt.% (n=141) and ash yield of 11.65 wt.% (n=91). The Fire Clay coal bed may occur as a single coal bench, or multiple benches containing a flint clay and shale parting, locally known as the `jackrock' parting (Fig. 1B). The flint clay part of the parting contains sanidine phenocrysts, beta-quartz paramorphs, euhedral zircons, and Fe–Ti minerals such as ilmenite and rutile, which have been used to interpret the flint clay as a tonstein of volcanic origin (Bohor and Triplehorn, 1981; Chesnut, 1983; Lyons et al., 1992, Lyons et al., 1994). Samples of sanidines from the flint clay have been age-dated at 312±1 Ma (Hess and Lippolt, 1986; Rice et al., 1990; Lyons et al., 1992). The tonstein is the only radiometrically dated layer in the coal field, making it a useful datum for stratigraphic correlations, including correlations of individual coal benches and rock strata above and below the Fire Clay tonstein.

Section snippets

Regional setting

The study area consists of 15 7.5-min quadrangles in the Eastern Kentucky Coal Field (Fig. 2), which is part of the Central Appalachian Basin. This basin formed as a series of foreland basins, initially above the Rome trough, a Precambrian aulocogen (RT in Fig. 2), and then enlarged in response to collisional tectonics along the eastern margin of North America during the Taconic, Acadian, and Alleghenian/Hercynian Orogenies. Within the study area, gravity studies define two dominant features, a

Data bases

To accurately assess thickness variation at the scale of coal benches, more than 900 mine maps from 15 7.5-min quadrangles were examined. Thickness points from more than 3800 locations within a 1860 km2 area of the coal field were collected (Fig. 3). Stratigraphic and sedimentologic data from more than 300 outcrops and cores were used to map roof facies and illustrate controls on coal-bench thickness that might be caused by changing roof and floor conditions. Previously, 21 vertical profiles

Purpose

The purpose of this paper is to use bench-scale isopach maps, cross sections and regional roof facies maps in conjunction with previous petrographic, palynologic, and geochemical studies to interpret the depositional history of the Fire Clay coal bed. The combination of these varied data sets makes it possible to more accurately place previous data in the context of the temporal and spatial distribution of each coal bench. This, in turn, allows for a better understanding of (1) how the

Leader coal benches

Several isolated, thin, leader coal benches were mapped within 3 m of the base of the mined Fire Clay coal bed (Fig. 4A). Often the leaders drape underlying sandstones and are separated from the lower bench of the Fire Clay coal bed by dark shales or shales and siltstones (Fig. 4B,C). Locally they merge with the lower coal bench (Fig. 4B,C). In some cases the leaders appear to split from the lower bench (Fig. 4B), but in others the lower bench does not retain the thickness of the combined coals

Roof geology

A regional geologic map of the rocks within 3 m of the top of the coal shows a series of sandstone belts flanked by wedges of shaly sandstone, interbedded sandstone and shale, and shale (Fig. 10A). Thick sandstones in the roof are generally more widespread to the west and southwest, and thin eastward and northeast as narrower belts. Sandstones follow northwest–southeast and northeast–southwest orientations similar to the thickness trends in the upper coal bench (Fig. 10A). The belts consist of

Depositional history

The flint clay parting in the Fire Clay coal bed allows separation of several distinct benches of coal. Coal benches show significant differences in thickness, palynology, total sulfur content, and ash yields, which suggest the benches represent distinctly different mires rather than a single mire, interrupted for a short period of time by clastic influx. Hence, rather than a single succession, the Fire Clay coal bed is probably best thought of as a series of mire successions that combined to

Summary

The Fire Clay coal bed, like many Appalachian basin coal beds, is a multiple-bench coal seam. Regional mapping and analysis of the benches show that each represents a distinctly separate mire, rather than splits of the same mire. This is critical to application of depositional models and to projections of coal characteristics beyond data points, because it shows that models and projections should be made from the bench scale, rather than to the seam as a whole. The interaction of the benches

Acknowledgements

The authors wish to thank the many individuals from the mining community that gave us access to their property, provided data, and shared their knowledge of the Fire Clay coal. Some of the data used in the study was collected under a grant from the U.S. Geological Survey, National Coal Resource Data System (NCRDS). John Hiett of the Kentucky Department of Mines and Minerals was a major help in accessing underground mine maps for coal data. Gerry Weisenfluh and Robert Andrews at the Kentucky

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