Petrology, palynology, and geochemistry of the Pond Creek coal (Pennsylvanian, Duckmantian), Pike County, Kentucky: Overprints of penecontemporaneous tectonism and peat doming

Highlights

• High volatile A bituminous Pennsylvanian Pond Creek coal was studied in Pike County, KY

• The coal thins from east to west, approaching an anticline with penecontemporaneous uplift

• Mid-seam bright zone has an upwards decrease in vitrinite and ash and increase in floral diversity

• The zone shows greater floral stability towards the anticline

• In ombrogenous zone, peat growth led to the collapse of peat and deposition of bony coal

Abstract

The petrographic, palynologic, and geochemical character of the high volatile A bituminous Pennsylvanian (Bashkerian, Duckmantian) Pond Creek coal was evaluated for a series of mine samples in the Lick Creek and Jamboree 7 ½’ quadrangles in Pike County, Kentucky. In its prime years of production, the Pond Creek coal was among the most important coal resources in the leading coal producing state in the U.S. The thickness and quality of the Pond Creek coal was closely related to its position relative to the northwestward-plunging, N20°E-trending Belfry anticline with the main bed of the coal thinning and the thickness of rock between the latter and the high-S Pond Creek rider bed thinning across the anticline. This suggests that the Belfry anticline and the Pond Creek mire were contemporaneous features, with the rising anticline influencing the lithology and chemistry of the peat. Within the study area, the lower three lithotypes fill the original topography and the next higher lithotype pinching out along the rise towards the anticline. The next three lithologies, a thick bright zone (zone 4 in the nomenclature of this study), a bone coal, and a thinner bright zone are continuous through the study area, although they all thin towards the anticline. The upper durain, the termination of the coal seam, also pinches out towards the anticline. Zone 4 shows a decrease in total vitrinite towards the uppermost bench of the zone, an upwards increase in ash yield, the upwards increase in the floral diversity, and a higher concentration of Mn, CaO, and Ba + Sr in the lower lithologies of the zone. Towards the anticline, there is a greater floral stability within zone 4 compared to the relatively subsiding area to the east. Within the zone 4 bright lithologies, peat growth outpaced the level of the water table, but at the cost of the turnover of the arborescent lycopod forest to a more diminutive flora, the collapse of the ombrogenous peat, and the influx of deeper water and the consequent deposition of the overlying relatively high-ash bone coal.

Introduction

The Pennsylvanian (Bashkerian, Duckmantian) Pond Creek coal (Fig. 1) has long been one of the most important coals in eastern Kentucky. In the later 1980's, the peak of coal production in Kentucky, Pike County trailed only Campbell County, Wyoming, the home of the largest Powder River Basin mines, in U.S. coal production (from 1971 to 1988, Kentucky was the leading coal producing state). At that time, the high volatile A bituminous, low-S Pond Creek coal accounted for 35–40% of the production in Pike County (Hower et al., 1991).

As part of the effort by the University of Kentucky's Center for Applied Energy Research to characterize the important coals in the state, a comprehensive sampling of several of the major Pond Creek coal mines was undertaken (Fig. 2). Studies of the petrology and lithology (Hower and Pollock, 1988; Hower et al., 1990), geochemistry (Hower and Bland, 1989), palynology (Helfrich and Hower, 1991), and the relationship of the coal lithology to the regional structure (Hower et al., 1991) were the products of the sampling.

As a broad summary, Hower et al. (1991) found that the thickness and quality of the Pond Creek coal was closely related to its position relative to the northwestward-plunging, N20°E-trending Belfry anticline (Chesnut Jr., 1988, Coskren and Rice, 1979, Hunt et al., 1937, Rice et al., 1977). The coal to the northwest of the anticline has a high-TiO₂, high-Zr durain and dull clarain lower bench; a middle bench with relatively high-CaO with enrichment in Ba and Sr and, to a lesser extent, Mn, Cu, and Ni; and a high-S, high K₂O/Al₂O₃ dull lithotype zone at the top of the coal. The main bed Pond Creek coal zone thins across the Belfry anticline accompanied by the thinning of the interval between the main bed and the rider coal. Consequently, the relatively high-S rider was often mined along with the main coal bed, resulting in the reporting of greater thicknesses of mined Pond Creek coal compared to the northwest and southeast of the anticline (Rice, 1963, Wolcott and Jenkins, 1966). The region to the southeast of the anticline, arguably representing the highest quality (lowest S, lowest ash yield in the beneficiated coal), has several lithotypes that pinch out towards the anticline (four at the base of the coal and a thin durain at the top of the coal bed) and three lithotypes that are present in all of the sampled sites to the southeast of the anticline (Fig. 3; Table 1). Collectively, the nature of the coal throughout the region indicates that the Belfry anticline and the Pond Creek mire were contemporaneous features, with the rising anticline influencing the lithology and chemistry of the peat/coal. Similar relationships between penecontemporaneous tectonism and coal lithology have been noted for other coals (the Taylor through the Hazard No. 8) in eastern Kentucky (Hower et al., 1992); the Stockton coal in Martin County, Kentucky (Greb et al., 2005); and the Upper Elkhorn No. 2 coal in southern Pike and Floyd counties, Kentucky (Raione et al., 1991).

In this study, the petrographic nature of the coal in the sections to the southeast of the anticline, specifically in the Lick Creek (McKay and Alvord, 1969) and Jamboree 7.5′ (Outerbridge and Van Vloten, 1968) quadrangles, is re-examined using revised petrographic nomenclature (ICCP, 1998; 2001; Pickel et al., 2017) and revised interpretations of depositional environments (Dai et al., 2020). Following the philosophy of the discussions by Dai et al. (2020), we do not rely on the maceral-ratio approach to the interpretation of the coal facies; rather, this work is based on direct interpretations of the megascopic and microscopic petrology, the palynology, and the geochemistry (ash, major oxides, minor elements) of the coals.