Sequence stratigraphic interpretation of a Pennsylvanian (Upper Carboniferous) coal from the central Appalachian Basin, USA

Abstract

Peat mires retain a sensitive record of water‐table (base‐level) fluctuations throughout their accumulation. On this basis, coals provide one of the best opportunities to interpret high‐resolution base‐level change in ancient non‐marine deposits. The petrographic composition of 275 samples collected from 11 localities along a 100 km south‐west to north‐east transect across the regionally extensive (>37 000 km2) Pennsylvanian (Upper Carboniferous) Fire Clay coal of the Central Appalachian Basin, USA was analysed to determine its internal stratigraphy. The coal is positioned within the late lowstand/early transgressive systems tract of a fourth‐order depositional sequence. The results of the petrographic analyses reveal a cyclicity in the composition of the Fire Clay coal, which defines six units that are correlated over more than 100 km. Each coal cycle is characterized by a gradual upward transition from vitrinite‐dominated to inertinite‐dominated coal, which represents a ‘drying‐up’ succession. Increased concentrations of resistant peat components at the top of the drying‐up successions indicate reduced peat accumulation rates associated with slowing rate of water‐table rise, and may represent a residue of peat remaining from a phase of exposure and erosion resulting from a falling water table. These drying‐up successions are bound by surfaces that display an abrupt coal facies shift from inertinite‐rich to vitrinite‐rich coal, representing a rapid water‐table rise. Each cycle represents markedly different mire conditions with different aerial distributions, which supports the notion of temporal disconnection between each unit of coal, and suggests that considerable time may be ‘locked‐up’ in unit bounding exposure surfaces. Recognition that the rate of peat accumulation in a mire may vary considerably through time, has important implications for studies which assume that peat and coal successions provide continuous and time‐invariant records of base‐level fluctuations or palaeoecological change.

Wetland-Dryland Vegetational Dynamics in the Pennsylvanian Ice Age Tropics

“…Where it has been possible to determine it, Pennsylvanian coal beds appear to represent peat bodies that were contemporaneous throughout their distributions (this is true whether they formed as one temporally unbroken swamp or as several stacked peat bodies separated by hiatuses; Jerrett et al 2011). That is, they are not directionally time-transgressive deposits, significantly older at the shelf edge and progressively younger inland; they did not form in narrow coastal bands being pushed continuously inland by rising sea level ahead of other narrow bands of marine muds and limestones.…”

On the fundamental difference between coal rank and coal type

“…However, the subsequent planar peats have low amounts of fine-grained siliciclastics, suggesting limited clastic material in the background. Coals, or even benches within an apparently single coal bed, in their final phases of formation, may show changes in petrographic and palynological composition associated with the influx of fine-grained sediments (Greb et al, 1999;Jerrett et al, 2011), but without the introduction of a coarser siliciclastic fraction, again suggesting that such sediments either were not present in the depositional basin, or the flood intensities were too low to carry them out of the main drainage channels running through the mires. These interpretations are consistent with studies in modern tropical environments where streams draining humid to perhumid regions are virtually devoid of coarse siliciclastic sediment loads (Cecil et al, 2003a;Harris et al, 2008).…”

Compaction of lignite: a review of methods and results

“…It has been estimated that the lignite seams were deposited over thousands of years (e.g. Kojima et al 1998;Petersen et al 2003), through tens or hundreds of thousands of years (Volkov 2003;Jerrett et al 2011;Flores 2013), to as much as 7 Ma in the case of the lignites in the Lower Rhine Basin in northwest Germany (Schäfer et al 2004(Schäfer et al , 2005. So, when the top layers of the peat were formed, the basal layers were already highly decomposed and dewatered; that is, (auto)compacted (Falini 1965;McCabe 1984;Volkov 2003).…”

“…Collinson and Scott 1987;Kojima et al 1998), 'consolidation coefficient' (Widera 2002;Widera et al 2007), or 'compaction ratio' (e.g. Ting 1977Ryer and Langer 1980;DeMaris et al 1983;Law et al 1983;McCabe 1984McCabe , 1987Elliot 1985;White 1986;Winston 1986;Courel 1987;Salinas et al 1990;Gayer and Pešek 1992;Nadon, 1998;Greb et al 2003;Petersen et al 2003;Rajchl and Uličný 2005;Rajchl et al 2009;Jerrett et al 2011;Flores 2013). As the most common, the term 'compaction ratio' will therefore be employed throughout this paper, particularly in the final sections.…”