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



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.