PALYNOLOGY, PETROGRAPHY, AND GEOCHEMISTRY OF DUNKARD GROUP COAL BEDS: INSIGHTS INTO THE ORIGIN OF LATE PALEOZOIC PALEOMIRES IN THE APPALACHIAN BASIN

The youngest Paleozoic strata in the Appalachian Basin is assigned to the Dunkard Group, an interval of (mainly) clastic rocks intercalated with thin, discontinuous coal beds. This report documents the palynology, petrography, and geochemistry of some of these coal beds, in an effort to better discern the nature of the paleomires these coal beds were derived from.

Palynologically, all of the Dunkard coals are dominated by tree fern spore taxa, namely Punctatisporites minutus, Punctatosporites minutus, Laevigatosporites minutes, and Thymospora spp. Calamite spores, mainly Calamospora and Laevigatosporites spp., are subdominant in some samples. Cordaite and pteridosperm pollen, Florinites and Vesicaspora respectively, occur consistently, but in minor amounts. Other gymnosperous pollen, Pityosporites and bisaccate-striate genera (e.g., Hamiapollenites, Striatopodocarpites, Protohaploxypinus) occur sporadically. Collectively, Dunkard coal spore/pollen assemblages are indistinguishable from underlying Monongahela Group coal palynofloras.

Petrographically, Dunkard coal beds are largely dominated by vitrinite macerals, with relatively minor amounts of liptinite and inertinite macerals. This type of maceral distribution is also very similar to that of Monongahela Group coal beds. Geochemically, Dunkard Group coals are highly impure, containing high amounts of ash (≥ 25 percent) and sulfur (≥ 3 percent). By contrast, mineable Monongahela Group coal beds are lower in both ash (≤ 15 percent) and sulfur (≤ 3 percent).

Based on the three sets of data, Dunkard Group coal beds are interpreted to have formed in small, discontinuous mires that were dominated by tree ferns with subdominant calamites. The mires were planar and topogenous, which resulted in coal beds with high ash and sulfur contents. The thin nature and ephemeral occurrence of Dunkard coal beds is most likely the result of a “drying” trend in the Late Pennsylvanian/Permian, which greatly curtailed the availability of moisture necessary for the more extensive peat formation observed in the underlying Monongahela Group.