Dunkard Group coal beds: Palynology, coal petrography and geochemistry

• Dunkard Group coal beds are thin, and high in ash and sulfur.
• Dunkard Group coal beds are dominated by vitrinite macerals and tree fern miospores.
• Dunkard coal miospore assemblages are similar to Late Pennsylvanian coal assemblages.
• Dunkard coals accumulated during brief wet intervals in an overall dry climate.

Coal beds that occur in the Dunkard Group are typically thin (avg. 0.3 m), and are high in ash yield (avg. 27.8%) and total sulfur content (avg. 5.1%). Petrographically, Dunkard coals are high in vitrinite (avg. 83.1%, mmf), with correspondingly low to moderate amounts of liptinite (avg. 2.1%, mmf) and inertinite (avg. 13.8%, mmf). Palynologically, Dunkard coal beds are all dominated by tree fern spore taxa (avg. 96.3%), especially Punctatisporites minutus (avg. 82.8%). Calamite spores are the second most abundant plant group (avg. 2.4%), with others (lycopods, small ferns, Cordaites and other gymnosperms) having very minor representation. Overall, the coal palynofloras are strongly dominated by lowland plants (ferns and Calamites), with only rare occurrences of bisaccate–striate conifer pollen.Coal beds of the underlying Monongahela Group (Pittsburgh, Redstone, Sewickley and Waynesburg) are thicker (avg. 1.6 m), and are lower in ash (avg. 13.4%) and sulfur (avg. 3.3%) than their Dunkard counterparts. Petrographically, Monongahela Group coals are very similar to Dunkard Group coals, when compared on a mineral matter free basis; coal beds that occur in both groups are strongly dominated by vitrinite macerals. Monongahela Group coals are also palynologically very similar to Dunkard Group coals, with tree fern spore taxa dominating. A coal bed of early Permian age from west-central Texas is also similar, petrographically and palynologically, to Dunkard and Monongahela coals, being vitrinite and tree fern spore dominant.The almost complete lack of coniferous pollen in Dunkard coals led earlier workers to conclude that the Dunkard was entirely Late Pennsylvanian, and not Permian, in age. However, it is now known that Late Pennsylvania lowland floras persisted into the Dunkard, especially during wet intervals. As earlier interpretations were based primarily on spore and pollen floras from coal beds, the lack of conifer pollen in Dunkard coals is probably the result of a sampling bias. Because of this ecological bias, coal palynology is a poor proxy for age dating Dunkard Group coals.Collectively, Dunkard swamps were all planar and topogenous, their formation being controlled by topography and moisture availability. A progressive decrease in wet intervals, both in terms of frequency and duration, during the Dunkard was the major control on peat accumulation and preservation. Collectively, moisture limitation appears to be the principle factor that controlled the formation of Dunkard Group coals.