ARE COAL BALLS RARE? A CYCLOSTRATIGRAPHIC ANALYSIS

In the context of the entire Phanerozoic, coal balls are clearly rare. However, over a 24 m.y. interval in the Pennsylvanian and earliest Permian (323 – 299 Ma) coal balls formed frequently. Here, we analyze coal-ball frequency using 84 major, intermediate and minor transgressive-regressive cycles identified by Heckel (2008, 2013) in the latest Atokan through mid-Virgilian of North America. Of these, 32% (27/84) have coal balls. Coal ball frequency by cycle hierarchy in the Mid-continent, Illinois and Appalachian basins is 61% of major cycles, 32% of intermediate cycles and 19% of minor cycles. The frequency of coal ball occurrence in the Donets Basin is similar: Over an interval of about 4 m.y. (~315 – 311 Ma, latest Bashkirian to mid-Moscovian), 39% of transgressive/regressive cycles (11/28) have coal balls.

Through the Desmoinesian/Missourian boundary interval, North American paleotropical climates became drier and tree ferns replaced lycopsids as the dominant plants in paleomires. Significantly more cycles dominated by lycopsids or cordaiteans have coal balls than cycles dominated by tree ferns. Overall, 44% of cycles (18/41) with lycopsid or cordaitean dominance have coal balls, whereas 21% (9/43) of cycles with tree fern dominance have coal balls (p < 0.01). While 25% (5/20) of minor cycles with lycopsid or cordaitean dominance have coal balls, no minor cycle with tree fern dominance has coal balls.

This pattern may reflect the relative abundance of coal in latest Atokan – Desmoinesian vs Missourian – Virgilian cycles, with drier paleotropical climates leading to less paleotropical coal and fewer coal balls. Rygel et al. (2008) reported increased erosional relief in clastic facies of Missourian-Virgilian cycles, which would have increased the amplitude and rate of sea-level change during each glacial-eustatic cycle. Faster sea-level rise could affect coal-ball abundance, particularly if coal balls formed in marine paleomires, with rapid sea-level rise leading to thinner coals, or freshwater coals directly overlain by marine sediments. Conversely, as antecedent topography filled with sediment in the Missourian-Virgilian, producing a broader flatter shelf, similar amounts of glacial-eustatic sea-level rise may have mimicked the effect of increasing the amplitude and rate of sea-level rise.