MG/CA RATIOS OF ECHINODERM OSSICLES IN PENNSYLVANIAN COAL BALLS: IMPLICATIONS FOR RECONSTRUCTING MG/CA RATIOS OF PENNSYLVANIAN SEAWATER

The informally named Dalton coal is an organic rich shale deposit at the top of the Wolf Mountain Shale (underlying the Winchell Limestone) assigned to the base of the Stanton major cycle near the top of the Missourian (Late Pennsylvanian). Coal balls from the Dalton coal contain echinoderm ossicles and corals as well as permineralized wood and stems including Calamites, tree ferns, and cordaiteans. The first-formed carbonate minerals in Dalton coal balls are small amounts of disordered dolomite (42.8 – 44.8 mol % MgCO₃, avg = 43.9 mol % MgCO₃) and HMC (6.73 – 11.11 mol % MgCO₃, avg = 10.22 mol % MgCO₃). Subsequent generations of carbonate in Dalton coal balls are LMC (1.50 – 2.05 mol % MgCO₃, avg = 1.68 mol % MgCO₃). In coal balls, the pattern of early HMC followed by later LMC has been interpreted as: 1, evidence for the initiation of coal balls in marine mire with subsequent diagenesis in freshwater, or 2, CO₂ degassing and rapid precipitation of HMC from meteoric pore water supersaturated with carbonate. In this scenario, chemical weathering in the watershed supplied the Ca²⁺ and Mg²⁺ ions required for coal-ball formation.

Echinoderm ossicles in Dalton coal and other marine invertebrates were likely transported to the paleomire by wave action and provide direct evidence of Mg concentrations in adjacent marine environments. Preliminary analyses suggest that echinoderm ossicles in the Dalton coal retain their original microstructure, including labyrinth and rectilinear stereom and have estimated Mg concentrations ranging from 6 – 13 mol % MgCO₃ (avg = 10 mol % MgCO₃, N = 35). These values fall within the range of values reported by Dickson for contemporaneous crinoid ossicles from North America (9.9 – 12.5 mol % MgCO₃) and are very close to the Mg concentration of HMC associated with permineralized plant fossils in Dalton coal balls. The similarity of Mg content in crinoid ossicles and permineralizing HMC in Dalton coal balls is consistent the marine hypothesis for the formation of coal balls. Coal balls, which occur commonly in Pennsylvanian coals from North America and the Donets Basin, could provide additional data to reconstruct the Mg/Ca ratios of Pennsylvanian seas.