Carbonate petrology and geochemistry of Pennsylvanian coal balls from the Kalo Formation of Iowa
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Coal balls are carbonate and pyrite concretions enclosing uncompressed peat, primarily found in Pennsylvanian and earliest Permian paleotropical coals. Petrographic and microprobe analysis of coal balls from the mid-Moscovian (latest Atokan or earliest Desmoinesian) Williamson No. 3 Mine (Kalo Formation) from Lucas County, Iowa indicates that pyrite was the earliest mineral to form, followed by high-Mg calcite (average 13.9mol% MgCO 3 ), which is also high in Sr (700-1500ppm), suggesting that the Kalo Formation coal balls formed in response to the incursion of marine water into the mire. The high-Mg calcite has an unusual growth habit, radiating arrays of bladed polycrystals, which look like distorted triangles with elongated apices when cut parallel to the basal plane. These high-Mg calcite polycrystals have low-Mg, non-ferroan calcite rims, consistent with diagenesis in meteoric water. The presence of early high-Mg and low-Mg, non-ferroan calcite in the Williamson No. 3 coal balls suggests that these coal balls formed in a hydrologically dynamic environment. Most of the remaining cements in the Kalo Formation coal balls are low-Mg ferroan calcite, consistent with burial diagenesis. These cements retain regions of low-Mg and high-Mg calcite, suggesting that they are neomorphs of the early high-Mg calcite cement. Fibers in the vascular bundle supports of Cordaites principalis leaves have narrow rims of dolomite that appear unique to this species. The presence of high-Mg calcite in the Williamson No. 3 coal balls supports the Stopes and Watson (Phil. Trans. Roy. Soc. London B (1909) 200:167-218) model of coal-ball formation as a result of incursions of marine water into coastal mires. It conflicts with stable isotopic data, which suggests that most coal-ball cements formed in meteoric water. However, in our samples, the most common cement is diagenetically altered, low-Mg ferroan calcite, which might yield 'meteoric' oxygen isotopic values. Excellent preservation of plant fossils in coal balls is not necessarily correlated with the presence of original cement. 2012 Elsevier B.V.
