Greb, S.F., Eble, C.F., Chesnut, D.R., Jr., Phillips, T.L., and Hower, J.C., 1999, An in situ occurrence of coal balls in the Amburgy coal bed, Pikeville Formation (Duckmantian), Central Appalachian Basin, U.S.A.: Palaios, v. 14, p. 433-451.

Carbonate concretions containing permineralized peat, commonly called coal balls, were encountered in the Amburgy coal, a generally low-ash (9.4 %), but commonly high-sulfur (3.6 %), Middle Pennsylvanian coal of the Eastern Kentucky Coal Field. These are the first coal balls from the Amburgy coal, and one of only a few reported occurrences from the central Appalachian Basin. The coal balls occur in the upper part of the coal, between two paleochannel cutouts at the top of the Pikeville Formation, and immediately beneath a scour with a marine fossil lag at the base of the Kendrick Shale Member, Hyden Formation. The coal is thickest (1.3 m) in a narrow (< 300 m), elongate depression between the bounding paleochannels, and thins toward the occurrence of coal balls. Total biovolume as measured from acetate peels of coal balls indicates cordaites or lycopsids (36.1 % each) dominance. Vertical sampling through one coal-ball aggregate shows zoning from a lower cordaites-dominant (88.6%) assemblage, to a middle, degraded, sphenopsid-rich assemblage, to an upper lycopsid-dominant (88.6%) assemblage. Beneath the coal balls, palynologic and petrographic analyses indicate the basal and middle portions of the bed are dominated by arborescent lycopsid spores and cordaites pollen, and by vitrinite macerals. The top part of the bed, above the coal balls, contains increased inertinite macerals, increased percentages of small fern spores, and variable ash yield (5-21%).

Thickening of the Amburgy coal along a structural low, in combination with basal high-ash yields, vitrinite-dominance, and heterogenous palynoflora, indicate paleotopographic control on initial peat accumulation. Abundant lycopod spores in the basal and middle part of the coal suggest rheotrophic conditions consistent with accumulation in a paleotopographic depression. Apparent zonation preserved in one of the coal-ball masses may document plant successions in response to flooding. Similar percentages of cordaites and lycopods respectively in the zones above and below the degraded incursion interval suggest development of a mixed, successional pattern in response to the flooding. Coal-ball formation may have been facilitated by channeling along the Kendrick ravinement, within a paleotopographic depression, at the split margin of the Amburgy peat, either through direct transmittal of carbonates and marine waters into the peat, or through degassing of the peat beneath the scour.

The Amburgy coal occurs beneath the Kendrick Shale

Coal balls beneath calcareous shale rolls

Microscopic view of coal-ball thin sections showing cellular structure of plant fossil tissues. On the left, cell lumins are infilled with calcareous cement. On the right cell lumins are open.