OCCURRENCE OF PYROLITIC CARBON IN CLAIBORNE GROUP LIGNITES, KENTUCKY

The petrology of low-rank, clay-rich lignites from the Claiborne Group in the Jackson Purchase Region of Western Kentucky is being examined using oriented polished blocks. In the course of this investigation, circular to ovoid anisotropic pyrolitic carbon grains, some with red-brown fluorescing centers, were encountered. Their distribution in clay-rich and attrinite-rich areas of the blocks appears to be random. Pyrolitic carbon in coal is known to have five major sources: igneous intrusives, coal metamorphism, tectonic deformation, paleo-oxidation, and combustion. An intrusive, metamorphic, or other tectonic source for pyrolitic carbons in has been ruled out due to the low rank (0.23 Ro) and the lack of folds or large-scale faults intersecting the deposit. Paleo-oxidation is unlikely as the majority of the constituents in the deposit show no evidence of oxidation. Combustion is the most likely source of these grains, although there is little associated inertinite. Low levels of inertinite may be indicative of crown fires, where leaves and twigs are often vaporized and resins burn at very high temperatures. Areas of weak red-brown fluorescence in the center of the pyrolitic carbon grains may support this, as extremely oxidized resins are known to fluoresce close to the red end of the spectrum. Burned and burning particles can be water- or wind-transported for considerable distances; inertinite from charcoal may be not be incorporated into the deposit while heavier, resin-sourced pyrolitic carbon could be. This interpretation of a crown-fire source for pyrolitic carbon in Claiborne Group lignites is consistent with the standard hypothesis that the lignites, which formed on top of clay that have been interpreted to be oxbow-lake fills, were deposited in open-water settings with trees near, but not in, the mire.