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Stratigraphic analysis of the Lower and Middle Pennsylvanian rocks of part of the central Appalachian basin reveals two orders of cycles and one overall trend in the vertical sequence of coal-bearing rocks. The smallest order cycle, the coal-clastic cycle, begins at the top of a major-resource coal bed and is composed of a sequence of shale, siltstone, sandstone, seat rock, and overlying coal bed which, in turn, is overlain by the next coal-clastic sequence.

The major marine-transgression cycle is composed of five to seven coal-clastic cycles and is distinguished by the occurrence of widespread, relatively thick (generally greater than 5 m) marine strata at its base. The Breathitt coarsening-upward trend describes the general upward coarsening of the Middle Pennsylvanian part of the Breathitt Group and includes at least five major marine-transgression cycles.

Chronologic analysis, based on averaging relative age dates determined in previous investigations, provides a duration of 20 my for the deposition of Lower and Middle Pennsylvanian strata of the central Appalachian basin. The eight major marine-transgression cycles that occurred in this interval are calculated to represent an average of 2.5 my each. The average duration of the coal-clastic cycle, in contrast, is calculated to be only about 0.4 my.

The average duration of coal-clastic cycles is of the same order of magnitude (105 yr) as the Milankovitch orbital-eccentricity cycles and matches the 0.4 my second-order eccentricity cycle (Long Earth-Eccentricity cycle). These orbital periodicities are known to modulate glacial stages and glacio-eustatic levels. The calculated periodicities of the coal-clastic cycles can be used to support glacio-eustatic control of the coal-bearing rocks of the Appalachian basin. The 2.5-my periodicity of the major marine-transgression cycle does not match any known orbital or tectonic cycle. The cause of this cycle is unknown, but might represent episodic thrusting in the orogen, propagation of intra-plate stresses, or an unidentified orbital cycle. The Breathitt coarsening-upward trend represents the increasing intensity and proximity of the Alleghanian orogeny.

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