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Clay mineral analyses of underclays directly below the Upper Elkhorn Coals (eastern Kentucky) and the Lower Kittanning Coal (western Pennsylvania) demonstrate systematic lateral and vertical variations that include changes in the kaolinite-illite ratio, mica loss ratio, weathering ratio, apparent thickness of mica, and distribution of chlorite. Clay mineral analyses of associated, unweathered shales indicate that approximately 30 percent of the regional variation in Lower Kittanning underclay mineralogy is inherited from the parent material. The remaining variation is attributed to in situ pedogenesis.

Petrographic analysis of thin sections from a fluvial sandstone subjacent to the Upper Elkhorn underclay suggests that position of the ground-water table controlled pedogenesis. Two distinct alteration zones separated by a diffuse, subhorizontal boundary are present in the sandstone: an upper zone characterized by kaolinization of feldspars, dissolution of chlorite and detrital dolomite, and absence of siderite; and a lower zone characterized by ferron dolomite replacement of both detrital feldspar and detrital dolomite, and authigenic pore fillings of chlorite and siderite.

These systematic changes in sandstone and underclay mineralogy are consistent with a pedogenic model in which the process of podzolization was controlled by position of the ground-water table and topography. In this model, the main phase of organic material accumulation occurred above the underclay after water-table levels intersected the land surface as a result of compaction, subsidence, or marine transgression. Regional gravity and structure data in western Pennsylvania further suggest that syntectonic movements were the fundamental controls on regional topography and ground-water levels, and thus, pedogenesis.

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