Coal rank trends in the Central Appalachian coalfield: Virginia, West Virginia, and Kentucky

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Coal rank, determined from vitrinite maximum reflectance and from chemical rank parameters, increases from high volatile C bituminous in portions of the Eastern Kentucky coalfield to low volatile bituminous in Buchanan County, Virginia, and McDowell County, West Virginia, and then decreases significantly to the Dry Fork anticline, 20 km to the southeast. The latter rank decrease, as noted when rank data from individual coals is used across the fold (400–500 m structural relief), can be accounted for by post-tectonic coalification at a paleogeothermal gradient of about 40–45°C/km. Reflectance gradients decrease from about 0.09% R max /100 m in central Buchanan County, Virginia, to 0.06% R max/100 min southeastern Pike County, Kentucky, to 0.03% max/100 m and less among the high volatile A and B coals to the nortwest of Pike County. The reflectance gradients are likely an indication of a decrease in paleogeothermal gradient to the northwest. Depth of burial may have also decreased to the northwest. Northward-trending rank anomalies superimposed on the regional trend in Kentucky may have been caused by subtle changes in the geothermal gradients associated with basement discontinuities or, alternately, by a depth of burial not accounted for at present, perhaps under now-eroded thrust sheets.