Coal rank trends in eastern Kentucky

Examination of coal rank (by vitrinite maximum reflectance) for eastern Kentucky coals has revealed several regional trends. Coal rank varies from high volatile C (0.5% Rmax) to medium volatile bituminous (1.1% Rmax), and generally increases to the southeast. One east-west-trending rank high and at least four north-south-trending rank highs interrupt the regional increase. The east-west-trending rank high is associated with the Kentucky River faults in northeastern Kentucky. It is the only rank high clearly associated with a fault zone. The four north-south-trending rank highs are parallel with portions of major tectonic features such as the Eastern Kentucky syncline. Overall, though, the association of north-south-trending rank highs with tectonic expression is not as marked as that with the anomaly associated with the Kentucky River faults. It is possible that the rank trends are related to basement features with subdued surface expression. Rank generally increases with depth, and regional trends observed in one coal are also seen in overlying and underlying coals. The cause of the regional southeastward increase in rank is likely to be the combined influence of greater depth of burial and proximity to late Paleozoic orogenic activity. The anomalous trends could be due to increased depth of burial, but are more likely to have resulted from tectonic activity along faults and basement discontinuities. The thermal disturbances necessary to increase the coal rank need not have been great, perhaps on the order of 10-20 degrees C (18-36 degrees F) above the metamorphic temperatures of the lower rank coals.