Geologic controls on thermal maturity patterns in Pennsylvanian coal-bearing rocks in the Appalachian basin

Thermal maturation patterns of Pennsylvanian strata in the Appalachian basin were determined by
compiling and contouring published and unpublished vitrinite reflectance (VR) measurements. VR isograd
values range from 0.6% in eastern Ohio and eastern Kentucky (western side of the East Kentucky coal field) to
greater than 5.5% in eastern Pennsylvania (Southern Anthracite field, Schuylkill County), corresponding to
ASTM coal rank classes of high volatile C bituminous to meta-anthracite. VR isograds show that thermal
maturity of Pennsylvanian coals generally increases from west to east across the basin. The isograds patterns,
which are indicative of maximum temperatures during burial, can be explained by variations in paleodepth
of burial, paleogeothermal gradient, or a combination of both. However, there are at least four areas of
unusually high-rank coal in the Appalachian basin that depart from the regional trends and are difficult to
explain by depth of burial alone: 1) a west-northwestward salient centered in southwestern Pennsylvania;
2) an elliptically-shaped, northeast-trending area centered in southern West Virginia and western Virginia;
3) the eastern part of Black Warrior coal field, Alabama; and 4) the Pennsylvania Anthracite region, in
eastern Pennsylvania. High-rank excursions in southwest Pennsylvania, the Black Warrior coal field, and the
Pennsylvania Anthracite region are interpreted here to represent areas of higher paleo-heat flow related to
syntectonic movement of hot fluids towards the foreland, associated with Alleghanian deformation. In
addition to higher heat flow from fluids, the Pennsylvania Anthracite region also experienced greater depth
of burial. The high-rank excursion in southwest Virginia was probably primarily controlled by overburden
thickness, but may also have been influenced by higher geothermal gradients.