Coalification patterns of the Pennsylvanian coal measures in the Appalachian foreland basin, western and south-central Pennsylvania

Detailed vitrinite reflectance (R_o) profiles have been constructed for nine boreholes penetrating the Pennsylvanian coal measures of the Appalachian foreland basin in western and south-central Pennsylvania. A revised regional coal rank map was compiled with new and pre-existing data. By comparing the calculated R_o profiles with those of measured R_o maximum burial depth, coalification temperature, heat flow, and geothermal gradient were estimated for the coal measures using a one-dimensional transient heat-conductive model combined with the Lopatin-Waples, diagram. The results indicate that increase in coal rank (0.7%-1.9% R_o) and in R_o gradient (0.38%-1.45% R_o/km) across the study area coincides regionally with variation in calculated maximum burial depth (3-5 km), paleotemperature (90-155 °C), paleoheat flow (53 to >75 mW/ m²), and geothermal gradient (26 to >33 °C/ km), and that the region represents different burial and thermal regimes as well as different structural provinces.

At the western margin of the Intraplateau Structural Front, coalification continued after the Alleghanian orogeny. Regional geothermal heating due to depositional burial is thought to have been the dominant heat source responsible for the coalification level in the area. Closer to the eastern edge of the Intraplateau Front, both regional geothermal heating due to burial and advective heating due to the fluid flow accompanying the Alleghanian orogeny were important in creating the observed coalification patterns. A higher coal rank and reflectance gradient, together with a concentric isorank pattern centred in northern Somerset County, appear to represent an area where advective heating due to fluid flow was the dominant heat source.