Recent studies of vitrinite reflectance, sedimentary compaction, and fluid inclusions indicate that coal-bearing strata of the Pennsylvania Anthacite region were once buried at depths ranging from 6 to as much as 9 km. Vitrinite reflectance anisotropy suggests that anthracite rank was attained by normal geothermal heating largely prior to and during Alleghanian folding. Paleomagnetic ages for the folding and paleobotanical ages of the youngest extant sediments delimit the maximum burial and deformation to a relatively short interval (285 to 270 Ma). In comparison with typical molasse sequences, the implied thicknesses and rates of deposition are excessive, suggesting that much of the former overburden was emplaced tectonically or by extremely rapid sedimentation.
The total implied thickness (about 20 km) of Paleozoic strata in the region is abnormally large for an epicontinental basin. An inferred body of mafic rock embedded in the Precambrian basement, evidenced by the Scranton gravity high (SGH), may have influenced crustal subsidence, especially during the Allegheny orogeny, and produced a large downwarp accounting for the high coal rank. The fact that isorank contours, Paleozoic sediment isopach contours, and a large structural sag all lie roughly parallel to the boundaries of the SGH, oblique to the principal Alleghanian fold trend, lends support to this idea.