A COMPARATIVE SANDSTONE PETROGRAPHIC AND HEAVY MINERAL STUDY OF THE LOWER PENNSYLVANIAN POTTSVILLE FORMATION FROM THE BITUMINOUS AND ANTHRACITE FIELDS IN PENNSYLVANIA

The Pottsville Formation represents the clastic wedge, deposited in the Appalachian foreland basin during the Alleghenian orogeny in response to the collision of Laurentia with Gondwana. These siliciclastic sediments which were deposited near the paleo-equator, consist of predominantly interbedded sandstone, siltstone, conglomerate, conglomeratic sandstone, shale, limestone, and coal beds. The formation was deposited mainly in two locations in Pennsylvania; at the western bituminous field (20-250 feet) and the eastern anthracite field (100-1600 feet). The present study provides sandstone petrographic and heavy mineral analyses to compare the sandstones of these two areas and reveal the provenance of these fluvio-deltaic to shoreface deposits.

Pottsville sandstones from both locations mainly consist of monocrystalline and polycrystalline quartz, sedimentary lithics (chert, shale), metamorphic lithics (phyllite, schist, and quartzite) with very little feldspars. No volcanic lithics have been identified. Sandstones in eastern anthracite field contain relatively larger-grain sediments and more lithic fragments than those of the western bituminous field. The lower part of the Pottsville sandstones is more lithic arenites in anthracite field whereas the sandstones are more quartzose in the bituminous field. Granoblastic textures are common in polycrystalline quartz of both fields which indicate a medium to high-grade metamorphic source terrane. The predominance of quartz and the scarcity of feldspars indicate relatively high sediment maturity. Compositions of these sandstone suites (Qt₇₀F₃L₂₇ in western bituminous field and Qt₅₅F₅L₄₀ in eastern anthracite field) reflect a recycled orogenic provenance, which resemble closely with the composition of the Pottsville sandstones (Qt₆₀F₇L₃₃) from the greater Black Warrior basin in Alabama. The abundance of sedimentary and metamorphic lithic fragments indicates that the source areas were mostly sedimentary and metamorphic rocks. Moreover, the low abundance of heavy minerals and the presence of stable heavy minerals (ZTR-20%) in both locations indicate high sediment maturity and intense chemical weathering. The predominance of rutile and garnet also suggest derivation from medium-grade metamorphic source terranes in the Appalachians.