HEAVY MINERAL CONSTRAINTS ON DETRITAL HISTORY OF THE PENNSYLVANIAN POTTSVILLE FORMATION IN THE NORTHERN APPALACHIAN BASIN, PENNSYLVANIA

The Carboniferous Pottsville Formation of Pennsylvania in the northern Appalachian foreland basin is considered as a classic orogenic molasses. The formation is characterized predominantly by sandstone, shale, conglomerate, and a minor amount of limestone and coal. Much of the formation thins into eastward anthracite fields where the Pottsville Formation contains the Tumbling Run, Schuylkill, and Sharp Mountain members, ranging in thickness from 30 to 488 meters. These units are dominated by conglomerate and sandstone, with lesser amounts of shale, siltstone, and coal.

The current research describes the quantitative heavy mineral analysis of the Pottsville sandstones from the bituminous field of western Pennsylvania which helps in locating the sediment source terranes. Pottsville sandstones have a low abundance of heavy minerals (0.2%- 1.1% of total sample weight). Most of the grains (89%) fell in the highly magnetic fractions. Mineral identification has predominantly yielded garnet, amphiboles, rutile, zircon, tourmaline, pyroxenes, and aluminosilicates with abundant opaque minerals. Additionally, chlorite/chloritoid, muscovite/biotite and oxide/hydroxide minerals are present in lesser amount. The low abundance of heavy minerals in the Pottsville sandstone and the abundance of ultrastable minerals reflect high sediment maturity, and indicate intense chemical weathering as the basin was closer to the equator during deposition of the Pottsville Formation. The common occurrence of rutile, garnets, and chlorite/chloritoids suggests derivation from medium– to high-grade regionally metamorphosed terranes. Presence of amphibole, zircon and tourmaline suggest source rocks of both igneous and metamorphic terranes in the Appalachians. Zircon and tourmalines are less abundant than the rutiles suggesting that the contributions from igneous source terranes were not substantial for the Pottsville Formation in Pennsylvania.