Petrofacies and Detrital Geochronology of the Lower Pennsylvanian Pottsville Formation from the Northern Appalachian Basin, Pennsylvania

Abstract

The Appalachian foreland basin preserves a late Paleozoic synorogenic clastic wedge formed during the Alleghenian orogeny in response to the collision between Laurentian and Gondwanan continents. The Lower Pennsylvanian Pottsville Formation is one of the Paleozoic clastic wedges that is the focus of this study on the provenance and unroofing history of the Alleghenian orogenic belt. An integrated study of sandstone modal composition, heavy mineral analysis, mineral chemistry, and 40Ar/39Ar geochronology was made on samples collected from both surface exposures in the eastern anthracite basin (thickness ranges from 100-1600 feet) and eight cores from the western bituminous fields (thickness ranges from 20-250 feet) in Pennsylvania. Modal analyses show that sandstones in the western bituminous fields range from quartzarenite to sublitharenite with a mean composition of Qt84F1L15, and that the majority of the sandstone units are texturally and compositionally mature. In contrast, the sandstones from the eastern anthracite field are relatively compositionally and texturally immature. There, sandstones are dominated by sublitharenite to litharenite with a mean composition of Qt70F2L29. The majority of samples from both the anthracite and bituminous fields fall within the “recycled orogenic” provenance field on Qt-F-L and Qm-F-L ternary diagrams. The exceptions occur in the western bituminous fields.

Heavy mineral assemblages in the Pottsville Formation from both fields are dominated by a few groups of minerals, including ultrastable minerals (zircon, rutile, and tourmaline), apatite, sphene, spinel, siderite, and abundant opaques. These mineral assemblages indicate source terrane(s) dominated by metasedimentary/sedimentary units with low- to medium-grade metamorphic, and silicic igneous terranes. Sediments may have been derived from the crystalline, metavolcanic, and/or metasedimentary strata of Alleghenian orogenic belts. However, the presence of chromium- and zinc-rich spinel in samples from both fields suggests exhumation of an arc terrane and ophiolitic belts with ultramafic igneous rocks. Laser 40Ar/39Ar analyses on detrital muscovites of the Sharp Mountain Member from the eastern anthracite field yield mainly Middle Ordovician ages with a mode at 460 Ma. Muscovites from eastern anthracite field were likely derived from Taconic source terrane(s) of the Appalachian orogenic belt further northeast. In contrast, Pottsville sandstone from the western bituminous field contains muscovites dominated by Late Ordovician ages with a mode at 440 Ma and a Late Devonian age with a mode at 360 Ma. Age data from the western bituminous field are interpreted to have probable Appalachian source terrane(s) (e.g., late Taconic and Neoacadian) to the southeast of the basin.