Origin of Pottsville Conglomerates (Pennsylvanian) in the Central Appalachians

Abstract

During Early and Middle Pennsylvanian time, two major and strikingly different conglomerates were deposited in the northern part of the Central Appalachians. These are the Pottsville Formation, which crops out in the anthracite area, and the Olean (Sharon) Conglomerate, a member of the Pottsville Group, which occurs along the northern escarpment of the Allegheny Plateau. The thin Olean (Sharon) Conglomerate is equivalent to only a part of the thicker Pottsville Formation. An integrated stratigraphic, petrologic, and paleocurrent study was made of these conglomerates to reconstruct the clastic dispersal system, the geometry of the basin, and the conditions of deposition. Isopach, lithofacies, cross-bedding, maximum pebble size, and petrographic data were used to attain these objectives. The depositional basin, an asymmetric, elongate trough which trends northeast-southwest, consists of (1) a narrow zone of maximum subsidence (trough) along its southeastern margin and (2) a broad, stable platform area to the northwest. The basin was bounded on the southeast by a tectonic source land composed dominantly of metamorphic and sedimentary rocks and on the north by a stable cratonic area consisting largely of sedimentary and low grade metamorphic rocks. Both areas contributed coarse gravels to the depositional basin. A thick (as much as 1300 feet) wedge-shaped conglomeratic sequence (Pottsville Formation) was deposited along the tectonically active southeastern margin of the basin by streams (alluvial fans) emerging from the adjacent highland. Initially, deposition was restricted to the trough area, where deposition was uninterrupted from the Mississippian to the Pennsylvanian, but later spread to the platform. The transport direction was to the northwest, transverse to the axis of the basin and down the paleoslope. The depositional strike paralleled the axis of the basin. The stratigraphic section thins downslope from the fall line, located near what is now Philadelphia. This “tectonic” dispersal system deposited orthoquartzitic conglomerates and lithic sandstones (protoquartzites). The thin (generally less than 50 feet), sheet like conglomerate deposit (Olean-Sharon) along the stable northern margin of the basin was deposited by two contemporaneous fluvial systems: one located in north-central Pennsylvania, which dispersed material to the southwest, and one in northeastern Ohio, which dispersed material to the south. These systems transported material obliquely and parallel to the axis of the basin across a Mississippian erosional surface. These orthoquartzitic conglomerates and sandstones were deposited by the “cratonic” dispersal system. In the central part of the basin, three laterally extensive fluvial sand bodies (protoquartzite and orthoquartzite) were deposited. The Lower Con-noquenessing Sandstone was deposited by the “cratonic” system and is related to the Olean (Sharon) Conglomerate. The Upper Connoquenessing and Homewood Sandstones were deposited by the “tectonic” system and are downcurrent extensions of the upper Pottsville deposition in the anthracite area.