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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 48 (1964)

Issue: 4. (April)

First Page: 538

Last Page: 538

Title: Pottsville Conglomerates in Pennsylvania: Paleocurrents and Origin: ABSTRACT

Author(s): L. D. Meckel, Jr.

Article Type: Meeting abstract


During Pottsvillean 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 which occurs along the northern escarpment of the Allegheny Plateau. The Olean (Sharon) is equivalent stratigraphically to only 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 shelf area at 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 (up to 1,300 feet) wedge-shape conglomeratic sequence (Pottsville formation) was deposited along the southeastern margin of the basin by alluvial fans emerging from the southeastern highland. Initially, deposition was restricted to the trough area, where deposition was uninterrupted from the Mississippian to the Pennsylvanian, but later spread to the shelf. The transport direction was northwest (300-360°), 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 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 toward the southwest, and one in northeastern Ohio, which dispersed material toward the south. These systems transported material obliquely and parallel with the axis of the basin across an erosional Mississippian surface. These orthoquartzitic conglomerates and sandstones were deposited by the "cratonic" dispersal system.

In the central part of the basin, three sheet-like fluvial sand bodies (protoquartzite and orthoquartzite) were deposited. The lower Connoquenessing sandstone was deposited by the "cratonic" system; this sandstone and the Olean conglomerate appear to be part of the same rock stratigraphic unit. The upper Connoquenessing and Homewood sandstones were deposited by the "tectonic" system.

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Copyright 1997 American Association of Petroleum Geologists