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Abstract: Pennsylvanian Lacustrine Limestones in Continental Cyclothems of the Northern Appalachian Basin


The Pennsylvanian of the northern Appalachian Basin is composed of cyclothems with repetitive sequences containing shales, mudstones, claystones, sandstones, coals, and limestones. Four main groups comprise the Pennsylvanian: the Pottsville (Lower to Middle Pennsylvanian), the Allegheny (upper Middle Pennsylvanian), the Conemaugh (lower Upper Pennsylvanian), and the Monongahela (upper Upper Pennsylvanian). The first appearance of freshwater limestones in continental cyclothems occurs within the Allegheny Group. Marine to brackish cyclothems dominate the Conemaugh; continental cyclothems with many freshwater limestone deposits are found in the Monongahela. The paleoenvironment of one continental cyclothem in the Allegheny of western Pennsylvania, the Upper Freeport cyclothem, based on sedimentologic, isotopic, and paleontologic data, is envisioned as an anastomosed river floodplain containing wetlands (carbonate lakes, siliciclastic lakes, ponds, bogs) and soils within a foreland basin. With the presence of both siliciclastic and carbonate lakes in one depositional system, the source area for the lacustrine and fluvial sediments was probably the equatorial Allegheny Mountains to the paleosouth comprising metamorphics, plutonics, as well as Early Paleozoic carbonates. The deposition of nonmarine carbonate sediments during the Pennsylvanian was formerly attributed to a more arid climate. Data from the Alleghenian Upper Freeport cyclothem point exclusively to freshwater deposition in a more humid climate with tectonics as a major controlling factor in sedimentation patterns. The nonmarine limestones of the lower Monongahela Group are also postulated to have been deposited during a more arid phase. Facies and fabric comparisons may indicate whether large-scale climatic patterns indeed affected Upper Pennsylvanian sedimentation.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah