LITHOLOGIC AND FAUNAL PATTERNS RELATED TO SHORELINE POSITION IN THE AMES MEMBER OF THE GLENSHAW FORMATION (VIRGILIAN, UPPER PENNSYLVANIAN)

The Ames Member of the Glenshaw Formation (Virgilian, Upper Pennsylvanian) was deposited during the last significant marine incursion of the Paleozoic Era in eastern North America. Exposures of the Ames Member show significant variation in lithologic and paleontological characteristics from the northwestern margin of the Ames outcrop belt in eastern Ohio, to the southeastern edge of the Ames outcrop belt in central West Virginia. In eastern Ohio, outcrops of the Ames are dominated by carbonate lithologies; contain an open marine fauna composed mostly of brachiopods, crinoids, bivalves, and corals; and have an average thickness of less than one meter. In north central West Virginia, outcrops of the Ames are dominated by fine grained clastic lithologies including black shales, contain faunal assemblages that range from open marine to restricted, and have an average thickness of greater than 3 meters. In addition, many faunal groups in the southeastern outcrop belt exhibit miniatureism, indicative of lowered oxygen conditions.

Previous research has shown that syndepositional tectonics affected the thickness of the Ames Member in the southern portion of the Appalachian Basin. The variations listed above within the Ames Member in the Appalachian Basin is also thought to be controlled by processes related to syndepositional tectonics. Increased subsidence and/or sedimentation rates have been related to the proximity of the eastern margin of the Appalachian Basin to the recently uplifted Appalachian highlands during Late Pennsylvanian times. Differential subsidence would have depressed the eastern part of the basin and the Ames seafloor would have the asymmetric profile similar to a foreland basin. Increased subsidence and sedimentation rates in the southeastern Appalachian Basin would have (1) promoted greater outcrop thickness; (2) inhibited carbonate production; and (3) increased variation in depth-related benthic faunal assemblages. Decreased subsidence and/or sedimentation rates in the northwestern Appalachian Basin would have (1) limited outcrop thickness; (2) promoted carbonate production; and (3) reduced variation in depth related benthic faunal assemblages.