COMPILATION MAPPING AND STRATIGRAPHIC REVISION IN THE CENTRAL PENNSYLVANIA VALLEY AND RIDGE PROVINCE: AN EXAMPLE FROM UPPER DEVONIAN MARGINAL MARINE STRATA

The Pennsylvania Geological Survey completed bedrock geologic mapping at 1:24k scale for eight quadrangles (McAlevys Fort, Barrville, Allensville, Belleville, Lewistown, Newton Hamilton, McVeytown, and McCoysville) in central Pennsylvania between 2004 and 2011. These maps are currently being compiled into a single, seamless map product in line with the US GeoFramework Initiative. As such, new mapping is being conducted to rectify discrepancies between individual quadrangles and evaluate the applicability of the stratigraphic framework applied by the previous authors to the study area.

Contact placement and nomenclature usage for Upper Devonian marginal marine and transitional strata are problematic, largely owing to the dynamic and highly heterogeneous nature of these depositional environments. In Virginia, West Virginia, Maryland, and portions of south-central Pennsylvania, the Foreknobs Formation occupies this interval, while in central Pennsylvania, this interval is occupied by the Irish Valley Member of the Catskill Formation. These units are similar but differentiable. The principal difference between the Foreknobs Formation and the Irish Valley Member is a relative abundance of conglomerate and marine fossils within the Foreknobs Formation.

McElroy and Hoskins (2010; 2011) apply the name Irish Valley Member for fossiliferous sandstones interbedded with mudstones and conglomerate cropping out in the Tuscarora syncline in the McCoysville and McVeytown quadrangles. Recent mapping along strike by Bierly (2023) in the Fannettsburg quadrangle demonstrates that the strata within this strike belt are more consistent with the Foreknobs Formation than the Irish Valley Member. Within this context, outcrop and float material were examined south of the McVeytown quadrangle as part of this investigation and revealed that conglomerates, conglomeratic sandstones and fossiliferous sandstones are abundant in this area. This investigation demonstrates that these strata are more appropriately assigned to the Foreknobs Formation and that the lateral transition between facies consistent with the Foreknobs Formation and the Irish Valley Member occurs between the Juniata and Susquehanna Rivers in central Pennsylvania.