A MAPPING PERSPECTIVE ON DEVONIAN SEDIMENTARY ENVIRONMENTS IN SOUTH-CENTRAL PENNSYLVANIA: EXAMPLES FROM THE WESTERN LIMB OF THE BROAD TOP SYNCLINORIUM

Middle to Late Devonian strata crop out in a northeast-southwest striking belt along the western limb of the Broad Top Synclinorium in south-central Pennsylvania. New mapping by the Pennsylvania Geological Survey provides refined descriptions of these units and applies recently revised contact criteria for several intervals in this section. The Tully Member of the Mahantango Formation is the lowest stratigraphy within the study area, consisting of mixed carbonate-clastic facies that vary substantially over short along-strike distances. Intercalated laminated limestone and oolitic grainstone with siltstone suggest distal carbonate platform deposition. Black shale and sparsely fossiliferous gray shale of the Harrell Formation overly the Tully and signal the onset of clastic deposition that persists uninterrupted into the Mississippian. These shales, and the overlying siltstone and shale of the Brallier Formation, are interpreted as sudden deepening and gradual infilling of the Acadian Foreland Basin by deposition in shelf and lower shoreface environments.

Quartz-rich sandstones and quartz pebble conglomerates with discoidal clasts within the Foreknobs Formation indicate the onset of shoreface deposition within the basin. These deposits grade upwards into alternating fossiliferous sandstones, rooted grayish-red mudstones, and olive-gray shale and sandstone, indicating the interfingering of marginal marine Foreknobs and alluvial Catskill Formation environments.

Alluvial deposition continues in the Rockwell Formation, manifested by fining-upward packages of sandstone capped by grayish-red mudstone. The Devonian ends in south-central Pennsylvania with a transgression that places dark-gray, marine-fossil-bearing shale (the Riddlesburg Member) with the mixed alluvial and marginal marine Rockwell Formation.

The Devonian strata of south-central Pennsylvania record significant events in geologic history, including an extinction event and the transition from global greenhouse to icehouse climatic conditions. Detailed mapping of these units and refined biostratigraphy from palynology and conodont analysis establishes a robust stratigraphic framework upon which more detailed studies of this important interval in Earth’s history may be built.