Isopach and lithofacies maps of eight Silurian formations illustrate that during Middle Silurian time a broad shelf emerged from southeast to northwest across southern West Virginia, separating an adjacent basin on the north from the rest of the Appalachian trough in Ohio, Kentucky, and Tennessee. This subdivision of the once-continuous Appalachian trough produced Middle and Late Silurian sedimentation patterns which are quite different from those of the Late Ordovician and Early Silurian.

The Lower Silurian Tuscarora Sandstone (as well as the Upper Ordovician Juniata Formation) was deposited throughout an extensive, unbroken area from Tennessee to New York under fluvial to shallow-marine conditions--an onshore-offshore complex from east to west. Likewise, the Middle Silurian Rose Hill Formation was laid down in this same linear trend, with shallow-water ferruginous sandstones confined to the eastern margin. The Rose Hill isopach map indicates the beginning of differential subsidence with a major thickening of the formation in central West Virginia. The broad shelf and adjacent basin developed with sedimentation of the overlying Keefer Sandstone--high-energy sandstone on the shelf surrounding a central region of shale. During deposition of the Middle Silurian McKenzie F rmation and Upper Silurian Williamsport Sandstone, the same basin-shelf relation continued. Distinct members of the McKenzie indicate subaerial to shallow-subtidal conditions on shelves on the east, south, and west; greater subsidence identifies the enclosed basin. Clean Williamsport Sandstone accumulated on the southern shelf while immature sandstone was deposited in the northeast on mud flats associated with the Bloomsburg delta; simultaneously dolomite formed within the basin. The Wills Creek Formation represents uninterrupted deposition of fine clastic and carbonate materials in the basin and on mud flats in the northeast. Restriction of the sea in Late Silurian time is shown by the basin-centered Salina evaporites (anhydrite and halite) and the surrounding-shelf carbonate rocks (Ton loway Limestone). Shelves on three sides of this basin essentially isolated it from a Salina basin in Ohio.

Higher energy environments were associated with the Silurian shelves during deposition of both sands and carbonate material, whereas shales, carbonate sediments, and evaporites accumulated in the adjoining basin under low-energy conditions. Known hydrocarbon reservoirs in the Keefer, McKenzie, Williamsport, and Salina are limited to the respective shelves, and future exploration for gas should be concentrated in those areas.