The Huntersville Chert (Onesquethawan Stage) of the central Appalachians was deposited in a detrital sediment-starved basin where a restricted sea hosted mainly silicisponges of probable Demospongid fauna. The Huntersville Chert grades into the Onondaga Limestone to the west and the Needmore Shale to the east. These Onesquethawan rocks record an initial transgression followed by regression, maximum transgression, and a final regression. Basement sub-blocks as growing structures influenced sedimentation. Differential subsidence of basement sub-blocks and eustatic changes in sea level periodically exposed the basin margins or allowed reworking of low-energy shoreline and shoal sediments. Five mappable stratigraphic units are recognized in the Onesquethawan Stage in the chert-filled basin covering the Rome trough in northern West Virginia, southwestern Pennsylvania, and northeastern Ohio. Shale tongues extending from the east interbed with the chert where the eastern West Virginia arch was low in northern West Virginia and western Maryland, but do not to reach the basin center. Carbonates accumulated over the shelf to the west of the chert basin and periodically over parts of the southern, eastern, and central arches. A moderately steep ramp is interpreted for the facies change from chert to limestone in proximity to the western margin of Rome trough. Are ancient reefs located at this shelf margin? The upper chert lithofacies is the most productive reservoir within the Huntersville.

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