Stratigraphic Framework of Cambrian and Ordovician Rocks Across Rome Trough, Central Appalachian Basin

Robert T. Ryder

Restored stratigraphic cross sections drawn primarily through the subsurface of parts of Pennsylvania, Ohio, West Virginia, Kentucky, and Tennessee provide new detailed information to further the understanding of Cambrian and Ordovician sedimentation and tectonics associated with the Rome trough sector of the Appalachian basin. Drilled thickness of the Cambrian and Ordovician sequence ranges from a maximum of about 14,500 ft (4.5 km) along the axis of the trough to a minimum of about 3,500 ft (1 km) on the western flank.

The Lower through lower Upper Cambrian Rome Formation and overlying Conasauga Group, which accumulated primarily during the rifting stage of the Rome trough, occupy as much as two-thirds of the Cambrian and Ordovician sequence. Previously named limestones and shales of the Conasauga that crop out in eastern Tennessee are also recognizable in parts of the Rome trough. In West Virginia, the Maryville Limestone, the middle limestone unit in the Conasauga, thickens at the expense of the overlying Nolichucky Shale, the upper shale unit in the Conasauga, and correlates with a dolomite unit in Ohio, identified there as the Rome Formation.

Throughout the study area, the Upper Cambrian through Lower Ordovician Knox and Beekmantown Groups thicken abruptly across the western margin of the Rome trough. Thickening of this dolomite sequence is controlled by the flexural bending stage of the Rome trough and is particularly abrupt in Pennsylvania and West Virginia, where approximately 1,000 ft (0.3 km) of anhydritic dolomite and limestone of earliest Middle Ordovician age has been added to the upper part of the Beekmantown Group. The well-known regional unconformity at the top of the Beekmantown-Knox sequence along the western margin of the Rome trough can be traced across the trough in West Virginia, Kentucky, and possibly Pennsylvania, on the basis of a thin sandstone unit in the overlying Middle Ordovician rocks. In the Rome trough of Pennsylvania and West Virginia, this hiatus is located within the Beekmantown-Knox sequence rather than above it.

Subsidence of the Rome trough waned significantly in Middle and Late Ordovician, and control of thickness and facies patterns of the limestone and shale sequence of that age shifted primarily to the rapidly subsiding foreland basin adjacent to the Taconic orogene.

AAPG Search and Discovery Article #91041©1987 AAPG Eastern Section Meeting, Columbus, Ohio, October 7-10, 1987.