Depositional Setting of Ordovician and Cambrian Rocks in Central Appalachian Basin along a Section from Morrow County, Ohio, to Calhoun County, West Virginia

Robert T. Ryder

A 200-mi (320 km) long restored stratigraphic section from Morrow County, Ohio, to Calhoun County, West Virginia, contrasts Ordovician and Cambrian rocks deposited on a relatively stable shelf with those deposited in rift and postrift basins. Lithologic data are from commercial logs and from detailed descriptions of cores in five of the nine drill holes used to construct the section. Particularly instructive was the 2,352 ft (717 m) of core from the Hope Natural Gas 9634 Power Oil basement test in Wood County, West Virginia.

Rift basin deposits are dominated by medium to dark-gray argillaceous limestone, argillaceous siltstone, and by green-gray to black shale of probable subtidal origin. Approximately 10% of the subtidal sequence is fine to coarse-grained, glauconitic, quartzose sandstone. Red shale is uncommon except near the northwestern margin of the rift basin where tidal-flat conditions probably existed.

Dolomite is the dominant rock type in the postrift basin and adjacent stable shelf deposits. The Lower Ordovician part of the Beekmantown Group in the postrift sequence is characterized by a very fine to medium crystalline dolomite that has abundant features indicative of tidal-flat deposition. Zones composed of thrombolitic stromatolites and/or stromatolite-derived debris exhibit fair to good vuggy porosity, particularly where they have been exposed to subaerial processes. Anhydritic dolomite and interbedded dark-gray shale with a moderate organic content in the lowest Middle Ordovician part of the Beekmantown grade northwestward into green-gray shale and very fine crystalline dolomite of the Middle Ordovician Wells Creek Formation on the stable shelf.

The upper part of the postrift sequence, composed of the Middle Ordovician Black River Limestone, the Middle Ordovician Trenton Limestone, and Middle and Upper Ordovician Antes (Utica) Shale with a high organic content, represents deposition in gradually deepening water on an open shelf.

AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.