REVISING THE LOWER MISSISSIPPIAN STRATIGRAPHIC NOMENCLATURE IN THE PORTSMOUTH, OHIO REGION

Dating back to 1838, numerous geologic studies have evaluated lower Mississippian strata in Ohio, resulting in a varied and diverse nomenclature for these units throughout the state. In the Portsmouth region of southern Ohio, lower Mississippian strata are identified on the statewide Bedrock Geologic Map of Ohio as Logan and Cuyahoga Formations undivided, whose type localities are respectively 55 and 175 miles away in the central and northeastern regions of the state. In 2015, the Ohio Department of Natural Resources, Division of Geological Survey (Ohio Geological Survey) began constructing a detailed lithologic bedrock map of four 7.5-minute quadrangles in the Portsmouth region using a combination of intensive field study, digital elevation models (DEM), and examination of previously existing measured sections and geophysical core records for the area.

In the Portsmouth region, the lower Mississippian section is composed of interbedded siltstones and shales, as well as massive siltstones. This study found that these units are sedimentologically dissimilar to both the shales of the Cuyahoga Formation and sandstones of the Logan Formation at their type localities. These strata are instead correlative to members of the Borden Formation in adjacent north-central Kentucky. Reconciling the Portsmouth area strata with the Borden Formation provides a logical stratigraphic framework for Mississippian units in southern Ohio. Consequently, for the detailed lithologic bedrock map of the Portsmouth region, the Ohio Geological Survey replaced the Cuyahoga and Logan Formations with Borden Formation members: Henley, Farmers, Portsmouth, Nancy, and Cowbell. The nomenclature of this study will be applied to strata in adjacent areas as bedrock mapping continues to the north and east along the strike of the Mississippian section. The Ohio Geological Survey anticipates encountering the “traditional” Cuyahoga and Logan Formations in central Ohio as the Borden Formation members pinch out or as facies change.