THE FIRST STABLE CARBON ISOTOPE AND ELEMENTAL (PXRF) CHEMOSTRATIGRAPHIC PROFILES OF THE COLUMBUS AND DELAWARE LIMESTONES (MIDDLE DEVONIAN, EIFELIAN – GIVETIAN) OF OHIO

New geologic mapping and critical minerals studies provide an opportunity to advance understanding of the Onondaga-Marcellus (Eifelian) interval across the Appalachian Basin and into the midcontinent. The Onondaga Limestone serves as an important datum for subsurface mapping in the Appalachian Basin. It extends from its type area in central New York, south through the central Appalachian Basin and westward toward the distal basin margin in central Ohio where it is considered age-equivalent to the Columbus Limestone. Across this transect part of the shaly Marcellus Subgroup grades into the Delaware Limestone. To further fine tune correlations, carbon isotope (δ¹³C_carb) and pXRF elemental profiles of the Columbus-Delaware limestone succession were obtained from OGS Core 2648 from Delaware County as part of a collaboration between the Illinois State and Ohio geological surveys.

In central Ohio the Columbus-Delaware (limestone-on-limestone) contact is difficult to distinguish using geophysical logs. Preliminary work indicates there may be subtle petrophysical indicators useful for separating the units but will require careful comparison of several cores from central Ohio to ensure that well log-based and lithostratigraphic correlations remain accurate farther away from the Devonian outcrop area. Lithostratigraphic correlations may be improved by integration of pXRF data.

The stable carbon isotope profile presented here is the first obtained from Ohio in a project aimed at advancing regional chronostratigraphic correlation of the Eifelian from the Appalachian Basin across the midcontinent. Comparison of the δ¹³C profile from Ohio to profiles from Pennsylvania and New York indicate that stable carbon isotopes have some utility for intra-basin correlation and provides some tentative evidence that the uppermost Onondaga Limestone in New York may be equivalent to the basal Delaware Limestone. More work is planned to further document variability in the regional Eifelian δ¹³C record.