HIRNANTIAN(?) THROUGH WENLOCK (LATE ORDOVICIAN THROUGH LLANDOVERY) CONODONTS AND STABLE CARBON ISOTOPES FROM THE MEDINA AND CLINTON GROUPS, SUBSURFACE CENTRAL OHIO, DISTAL APPALACHIAN BASIN, USA

The chronostratigraphic correlation of the latest Ordovician through early Silurian Medina and Clinton groups is well studied in outcrop, with decades of research covering the litho/sequence stratigraphy, conodont biostratigraphy, and carbon isotope stratigraphy where the units are exposed or in the nearby subsurface. However, farther away from outcrop belts the subsurface distribution of these units is not understood at precise chronostratigraphic resolution. This lack of understanding is due in part to entrenchment of informal drillers terminology and a general disconnect between subsurface researchers utilizing geophysical well logs and geologists studying the biostratigraphy, chemostratigraphy, and litho/sequence stratigraphy at outcrop. As part of a broader project to bring formal, member-scale correlation and nomenclature of strata comprising the Medina and Clinton groups to the subsurface of Ohio, Ohio Geological Survey core 3241 from Pickaway County (central Ohio) was sampled for conodont and carbon isotope biochemostratigraphy.

Strata comprising the Medina and Clinton Groups in Pickaway County consist of alternating limestone-rich and shale-rich units, with a few iron-rich limestone and ironstone beds. In contrast to outcrops in Ohio, the carbonates in this core are completely non-dolomitized. Forty limestone and calcareous mudrock intervals were sampled for conodonts. These samples were processed using formic acid dissolution, magnetic separation, and heavy liquid density separation. The remaining residue was picked for conodonts using a light microscope. Recovered conodonts include the genera Distomodus, Ozarkodina, Pterospathodus, Kockelella, Pseudooneotodus, Panderodus, Walliserodus, Pseudolonchodina, and Oulodus(?).

Micritic matrix was drilled at ~1 to 2-ft (0.3 to 0.6-m) intervals and analyzed for stable carbon isotope values. The resulting isotope profile is unusually muted, with δ¹³C_carb variation less than 0.5 ‰ throughout the Medina Group. The Ireviken excursion is present in the upper Clinton, although smaller (only ~+2 ‰ shift) than other examples from Ohio. The unusually attenuated isotope values may be related to processes that prevented dolomitization of the strata in Pickaway County, but this hypothesis warrants further investigation.