HIGH-RESOLUTION SUBSURFACE CORRELATIONS OF HIRNANTIAN (LATE ORDOVICIAN) AND SHEINWOODIAN (WENLOCK, SILURIAN) STRATA IN SOUTHERN AND SOUTHEASTERN OHIO, USA

In Ohio, Upper Ordovician and lower Silurian strata are a major source of aggregate and cement material, and contain extensive natural gas and oil. Despite their importance, the fine-scale subsurface correlation of these units is poorly understood. Subsurface correlations are complicated by several factors, particularly the use of several formal nomenclature schemes in different parts of the state, incorrect and/or inconsistent application of formal unit terminology in subsurface studies, and difficulty comparing log-based subsurface units with the lithologically and paleontologically-defined surface units. To facilitate comparison between surface and subsurface units, a handheld gamma-ray spectrometer was used to create composite gamma curve of uppermost Ordovician and lower Silurian exposures in Adams County, Ohio. Core and geophysical logs from the Aristech #4 well in southeastern Scioto County, Ohio provided additional lithological control for the units in deeper, more clastic-rich environments. All of the members of the Brassfield, Drowning Creek, Alger Shale, and Bisher formations can be distinguished by geophysical logs, and were correlated in 140 wells throughout 10 counties in southern/southeastern Ohio. Five cross sections (two approximate depositional strike, and three follow depositional dip) were created from these wells. These correlations place some commonly used subsurface terminology into the more precise, surface-unit framework of southern Ohio. The “Medina sand” correlates to the Whirlpool Sandstone of New York, and to the Centerville Formation of southern Ohio. The lower Cabot Head extends from the top of the Brassfield Formation or Whirlpool Sandstone to the base of the “red Clinton” sand. The “Clinton” sands interval in southeastern Ohio generally correlates with the thin-bedded and shaly members of the Brassfield Formation. The upper Cabot Head spans from the top of the “stray Clinton” or “red Clinton” sand through the Plum Creek Shale. The “Packer Shell” in southern Ohio is equivalent to the Oldham Limestone where the Lulbegrud Shale is present, and to the Oldham Limestone and Waco limestone where the Lulbegrud is thin or absent. The “Rochester Shale,” as generally used in most subsurface correlations, corresponds to the base of the Lulbegrud to the top of the Estill Shale.