USING HIGH-RESOLUTION GAMMA RAY LOGGING AS A METHOD TO REFINE REGIONAL CORRELATIONS: EXAMPLE FROM THE WAYNESVILLE FORMATION OF OHIO

Gamma ray logs are a useful but under-utilized tool for correlation of Cincinnatian aged strata in Ohio. Most beds of the Upper Ordovician in the subsurface are logged as “undifferentiated Cincinnatian.” However, through detailed stratigraphic examination of drill cores and corresponding Gamma ray logs, recognizable Cincinnatian units can be found in the subsurface. For this study a high-resolution Gamma log was generated for a drill core through the upper Cincinnatian from Fairborn, Ohio. The Gamma ray log was created with a core scanner, which collected high-resolution total gamma ray data logged directly from the drill core. The Upper Ordovician Waynesville Formation was the targeted stratigraphic unit for this study due to an existing detailed framework of identifiable marker beds and its unique Gamma ray log signature. Presently, the Waynesville Formation has a poorly understood subsurface record within the region. The high-resolution Gamma ray log from Fairborn Ohio was compared with other Gamma logs generated from boreholes across Ohio. The primary Gamma ray log used for comparison was taken from Cedarville, Ohio about 13 km to the east of Fairborn. Owing to the proximity of the two drill cores the Waynesville Formation was found to be similar lithologically. Correlations of key lithologic units within the Waynesville Formation were easily made despite the lower resolution of the borehole Gamma logs compared to the Fairborn Gamma log. This comparison led to noticeable patterns in the Waynesville Formation, which can be used to better correlate through the subsurface of Ohio and into Indiana and Kentucky. These results, in tandem with lithostratigraphic patterns and biostratigraphic markers that were observed in the drill cores during this study led to refinements of correlation. The data points to both abrupt westward thinning of the section and to the presence of two subtle, but field-identifiable disconformities. These disconformities truncate units below recognized sequence boundaries from east to west. Data from multiple stratigraphic sources, including the Gamma ray logs, permit stronger correlations of the Waynesville Formation both regionally and within Ohio. Thus, high-resolution Gamma ray logging may be used to successfully correlate sections even in the absence of key marker beds and useful biostratigraphic tools.