CHARACTERIZING FORCED REGRESSIONS WITHIN THE BEREA INTERVAL, ATHENS COUNTY, OHIO

The Berea interval (Upper Famennian) in the subsurface of Athens County, Ohio is composed of two fine- to very fine-grained sandstone units separated by a gray to red mudstone and is bracketed in many areas by black shales all of which were deposited in the back-bulge zone of the Appalachian foreland basin. Cross sections as well as isopach and log motif maps constructed from 300 gamma-ray logs reveal two high frequency sequences within the interval. The lower sequence averages 15 m in thickness and is composed of the Cleveland Shale and the overlying Cussawego-Second Berea Sandstone that both pinchout to the southeast. The upper sequence is approximately 11 m thick and consists of the Berea Sandstone capping the Bedford Shale. Log motifs within the lower sequence form primarily coarsening upward trends. A gross sandstone isopach map of the interval using a 50% log cut-off shows that the sandstone forms two NE-SW oriented bands, that overall thicken to the northwest, separated by an area of zero sandstone. The sandstones of the upper sequence also thicken to the northwest but are more continuous laterally and are composed primarily of blocky log motifs. The log patterns and isopach maps within both sandstones are interpreted as a product of forced regression with detached shoreface sandstones in the lower sequence and attached shorelines in the upper sequence. The differences between the two sequences suggests an increase in the rate of formation of accommodation during transgression of the upper sequence that preserved more of the sandstones.