Forced Regressive, Wave-Dominated Deltas Within the Berea Sandstone, Athens County, Southeastern Ohio

Abstract

The Berea interval (Upper Famennian) is a widespread siliciclastic unit in the Appalachian basin, which in the subsurface of Athens County in southeastern Ohio, varies in thickness from less than 3 to more than 30 m and may contain one or two sandstones. These sandstones, which are informally referred to as the Berea (upper) and Second Berea (lower), were examined with limited core data augmented with cross sections, isopach maps, and log motif maps constructed using 294 gamma-ray logs. The data show that the Berea sandstones prograded from the southeast as two high frequency sequences. The lower sequence averages 15 m in thickness and is composed of the Cleveland Shale and the overlying Second Berea Sandstone. The gross sandstone isopach map shows that the interval thickens to the northwest, pinches out to the southeast, and contains two NE-SW oriented sandstone belts separated by an area of zero sandstone thickness. The log motif patterns are primarily coarsening upward. The upper sequence is averages 11 m thick and consists of the Berea Sandstone capped by the Bedford Shale. The gross sandstone isopach of the upper sequence also thickens to the northwest but is laterally continuous. The log motifs in this sequence are primarily blocky. The log patterns and isopach maps of both sandstones are consistent with deposition by wave-dominated deltas broadly similar in scale to the modern Grijalva Delta during forced regressions. Both sequences contain fining upward log motifs that can be oriented perpendicular to the main sandstone trends and which are interpreted as products of distributary channels that flowed from a source to the southeast. The lower sequence formed detached shoreface sandstones, whereas the shoreface sandstones of the upper sequence are attached. The difference in offlap patterns suggests that the rate of formation of accommodation during the transgression that capped the upper sequence was greater and preserved more of those sandstones than that capping the lower sequence.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015