SEDIMENTOLOGIC ANALYSIS OF THE BENWOOD LIMESTONE (MONONGAHELA GROUP) IN MORGAN COUNTY, OHIO

The Benwood Limestone (Monongahela Group, Pennsylvanian) was deposited in the Dunkard Basin of Ohio, West Virginia, and Pennsylvania. This basin was situated within the equatorial zone approx.10 degrees S in the Late Pennsylvanian. The Benwood is part of the Benwood cyclothem which includes coal and siliciclastics and is 8.5 m thick in Morgan County of southeastern Ohio. The limestone has been previously interpreted as marine dry mudflat, marine tidal flat, as well as open shallow to deep lacustrine in origin. Petzold (1990) (PhD – Indiana Univ., 213p) most recently described the Benwood, mostly from localities in Pennsylvania and West Virginia, as containing three main facies sequences: (1) a deep-water lacustrine to terrestrial sequence; (2) a sequence of shallow lake to shoreline carbonate-siliciclastic packages, and (3) a lacustrine to dry mudflat sequence.

With little work done to date on the Benwood Limestone in Ohio, two sections in Morgan County were measured and sampled. These two sections, 8.4 m and 4.7 m long, contained thick carbonate beds interbedded with thinner shales. Three main facies, averaging dm in thickness, were recognized: (1) gray to green shales, (2) mostly massive biomicrites containing rare clay-lined tubules and intraclastic textures, and (3) quartz-rich biograinstones containing bivalves, ostracods, gastropods, and spirorbid worm tubes. Facies (1) is interpreted as a quiet water paleoconditions while facies (2) may represent lacustrine to palustrine conditions with evidence for subaerial exposure and rooting. In facies (3) angular quartz grains are matrix-supported in micrite mixed with a traction-load texture of bioclastic to intraclastic debris grainstone. This mixture of marine and non-marine textures and fossils suggests that the Benwood Limestone in Ohio was an open lake system with intermittent marine influence. A freshwater to brackish coastal lake setting affected by marine storm surges is envisioned.