LARGE TRACE FOSSILS (DOMICHNIA AND REPICHNIA) IN A KEY EXPOSURE OF THE BENWOOD LIMESTONE OF THE MONONGAHELA GROUP (VIRGILIAN) IN SOUTHEAST OHIO

A half dozen, large (±1 m long), elongated, well-preserved, apparent burrow mounds (domichnia) occur together with long (>2 m), shallow trails (repichnia or natichnia) on a well-exposed bedding surface within the Benwood “limestone” of the Monongahela Group (Virgilian, Gzhelian) in Washington County, Ohio. At this 275-m-long exposure, the Benwood is greenish-gray and light-olive-gray to dark-gray, thin- to medium-bedded, calcareous siltstone or mudstone to argillaceous and carbonaceous limestone, with few yellowish-gray, calcareous, silt-shale interbeds. Parallel bedding dominates, but some wavy beds occur. Pits and voids sized a few mm to several cm are common (weathered-out pisolites). Dark, carbonaceous limestones contain few ostracode fragments and Spirorbis.

The large, elongated mounds are bulbous, with smooth, slightly mammillated surfaces, and appear to have been drawn out by a gentle current, approximating crag and tail structures or greatly oversized current crescents. Leading faces of mounds are subvertical, rising about 10 to 20 cm above adjacent bedding. Lee ends taper away at a gentle angle to bedding. Mounds stretch 0.8 to 1.0 m and have long axes oriented north-south. They are about 0.4 m wide. Broken surfaces on mounds show no internal concentric or hemispheric laminations as in stromatolites. The mounds are unlike any abiotic structure. They do not displace subjacent strata and are not composed of differing secondary minerals as in concretions. The mounds may be similar in origin to Monongahela burrow complexes of Hembree and others (2011). Shallow, lacustrine deposition is educed, and the burrow mounds are likely domichnia of large arthropods, amphibians, or fish—perhaps for aestivation.

The shallow trails on the same bedding exposure are 12 to 15 cm wide, about 1 cm deep, and over 2 m long. Lateral edges are subdued. The trails are remarkably straight, and have NNE, NE, and ENE orientations in the outcrop. These are not “gutters” that form in marine shelf environments. They are similar to body-drag grooves of tetrapod trackways in Givetian rocks of Valentia Island, western Ireland. But these trails completely lack any lateral footprints. These large trails are enigmatic, but close association with burrow mounds in the same exposure hints at amphibian or fish repichnia or natichnia.