SIGNIFICANCE OF MARINE ICHNOFOSSILS IN THE MISSISSIPPIAN BLACK HAND SANDSTONE OF CENTRAL OHIO

The Black Hand Sandstone of central Ohio is a conglomeratic, crossbedded quartz sandstone encased in fine-grained, shallow marine sediments. It is interpreted as a braided-stream deposit within an incised valley. Cross beds in the Black Hand are primarily trough or planar in geometry; bedsets are commonly 1-3 m thick. The unit displays little vertical variation in lithology. Compared to typical valley fill deposits, the Black Hand appears anomalous in its lack of an upper transgressive component. However, a few marine fossils have been reported from a single exposure near Pleasant Hill Dam.

Bioturbation is also rare. One exposure within Rockbridge State Nature Preserve has a diverse ichnofossil association restricted to a 1 m bed in the upper Black Hand. This bed is coarse-grained and cross bedded but not conglomeratic. Bedforms are generally ripple- rather than sand wave- and dune-scale as seen in the rest of the Black Hand. The upper surface of the bed is scalloped with 1-5 cm of relief. Ichnofossils are a mixture of both vertical and horizontal marine traces including Diplocraterion, Thalassinoides?, and Bergaueria?. No body fossils have been observed here. A single example of runzelmarks suggests the presence of a thin algal layer on the bedding surface.

The relationship of this stratigraphic unit to the rest of the Black Hand is unclear. The bioturbated bed at Rockbridge may correlate to the unit at Pleasant Hill Dam reported to contain marine invertebrate remains; no intervening outcrops with recognizable marine influence have been identified. The relatively sudden appearance of a diverse and abundant collection of marine ichnofossils in the upper Black Hand suggests a change to marine conditions late in its depositional history. This marine incursion appears to have been very short lived, quickly superceded by additional fluvial deposits in the uppermost Black Hand. The presence of this thin accumulation confirms the presence of a transgressive component in the valley fill sequence.