PROBABLE ORGANIC-WALLED MICROFOSSILS FROM THE MIDDLE RUN FORMATION (NEOPROTEROZOIC?) OF OHIO, U.S.A

The Middle Run Formation was discovered and described in the early 1990’s by the Ohio Department of Natural Resources, Division of the Geologic Survey, while drilling a deep-core in the Warren County (DGS 2627). The unit is composed of roughly 1800m of arkosic sandstone with some siltstone intervals and containing cross-bedding and some small-scale ripple marks throughout. Stratigraphically, the unit is located below the Cambrian-aged Mt. Simon Sandstone and above the Precambrian basement. Sedimentologically, the unit is believed to have been deposited as rift-fill which may be associated with the break-up of Rodinia. Seven samples were taken as a strictly exploratory project from some of the finer lithologic intervals in the middle part of the cored unit. The samples yielded a low-diversity, moderately abundant assemblage of probable organic-walled microfossils. The specimens show a variety of vesicle shapes ranging from round to ovate to sub-triangular. Some of the specimens show slight equatorial thickening, while are simple spheroids showing a granular structure but not show a distinctive cell wall. A number of the specimens have evidence of some ornamentation, contain short, rounded processes. The specimens suggest that early prokaryotes were starting to inhabit a variety of environmental niches during the Precambrian.