THE MOLLUSK-RICH ORDOVICIAN MIAMITOWN SHALE MAPPED FROM CINCINNATI TO THE BLUEGRASS: PROBING CONTEMPORANEOUS PERITIDAL DEPOSITS TO DECIPHER THE PALEOBATHYMETRIC PROBLEM OF A PUZZLING PELITE

While most Upper Ordovician (Katian stage) strata in the Cincinnati region can be characterized as mixed carbonates and mudstones, there are a few carbonate-poor silty mudstone units including the Maysvillian Miamitown Shale and the Richmondian Waynesville Formation. These are characterized by molluscan faunas that contrast with the typical brachiopod-bryozoan fauna of the Cincinnatian, or, for that matter, of the Lower Paleozoic in general. The paucity of more widespread common taxa makes it difficult to use such assemblages in determining paleoenvironmental conditions, particularly water depth. Thus, combined with the lack of distinctive sedimentological indicators, it is difficult to say if these units formed in deeper, shallower, or comparable water depths to the underlying and overlying strata.

We address this problem by correlating the interval that includes the Miamitown Shale, the underlying Fairview Formation, and the overlying Bellevue Member from the Cincinnati area (where all units are subtidal) into the equivalent subtidal-to-peritidal Calloway Creek and Ashlock Formations of central Kentucky, where slight changes in water depth should be enhanced by the overall shallowness. This effort incorporates biostratigraphy, magnetic susceptibility, and petrography from multiple localities along with cores and geophysical logs from boreholes along the northern edge of the Cincinnati Arch outcrop belt in Cincinnati and southeastern Indiana, the western edge of the arch south of Bardstown, Kentucky, and the southern part of the arch between Danville, Kentucky and Richmond, Kentucky.

The Miamitown shale appears to be absent on the Eastern Limb of the Cincinnati Arch, meanwhile, on the Western Limb the Miamitown is present as peritidal-to-lagoonal dolomitic calcareous mudstones exhibiting variations in water depth evidenced by alternations between beds with mud cracks and limestone beds with marine fossils. The overlying and underlying units are more characteristically limestone rich and contain brachiopods and bryozoans of the normal marine community. This implies that the Miamitown Shale and perhaps other mollusk-rich mudstone units were deposited during a period of shallowing with small-scale-higher-frequency fluctuations in sea level.