SEQUENCE AND EVENT STRATIGRAPHY OF THE MEDIAL SILURIAN OF EASTERN NORTH AMERICA: COMPARISON OF APPALACHIAN BASIN AND CINCINNATI ARCH SUCCESSIONS

Depositional sequences in upper Llandovery to Wenlock strata of southern Ohio-northern Kentucky exhibit marked similarities to sequences S-IV through S-VI recognized in the northern Appalachian Basin (Ontario, NY, PA). As with the coeval (mid Telychian) basal transgressive (TST) carbonates in Ontario, the Dayton Dolostone in Ohio oversteps a regionally angular unconformity that bevels older Silurian strata NW onto the Findlay-Algonquin Arch. The overlying gray Estill-Osgood shale (P. amorphognathoides Zone) records a major late Telychian S-IV highstand systems tract (HST) represented by the Willowvale-upper Rose Hill shales in the Appalachian Basin. Newly discovered K-bentonites in the upper Estill may correlate with those in the Willowvale. Sandy, crinoidal dolo-packstones in the lower Bisher Fm. (lower Wenlock; K. ranuliformis Zone), that sharply overlie the Estill, are coeval with the Irondequoit Limestone (NY, Ont.) and Keefer Sandstone (PA, MD), the TST of S-V. In southern Ohio, these dolostones are overlain by ~3 m of gray, sparsely fossiliferous "Massie Shale", a tongue of Rochester Fm., the HST of S-V. However, unconformities truncate this shale both to the SE and NW. A 0.5 m, sandy, contorted dolostone that locally overlies the shale, may correlate with the DeCew Dolostone, a widespread (>150,000 km2) deformed interval (seismite?) in NY and Ontario. The sharp base of the overlying Lilley Formation (Wenlock;O. sagitta rhenana Zone) represents the sequence SV-VI boundary, the base of the Lockport Group. Crinoidal dolostones (TST of S-VI) are overlain by argillaceous, biohermal dolostones comparable to the Gasport Formation reef zone (HST of S-VI) in NY. Thin K-bentonites in the Lilley Fm. (OH) also may also be present in the Gasport. Thus, sequences recognized in the Appalachian Basin can be extended into the Cincinnati Arch area, indicating eustatic fluctuations. However, sequences were locally modified or truncated by epeirogenic events associated with early phases of the Salinic Orogeny.