Richly fossiliferous strata of the Cincinnati area have a long history of study. An abundance of lithologic names have been applied to various shallow marine depositional facies across the Cincinnati Arch in Indiana, Kentucky and Ohio, U.S.A. The current study builds upon recent, high-resolution stratigraphic studies of the Edenian Stage, Kope Formation, and applies a similar approach to the Maysvillian Stage strata of the Cincinnati Arch. Strata of the Maysvillian Stage were deposited in shallower water conditions than the underlying Kope Fm., resulting in an increased localization of facies. Maysvillian Stage strata have previously been treated as a mosaic of local facies with little to no continuity or small-scale cyclicity; however, correlation of time-synchronous marker horizons and sequence stratigraphic units the Maysvillian Stage throughout the Cincinnati Arch demonstrates that this is not the case and suggests widespread, allocyclic controls on sedimentation.
Based upon the documentation of widely traceable horizons the current study establishes a fourth order sequence stratigraphy for the Cincinnati Arch. As an independent test of lithologic-cycle correlation, the current study has utilized magnetic susceptibility in vertical successions, and attempts to quantify small scale cycles within the Maysvillian Stage. Magnetic susceptibility in combination with field observations has revealed previously overlooked unconformities, such as the basal Bellevue Mbr. unconformity, which oversteps the Miamitown Shale to the east of Cincinnati. In addition, intervals of relatively thick siltstone beds, commonly with soft-sediment deformation, are identified as an indicator of falling stage systems tracts, formed during forced regressions. In contrast, intervals of relatively compact skeletal pack and grainstones with evidence of condensation, such as hardgrounds indicate transgressive systems tracts. Using these criteria the current study provides a newly established fourth order sequence stratigraphic framework, and a reevaluation of third order depositional sequences on the Cincinnati Arch. Revisions include reinterpretation of key surfaces, some of which record flooding surfaces, previously identified as sequence boundaries, identification of previously unrecognized sequence boundaries, and replacement of “shazam” lines of imaginary facies transition by a more realistic framework of small-scale cycles.
Empirical evidence of sea level variation within small-scale cycles, based upon sedimentary structures in shallow water facies, e.g., changes from mud-cracked strata to fully marine carbonate deposits, suggests sea-level change as the principal mechanism for small-scale cycle generation; widespread correlation of these cycles indicates an allocyclic forcing mechanism, probably eustasy. Differences in thickness of mudstone deposits between Cincinnatian units are attributed to tectonic and climatic changes occurring in the orogenic hinterland during the Late Ordovician, affecting the supply of fine-grained siliciclastics to offshore areas. Synthesizing sequence stratigraphy of the Cincinnati Arch with observation of coeval strata in New York, and the Southern Appalachians the current study also recognizes variations in the magnitude of base-level change to peripheral foreland bulge migration occurring during waning phases of the Taconic orogeny. Throughout the Cincinnatian, and other Paleozoic strata a three-fold motif of lithologic units has been observed: the current study offers a discussion of the naming of lithologic units and the hierarchical nature of depositional cycles associated with this phenomenon.