REVISED SEQUENCE STRATIGRAPHIC FRAMEWORK IN AERONIAN AND TELYCHIAN STRATA (LLANDOVERY; SILURIAN) OF THE EAST CINCINNATI ARCH

Six unconformity-bound packages of strata have been identified in the lower Silurian of eastern North America, representing third-order cycles of sea level rise and fall. These sequences, S-I to S-VI in ascending order, are roughly equivalent to group-level lithostratigraphic units in the Appalachian Basin. Previous westward extrapolations of this framework into the Cincinnati Arch argued that S-IV overlies S-II at a regionally angular unconformity. In this model, the basal transgressive systems tract (TST) unit of S-IV is a tabular carbonate: the Dayton Formation of Ohio and the presumptively coeval carbonate horizon at the base of the Waco Member (Noland Formation) of Kentucky. The classic work of A. F. Foerste indicated that these units trace into one another and never appear as distinct units in a single outcrop, but detailed correlation has been hindered by the rarity of index fossils and heavy dolomitization. A preliminary reexamination of these units indicates a faunal and lithologic dichotomy between the Dayton and Waco. The former is characterized by tabular, stacked hardgrounds with abundant glauconite, pyrite, and phosphate; the latter contains comparatively less authigenic mineralization and abundant favositids. Moreover, distinct conodont assemblages have been reported in each unit. An alternative depositional model accounting for these discrepancies is described herein.

The Waco Member overlies a major, regionally angular unconformity; its basal carbonate bed represents deposition in basinal settings during the TST of S-III. In these environments the Dayton is expressed as a sediment-starved, glauconitic horizon within the shales of the upper part of Waco Member. In shallower depositional environments to the northwest, the Dayton becomes thicker and more calcareous. Both units are present as massive, tabular carbonates in south-central Ohio. However, in the type area of the Dayton the basal Waco bed, and much of the underlying S-II succession, is absent. The basal contact of the Dayton apparently represents a second major erosive surface that intersects the lower boundary of S-III high on the proto-Findlay-Algonquin Arch. Although further work is required to test the feasibility of these hypotheses, these observations suggest a depositional framework that is both subtle and dynamic.