Faunal epiboles in the Upper Ordovician of north-central Kentucky: Implications for high-resolution sequence and event stratigraphy and recognition of a major unconformity

Abstract

Mixed carbonate-siliciclastic strata of the lower Richmondian (Upper Ordovician, Cincinnatian) in the Cincinnati Arch region represent deposition along a gentle epeiric ramp in the Taconic foreland basin. Previous stratigraphic investigation of this interval largely emphasized local facies, with little consideration of regionally persistent patterns and widespread marker horizons. The current study describes a distinctive package within the lower Rowland Member of the Drakes Formation of northern Kentucky; this interval comprises a four-part succession of units, several of which are characterized by unusually high abundance of particular taxa (proliferation epiboles). Similar (to nearly identical) successions, including epiboles in corresponding positions, are present within the undifferentiated Bull Fork Formation of central Kentucky and the Fort Ancient Member of the Waynesville Formation in southwestern Ohio. Further, progressive erosion of the widely traceable “lower Rowland” succession beneath a distinctive, prominent marker bed indicates the presence of a previously undocumented, regionally angular unconformity within the middle Rowland Member (the “mid-Richmondian unconformity” predicted by some earlier workers). Hence, a widely persistent, nearly layer-cake pattern can be documented over 150km across the Cincinnati Arch. Although the mechanism(s) responsible for such widespread epiboles is (are) enigmatic, this study demonstrates the significant value of epiboles in regional stratigraphic correlation and recognition of unconformities.