REVISED SEQUENCE STRATIGRAPHY FOR THE UPPERMOST ORDOVICIAN STRATA OF THE CINCINNATI REGION

The richly fossiliferous carbonates and shales of the Cincinnatian Series (Katian) in the Cincinnati, Ohio region record details of physical and biotic change in the run up to the End Ordovician crises. Extensive field study has clarified the detailed stratigraphy of the upper part of the Cincinnatian, until now incompletely known. This ~1.5-2 myr interval is divisible into parts of three 10-20m (3^rd order) sequences as well as several 2-5m small-scale (5^th order) sequences, each comprising a basal (transgressive) skeletal-peritidal limestone bundle and an overlying shale/thin limestone interval (highstand).

The Liberty Formation (C6B) serves as an important baseline with key index fossils and widely consistent subdivisions; it can be split into four small-scale sequences showing a slightly progradational pattern. The overlying Whitewater Formation (C7) shows strong changes in thickness and facies in its southwesterly transition into the dolomitic, peritidal Saluda Formation suggesting tectonic overprint. Previously undescribed sections exposed the entirety of an 18m Whitewater Fm., including middle peritidal “Saluda” facies and eight 5^th order sequences, forming an overall shallowing-deeping stacking pattern. The highest Katian unit, the shaly Elkhorn Formation (C8), is little studied due to its limited exposures but recent exploration has identified at least four widely traceable 5^th order sequences. These beds are locally truncated by a major lower Silurian unconformity, which may cut down into the Whitewater. However, in scattered localities, the upper Elkhorn is overlain by a thin (1-5m) package of shaly limestones, siltstones, and dolostones now referred to as the Whippoorwill Formation. This unit, formerly included in the basal Brassfield Formation, has long been considered to be of earliest Silurian age. Recent chemostratigraphic and faunal evidence, however, suggests that this package is actually of late Hirnantian age, and its two 5^th order sequences probably represent the final two cycles of the Ordovician. Its sharp, albeit cryptic, basal contact is the H-1 or Cherokee unconformity recording a glacioeustatic lowstand. These results provide a high-resolution allostratigraphic framework for the critical interval preceding and immediately following the Hirnantian biocrisis.