Bryozoans in Ordovician depositional sequences, Cincinnati Arch region, USA

Various biostratigraphic and lithostratigraphic models are used to interpret the complex facies and age relationships of the Ordovician Lexington Limestone (Trentonian) and Cincinnatian strata of the Cincinnati Arch region. This famous stratigraphic succession is rich in fossils, including bryozoans. Regional unconformities separate the succession into at least four, and perhaps as many as twelve, depositional sequences and provide the framework in which bryozoan assemblages are analyzed. At least five bryozoan assemblages characterize these depositional sequences. The Lexington Limestone rests unconformably on a regional erosiona! surface which is a widely recognized sequence boundary event throughout the eastern North American craton. The upper Lexington Limestone forms a shallow carbonate bank on the Jessamine Dome and indicates a regional lowering of sea-level. Off the bank’s flanks, different facies expand, contract, and change abruptly into a siltstone-calcisiltite-wackestone of the Clays Ferry Formation. Tongues of the upper Lexington’ Limestone extend northward into the lowest part of the Cincinnatian Kope Formation (i.e. Point Pleasant Member). Most of the Kope Formation is shale and siltstone deposited during high sea-level and has a low bryozoan diversity. General shallowing near the top of the Kope brings a return to more carbonate-rich shales and siltstones (McMicken Member) and increased bryozoan diversity. Gradual shallowing continues through the Maysvillian, accompanied by the introduction of many additional bryozoan species; Most of these range upward into the Arnheim Formation where they become extinct. Only a few bryozoans survived into the overlying Waynesville, Liberty, and Whitewater formations.