Calibrating water depths of a Late Ordovician ramp, southern Ohio and north-central Kentucky, USA

Limestones and shales of the Grant Lake Formation (Upper Ordovician; Katian; Maysvillian) are well exposed in the Cincinnati Arch of southern Ohio and north central Kentucky, and highly fossiliferous. These rocks document gradual changes in lithofacies and biofacies from offshore nodular, phosphatic, brachiopod-rich limestones and marls to very shallow olive gray platy, laminated dolostones with desiccation cracks, southward along a gently sloping ramp. This study uses facies analysis in outcrop to determine paleoenvironmental parameters, particularly those related to water depth. Within a tightly correlated stratigraphic interval, we document the occurrence of paleoenvironmental indicators, including desiccation cracks, and light-depth indicators, such as red and green algal fossils, and oncolites. Mapping of facies on both sides of the Cincinnati Arch suggests an approximately E-W strike. The deepest water environments, exposed near Hamilton, Ohio, comprise nodular bedded, phosphatic limestone facies, dominated by the robust brachiopods. These facies were deposited within the shallow euphotic zone, as indicated by the presence of cyclocrinitid green algae and microendolith assemblages in brachiopod shells, and thus, approximately 20-40 m of water for semi-turbid water in subtropical shelf settings (average depth of 30 m used in calculations). The northernmost appearance of desiccation cracks in age-equivalent tidal flat facies (Terrill Member), just north of Richmond, Kentucky (200 km to the south), constrains the position of the contemporary shoreline and allows for the first semi-quantitative assessment of the slope of the Ordovician seafloor at this time. This implies a very gentle ramp with a gradient of about 10-20 cm of water depth increase per kilometer in a northward direction. By determining such ecological parameters, this study provides a better understanding of paleogeography and environments during the time of important ecological change