BRIDGING THE LIMESTONE TO SHALE TRANSITION: ADVANCES IN PLATFORM TO BASIN FACIES MODELS USING AN INTEGRATED CHRONOSTRATIGRAPHIC APPROACH (UPPER ORDOVICIAN; KENTUCKY AND OHIO)

Correlation of shallow marine limestones into deeper shale-dominated settings remains an area of great uncertainty in many sedimentary basins impairing the accuracy of depositional models. The Lexington Platform-Sebree Trough transition (Kentucky-Ohio) represents a well preserved Upper Ordovician example of this problem. In this study, outcrops and cores in central Kentucky and new drill cores from Cincinnati, Ohio were logged using litho-, bio-, and chemo-stratigraphy to develop detailed correlation of chrono-stratigraphic packages across a major facies transition from a the carbonate-dominated Lexington Limestone platform succession to the dark, mudrock-dominated deposits at the transition to the Sebree trough. A number of facies-crossing markers provide useful, first-order controls on stratigraphy: these include abrupt facies offsets, K-bentonites, fossil epiboles (e.g. Prasopora bryozoans), and deformed beds, probably representing regional seismites. These correlations are corroborated by gamma ray, carbon isotope curves, and elemental abundance profiles. This research expands the regional correlation of the Lexington Formation and its members into more distal settings and provides strong evidence that depositional sequences, and distinctive faunal epiboles, and chemo-stratigraphic signatures are of regional extent. Thus, despite local abrupt facies variations, associated with far field tectonics, there is strong evidence for allocyclic, probably eustatic control of large and small-scale cycles. However, the degree of environmental change across this gradient varies strongly with stratigraphic levels. Thus, lower units (Curdsville, Logana members) persist across the profile with relatively little change. In contrast, the upper Lexington units display much more abrupt northward change to shaly facies, suggesting increasing rates of subsidence in the Sebree trough through deposition of the Lexington sediments.