ESTABLISHING A TEMPORAL/CORRELATIVE FRAMEWORK FOR THE UPPER ORDOVICIAN (TRENTON) LEXINGTON LIMESTONE, CENTRAL KENTUCKY

The Lexington Limestone is an Upper Ordovician (“Trenton,” Chatfieldian) unit in central Kentucky, which has been known since the pioneering work of Campbell in the late 1800s. Except for dated bentonites at its base, at this point very few means exist for correlation within the unit, which in central Kentucky is nearly 100-m thick. Lower parts of the Lexington Limestone contain a number of through-going members, which are notable for their lithologic similarity to units of similar age on the Nashville Dome. Upper parts of the Lexington, however, form a facies mosaic of limestones, shales and shaly limestones, formerly part of the Cynthiana Formation, which was divided into members differentiated by key taxa. In the mid 1960s, the Cynthiana Formation was subsumed into the Lexington Limestone, the use of fossils for member discrimination was halted, and lithology alone was used to define members of unusually complex geometry. We now understand that much of the complexity was caused by repeated structural reactivation and structural control of facies, but correlations are no less difficult. Our ongoing work in the Lexington, however, has shown that subtle means of correlation are apparently present. Certain hardgrounds (e.g., Grier/ Brannon; sub-Sulphur Well, sub-Millersburg) appear to be nearly universally present and seem to represent widely correlative events. Other events such a seismite horizons (Brannon, middle and upper tongues of the Tanglewood) and prominent flooding events (certain Millersburg and equivalent Clays Ferry tongues) also have correlative value. Finally, those fossils used to subdivide the former Cynthiana Formation, ( Constellaria , Ambonychia, Sowerbyella, and Cyclonema, seem to have at least local correlative value. It is our hope that these event and biostratigraphic markers, when combined with graphic correlation, stable isotopes and the dating of newly found bentonites in the unit, will provide the temporal and correlative framework that has been missing for so long from the Lexington Limestone.