GEOLOGY, PETROGRAPHY, CATHODOLUMINESCENCE, AND GEOCHEMISTRY OF AN UPPER ORDOVICIAN HARDGROUND FROM CENTRAL KENTUCKY: IMPLICATIONS FOR EARLY SEAFLOOR DIAGENESIS

The Upper Ordovician strata of the Cincinnati Arch contain an exceptional abundance of carbonate hardgrounds. These surfaces, which were lithified near the seafloor prior to a period of submarine erosion and exhumation, contain an array of morphologies from simple, planar beds to complex, high-relief topographies. Hardgrounds expanded in abundance and complexity throughout the Ordovician and provided increased hard substrate niches. Sclerobionts (encrusting organisms) rapidly diversified to occupy these niches and contributed to the Great Ordovician Biodiversification Event. Hardgrounds, which may be traceable over hundreds of km², may also provide important details about basin dynamics and global environmental fluctuations, however the genesis of these surfaces and their topographic heterogeneities remains controversial.

In this study, we document and describe a complex hardground from the Upper Ordovician of central Kentucky and investigate its genesis and modification using an integrated approach including detailed stratigraphic, petrographic, cathodoluminescence, and geochemical analyses. The hardground occurs in the Curdsville Member of the Lexington Limestone (Upper Ordovician, Sandbian-Katian) and exhibits considerable topographic heterogeneity which hosted a high richness of encrusting organisms. The hardground surface is well exposed over at least 100 m² on a highway roadcut bench, providing a detailed look at its morphology. The hardground is developed on a 40–50 cm grainstone bed with localized patches of heavy bioturbation (Thallasinoides­­-type burrow galleries). In places are high-relief, domal structures 10–15 cm tall that we interpret to represent erosional remnants of an originally continuous layer. These structures, undercut in places, formed cantilevered ledges with rounded margins. In this study, we demonstrate the early lithification of this surface by analyzing vertical gradients in cement types, carbon-isotope geochemistry, and diagenetic history of the hardground bed and subjacent strata. We also propose a mechanism for the early lithification of the hardground surface, assess the formation of high-relief surface structures, and discuss implications for basin analysis and effects on local encrusting communities.