LATE ORDOVICIAN TRACE FOSSILS LEFT BY FIXOSESSILE CORAL COLONIES AT A PRE-BENTONITE DIASTEM, CENTRAL KENTUCKY, USA

The Deicke K-bentonite, dated at about 457 Ma, is a prominent Late Ordovician (Sandbian; Turinian) regional marker horizon across several coeval formations in the central United States. In central Kentucky, this bentonite occurs in the peritidal Tyrone Formation and overlies a widespread, flat, hardground/firmground surface that experienced sediment starvation for periods long enough to support colonies of the fixosessile tabulate coral, Phytopsis cellulosum. An exposure near Boonesboro, Kentucky, which was benched just below the bentonite, reveals the firmground/hardground surface with 41circular traces, ranging from 14 cm to 1 m in diameter. The traces are slightly impressed into formerly underlying carbonate muds with irregularly linear to anastomosing grooves radiating from the center of each. Fragments of tabulate coralla within the grooves, as well as whole fossil colonies from the Tyrone and overlying Lexington limestones, indicate that the grooves reflect axes of growth and basal parts of the colonies that were cemented within underlying carbonate muds. The traces are generally distributed evenly across the surface, although some occur in loose groupings. In up to five of the traces, individuals are apparently conjoined, producing multilobed traces. Three crinoid holdfasts were found encrusting the surface on top of individual traces and three internal gastropod molds were found in similar situations. Apparently, this Tyrone diastem surface reflects a brief marine inundation of Tyrone carbonate tidal flats and the coeval colonization of the surface by Phytopsis colonies. The colonies were apparently abruptly removed and fragmented, perhaps during a major storm, and the surface was later recolonized by a sparse marine fauna. That fauna was subsequently snuffed out and buried by the Deicke volcanic ash fall, which preserved the traces and the sparse fauna that had lived on them. The surface not only reflects a pre-ashfall diastem and an interesting taphonomic sequence of events, but also what may be the first reported occurrence of trace fossils produced by skeletal coral colonies. The traces themselves are best classified as fixichnia.