Paper No. 9
Presentation Time: 11:10 AM
TRACE FOSSILS PRODUCED BY TABULATE CORAL COLONIES, UPPER ORDOVICIAN (MOHAWKIAN) TYRONE FORMATION, CENTRAL KENTUCKY, U.S.A
<1><1><1><1><1>The Tyrone Formation of central Kentucky is composed of peritidal, birdseye calcilutites with mudcracks, vertical burrows and a few interbedded shale layers, most of which are bentonitic in origin. The largest shale layer is the Pencil Cave bentonite, a prominent regional marker, and at Boonesboro, Kentucky, the exposure was benched just below the bentonite, revealing a bedding-plane surface, 105-m long by 6-m wide. The surface exhibits 41 circular to elliptical traces, ranging from 14 cm to 1 m in diameter, with linear to branching grooves radiating from their centers. The traces are distributed evenly on the surface, although some loose groupings occur. In five traces, individuals are apparently conjoined, producing multilobed patterns. Three gastropod molds and three crinoid holdfasts were found on or encrusting the surfaces of individual traces. Close surface examination revealed many shredded fragments of the tabulate coral <1>Phytopsis cellulosum1>, and similar corallum fragments were embedded within the grooves of some traces, suggesting that each trace is an impression of a colony base in soft mud and that the radiating grooves reflect axes of colony growth. Actual, in-situ domed colonies of <1>Phytopsis1> sp., preserved on bedding planes in the overlying Curdsville Member of the Lexington Limestone at other localities, serve as models for the way the surface must have once appeared. Apparently, the surface reflects a brief marine inundation of Tyrone carbonate tidal flats and the colonization of the surface by <1>Phytopsis1> colonies. The colonies were apparently abruptly removed and fragmented by a major storm, and the surface, by now a firmground or hardground, was later recolonized by a sparse marine fauna. That fauna was subsequently snuffed out and buried by a major volcanic ash fall, which preserved the traces and the sparse fauna that had encrusted or lived on them. Although the surface reflects an interesting taphonomic sequence of events, the traces may be the first reported occurrence of trace fossils produced by skeletal coral colonies. The ethological or behavioral classification of the traces is also enigmatic. It is tempting to call them resting traces or <1>cubichnia1>, but inasmuch as <1>cubichnia1> are produced by vagile organisms and the coral colonies were never vagile, their ethological classification remains uncertain.