EPIBOLE OF THE STROMATOPOROID LABECHIA HURONENSIS FROM THE UPPER LEXINGTON LIMESTONE (SHERMANIAN), NORTHERN KENTUCKY

The Stamping Ground Member and the probably equivalent Strodes Creek Member of the upper Lexington Limestone (Middle Ordovician: Shermanian) is a 3-4 meter thick package of interbedded shale and limestone locally containing abundant stromatoporoids of the species Labechia huronensis. The basal contact of the Stamping Ground with underlying grainstones (“Tanglewood”) is a flooding surface, sharply demarcated by a rusty hardground. The Stamping Ground commences with shaly, nodular wackestones and coarsens upward to a thick grainstone. Faunal associations include Solenopora (red algae), Tetradium, ramose and sheet-like bryozoans, and the brachiopods, Rhynchotrema, Zygospira, and Rafinesquina. Labechia is not restricted to a specific lithofacies. However, smaller coenostea occur in shaly nodular wackestones, while larger domal Labechia typify more proximal grainstones. Morphologies include the full range described by Kershaw and Brunton (1999), but appear to be dominated by tall/domal and flat/convex forms. Taphonomic signatures include breakage and reorientation due to storm reworking and diagenesis. Labechia biostromes occur at the caps of smaller, meter-scale cycles. The stromatoporoids are best developed in sections near the crest of the “Tanglewood buildup”, near Frankfort and Lexington. In the shallowest sections the stromatoporoids occur near the base of the Stamping Ground and are absent in the coarsest upper beds. In downramp sections the stromatoporoids are absent in deeper, shaly facies near the base and their lowest occurrence is in successively higher cycles. At the northernmost section about 40km to the north the stromatoporoids are confined to the shallowest, upper bed of the Stamping Ground. The Labechia epibole ends at the top of the Stamping Ground as the stromatoporoids were drowned out at a flooding surface that begins the overlying highstand. This stromatoporoid biostrome occurs in patches over 6000 km² on the Lexington platform. The restriction of Labechia to this interval appears to be dependent on a higher order control (e.g. climatic warming).