FAUNAL EPIBOLES IN THE UPPER ORDOVICIAN OF NORTH-CENTRAL KENTUCKY: IMPLICATIONS FOR HIGH RESOLUTION SEQUENCE AND EVENT STRATIGRAPHY

A succession of distinctive marker beds and epiboles in the Richmondian Group (upper Katian) has been traced over 150 kilometers around the eroded center of the Cincinnati Arch in north central Kentucky. The lower Rowland Member of the Drakes Formation on the west side of the Arch correlates closely with a distinct interval of the Bull Fork Formation on the east side, providing strong evidence of a nearly "layer cake" pattern of strata the can be correlated along depositional strike. This interval includes: a) a basal phosphatic limestone; b) a lower "barren" shale and cherty calcisiltite; c) a rhythmic succession of fossiliferous wackestones and dark, dasyclad algae-rich shales, with locally abundant Tetradium and other corals (“Fisherville coral beds”); and d) an upper olive-gray mudstone with abundant hemispherical Cyphotrypa bryozoans. The meaning of the distinct epiboles remains to be explored. The Fisherville beds record the first appearance of Columnaria corals in the Cincinnati region; these occur at several higher levels and record an incursion of warm water taxa (i.e., initial "Richmondian invasion") associated with a late Katian sea level highstand and positive carbon isotope excursion ("Waynesville excursion"). A rhythmic lagoonal depositional system appears to consist of two end members: an organic-rich clay setting that favored abundant non-calcified algae, and a carbonate mud setting with corals, mollusks and ostracodes. Abundant dasyclad algae indicate relatively shallow water depths, probably less than 15 m. In addition, the abundance of chert and the proliferation of gastropods and opportunistic bryozoans suggests incursion of nutrient rich waters. Regardless of cause, these distinctive marker beds provide an excellent framework that permits characterization of a critical, previously unrecognized unconformity, beneath a middle Rowland Skolithos-bearing, coral biostromal limestone bed, that ultimately removes successively lower beds in approximately southward direction. This mid Richmondian unconformity may record local tectonic flexure of a north-sloping ramp.