SEQUENCE STRATIGRAPHY AND BIOTIC PATTERNS OF AN UNUSUAL INTERVAL IN THE UPPER ORDOVICIAN LEXINGTON FORMATION, CENTRAL KENTUCKY

The Upper Ordovician (Katian: Mohawkian; ~451-453 million year-old) Lexington Formation of northern Kentucky was deposited in a shallow platform sea in the southern subtropics (~25ºS). A general lack of corals and sponges and enrichment in granular phosphates has suggested that these carbonates accumulated under cool conditions and upwelling of nutrient-rich waters associated with a major tectonic episode. However, a middle interval (Sulfur Well, Stamping Ground and Strodes Creek) contains an exceptionally rich fauna of stromatoporoid sponges, corals, and nodular red algae. In this study several new highway cuts in this interval, near Winchester, KY, were studied in detail to document patterns of microstratigraphy, facies, and fauna. This interval comprises a depositional sequence (M6A) bounded by a regionally angular unconformity at the base of massive grainstones and rudstones of the Sulphur Well Member and showing a moderate deepening into the overlying shaly nodular packstones and thin grainstones. A maximum flooding zone near the base of the Stamping Ground Member is associated with the proliferation of stromatoporoids and tetradiids; intercalated black shales show abundant carbonized dasclad? algae. Upward shoaling culminates in a minor (5th order) sequence boundary overlain by transgressive systems Strodes Creek grainstone and Greendale shaly facies. In addition, detailed study permits delineation of several smaller-scale cycles within the interval. Comparison of fauna, facies and stratigraphic patterns of this interval with other well-documented, warm episodes and biotic incursions (e.g. Richmondian invasian) suggests a previously unrecognized brief period (<500,000 years) of abrupt warming which produced regional transgression and permitted a proliferation of stromatoporoids, corals and other organisms otherwise rare or absent in the Lexington-early Cincinnatian interval.