SEQUENCE AND EVENT STRATIGRAPHY OF AN EARLY KATIAN (UPPER ORDOVICIAN) STROMATOPOROID-BRYOZOAN-RICH INTERVAL: EVIDENCE FOR REGIONAL CLIMATIC AND BIOTIC CHANGE

The Upper Ordovician (Katian: Mohawkian; ~451-453 million year-old) Lexington Formation of northern Kentucky was deposited in a locally variable shallow platform sea in the southern subtropics (~25ºS) between the Taconic foreland basin and the deep intracratonic Sebree Trough. In this study, outcrops in the type area and new drill cores from Cincinnati, Ohio were logged on a centimeter scale lithologically as well as with spectral gamma ray analysis to aid in linking subsurface data new and previously studied sections near Lexington, Kentucky. This research expands the regional sequence stratigraphy of the Lexington Formation to ramp settings distal to the well-known Lexington platform sections and provides strong support that depositional sequences, as well as distinctive faunal epiboles, are of regional extent and not merely related to local tectonics. Specifically, the mid Lexington 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 abrupt, moderate deepening into overlying shaly nodular packstones and thin grainstones. A maximum flooding zone near the base of the Stamping Ground and correlative Strodes Creek members is associated with the proliferation of stromatoporoid sponges, tetradiid corals, ramose bryozoans, and nodular red algae; black shales intercalated with micritic wackestones also show abundant carbonized dasyclad algae. Upward shoaling culminates in a minor (4th order) sequence boundary overlain by transgressive systems tract grainstone and shaly facies. These patterns transcend local facies variations related to tectonics. Strong parallels of biofacies and stratigraphic patterns between this interval and another well-documented biotic incursion and coral proliferation, the upper Katian Richmondian invasion, suggest a previously unrecognized relatively brief period (<400,000 years) of warming in the early Katian, which produced regional transgression and permitted a proliferation of stromatoporoids, corals and other organisms otherwise rare or absent in the Lexington-early Cincinnatian interval.