REVISITING A CLASSIC AREA: REVISED STRATIGAPHY AND PALEONTOLOGY OF THE UPPER ORDOVICIAN (MOHAWKIAN/LOWER KATIAN) MILLERSBURG AND NICHOLAS MEMBERS, LEXINGTON FORMATION, IN NORTH CENTRAL KENTUCKY

Integrative stratigraphy requires the reexamination of reference localities and previous interpretations in light of new data. Documentation of a series of fresh highway cuts near Millersburg (Bourbon County) and Myers (Nicholas County) in north-central Kentucky, supplemented by two nearby drill cores, has provided new insights into the stratigraphy and paleontology of the famous, but poorly understood, upper Lexington Formation (Upper Ordovician, lower Katian) representing a majority of 4th order sequences M6B-M6C. Historically, the “Millersburg” and “Nicholas” members have been used in varied and often confusing ways. New complete sections of the Millersburg Member are present in both areas, affording bed by bed correlations over 20 km and considerable refinement of the overall succession. This unit comprises ~10-12 m of alternating shale and argillaceous, nodular packstone with a distinctive brachiopod fauna (small Hebertella, Vinlandostrophia and Orthorhynchula), recording the highstand systems tract (HST) of M6B that conformably overlies ~4 m of medium to thick bedded grainstone with abundant ramose bryozoans (Sulphur Well Member; inferred transgressive systems tract [TST] of M6B). A biostromal interval, up to 5 m thick, referred to as the Strodes Creek Member (type section ~15 km to the south), packed with stromatoporoids, solenoporids, and Tetradium, gradationally overlies the type Millersburg and persists >35 km northward from Winchester, Kentucky. It appears to correlate with an even more widespread stromatoporoid epibole in M6B in the mid-upper Lexington Formation in central KY. Overall, the Millersburg-Nicholas succession carries a distinctive faunal assemblage reminiscent of later, Cincinnatian faunas, suggesting conditions that presage the latter interval. The overlying 3-4 m thick unnamed grainstone interval (5^thorder TST) is in turn separated from a higher, massive grainstone-bryozoan rudstone (TST of M6C) package by ~2 m of fossiliferous shaly nodular beds (5th order HST). To the northeast, near Myers, this intervening shale appears to be cut out at the M6B/M6C boundary, stacking the upper grainstone upon the lower and producing a massive unit that has been referred to as the “Nicholas Member”. This amalgamation of grainstones suggests a previously unsuspected local high.