INVESTIGATION OF DRIVING MECHANISMS FOR TWO BIOTIC INCURSIONS OF THE MOHAWKIAN SERIES (LOWER KATIAN STAGE) INTO CINCINNATIAN SERIES, NORTHERN KENTUCKY

New exposures in northern Kentucky allow for detailed reinvestigation of the stratigraphy/faunas of the upper Mohawkian Series (lower Katian Stage). Millersburg, Nicholas, Bromley, and Rogers Gap members, as well as the lower Kope Formation of the Cincinnatian Series, represent the 4^th order sequences M6B to lower C1. This section in Kentucky consists primarily of bioclastic calcarenites and siliciclastic siltstones and shales with distinctive, well-preserved shallow marine faunal assemblages. The M6B-M6C sequences (Millersburg, Nicholas members) include assemblages typical of warm water, including biostromes of stromatoporoids, solenoporids, and corals, with distinct rhynchonellid brachiopods. This biota extends through M6B into M6C when it abruptly disappears as the platform deepens with increased siliciclastic influx during the HST of M6C (Bromley Mbr). The shaly packstones of the Bromley, previously referred to as the upper Millersburg “tongue” or upper Clays Ferry Formation, contain a generalized, low diversity biota of brachiopods (Hebertella, Rafinesquina, and Zygospira), bivalves, gastropods and an extreme abundance of the trilobite Isotelus. The overlying and final member of the Point Pleasant, the Rogers Gap Member, consists of mixed skeletal grainstones and shaly packstones. These beds show an incursion of a varied, taxa that presage assemblages of the overlying Cincinnatian (C1). Kope Formation with varied brachiopods (Sowerbyella, Leptaena, Eridorthis), crinoids (Ectenocrinus, Merocrinus), and bryozoans but a nearly complete absence of stromatoporoids and corals. These skeletal limestones represent the TST of C1, that includes the Kope HST.

The occurrence of two distinctive types of biotas, appearing close together stratigraphically provides a unique opportunity for study of incursions forces. Documentation of the fauna and stratigraphic/paleontological patterns in a sequence stratigraphic context, provides insights into the driving mechanisms of these incursions. Initial comparisons with much better known Ordovician Richmondian Invasion, a putatively warm water incursion, suggests patterns most similar to those of the M6B. We hypothesize that two distinctive types of incursions are due to differences in climate/ocean temperature and/or sediment supply.