A CONTRIBUTION FROM INVERTEBRATE PALEOECOLOGY TOWARD UNDERSTANDING LATE CHESTERIAN (MISSISSIPPIAN; SERPHUKOVIAN) EUSTASY: MENARD FORMATION IN HANCOCK COUNTY, KENTUCKY

The mostly marine Menard Formation (Upper Chesterian) near Hawesville, Kentucky (Hancock County) contains a varied invertebrate fauna that provides clues to the nature of Late Mississippian eustatic submergence-emergence cycles in this area. The macrofauna includes brachiopods, bryozoans, pelecypods, corals, gastropods, conulariids, and nautiloids (in approximate order of abundance). Echinoderm debris is common to profuse, but it is mostly disarticulated, so relative population sizes for crinoids, blastoids, and echinoids are difficult to assess. Ostracodes, fusulinids, and conodonts make up the majority of the microfauna.

Certain brachiopods, including Composita, Anthracospirifer, Productus, and Orthotetes occur both in limestones and shales throughout the formation. Varied fenestrate bryozoans (Archimedes, Fenestella, Polypora, and possibly others) are also abundant both in limestones and shales. Pelecypods including Modiolus and one species of Aviculopecten are common in some limestones, and other pelecypods are common in shales (a different species of Aviculopecten, Phestia, Prothyris, and Nucula) where they are the only macrofauna at certain levels. Other faunal groups tend to be restricted to particular limestone beds (corals, gastropods, blastoids, and crinoids with articulated calyces), and others to particular shale beds (conulariids).

Assemblages range from very diverse, including stenohaline marine taxa, to monospecific (only pelecypods), suggesting a contrast in living conditions for marine organisms from one bed to another. Progression from a diverse marine fauna to a less diverse, or even monospecific, fauna suggests environmental changes associated with regression--either decreasing salinity or increasing turbidity, or a combination of both. The opposite pattern was probably associated with transgression, bringing greater distances from points of sediment and freshwater influx, and possibly greater water depth. In the Menard, shaly intervals bearing mainly (or only) pelecypods and lacking stenohaline marine organisms were probably deposited closest to shore. Faunal changes occurring within several shaly intervals in the Menard have allowed us to reinforce previous inferences for Late Chesterian eustatic trends in this area, and to better characterize their development.