STRATIGRAPHY, PALEOENVIRONMENTS, AND CORRELATION OF THE UPPER ORDOVICIAN TATE MEMBER OF THE ASHLOCK FORMATION AT POINT LEAVELL, KENTUCKY

The Tate Member of the Ashlock Formation in SE central Kentucky has long been correlated with the McMillan Formation in Cincinnati. Most of the Tate is a barren, laminated and mud-cracked greenish dolomitic siltstone. Finer correlations between the intertidal Tate and the subtidal McMillan Formation have not been made, but The Back Bed, a thin fossiliferous carbonate within the Tate, suggests intertonguing. The goal of this research is to search for correlatable stratigraphic signals in the Tate.

The Tate is well exposed in a series of fresh road cuts stretching approximately 11 km along Kentucky Hwy. 52 in Garrard County Kentucky. Here we report on the stratigraphy of one of these immediately west of the East Fork of Back Creek, 1.6 km east of Point Leavell. The cut exposes 32 m of section, with the Calloway Creek Formation at 0-5.7 m the Tate at 5.7-29.9 m, and the Gilbert Member of the Ashlock Formation at 29.9 m to the top of the exposure at 32 m+.

Along with the ubiquitous brachiopod taxa Vinlandostrophia and Hebertella, The Tate is bracketed by potential index fossils with the brachiopod Orthorhyncula, an index fossil of the Cincinnati Fairmount Member of the Fairview Formation occurring along with some unusual cephalopods in the uppermost Calloway Creek, and the sponge Dystactospongia, a normally rare taxon, occurring in abundance in the lower beds of the Gilbert. Furthermore, the Back Bed, which occurs at 7.8-8.8 m, is not the only fossiliferous carbonate within the Tate; others occur at 19.7-20.3 m, at 22.3-23.1 m, and at 24.5-26.3 m. These provide possibilities for faunal correlations.

Fossil collections are currently under study, but, while the uppermost of these units contains a variety of mollusks, most of the units are dominated by robust ramose bryozoans. Because of their abundance in otherwise barren rocks, bryozoans taxonomy may be a key to the correlation of the Tate and similar peritidal carbonates in the upper Ordovician of Kentucky.

These fossiliferous carbonates, obviously deeper-water deposits, suggest depth fluctuations that might be correlated with similar depth fluctuations in the McMillan Formation. Other potentially useful horizons display variations in color, mineralogy, and bedding. Future work along the 11 km transect will show whether any of these horizons can be traced laterally.