Trace Fossils and Depositional Environments of Upper Devonian Black Shales, East-Central Kentucky, U.S.A.

Trace fossil assemblages are recognized in “anoxic” black shales and these assemblages are valuable in interpreting paleodepositional environments in shaly basins. Trace fossils in the Upper Devonian black shales of east-central Kentucky (Ohio or New Albany Shale) occur within and below dolomites, gray shale, and in black shale. A diverse suite of epifaunal and infaunal deposit-feeding traces is preserved in interbedded dolomite and shale of the lower part of the Huron Member. Planolites is the most common trace fossil and is associated with Chondrites (Type A), Zoophycos. Phycodes, Cruziana, Trichichnus, Teichichnus, Laevicyclus, and larged ribbed burrows. In interbedded gray and black shale of the lower part of the Huron Member, Planolites, Chondrites (Type B), Zoophycos. Teichichnus, and Rhizocorallium are common on black shale bedding planes. In gray and black shale interbeds of the Three Lick Bed, a less diverse assemblage of traces occurs, including Planolites-Wke burrows, Chondrites (Type C and Type D), and Zoophycos. These ichnogenera are included in Seilacher's (1967) Cruziana and Zoophycos ichnofacies.

Devonian black shales were deposited in an environment of deposition that was transgressive over the axis of the present Cincinnati Arch in east-central Kentucky. Sediments representing the shallow-water carbonate environment of the Middle Devonian Boyle Dolomite interfinger with Upper Devonian shales. Upsection, carbonates and black shales are replaced by black and gray shales. Black shales represent total anoxic conditions while gray shale represents periodic oxygenation events (possibly movement of the dysaerobic zone) which allowed burrowing to occur for short time intervals. At the beginning of the Early Mississippian, the depositional regime changed, and deltaic sands and prodeltaic muds replaced black mud deposition.

The depositional origin of Cincinnatian shales (Upper Ordovician) of Ohio has long been problematical. Some of these shales are characterized by a patchy distribution of Chondrites burrows, a lack of fissility, and an abundant, well-preserved trilobite fauna (mostly Flexicalymene and Isotelus). The spatial distribution and density of biogenic structures in these “trilobite shales” help clarify the relative timing of deposition and burrowing of the shales. The excellent preservation of the trilobite body fossils indicates that their burial by the mud was instantaneous. Petrographic thin-sections of these shales show decreasing density of fossil fragments upward within some shale horizons. These observations indicate rapid deposition of the “trilobite shales.” The Cincinnatian shales compare favorably with other fossil localities, famous for excellent preservation of a fossil fauna in shale, which have been interpreted as turbidites. The full suite of sedimentary structures characteristic of classical turbidites is absent in the “trilobite shales.” The ichnologic, taphonomic, and sedimentologic features of these shales, however, provide data that bear on new views of the rapid deposition of fine-grained sediment.

The Cincinnatian shales compare favorably with other fossil localities, famous for excellent preservation of a fossil fauna in shale, which have been interpreted as turbidites. The full suite of sedimentary structures characteristic of classical turbidites is absent in the “trilobite shales.” The ichnologic, taphonomic, and sedimentologic features of these shales, however, provide data that bear on new views of the rapid deposition of fine-grained sediment.