Abstract 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.