Distinctive blue-gray shales characterized by a homogeneous texture, blocky weathering, and an abundant trilobite fauna (Flexicalymene, Isotelus) are found at various stratigraphic positions in the Upper Ordovician near Cincinnati, Ohio. Whole and fragmentary trilobite body fossils are randomly oriented within the shale, bivalves are common, and biogenic structures (Chondrites) are locally abundant. Limestones and siltstones above and below the shales are more extensively burrowed and contain at least two ichnogenera, Chondrites and Diplocraterion.

The absence or relatively low density of biogenic structures in shale traditionally has been interpreted as (a) an inhospitable environment for burrowing organisms during shale deposition or (b) rapid deposition of the shale, which prevented disruption of the original fabric by burrowers. The abundance of trace fossils in lithologies below and above the "trilobite shales" implies favorable conditions for burrowers before and after shale deposition. The abundant pelecypod fauna and low organic content of these shales suggest the muds were well-oxygenated. The relatively low density and localized occurrence of biogenic structures in these shales, then, reflect rapid deposition of the shales. Paleontologic criteria, including the excellent preservation and random orientation of trilobite , support the interpretation of rapid deposition for the shales.

The "trilobite shales" represent events in which a turbid layer of silt and clay swept over the shelf, killing and rapidly burying the trilobites. Burrowers, also disturbed during the event, reestablished themselves and exploited the new substrate after it had stabilized. Rapid, recurrent influxes prevented extensive disturbance by burrowers.

End_of_Article - Last_Page 680------------