Trilobite Taphonomy and Temporal Resolution in the Mt. Orab Shale Bed (Upper Ordovician, Ohio, U.S.A)

Clay-rich units, locally termed “butter shales,” contain the best-preserved trilobites in the richly fossiliferous Cincinnatian Series and likely provide the highest temporal resolution available within these rocks. Sedimentological and taphonomic evidence indicates that the 0.46-m-thick Mt. Orab “butter shale” bed of the Arnheim Formation is composed of a series of stacked event beds, each representing rapid deposition from a flow bearing fine-grained sediment, most likely associated with distal storm processes below storm-wave base. It contains sedimentary structures similar to those of distal mud turbidites, and comprises a total of at least seven, and possibly many more, alternating silt and clay couplets. These clay and silt layers are interpreted to represent the products of different energetic regimes in a series of discrete depositional events accumulated within a common depositional regime. Trilobites within individual clay beds represent census assemblages of animals alive at the same time, and evidence from sedimentology, taphonomy, and stratigraphic architecture are consistent with accumulation of the whole bed within a period from 101 to 103 years. Silt layers of the Mt. Orab events beds are interpreted to represent parautochthonous assemblages, while clay layers, although displaying reorientation of specimens, are interpreted as autochthonous assemblages. Both layers are deposited in a shallower-water environment than the comparable “granulosa” trilobite cluster of the Kope Formation, which represents an autochthonous assemblage with in-situ burial of trilobites.