THE STRATIGRAPHIC DISTRIBUTION OF THE LATE ORDOVICIAN ‘BUTTER SHALE’ LAGERSTäTTEN IN THE CINCINNATI ARCH REGION

The Upper Ordovician Cincinnatian strata of Ohio, Kentucky, and Indiana contain multiple claystone units called “butter shales” by local collectors. These shales have long been noted for their abundantly well preserved trilobites, cephalopod, and bivalve fauna. The most well studied butter shale is the Treptoceras duseri shale of the lower Fort Ancient Member of the Waynesville Formation. The Waynesville also hosts at least two or three other claystone units, the Lyrodesma major or Isotelus shale and the newly discovered Oldenburg shale. The slightly older Arnheim Formation contains the Mt Orab butter shale, and the slightly younger Liberty Formation consists of the Minuens shale, named for its unusually small Flexicalymene trilobites, along with some other potentially unnamed trilobite shales. These butter shales may all represent an example of time-specific facies.

The change from brachiopod-bryozoan dominated fauna of the surrounding Cincinnatian to the mollusk-trilobite rich fauna of the butter shales indicates an increased rate in sedimentation allowing for mobile organisms to dominate where sessile fauna no longer can. This environment was episodically overwhelmed due to storm events and submarine mud flows, rapidly smothering even the mobile organisms providing the exceptional preservation. The butter shales are also punctuated with lenses of limestone and siltstone, allowing for the return of brachiopods and bryozoans. In contrast, the Moranburg shale of the Maysvillian aged Grant Lake Formation, is dominated by articulated Glyptocrinus and likely represents a time of low sedimentation rate punctuated by rapid sedimentation. The differences in these lagerstätten may be explainable in terms of sequence stratigraphy. Here we present a preliminary comparison of butter shale stratigraphy, paleontology, sedimentology and taphonomy.