A SEQUENCE STRATIGRAPHIC MODEL FOR RECURRING TRILOBITE-RICH ‘BUTTER SHALES’ IN THE UPPER ORDOVICIAN (KATIAN) OF THE CINCINNATI ARCH

The Upper Ordovician (Katian: Cincinnatian Series) in the Ohio-Kentucky-Indiana tristate is host to several fossiliferous, blue-gray claystone deposits, commonly referred to as ‘butter shales’. These shales, composed of illite, chlorite, and fine-grained calcite, quartz and pyrite, can range up to 2m thick, generally with few interbeds. Some of these butter shales have been traced for over 135km, indicating that these do not represent simple localized events, but rather broad scale depositional intervals. Also referred to as ‘trilobite shales’, these deposits are sought after by fossil collectors because they contain relatively abundant, articulated trilobites and well-preserved mollusks, including uncrushed and commonly in situ bivalves. These beds contrast strongly with typical Cincinnatian strata, comprising mixed thin shale and limestones composed of disarticulated and/or fragmented brachiopod, crinoid, and bryozoan debris.

Butter shales represent low-energy environments with a moderate background influx of mud-dominated sediment, which favored vagrant trilobites and infaunal deposit- and suspension-feeding annelids and mollusks. Rapidly deposited mud layers up to several centimeters thick episodically smothered benthic communities. Multiple butter shale intervals occur within the Waynesville Formation, and detailed regional analysis of this unit indicates that these deposits recur in a consistent position with respect to sequence stratigraphy. Here we present the occurrence of butter shales in the context of eustatic fluctuations at multiple scales, resulting in a predictive model for development of such bodies based on the coincidence of specific systems tracts of different orders. Sedimentological and stratigraphic data from all additional butter shales in the upper Cincinnatian were then used to test this model.

Thicker butter shales occur in the late transgressive to early highstand systems tracts (HSTs) of certain sedimentary cycles. We suggest that trilobite-bearing butter shales form preferentially where the HST of a higher-order sequence is superimposed upon a longer-term HST to falling state systems tract, amplifying offshore mud sedimentation.