A PRELIMINARY COMPARISON OF THE LATE ORDOVICIAN BUTTER SHALES OF THE CINCINNATI ARCH

The Upper Ordovician Cincinnatian strata of Ohio, Kentucky and Indiana contain several claystone units, colloquially referred to as “butter shales” or “trilobite shales”. These units are widely known for their relative abundance of well preserved trilobites Isotelus and Flexicalymene. Previous studies have focused on trilobite taphonomy and bed characteristics of individual butter shales; however, there has been little comparison between butter shales to assess lateral variation, or broader stratigraphic/facies context. This study takes a comparative approach by examining the geographic extent of each claystone unit, as well as lateral variations in bed thickness, paleoecology, taphonomy and clay sedimentology within and between various butter shales.

Field study and review of literature has identified three major butter shale units in the Waynesville Formation and several minor ones in the older Grant Lake and Arnheim formations. The Treptoceras duseri shale, the most extensively studied of the claystones, has produced a mollusk-dominated fauna adapted for environments characterized by muddy substrates, high turbidity and rapid sedimentation in contrast to the brachiopod dominated fauna of the surrounding units. Surprisingly, however, at least three of the butter shales also contain zones with corals (Tetradium) and small stromatoporoids, unusual fauna for clay-dominated environments. These coral/sponges are frequently overturned and heavily bored and encrusted, indicating reworking in relatively shallow water conditions during pauses in sedimentation. In terms of sequence stratigraphy, butter shales appear to be consistently situated within highstand portions of third-order cycles, apparently amplified by analogous phases of higher-order cycles. We suggest that this common position within a 3^rd and 4^th order stacking reflects a sedimentational “sweet spot”, in which progradation during shallowing, regressive conditions permitted episodic pulses of mud deposition.