A GEOCHEMICAL AND CLAY MINERALOGICAL ANALYSIS OF THE BUTTER SHALES FROM THE WAYNESVILLE FORMATION (UPPER ORDOVICIAN) IN THE CINCINNATI ARCH

The Cincinnati Arch contains a unique suite of Upper Ordovician strata displaying a mixed siliciclastic-carbonate shallow marine system of alternating packages of packstones and shales containing extremely diverse fauna. The Waynesville Formation (C5 sequence) in Eastern Indiana has a very distinctive exposure of clay-rich shale, previously believed to be k-bentonites in origin, which is locally referred to as butter shale. These bluish-green claystones are very distinct in comparison to other shales because of their geochemistry, clay mineralogy, bulk composition, and interesting community of well-preserved fauna, which are commonly used as biostratigraphic markers.

This study focuses on one of the few outcrops with exposed butter shales from the Waynesville Formation at Hanna Creek in Brookville, Indiana. Conducted in two stages, the first stage involved field collections from three stratigraphic sections tens of meters apart. Bulk rock samples (including butter shales) were collected 30 cm deep into the outcrop, sampled at 1-5 cm intervals, and placed in sealed containers for further analysis. The second stage of the study involved analyzing the geochemical and clay mineralogical composition of the claystone using x-ray fluorescence (XRF) and x-ray diffraction (XRD) techniques. On average, the shales are 74% illite, 25% chlorite/kaolinite, and less than 1% smectite. Contrary to preexisting hypotheses, the butter shales are not bentonites as they lack expandable smectite. The bulk chemical analysis coupled with clay mineral analysis provide complimentary determinative characterization methods of understanding depositional processes in the butter shale beyond that of the biostratigraphic markers used traditionally. Through the implementation of multiple methods to investigate the butter shales of the Cincinnati Arch, we aim to better understand how the butter shales formed, the evolving geochemistry of the basin, diagenetic and authigenic impacts on the clay mineralogy, and what properties promote unique preservation of fauna within the shale. This in depth investigation will further aid in understanding the role of the butter shales with mass wasting events and its utility in urban planning.