A sedimentological approach to understanding the butter shales from the Waynesville Formation (Upper Ordovician) in the Cincinnati Arch using geochemical and clay mineralogical techniques

The Upper Ordovician of the type Cincinnatian is a mixed siliciclastic-carbonate shallow marine system of alternating packages of packstones and shales containing diverse Ordovician fauna. Of the many sequences in the Cincinnati Arch, the Waynesville Formation (C5 sequence) in Eastern Indiana and the Kope Formation (C1) in Northwestern Kentucky have a quite different exposure of clay-rich shale, which is locally referred to as butter shale. These butter shales are bluish-green claystones that are very distinct in comparison to other shales because of their sedimentology, geochemistry, mineralogy, and interesting community of well-preserved fauna, which are commonly used as biostratigraphic markers. This study focuses on Hanna Creek in Brookville, Indiana, one of the few outcrops with exposed butter shales from the Waynesville Formation. Conducted in two stages, the first stage involved field collections from three stratigraphic sections that occurred 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 is to understand the sedimentology by 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 hope to better understand how the butter shales formed, the geochemistry of the basin, and what properties promote unique preservation of fauna within the shale. This in depth investigation would further aid in understanding the role of the butter shales with mass wasting events and its utility in urban planning.