GEOCHEMISTRY, CLAY MINERALOGY, AND TAPHONOMY OF THE FOSSIL-RICH BUTTER SHALES

Abstract

The Cincinnati Arch contains a suite of Upper Ordovician, mixed siliciclastic-carbonate, shallow marine strata, with alternating packages of packstones and shales that contain exceptionally well-preserved articulated fauna. The Waynesville Formation (C5 sequence) in Eastern Indiana includes distinctive clay-rich shales, which are locally referred to as butter shale. These bluish-green claystones are distinct in comparison to other shales because of their geochemistry, clay mineralogy, bulk composition, and well-preserved and biostratigraphically important fauna. This study focuses on an outcrop with exposed butter shale from the Waynesville Formation at Hanna Creek in Brookville, Indiana. Bulk-rock samples were collected and the geochemical and clay mineralogical composition of the claystones were analyzed using x-ray fluorescence (XRF) and x-ray diffraction (XRD) techniques. On average, the butter shale clay mineralogy is 74% illite, 25% chlorite/kaolinite, and less than 1% smectite. The butter shale lacks expandable smectite, likely from post-depositional alteration of original smectite through illitization and chloritization. The presence of iron in the system as a result of diagenetic processes allowed for the immediate mineralization, and in some cases pyritization, of fossils. Bulk chemical analysis coupled with clay mineral analysis provides an explanation for the Fe substitution into chlorite for the butter shales and supports the interpretation of rapid sediment sealing that allowed for the pristine preservation of articulated fauna and mineralization observed in the butter shale.