PRESERVATION OF CRINOIDS IN THE UPPER ORDOVICIAN 'FLEXICALYMENE MINUENS BUTTER SHALE' OF SOUTHEASTERN INDIANA: PRELIMINARY RESULTS

Very distinctive units consisting of nearly pure siliciclastic clay occur sporadically throughout the Upper Ordovician (Katian) succession of the Cincinnati Arch region. Several of these so-called ‘butter shales’ are characterized by exceptional fossil preservation, most notably in the form of articulated trilobites. One such unit, located within the lower portion of the Richmondian-age Liberty Formation, is known as the ‘Flexicalymene minuens butter shale’ in reference to the abundance of articulated and commonly enrolled trilobites attributable to the diminutive calymenid F. minuens. However, little attention has been paid to the taphonomy of other organisms bearing multi-element skeletons from this interval, including crinoids. To assess the preservation of crinoids, 6 bulk samples were collected from three horizons throughout this interval at St. Leon, southeastern Indiana, and disaggregated. Samples were sieved at 0.125 and 0.074 mm. All crinoid material was present in the coarser fraction. Crinoids are preserved most commonly as brachials and articulated arm segments, with a subordinate amount of isolated columnals. All identifiable material belongs to minute disparids. Unlike other organisms in the residual fraction, nearly all crinoid specimens display some form of ferruginous mineralization, either coarsely crystalline pyrite overgrowth, resulting in a rusty brown color, or a microcrystalline replacing and pore-filling ferrous mineral, resulting in a medium gray color. The most important taphonomic conclusions from this analysis are: (1) crinoid skeletons are largely disarticulated but articulated arm segments persist; (2) elements of the arms are more abundant than elements of the column; and (3) crinoidal material is preferentially affected by diagenetic ferruginous mineralization. Ongoing and future studies will test whether these patterns are unique to the ‘F. minuens butter shale’ or indicative of a consistent taphofacies inherent to ‘butter shales.’