CRINOID STEREOM MICROSTRUCTURE AND CARBONATE DIAGENESIS IN THE LOWER DEVONIAN, BECRAFT FORMATION FROM NEW YORK
The Lower Devonian Becraft Formation of New York is well known for its whitish-pink crinoid ossicles and disc-shaped holdfasts. We petrographically and geochemically analyzed crinoidal stereom microstructures to better understand the role of carbonate diagenesis within stomal spaces of the original crinoid stereom. Petrographic analysis of crinoid fragments that exhibit a pinkish-white color show stereom pore spaces filled with an opaque Fe-bearing material. Acid digestion of individual ossicles exposed the residual finely-crystalline material filling in the stomatal network, thus revealing a three-dimensional stereom structure. SEM secondary imaging, and EDS backscatter imaging analysis of etched crinoid columnals and on the digested ossicles, reveal a labrynthic stereom microstructure with surrounding ferroan calcite cement and microdolomite infilled with microcrystalline clay minerals. EDS imaging on infilled stomal spaces by non-biogenic elements including; Al, Mg, Si, Ti, O, and Fe. Whole crinoid columnals that were exceptionally pink in color analyzed by EDS showed high amounts of iron oxide in addition to the ferroan calcite. These data suggest that the stereom infilled clay minerals were most likely chlorite, and potentialy corrensite, even though XRD analysis revealed no apparent peaks. The presence of fluid within the pore system at high pressures can allow for magnesium and non-biogenic elements to be transported and deposited. The occurrence of ferroan calcite with non-biogenic elements and microdolomite is indicative of low-temperature dissolution-reprecipitation diagenesis. It is also evident that the crinoid ossicles have undergone a type-2 transformation that involves the relegation of Mg-rich calcite cement to ferroan calcite cement, and in the process the original stereom structure is partially destroyed. Ferrous iron brought into the stereom ultrastructure may have experienced oxidation due to the redox conditions of the hydrothermal fluid. The iron present in pore spaces is stabilized by forming siderite, and is also incorporated into the structures of the surrounding dolomite and calcite, resulting in the pink color. Ultimately, the crinoid columnals in the Becraft Formation show evidence of a deep burial diagenesis under low temperature conditions.