MAPPING DIAGENETIC CALCITE VS. ORIGINAL ARAGONITE IN ORTHOCONIC NAUTILOIDS OF THE BUCKHORN ASPHALT (MIDDLE PENNSYLVANIAN) OF OKLAHOMA

The Buckhorn Asphalt quarry, south of Sulfur, Oklahoma in the Arbuckle Mountains, contains an asphalt-impregnated fossil Lagerstätte that preserves the original aragonite mineralogy of a diverse middle Pennsylvanian (Desmoinesian) marine assemblage dominated by molluscs. Orthoconic nautiloids are preserved as segments of the phragmocone and are very abundant in the deposit. Cameral deposits of the orthocone shells are present within and completely fill most preserved chambers. Given the generally high quality of Buckhorn preservation, these cameral deposits are potential targets for geochemical studies that seek to track movement of these cephalopods through different water masses during their lifetime. However, diagenetic alteration must first be assessed.

Orthoconic nautiloids were removed from bulk samples of Buckhorn material via chemical and mechanical means. Alteration of original aragonite material to diagenetic calcite within the cameral deposits of the nautiloids was investigated utilizing reflected light, scanning electron, and cold cathodoluminescence microscopy (CL); X-ray powder diffraction (XRD); energy dispersive spectrometry (EDAX); and laser ablation inductively coupled plasma optical emission spectrometry (LA-ICP-OES). Samples of outer shells and septa displayed well-preserved aragonite nacre. Cameral deposits were also generally well-preserved, showing fine-scale banding that probably reflects variations in grain size during their growth. Diagenetic calcite was found to be present in and around the siphuncle, in fractures through the cameral deposits, and along the outer edge of the cameral deposits. The location of the diagenetic calcite is inferred to represent fluid pathways. LA-ICP-OES transects across cameral deposits indicate intriguing repeated fluctuations in trace element abundances, such as Sr and Ba. Efficient identification of diagenetic and unaltered material within orthocone nautiloid cameral deposits can identify productive zones for microsampling for trace element and isotope ratios in future studies.