TAPHONOMY AND MAGNETIC SUSCEPTIBILITY SIGNATURE OF PYCNODONTE SP. ASSOCIATED WITH THE K-PG BOUNDARY, CENTRAL NEW JERSEY

Dense concentrations of gryphaeid Pycnodonte sp. oysters (P. convexa and P. mutabilis) in the uppermost Navesink Formation (Rancocas and Inversand sites, New Jersey) offer an insight into both biostratinomic and diagenetic conditions immediately prior to K-Pg extinction. The concave up attitude of large fragments and nested double-valve samples suggest either the original position of the large (up to 7-8 cm) left valve or relatively low-energy depositional conditions. Several healed suture lines indicate predation by durophagous crustaceans, whereas intricate clionid sponge borings suggest post-mortem shell exploitation prior to burial. In addition to glauconite-rich sediment adhering to and infilling the shells, the majority of fossil valves exhibit varying degrees of pyritization, with >80% of shell surfaces and internal elements coated with fully developed pyrite crystals. This diagenetic feature is especially well expressed in Pycnodonte and induces anomalous magnetic properties. Bulk low-field magnetic susceptibility (MS) values of most oyster shells range from 800-1200 μSI (max ~1800 μSI). These are conservative values due to valve curvature (reduced sensor-sample contact), however are still substantially higher than the background sediment MS of bioturbated K-Pg successions, which typically range between 500-800 μSI. Therefore, detailed magnetic susceptibility characterization of Pycnodonte shells not only documents the diagenetic history of these oysters, but also cautions that locally high susceptibility values must be considered during in situ measurements of glauconite-rich successions along the New Jersey Coastal Plain. In addition to constraining the paleoecological context, detailed analysis of Pycnodonte taphonomy and diagenetic history will shed light on the nature of benthic marine redox potential during this critical interval in the Earth history.