MAGNETIC SUSCEPTIBILITY PATTERNS IN THE K/PG BOUNDARY GLAUCONITE DEPOSITS AT INVERSAND, NEW JERSEY: REFINING THE METHODOLOGY

Low-field bulk magnetic susceptibility (MS) measurements were collected in situ from the glauconite-rich K/Pg boundary section at the Inversand Quarry in Sewell, NJ to assess its value as a lithological characterization and stratigraphic correlation tool. MS values range from ~60 to 70 (x 10^-5 SI units) in the Upper Cretaceous Navesink Formation, and reach a maximum of 66 to 84 in the Main Fossiliferous Layer (MFL), a deposit of latest Cretaceous vertebrate and invertebrate fauna in the basal Hornerstown Formation. MS values then decrease upward in the Paleogene Hornerstown Formation. The clay-rich matrix of the upper Navesink Formation has relatively low MS values (~60), but coarser-grained burrow fills typically exceed 70. These higher values may result from introduction of higher MS materials from the overlying MFL into the Navesink matrix by bioturbation. Alternatively, there may be an inherent lithological control on magnetic susceptibility, such as a texture-dependent fraction of paramagnetic and ferromagnetic minerals. MS values differ slightly among three separate transects across the K/Pg boundary. These differences may result either from variations in primary depositional conditions or diagenetic effects following the K/Pg. In contrast to the greensands, grey clay rip-up clasts in the MFL exhibit anomalously low MS values (26-30). Oxidation of the glauconitic material was found to have a significant impact on MS readings, which could lead to analytical errors when comparing samples measured in situ with those analyzed in the lab, or stored in core libraries. Representative samples were measured again in the laboratory four months after the field measurements using a dual-frequency MS meter. This second round of measurements revealed a substantial shift (by as much as 30 MS units) to lower values only for those sediments that initially had a relatively high MS, primarily the MFL sediments. After an additional three-month period the values stabilized, therefore emphasizing the importance of obtaining measurements on fresh samples within a short time interval.