ADVANCES IN THE APPLICATION OF LOW-FIELD MAGNETIC SUSCEPTIBILITY IN MARINE CRETACEOUS-PALEOGENE SUCCESSIONS OF NEW JERSEY

Complementing the granulometric analysis of the K-Pg section of central New Jersey, bulk low-field magnetic susceptibility (MS) is emerging as a new tool for: 1) characterization of lithologic differences between burrow fill and matrix of a bioturbated outcrop and 2) constraining the Navesink (NF) and Hornerstown (HF) formational boundary. This study synthesizes the trends within a compound Thalassinoides-bioturbated marine succession at the Rancocas Creek site, Medford, NJ. Laboratory-based MS measurements of stratigraphic subsamples at 5-cm intervals were performed on the sand fraction from the non-bioturbated matrix and burrow fill across the NF and HF from -110 cm to +65 cm (relative to an observable color change). Within the overlapping section (-30 to +30 cm), the sand fraction of the matrix shows an abrupt MS increase in the 0-15 cm interval. Higher maturity, hight content and textural characteristics of glauconite result in relatively high MS values. The Late Cretaceous NF ranges from 400-625 μSI. The matrix of the Paleogene HF (+15 cm and above) is 14-27% higher (625-720 μSI) and overlaps burrow fill measurements from all levels (582-696 μSI). Aside from the concentrations of low-paramagnetic (glauconite) and diamagnetic (quartz and carbonates) components, localized concentrations of highly paramagnetic minerals result in anomalously high MS values. Pycnodonte sp. oysters yield MS values >800 μSI due to a dense accumulation of pyrite crystals. Where these shells are concealed at the outcrop surface by <5 mm of sediment, they may increase in situ MS measurements. Thus, high matrix MS values within the -20 to -25 cm interval are likely attributed to dense concentrations of these pyrite-replaced mollusks. Our findings demonstrate that magnetic susceptibility trends support the placement of the NF/HF boundary at +15 cm relative to the observable color change, show the affinity of NF burrow fill with the Early Paleogene HF matrix, and offer a potential proxy for glauconite maturity. This study complements ongoing MS-based research at other Coastal Plain localities by differentiating the contribution of the matrix, burrow fill, and anomalous macrofossil-associated para- and diamagnetism to sample-scale signatures.