INTO THE MATRIX: SEDIMENTOLOGICAL CHARACTERIZATION OF BIOTURBATED SEDIMENTS ACROSS THE CRETACEOUS-PALEOGENE BOUNDARY AT RANCOCAS CREEK, NEW JERSEY, WITH IMPLICATIONS FOR LITHOSTRATIGRAPHY

The K-Pg boundary interval is preserved near the lithologic contact of the Navesink Formation (NF) and the overlying Hornerstown Formation (HF) in the Woodbury, Runnemede, and Mt. Holly quadrangles, New Jersey. The formational contact is typically placed at an observable color change ~20-25 cm below the Main Fossiliferous Layer (MFL), a ~20-cm-thick, regionally extensive bonebed containing Cretaceous macrofossils. A detailed granulometric analysis at the Rancocas Creek section, complemented with low-field magnetic susceptibility (MS) measurements, confirms that glauconite-rich Thalassinoides isp. burrow fills in the NF differ from enclosing NF matrix. These results have implications for lithologically constraining the NF/HF contact. Non-bioturbated matrix was point-sampled at 5 cm intervals (burrow fill: 10 cm) from -110 to +25 cm (~MFL base) relative to the currently assigned formational contact. A distinct lithologic shift occurs at +15 cm from the currently accepted formational boundary. Above this level, the mean grain size of the matrix sand fraction (>90%) becomes 6.7% coarser, sorting improves by 17.5%, and skewness decreases by 30.4%. Therefore, based on these shifts in sediment properties, we propose that the lithological NF/HF contact at Rancocas Creek outcrop should be raised to 15 cm above the color change. In addition, MS values of the matrix also rise abruptly by 27.3% at this level, matching the values of the burrow fill at all levels. Our preliminary findings provide independent evidence that deep burrows in the Late-Cretaceous Navesink Formation were filled with sediments from the Paleogene Hornerstown Formation. Together with growing geochemical and ichnological datasets along the New Jersey Coastal Plain, this research emphasizes the importance of point-sampling the matrix in bioturbated successions and has implications for refining the stratigraphic context of the complex depositional history associated with the extinction interval.