CORRELATIONS IN MAGNETIC SUSCEPTIBILITY ACROSS THE CRETACEOUS/PALEOGENE BOUNDARY IN CENTRAL AND SOUTHERN NEW JERSEY

Late Cretaceous and early Paleogene formations in the New Jersey Coastal Plain, deposited in gently eastward dipping shelf environments, contain strata rich in Maastrichtian and Danian fossils, including the Pinna and Main Fossiliferous Layers (MFL). Although these formations are well studied, various interpretations, including the position of the Cretaceous/Paleogene (K/Pg) boundary, are the basis of continued debate. Other workers have placed the K/Pg boundary at the contact between the Hornerstown and underlying Navesink/New Egypt Formations, at the top of the MFL (in the lower Hornerstown Formation), or higher in the Hornerstown Formation. This study presents a new data set of in-situ magnetic susceptibility (MS) measurements from five localities in central and southern New Jersey, and demonstrates the utility of low-field MS for rapidly assessing lithologic changes and correlating glauconite-rich K/Pg sequences across the New Jersey Coastal Plain. Samples were collected at 3 cm intervals in freshly exposed outcrops spanning from 1-4 m of section from the Cretaceous upper Navesink/New Egypt Formation, across the K/Pg boundary, and into the Hornerstown Formation. MS values range from 40 to 160 (x 10^-5 SI units) in glauconite-rich sediments, but are less than 30 in glauconite-poor sediments and non-glauconitic clay clasts in the MFL. MS values are generally higher in the Hornerstown than in underlying formations, peak at or above the MFL or Pinna layer, and gradually decrease above this interval despite increasing glauconite maturity.

Point counts of thin sections and magnetically separated sediment fractions do not reveal significantly different heavy mineral content across formations, or in the MFL proper. This suggests the source of the higher MS signal may be related to elemental changes in sediment composition during formation of the MFL, possibly from meteoritic input, biological activity, or relative sea level change, and confirming results from glauconite composition and maturity that the MFL is not a transgressive lag or the result of a greatly decreased sedimentation rate.