CHANGES IN MAGNETIC SUSCEPTIBILITY ACROSS THE CRETACEOUS/PALEOGENE INTERVAL AND MAIN FOSSILIFEROUS LAYER AT SEWELL, NEW JERSEY

Late Cretaceous and early Paleogene formations in the New Jersey Coastal Plain, deposited in gently dipping shelf environments, contain strata rich in Maastrichtian and Danian fossils. Although these formations/strata are well studied, various interpretations, including placement of the Cretaceous/Paleogene (K/Pg) boundary, are the basis of continued debate. Exposures of glauconite-rich strata in the Inversand quarry at Sewell, NJ cross the K/Pg boundary and reveal three distinct fossiliferous layers: one Maastrichtian fossil layer in the Navesink Formation, one composed of latest Cretaceous fossils in the basal Hornerstown Formation ca. 10 cm above the contact with the Navesink Formation (the Main Fossiliferous Layer or MFL) which contains Cretaceous macrofossils mixed with Paleogene microfossils, and one ca. 2 m above the MFL composed of Danian fossils. Various workers have placed the K/Pg boundary at the Navesink/Hornerstown Formation contact, the top of the MFL, or higher in the Hornerstown Formation. Magnetic Susceptibility (MS) may help position the K/Pg boundary.

In situ magnetic susceptibility (MS) data were collected at 5 cm intervals from freshly exposed outcrops spanning 3.5 meters of section from the upper Navesink Formation, across the K/Pg boundary, and into the Hornerstown Formation. MS values in outcrop range from 40 to 80 (x 10^-5 SI units), increase from the Navesink into the basal Hornerstown Formation, and continue to a maximum at the top of the MFL. MS values gradually decrease above the MFL, despite increasing glauconite maturity, and are similar to those in the Navesink Formation. The maximum MS values are clearly associated with the MFL. MS values above and below the MFL are much lower, suggesting elemental changes in sediment composition occurred during formation of the MFL, possibly related to meteoritic input, biological activity, or relative sea level change.