Detection of Late Cretaceous and Cenozoic sequence boundaries on the Atlantic coastal plain using core log integration of magnetic susceptibility and natural gamma ray measurements at Ancora, New Jersey

Abstract

Magnetic susceptibility (MS) and natural gamma ray (NGR) were measured on middle Miocene to Cenomanian coastal plain strata from a continuously cored (356 m total depth; 86% recovery) borehole at Ancora, New Jersey (Ocean Drilling Program Leg 174AX). These measurements were integrated with lithologic descriptions and a downhole NGR log. A simple linear model explains most of the variation of the downhole NGR and the core MS as a consequence of the previously described lithologic variation. Spectral NGR at selected levels shows that NGR activity is mostly due to 40K associated with glauconite and mud, sediment components that also tend to give high MS values. Abrupt deviations from the average values of NGR activity and MS determined by the linear model can be interpreted as due to singular lithologies. For example, an anomalous level with high NGR but not a parallel increase in MS was found at the Navesink/Mount Laurel formation contact and can be attributed to high uranium concentration in phosphorite. Most (27 of 33) of the sequence boundaries independently identified at the Ancora site have sufficient lithological contrasts to be expressed in the NGR and/or MS logs.