Stratigraphic correlation of Miocene to Plio-/Pleistocene sequences on the New Jersey shelf based on petrophysical measurements from ODP leg 174A

Abstract

Ocean Drilling Program (ODP) Leg 174A is the latest of a series of drilling operations forming a transect from the New Jersey shelf to the slope, and designed for analysis of sea-level changes and calibration of seismic profiles. Two sites on the outer shelf and one on the upper slope were drilled. Sand-prone units reduced core recovery and caused technical problems during drilling. Downhole measurements were undertaken at all three drill sites and the interpretation of logging data in terms of sequence stratigraphy is the focus of this study. Our logging data provide a continuous record; filling the gaps in core recovery and resulting in an enhanced image of the drilled sequences. The development of sequences and their lateral changes are characterized by changes in the physical properties of the sediments, such as total natural gamma radiation and resistivity. However, the interpretation of the total gamma ray log can be misleading, since glauconite, a potassium-rich clay, is locally enriched in all types of sediments. Therefore, it is essential to use the individual potassium, thorium and uranium logs, which contribute to the total gamma radiation. Within sequences, cyclic changes in grain size reflected in the spectral gamma ray and resistivity logs are interpreted to indicate small-scale sea-level changes. Lateral changes in sediment successions are well observed on the shelf and the slope. In addition to the logs, the magnetic susceptibility, measured continuously on the cores, is a helpful indicator of facies changes. The variation in magnetic susceptibility with depth is important for hole-to-hole correlation and complements the core and log analyses.