Spectral Gamma-Ray Logs in Relation to Clay Mineralogy and Sequence Stratigraphy, Cenozoic of the Atlantic Margin, offshore New Jersey

Abstract

The spectral gamma-ray logs for all sites drilled on Ocean Drilling Program Leg 150 can be subdivided into four spectral gamma-ray units characterized by distinct patterns of total gamma radiation; Th, U, and K content; and Th/K and Th/U ratios. These units (A-D) are of genetic significance in that they correspond to the major elements of the large-scale, "second-order" sequence stratigraphic architecture of the Atlantic continental margin, offshore New Jersey. Changes in the Th/K ratios throughout the succession can be explained by changing relative proportions of detrital clay minerals, particularly illite, mixed-layer illite-smectite, smectite, and kaolinite, except over intervals where glauconitization has been a significant process. Changes in Th/K ratios due to changes in detrital clay minerals do not in this case closely reflect distance from shoreline, but instead register mainly changes in transportation mechanisms and source area climate or topography. Glauconitic intervals show variable Th, U, and K content but in general have very low Th values. Variable U and K content means that the same glauconitic horizon when traced downslope may change from a total gamma-ray low to a total gamma-ray high. Total gamma-ray peaks are likely to be misleading indicators of maximum flooding surfaces in offshore settings containing significant quantities of glauconitic minerals.