Progressive change of pelagic clay microstructure during burial process: examples from piston cores and ODP cores

Abstract

Progressive change of microstructure in pelagic clays of different three orders of burial depths, several centimeter, several meters and several hundred meter depths, was observed by measurement of anisotropy of magnetic susceptibility (AMS) and observation by scanning electron microscope (SEM) and thin section. At depths of several centimeters (void ratio >3.0), peds of several 10 μm in diameter are linked by long and narrow connectors in edge-face (EF) contact and pore sizes between the peds are large. At depths of several meters (void ratio 3.0–2.0), the shape of the connectors changes from narrow to broad. The pore size between the peds decreases with burial depth. At depths of several hundred meters (void ratio <2.0), clay flakes are arranged in wavy parallelism in face-to-face (FF) contact. The pores of intra-peds are reduced in volume. The pelagic clays are consolidated due to rotation of clay flakes composed of the connectors and peds. The AMS suggests that parallel coordination is developed strongly at 150 m depth. Below this depth, zeolite occurs around radiolarian tests, and conjugated zeolite veins are observed with a normal fault sense of displacement. Due to cementation, the rotation of clay flakes in pelagic clays ceased and the resulting clay microstructure were further preserved through diagenesis.