Abstract

In a study of fabrics in clay-rich sediments from Site 799 of the Ocean Drilling Program (ODP), located in the Kita-Yamato trough (Japan Sea), we have investigated the relationships between diagenetic processes and evolution of petrofabric, magnetic fabric, and related pore-space reduction. Position fabrics, shape preferred orientations (SPO), and crystallographic preferred orientations (CPO) in the sediments were quantified using optical microscopy and X-ray texture analysis. Commonly, uniaxial vertical shortening resulting from progressive loading of the sediment pile by burial and loss of pore space by dewatering is well imaged in the building of preferred orientation fabrics. SPO of illite flakes and CPO of smectite (001) poles are axially symmetric around the core axis (the vertical), with fabric intensities increasing with depth. These fabrics are proposed to have evolved from initially random orientation distributions of flocculated clay fallout from an oxic bottom water column. Distortion of arrays of marker particles, such as organic detritus, biogenic microclasts, and framboidal pyrite, corroborate these results. Magnitudes of uniaxial vertical shortening calculated from loss of porosity and petrofabric parameters are roughly similar. Intergranular slip and distortion of grains facilitating pore-space collapse are the dominant deformation mechanism. Solution and precipitation of SiO 2 is an important additional fabric-building mechanism below the opal-A/opal-CT transition at 450 m depth. Below this depth, smectite fabrics are also randomized by recrystallization.

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