FABRIC EVOLUTION AND MECHANISMS OF DIAGENESIS IN FINE-GRAINED SEDIMENTS FROM THE KITA-YAMATO TROUGH, JAPAN-SEA

In a study of fabrics in clay-rich sediments from Site 799 of the Ocea n 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 qu antified using optical microscopy and X-ray texture analysis. Commonly , uniaxial vertical shortening resulting from progressive loading of t he sediment pile by burial and loss of pore space by dewatering is wel l imaged in the building of preferred orientation fabrics, SPO of illi te 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 rand om 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, corrob orate these results. Magnitudes of uniaxial vertical shortening calcul ated from loss of porosity and petrofabric parameters are roughly simi lar, Intergranular slip and distortion of grains facilitating pore-spa ce collapse are the dominant deformation mechanism. Solution and preci pitation of SiO2 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.