The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
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Volume 114, Issue 3
Displaying 1-5 of 5 articles from this issue
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Articles
  • Progressive change of microfabrics by compaction in superficial layers of ocean bottom sediments : examples from the Okinawa Trough
    Kawamura Kiichiro, Kawamura Noriko
    2008 Volume 114 Issue 3 Pages 97-108
    Published: March 15, 2008
    Released on J-STAGE: February 24, 2009
    DOIhttps://doi.org/10.5575/geosoc.114.97
    JOURNAL FREE ACCESS
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    This paper deals with the development of the microfabric of hemipelagic clay during the early compaction stage. Materials used are core samples ca. 30 cm long, composed of homogeneous hemipelagic clay, collected from the superficial layer of ocean bottom sediments in the Okinawa Trough, Japan. Architecture of constituent particles, together with void ratio, shear strength and anisotropy of magnetic susceptibility (AMS), was analyzed in the core samples. Void ratios decrease downward through ca. 6-4 to ca. 4-2 at ca. 5 cm in burial depth. Shear strengths increase exponentially through ca. 0 kPa to ca. 6 kPa downward at ca. 10 cm in burial depth. The AMS parameters are rather constant throughout the column, suggesting no significant particle re-orientation of magnetic minerals by compaction. The microfabric of the samples is characterized mainly by the combination of peds and connectors. A ped is an aggregation of clay particles, and a connector is train of particles which connect peds successively to each other. The particles of the connector in the superficial layer within 6 cm in depth are linked each other by edge-to-edge and edge-to-face contacts. The architecture of the connectors changes to edge-to-face and face-to-face linkage with burial compaction. Thus, the increase of shear strength and the decrease of void ratio are resulted from this change of the connector's architecture.
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  • Weathering mechanism of pelitic schist and its relationships with landslides:
    analysis of undisturbed drilled cores from the Sambagawa Metamorphic belt in Shikoku.
    Shintaro Yamasaki, Masahiro Chigira
    2008 Volume 114 Issue 3 Pages 109-126
    Published: March 15, 2008
    Released on J-STAGE: February 24, 2009
    DOIhttps://doi.org/10.5575/geosoc.114.109
    JOURNAL FREE ACCESS
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    Weathering mechanisms of pelitic schist are clarified by analyzing the vertical variations from fresh rock zone to weathered rock zone in densities, porosities, chemistry, mineralogy and petrological features of undisturbed drilled cores from a landslide body. An oxidation front is made within a weathering profile by the reaction between rock and oxidizing water percolating from the ground surface. At the oxidation front, chlorite is transformed into Al−vermiculite, goethite is formed, and pyrite and graphite are depleted by oxidation. Pyrite is also oxidized by Fe3+ just below the oxidation front. The sulfuric acid yielded by the oxidation of pyrite, percolates further downward and interacts with rock, which consequently decreases its strength in conjunction with fractures induced probably by unloading. The deteriorated rock mass could be easily sheared and be transformed into a shear zone, which is a sliding zone in other words. The sliding zone is less permeable and hence interrupts the downward filtration of groundwater through it.
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  • Metamorphism and timing of the nappe movement in the Asaji metamorphic area, eastern Kyushu
    Masahiro Fujii, Yasutaka Hayasaka, Kenji Horie
    2008 Volume 114 Issue 3 Pages 127-140
    Published: March 15, 2008
    Released on J-STAGE: February 24, 2009
    DOIhttps://doi.org/10.5575/geosoc.114.127
    JOURNAL FREE ACCESS
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    The pre-Upper Cretaceous basement rocks distributed in the Notsuharu area, eastern Kyushu, are classified structurally from bottom to top into the Chokai Unit, the Ultramafic Unit and the Hikata Unit. The Chokai Unit known as the Asaji metamorphic rocks consists mainly of pelitic, psammitic and mafic rocks, and chert. Metamorphic zonation and the chemical compositions of metamorphic garnet and biotite indicate that the metamorphism of the Chokai Unit is due to the thermal effects of the syntectonic intrusion of the Nioki granite. Metamorphic conditions are estimated to be approximately 610°C at 2 kbar. SHRIMP Zircon U-Pb dating of the Nioki granite shows a concordia age of 134.7 ± 2.8 Ma (2σ). The uppermost Hikata Unit consists of non-metamorphic sedimentary rocks, and forms a southeastern vergenced nappe structure (Hikata nappe). The Hikata nappe truncates the thermal structure within the Chokai Unit, and is unconformably overlain by the Upper Cretaceous Onogawa Group. Therefore, the movement of the Hikata nappe can be limited to the period between the intrusion of the Nioki granite (ca. 135 Ma) and initial sedimentation of the Onogawa Group (ca. 100 Ma).
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