The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 121, Issue 5
Displaying 1-3 of 3 articles from this issue
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  • Shin'ichi Mori, Naoki Takahashi, Kenichiro Shibata, Yuichiro Tanaka, D ...
    2015 Volume 121 Issue 5 Pages 161-166
    Published: May 15, 2015
    Released on J-STAGE: July 30, 2015
    JOURNAL FREE ACCESS
    Visual observations were conducted at the mouth of the Tokyo Submarine Canyon (TSC), at the junction of the TSC and the Misaki Depression, in August 2010, from the ROV Hyper-Dolphin. Examination of the topography and geology of the TSC and Miura Basin revealed four lithostratigraphic units in area, Units I to IV (in stratigraphically descending order). Unit I consists of Recent unconsolidated mud that thinly covers the bottom of the TSC and two deep-sea terraces. Unit II is a middle Pleistocene conglomerate observed at water depths of 1148-1096 m; the lithology and areal distribution of Unit II suggest deposition from a debris flow that surged down the canyon. Unit III is a middle Pleistocene semi-consolidated muddy deposit consisting of alternating horizontal beds of sandstone and mudstone, observed at water depths of 1120-809 m; Unit III is present throughout the Misaki Depression. Unit IV is a consolidated muddy deposit consisting of N-dipping alternating beds of sandstone and mudstone, observed at water depths of 1174-1120 m; the unit is partly eroded and is overlain by Unit II. The stratigraphic relationships and distributions of lithofacies at the mouth of the TSC help clarify the nature of tectonism in the area of the TSC and Miura Basin.
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  • Yuji Orihashi, Shunso Ishihara
    2015 Volume 121 Issue 5 Pages 167-171
    Published: May 15, 2015
    Released on J-STAGE: July 30, 2015
    JOURNAL FREE ACCESS
    We conducted U-Pb dating on zircon crystals separated from two drill core samples to ascertain whether a tonalite body discovered by drilling in Matsukawaura east of the Futaba Fault near the city of Soma, belongs to a 300-Ma Paleozoic granite, or to Cretaceous granitoids with age common to the Abukuma Highlands or the Kitakami Mountains. The zircons yield U-Pb ages of 115-114 Ma, which are slightly older than the age of Abukuma granitoids exposed to the area between the Hatagawa and Futaba faults (108-106 Ma). The Matsukawaura tonalite, having a typical adakitic signature, exhibits similarity to Cretaceous Takase Granite in Wariyama Sheared Granodiorite occurring along the east side of the Futaba Fault.
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