The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 113, Issue 1
Displaying 1-8 of 8 articles from this issue
Article
  • Hideo Takagi, Toru Ishii, Eri Tobe, Yusuke Soda, Kazuhiro Suzuki, Hide ...
    2007 Volume 113 Issue 1 Pages 1-14
    Published: 2007
    Released on J-STAGE: June 12, 2007
    JOURNAL FREE ACCESS
    Aso-4 pyroclastic flow is one of the largest volcanic activities in Japan and formed huge caldera of the Aso volcano at about 90,000 years b.p.. This pyroclastic flow brought about accidental clasts of granitic mylonite in the Aso-4 pyroclastic deposit in northern area of the caldera. These granitic mylonite clasts were investigated petrologically and geochronologically, and compared with the Nioki Granite in the Asaji area to the east of the volcano. The Aso-4 granitic clasts are characterized by garnet-muscovite bearing adamelite-granite, and all of them are moderately to strongly mylonitized and show porphyroclastic texture. Their SiO2 content ranges 71-74 wt.%. The range of garnet composition of the Aso-4 granitic clasts is Alm69~78, Sps14~27, Pyr0.2~3.3, Grs1.0~9.3. These mineral and chemical compositions are quite similar to those of the Nioki Granite. The K-Ar muscovite age for Aso-4 granitic clasts is 98.4±2.1 Ma, whereas Nioki Granite is 98.8±2.1 Ma. The heat of magma of the volcano did not affect the K-Ar closure system for muscovite when the pyroclastic flow took place, because heating has not fundamentally shortened the fission tracks within zircon grains in the dated clast. Accordingly, the Aso-4 granitic mylonite clasts are correlative with the Nioki Granite, which is also weakly to moderately mylonitized. The intrusive age of the Nioki Granite is also determined using CHIME monazite dating, and the result is 106±4 Ma. This suggests that subsurface ductile mylonite zone initiated around 100 Ma of the major fault extends from Asaji area to the Aso volcano. This fault coincides with the Oita-Kumamoto Tectonic Line, along which the active fault segment has been known. The occurrence of E-W extending mylonite zone is also significant evidence in considering the western extension of the Median Tectonic Line in Kyushu.
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Prompt Reports
  • Toshio Yasui, Noboru Okitsu
    2007 Volume 113 Issue 1 Pages 15-18
    Published: 2007
    Released on J-STAGE: June 12, 2007
    JOURNAL FREE ACCESS
    Late Devonian plant Leptophloeum rhombi-cum (Dawson, 1862) is newly discovered from the “Suberidani lenticular body” of the Kurosegawa Belt in Tokushima Prefecture, Shikoku, Southwest Japan. The twelve specimens are well-preserved in fine-grained sandstone, and the morphologic information may be useful for paleontological study of Leptophloeum. The presence of the Upper Devonian strata has been confirmed now by this fossil evidence in the Tokushima Prefecture. The bed composed of conglomerate, sandstone and shale, about 20 m thick, lies on the Siluro-Devonian Suberidani Group. The relationship between the Upper Devonian and the underlying “Bed G4” of the Suberidani Group seems to be unconformable.
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  • Shigeaki Yamada, Hiroki Matsuda, Akio Omura
    2007 Volume 113 Issue 1 Pages 19-22
    Published: 2007
    Released on J-STAGE: June 12, 2007
    JOURNAL FREE ACCESS
    This study provides new uranium-series dates of the Ryukyu Group in Ishigaki-jima Island, South Ryukyu Arc (SRA). Two fossil hermatypic coral samples taken from coral limestone in the northern and southern parts of the island were dated as 118.5±2.0 ka and 119.9±2.0 ka, respectively, which correlates to oxygen isotope stage 5.5. This fact suggests that a reef complex was formed during isotope stage 5.5 in Ishigaki-jima Island, just like in Hateruma-jima and Yonaguni-jima Islands, SRA. The dated coral in the northern part was collected at an elevation of 1.3 m. It is, therefore, inferred that the northern part of the island was tectonically stable since isotope stage 5.5. On the other hand, the dated corals in the southern part was collected at an elevation of 15 m, which indicated that the southern part of Ishigaki-jima has been uplifted at rate of about 0.10 m/ky since isotope stage 5.5. Our data clearly show that the western part of SRA has been situated in the tectonic setting of uprift for the last 125 ky.
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  • Hideshi Suzuki
    2007 Volume 113 Issue 1 Pages 23-26
    Published: 2007
    Released on J-STAGE: June 12, 2007
    JOURNAL FREE ACCESS
    Two fossil shark teeth found from the laminated gray sandy mudstone of the uppermost part of the Middle Miocene (15 Ma) Yokoo Formation (Northern Fossa Magna Region) in Ueda-city, Nagano Prefecture, central Japan, were described. In comparison with Recent specimens, these were identified as teeth of gulper shark, Centrophorus sp.. This report is the first description of the genus Centrophorus from Japan. The uppermost part of the Yokoo Formation is mainly composed of alternation of sandstone and mudstone (rich in sandy mudstone) which grades upward into glauconite sandstone. Therefore judging from the habitat of the Recent genus Centrophorus, the lithofacies,the molluscan and benthic foraminifer fossils, it is estimated that the uppermost part of the Yokoo Formation was deposited on the shelf slope at depths between 700 and 1500 meters under the reducing conditions.
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Note
  • Wataru Tanikawa, Toshihiko Shimamoto
    2007 Volume 113 Issue 1 Pages 27-32
    Published: 2007
    Released on J-STAGE: June 12, 2007
    JOURNAL FREE ACCESS
    For the purpose of understanding fluid flow regime in the focal area, the permeability structure and porosity distribution are critical factors. The effects of burial and subsequent uplifting on surface samples are evaluated by effective pressure cycling tests, and we propose a method to estimate permeability at depths. Permeability at Ashigara basin generally decreased with depth, and estimated permeability-depth relationship was plotted as log-linear curve, and that porosity-permeability relationship is described as a power law. Drastic permeability and porosity reduction with burial depths might be due to chemical cementation and time-dependent compaction rather than mechanical compaction.
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