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 124, Issue 6
Displaying 1-6 of 6 articles from this issue
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Excursion Guidebook: 125th Annual Meeting of the Geological Society of Japan, 2018 Sapporo
Regular Section
Article
  • Geochemical identification of individual glass shards from pyroclastic flow deposits intercalated in drill core at the Nagata River mouth in Kagoshima City, based on major and trace element compositions
    Akira Furusawa, Kimihiko Oki, Ri-ichirou Miyawaki
    2018 Volume 124 Issue 6 Pages 435-447
    Published: June 15, 2018
    Released on J-STAGE: August 18, 2018
    DOIhttps://doi.org/10.5575/geosoc.2018.0007
    JOURNAL FREE ACCESS
    Supplementary material
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    Drill core, known as the Taniyama boring core, from the mouth of the Nagata River in Kogoshima city, southwest Japan, contains five pyroclastic flow deposits. The Kakuto, Kobayashi, and Hiwaki (Shimokado) pyroclastic flow deposits were identified through particle composition analysis and the refractive index of glass shards in the flow deposits. However, two of the deposits do not correlate with the deposits reported in previous studies.

    Many pyroclastic flow deposits occur in Kagoshima City and the surrounding areas. Some of these deposits have not been analyzed in terms of lithological features such as the major element compositions and trace element contents of volcanic glass shards. In addition, many deposits have not been clearly defined stratigraphically. It is necessary to review the major element compositions and trace element ratios of volcanic glass shards within these deposits to reconstruct the tephrostratigraphy of southern Kyushu.

    In this study, five pyroclastic flow deposits, including the Kakuto, Oda, Kobayashi, and Hiwaki, pyroclastic flow deposits were sampled, and the major and trace element compositions of volcanic glass in the deposits were analyzed by Energy Dispersive X-ray spectrometry (EDX) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP–MS). The results show that the Kakuto, Oda, Kobayashi, and Hiwaki pyroclastic flow deposits can be identified from the major element composition of volcanic glass, whereas in the core, one pyroclastic flow deposit remains unidentified, although these deposits show diagnostic trace element compositions.

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  • Stratigraphy and age of the Permian to Triassic chert-dominant succession of the Mino Belt in the eastern part of the Funabuseyama rock mass, western Gifu Prefecture
    Hiroyoshi Sano
    2018 Volume 124 Issue 6 Pages 449-467
    Published: June 15, 2018
    Released on J-STAGE: August 18, 2018
    DOIhttps://doi.org/10.5575/geosoc.2018.0010
    JOURNAL FREE ACCESS
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    This study examines the lithostratigraphy and age of the chert-dominated succession in the eastern section of the Funabuseyama rock mass within the Mino Belt. The chert-dominated succession (~550 m thick) comprises (in ascending order) basaltic rocks, reddish chert, clastic dolomite, gray-to-black chert, black claystone, dark-gray-to-black chert, and alternating layers of siliceous micrite and chert. Radiolarian biostratigraphy indicates that the reddish chert, gray-to-black chert, and dark-gray-to-black chert are correlated with the middle to upper Cisuralian, lower Guadalupian to upper Lopingian, and upper Lower to upper Middle Triassic, respectively. The age of the alternating siliceous micrite and chert falls within the beginning to middle Late Triassic. The entirety of the chert-dominant succession ranges in age from the middle Cisuralian to the middle Upper Triassic.

    The lithostratigraphy and complete lack of coarse terrigenous grains and rocks within the chert-dominant succession indicate a formation through pelagic, deep-marine sedimentation on the lower flank of an ocean-floor basaltic seamount in an open-ocean realm. The clastic dolomite is interpreted to represent sediments displaced from a shallow-marine setting into a deep-marine setting where the chert accumulated.

    Lithostratigraphy and biostratigraphy show that the middle Cisuralian to upper Lower Triassic section of the chert-dominant succession is correlated with the previously defined Hashikadani Formation and partially emends the western section of the Funabuseyama rock mass. By adding the middle Triassic chert and upper Triassic alternating siliceous micrite and chert to the middle Cisuralian to upper Lower Triassic chert-dominated succession, we propose that a revision to the Hashikadani Formation is required. Accordingly, the Hashikadani Formation should be interpreted as a sedimentary unit recording a long-lived deep-marine and pelagic sedimentation from the middle Cisuralian to middle Late Triassic upon a basaltic basement in an open-ocean realm.

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