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 113, Issue 3
Displaying 1-5 of 5 articles from this issue
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Articles
  • Triassic-Jurassic unconformable boundary in the Kurosegawa Terrane, northern Itsuki area, western Kyushu
    Naoto Ishida
    2007 Volume 113 Issue 3 Pages 83-94
    Published: 2007
    Released on J-STAGE: October 16, 2007
    DOIhttps://doi.org/10.5575/geosoc.113.83
    JOURNAL FREE ACCESS
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    In the northern Itsuki area, western Kyushu, the Lower Jurassic Hirasawatsudani Formation (newly defined) overlying the Upper Triassic Matsukuma Formation was discovered from clastic rocks of the Kurosegawa Terrane. The Matsukuma Formation in the northern Itsuki area, correlated with the Carnian Stage, includes parallel-laminated mudstone with fine sandstone beds and massive mudstone. Lower part of the Hirasawatsudani Formation is composed of thick-bedded coarse sandstone beds with intercalations of conglomerate and turbiditic sandstone beds. Upper part of the Hirasawatsudani Formation is composed of black mudstone with thin sandstone beds. Based on the radiolarian fossils, it is highly possible that the Hirasawatsudani Formation is correlated with the Hettangian Stage. The boundary of the Matsukuma Formation and the Hirasawatsudani Formation is considered to be unconformable, because a certain stratigraphic-gap ranging from the Norian Stage to the Rhaetian Stage is recognized.
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  • Geology of the Yaeyama area, Kagoshima Prefecture and the detailed stratigraphy of the Pliocene Koriyama Formation
    Kimihiro Uchimura, Kimihiko Oki, Akira Furusawa
    2007 Volume 113 Issue 3 Pages 95-112
    Published: 2007
    Released on J-STAGE: October 16, 2007
    DOIhttps://doi.org/10.5575/geosoc.113.95
    JOURNAL FREE ACCESS
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    The authors carried out a geological survey of the Yaeyama mountain area located in the northwestern part of Kagoshima Prefecture and as a result of these findings 12 stratigraphic units were defined in the area. They established a stratigraphic framework for the Pliocene lacustrine sediments, such as the Koriyama Formation, which were difficult to correlate within the study area due to the sporadic distribution of those sediments. The Koriyama Formation is divided into four members: the Atsuchi Mudstone, the Toge Sandstone, the Yuya Mudstone and the Hanao Tuff Breccia in ascending order. Furthermore, the Koriyama Formation is accompanied by intrusive andesitic rocks (the Nasubida Andesites) while the Yuya Mudstone Member is interbedded with two pyroclastic flow deposits (the Kusakidan and the Miyawaki Pyroclastic Flows). K-Ar age obtained from the Miyawaki Pyroclastic Flow deposits is about 2.5 Ma. Fossil diatoms collected from the Koriyama Formation are divided into three assemblages within the Yuya Mudstone Member: the Stephanodiscus astraea assemblage in the lowest horizon, the Cyclotella michiganiana assemblage in the lower horizon and the Aulacoseira ambigua assemblage in the upper horizon. Accompanying these three subdivisions are the two previously mentioned pyroclastic flow deposits. It has been made clear that the Koriyama Formation sporadically distributed in the Yaeyama mountain area is synchronous lacustrine sediments based on correlations with the interbedded pyroclastic flow deposits and fossil diatom assemblages.
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  • Sedimentological history of Tertiary slope facies in the northern area of the Tsushima Islands, Japan
    Yuya Yamaguchi, Yukimasa Oho
    2007 Volume 113 Issue 3 Pages 113-126
    Published: 2007
    Released on J-STAGE: October 16, 2007
    DOIhttps://doi.org/10.5575/geosoc.113.113
    JOURNAL FREE ACCESS
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    This paper describes the sedimentological history of the bathyal slope facies deposits of the Tertiary Taishu Group in the northern part of the Tsushima Islands. The study area is underlain mainly by mudstone and alternative beds of sandstone and mudstone, and small amounts of submarine sliding deposits. Detailed mapping of this area makes it clear that the Taishu Group forms anticlinoria and synclinoria with half-wave lengths of about 0.5 km to 2 km. The axial traces trend NE and NNE. EW-trending folds had also be recognized in the northeastern part. The deposits in the Taishu Group are characterized by five large sand bodies surrounded by massive mudstone. Each sand body consists of more smaller bodies of sandstone and alternative beds of sandstone and mudstone. Comparing the stratigraphic levels of the succession in five sand bodies, five sedimentary cycles were recognized. Each sedimentary cycle exhibits coarsening- or fining-upward, which corresponds to each stage of relative sea-level change. Those stages are (a) rising to a highstand stable stage, (b) falling to a lowstand stable stage, (c) rising to a highstand stable stage, (d) falling to a lowstand stage, and (e) rising to a highstand stage, in ascending order. Likewise, sedimentary processes are considered to be as follows: In the first and second stages of (a) and (b), coarse sediment was supplied only to the east part. In the following stage (c), the supply of coarse sediment in the western part was larger than that of the eastern part. After these stages, coarse sediment was supplied all over the basin in stage (d). In the last stage (e), coarse sediment was supplied mainly to the east. Relative sea-level changes and the migrating of depocenter have played important roles in forming the stratigraphy of this area.
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Prompt Report
  • Fission-track ages of tuff beds from the Upper Cretaceous Mifune Group in Kyushu, Japan
    Naoki Ikegami, Hideki Iwano, Tohru Danhara, Harutaka Sakai
    2007 Volume 113 Issue 3 Pages 127-130
    Published: 2007
    Released on J-STAGE: October 16, 2007
    DOIhttps://doi.org/10.5575/geosoc.113.127
    JOURNAL FREE ACCESS
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    Fission-track (FT) dating was carried out on zircon crystals from four vitric tuff beds in the Mifune Group, middle Kyushu. FT ages of 93.1±4.4 Ma, 89.8±4.0 Ma for the lower part of the Lower formation, 83.6±3.1 Ma for the lower part of the Upper formation and 82.1±11.1 Ma for the upper part of the Upper formation were obtained. These data suggest that the Mifune Group ranges in age from Late Cenomanian to Early Campanian, though the upper age limit remains uncertain. The age of the vertebrate fauna from the middle part of the Upper formation is assigned to Coniacian to Campanian. This age is younger than previously estimated age obtained from paleontological studies. The Mifune Group is possibly correlated with the Onogawa Group on the basis of the present FT ages.
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