The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 119, Issue 9
Displaying 1-6 of 6 articles from this issue
Review
  • Yasuhiro Takashimizu
    2013 Volume 119 Issue 9 Pages 599-612
    Published: September 15, 2013
    Released on J-STAGE: February 07, 2014
    JOURNAL FREE ACCESS
    This paper reviews previous studies on tsunami deposits in Hokkaido, northern Japan. It presents a bibliography of relevant peerreviewed papers, reports, books, and digital maps. In particular, studies of three 17th Century tsunami deposits (i.e., deposits in the eastern Pacific coast area, deposits in the Iburi coast area related to an unknown earthquake, and deposits of the 1640 Hokkaido Komagatake tsunami), along with the geological features of the deposits, are described in detail to contribute to future tsunami mitigation in Hokkaido. Finally, three areas of research are proposed for future studies on tsunami deposits in Hokkaido: (1) clarification of the tsunami history in unsurveyed areas as part of a paleo-tsunami study, (2) detailed surveys in areas of social infrastructure (e.g., electric power plants, ports and harbors, main roads and railways, industrial and non-industrial waste disposal facilities, water supply network), and (3) identification of trigger events and the precise distribution of 17th Century tsunamis along the Pacific coast of Hokkaido.
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Articles
  • Kazuki Kikuchi, Nobuhiro Kotake
    2013 Volume 119 Issue 9 Pages 613-629
    Published: September 15, 2013
    Released on J-STAGE: February 07, 2014
    JOURNAL FREE ACCESS
    We have undertaken a detailed study of the geology and depositional environments of the Bandodani Formation in the Upper Cretaceous Izumi Group of Shimada-jima Island, north Tokushima, Shikoku, Japan. Shimada-jima Island is a well-known locality for the occurrence of the trace fossil Archaeozostera, which had treated as the ancestor of the Zosteracea seagrass. We have delineated the stratigraphic position of the Archaeozostera-bearing interval in the Bandodani Formation distributed in Shimada-jima. Furthermore, we have constrained the depositional conditions suitable for the habitat of the Archaeozostera-animals in submarine fan systems.
    Facies analysis has revealed that the Bandodani Formation mainly comprises repeating channel-fill and inter-channel deposits, which were deposited in the frontal splay setting of a submarine fan system. Furthermore, a large-scale coarsening-upwards trend is observed throughout the formation and can be interpreted as the result of a prograding submarine fan system. In the studied section, the occurrence of Archaeozostera is restricted to the mud-dominated lowermost interval. This implies that the Archaeozostera-animals preferred relatively stable bottom conditions that were free from severe physical disturbances caused by abrupt and frequent aggradation and/or degradation of the seafloor associated with the sedimentation of the turbidites. Moreover, the ‘Archaeozostera-bearing interval’ represents a useful marker horizon to identify the contemporaneous stratigraphic interval.
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  • Jun Hosoi, Kazuo Amano
    2013 Volume 119 Issue 9 Pages 630-646
    Published: September 15, 2013
    Released on J-STAGE: February 07, 2014
    JOURNAL FREE ACCESS
    Since the late 19th century, research on the Green Tuff of Japan has focused on its stratigraphy, which was finally summarized around 1990. Recently, the stratigraphy of rocks of the Oga Peninsula, which is the type locality of the Green Tuff of Japan, was revised. We sought to unravel details of temporal changes in the volcanism and sedimentary environment at Nishiwaga Town, Iwate Prefecture in the Ou Backbone Range, Northeast Japan, based on facies analyses of volcanic and clastic rocks, which is a relatively recent method used to reconstruct paleo-sedimentary environments. We reconstructed the paleo-sedimentary basin architecture, sedimentary processes, and paleo-volcanism. Two types of non-explosive submarine volcanoes (Types A and B) and three types of explosive eruptions (Types 1, 2, and 3) were identified. From the field investigations and detailed facies analyses, five tectonic stages were identified in the Miocene, as follows. (1) A half-graben formed and Type A submarine volcanoes erupted in the basin at about 20 Ma. (2) The basin was filled with turbidites, debrites, and hemipelagic sediments from about 18 to 15 Ma. (3) Large tectonic subsidence and explosive volcanism (Types 1, 2 & 3) occurred at about 15 Ma. Seawater depth increased from several meters to over 1000 m. (4) The increase in seawater depth corresponded with a decrease in the intensity of volcanism, resulting in the formation of Type B submarine volcanoes at about 14 Ma. (5) The basin was then covered with hemipelagic sediments from about 14 Ma.
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