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 115, Issue 6
Displaying 1-6 of 6 articles from this issue
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Articles
  • The experiment on dating of the tsunami deposits using the luminescence method: A case study of samples from Nemuro coastal lowland, eastern Hokkaido
    Futoshi Nanayama, Tsuneto Nagatomo, Yorinao Shitaoka, Ken-ichi Suzaki, ...
    2009 Volume 115 Issue 6 Pages 249-260
    Published: 2009
    Released on J-STAGE: October 29, 2009
    DOIhttps://doi.org/10.5575/geosoc.115.249
    JOURNAL FREE ACCESS
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    Since 1998, an increasing number of studies have examined late Holocene tsunami deposits within marsh deposits along the Pacific coastal zone of eastern Hokkaido. These works have contributed to the establishment of a stratigraphy of large tsunami traces extending between the Tokachi coast and the Nemuro coastal lowland, using AMS 14C dating of carbonized materials and tephrochronology; however, to establish a more accurate stratigraphy, it is desirable to obtain depositional data directly from the tsunami deposits themselves. In October 2005, we performed a trench survey of large tsunami traces at Gakkarahama beach and Nanbuto marsh in the Nemuro coastal lowland, and collected 18 samples for luminescence dating from 12 tsunami deposits (NS1–NS12), and 3 regional tephra layers (B-Tm + Ma-b, Ta-c, and Ma-d) within a peat bed. In comparing the results of luminescence dating with tephrochronology and AMS 14C data, we identify six distinct tsunami deposits and two tephra layers. Other luminescence results, however, yield erroneously old ages or ages older than the tephrochronology and AMS 14C data. These older ages are considered to mainly reflect insufficient zeroing of luminescence during the tsunami run-up process.
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  • Middle Miocene ostracods from the Furue Formation, Shimane Prefecture, Southwest Japan.
    Gengo Tanaka, Ritsuo Nomura
    2009 Volume 115 Issue 6 Pages 261-265
    Published: 2009
    Released on J-STAGE: October 29, 2009
    DOIhttps://doi.org/10.5575/geosoc.115.261
    JOURNAL FREE ACCESS
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    This report provides the first description of ostracod assemblages from the Middle Miocene Furue Formation, Shimane Peninsula, Southwest Japan. The assemblage consists mainly of Macrocypris sp., Palmoconcha saboyamensis, Falsobuntonia hayamii and Palmenella limicola. Based on a comparison among Recent ostracod assemblages, the reported fossil assemblage indicates a cool lower-shelf environment similar to the Recent off Shimane Peninsula.
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  • Genesis and evolutional processes of the Paleozoic oceanic island arc crust, Asago body of the Yakuno Ophiolite, Southwest Japan
    Yoshimitsu Suda, Yasutaka Hayasaka
    2009 Volume 115 Issue 6 Pages 266-287
    Published: 2009
    Released on J-STAGE: October 29, 2009
    DOIhttps://doi.org/10.5575/geosoc.115.266
    JOURNAL FREE ACCESS
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    A middle to lower crustal section of Paleozoic oceanic island arc is exposed in the Asago body of the Yakuno ophiolite. Based on field occurrence, petrography, and geochemical modeling, we investigated the evolution of the Asago body and relevant magmatic processes. The Asago body consists of two stages of rocks. The first-stage rocks consist of metagabbro and schistose amphibolite that represent the basement to the Permian Yakuno paleo-island arc. The second-stage rocks are mainly arc granitoids that intrude the first-stage rocks. Mafic migmatites occur in the lower crustal section of the Asago body. Field occurrences and petrographic data suggest that the migmatites formed by the anatexis of first-stage rocks, and that segregation and accumulation of the anatectic melt resulted in morphological changes in the migmatites toward the middle crustal section. Geochemical data indicate that the first-stage rocks were derived from a basaltic magma of back-arc affinity, suggesting in turn that the Yakuno paleo-island arc was developed within a back-arc basin. Moreover, a low-K series (hornblende gabbro, quartz diorite, and tonalite) and high-K series (quartz monzodiorite and granodiorite) within the second-stage rocks were generated by high and low degrees of partial melting of first-stage rocks, respectively. We conclude that the Asago body is an example of the transition from oceanic to continental crust, related to the anatexis of mafic lower crust in an oceanic island arc setting.
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Short Articles
  • Determination of slip system in olivine based on crystallographic preferred orientation and subgrain-rotation axis: examples from Ichinomegata peridotite xenoliths, Oga peninsula, Akita prefecture
    Takako Satsukawa, Katsuyoshi Michibayashi
    2009 Volume 115 Issue 6 Pages 288-291
    Published: 2009
    Released on J-STAGE: October 29, 2009
    DOIhttps://doi.org/10.5575/geosoc.115.288
    JOURNAL FREE ACCESS
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    In this study of peridotite xenoliths from Ichinomegata volcano, Oga peninsula, NE Japan, we used electron backscattered diffraction to assess the validity of two methods of identifying the olivine slip system that operated during deformation. The xenoliths are harzburgite in composition and contain a distinct foliation and lineation defined by aligned spinel and pyroxene grains. Using the first method, the dominant slip system in olivine was estimated based on the crystallographic preferred orientation (CPO) of olivine with respect to the foliation and lineation. The second method was based on an analysis of subgrain rotation: given that intracrystalline subgrain boundaries were observed within several olivine grains, we measured the misorientation angles across these boundaries to define sets of slip planes, slip directions, and rotation axes. The olivine slip systems estimated using the two methods are essentially identical. The CPO pattern provides an important constraint on the dominant slip system that operated during deformation, whereas the analysis of subgrain rotation yields the slip system responsible for the development of individual subgrain boundaries composed of edge dislocations. The two methods are therefore complementary.
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  • Upper Triassic reefal limestone from the Sambosan accretionary complex in Japan and its geological implication
    Tetsuji Onoue, Jérôme Chablais, Rossana Martini
    2009 Volume 115 Issue 6 Pages 292-295
    Published: 2009
    Released on J-STAGE: October 29, 2009
    DOIhttps://doi.org/10.5575/geosoc.115.292
    JOURNAL FREE ACCESS
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    Upper Triassic limestone within the Sambosan accretionary complex in Japan contains reefal facies that accumulated on a mid-oceanic seamount in the Panthalassa Ocean. Such limestone is found at the Inaba Cave locality along the Shimanto River, central Shikoku. The reefal limestone occupies part of a bioclast-rich limestone that occurs as an allochthonous tectonic slab. The reefal limestone is characterized by sponge-algal rudstone-floatstone and coral bafflestone rich in Retiophyllia. Megalodonts floatstone also occurs in association with the reefal facies. Although the lateral continuity and exact relationship between the Megalodonts floatstone and the reefal facies remain uncertain, this is the first report in which this unique facies association has been described in the Sambosan accretionary complex.
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