BULLETIN OF THE GEOLOGICAL SURVEY OF JAPAN
Online ISSN : 2186-490X
Print ISSN : 1346-4272
ISSN-L : 1346-4272
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Article
  • Hidetosh HARA
    Type: research-article
    2020 Volume 71 Issue 6 Pages 509-516
    Published: December 16, 2020
    Released: December 19, 2020
    JOURNALS FREE ACCESS

    U–Pb dating of detrital zircons were analyzed from a calcareous sandstone of the Lower Cretaceous Takaoka Formation, distributed in the eastern margin of the Kanto Mountains. The youngest single grain age and the youngest cluster age were estimated to be 109.7 ± 0.8 Ma and 110.0 ± 1.0Ma, respectively. Both ages represent the early Albian, suggesting that the Takaoka Formation deposited after the early Albian. In addition, the calcareous sandstone of the Takaoka Formation is characterized by abundant content of Permian to Triassic zircons (276–240 Ma). Based on detrital zircon U–Pb age and sandstone lithology, the Permian to Triassic granitic rocks and the Chichibu accretionary complex were widely distributed in provenance of the Takaoka Formation. The Takaoka Formation corresponds to the Sanyama Formation in the Sanchu Cretaceous System. However, both formations present different peak spectra of detrital zircon U–Pb ages, and are characterized by different provenances.

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  • Takahiro YAMAMOTO, Shun NAKANO, Yoshihiro ISHIZUKA, Akira TAKADA
    Type: research-article
    2020 Volume 71 Issue 6 Pages 517-580
    Published: December 16, 2020
    Released: December 19, 2020
    JOURNALS FREE ACCESS

    The younger Fuji pyroclastic fall deposits since 1,500 cal BC have been re-described with new geochemical data from representative outcrops. And, we measured minimum magma volumes for the fall deposits using the relationship between the area enclosed within an isopach and its thickness. From 1,500 cal BC to 300 cal BC, sub-Plinian eruptions took place at the summit and flanks, and ejected the S-10, Osawa, Omuroyama, S-13, S-18 and S-22 Pyroclastic Fall Deposits whose minimum volumes were about 1 × 10-1 km3 DRE (dense-rock equivalent volume). From 300 cal BC to 1,100 cal AD, all eruptions occurred at the flank, and produced the Gotenbaguchi 1 to 7, Subashiriguchi-Umagaeshi 1 to 7, Yoshidaguchi 1 to 4, Futatsuzuka and S-23 Pyroclastic Fall Deposits, etc., whose minimum volumes were less than 2 × 10-2 km3 DRE. Our revised stratigraphy has suggested that the Subashiriguchi-Umagaeshi 6´ Pyroclastic Fall Deposit and the Takamarubi Lava Flow are the products of the Jyohei eruption (AD 937).

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Report
  • Junko KOMATSUBARA
    Type: Report
    2020 Volume 71 Issue 6 Pages 581-586
    Published: December 16, 2020
    Released: December 19, 2020
    JOURNALS FREE ACCESS

    Postglacial deposits in four boring cores obtained from the Kujukuri Plain, central Japan, in 2015 were studied with grain size analysis. The results are discussed in terms of sedimentary facies. The majority of transgressive incised-valley deposits consists of mud and coarsens upward to lower shoreface deposits of very fine to fine, well-sorted sand. The proportion of medium, coarse and very coarse sand increases in upper shoreface deposits. The foreshore, backshore and dune deposits consist mainly of very fine and fine sand. The predominance of well-sorted sandy deposits is attributed to the supply of sand reworked from sandy and semi-consolidated Pleistocene nearby.

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