Earth Science (Chikyu Kagaku)
Online ISSN : 2189-7212
Print ISSN : 0366-6611
Volume 73 , Issue 3
Showing 1-6 articles out of 6 articles from the selected issue
Research Article
  • Toshiro MORIKIYO, Ryo MORITA, Yuuki OKADA, Yusuke KAMIYA, Kousuke KIMU ...
    Type: research-article
    2019 Volume 73 Issue 3 Pages 113-137
    Published: April 25, 2019
    Released: June 04, 2020
    JOURNALS FREE ACCESS

    Measurements of the oxygen and carbon isotopic compositions of calcite in nodules occurring in medium- to coarse-grained sandstone were conducted to verify the possibility that minerals comprising the stratum are isotopically altered due to the passage of groundwater of rainwater origin. The strata studied are the following Neogene formations: Oshimojo, Tamugigawa, Ronji, Abuzuri, Ichinose and other Formations. Except for the Oshimojo Formation, δ18O for nodule calcite in sandstone ranges from 17 to 24‰ vs. SMOW, which is 8 to 15‰ lower than the initial value (approximately 32‰) set during the early diagenesis. The nodule calcite that occurs in sandstone is oxygen-isotopically in equilibrium with groundwater of rainwater origin. Except for the calcite from the Oshimojo Formation, δ13C for nodule calcite ranges from −9 to +3‰. The observed δ13C is markedly higher than that for organic matter (ca. −25‰). It is considered that δ13C for nodule calcite was also determined by equilibration with carbonate ions dissolved in groundwater. The isotopic ratios of calcite nodules in the Oshimojo Formation indicate incomplete equilibrium with groundwater. The isotopic alteration in nodules resulted from the following two conditions: 1) the host rock of nodules is medium- to coarse-grained sandstone of Neogene in age with high permeability, and 2) the mineral comprising the nodules is calcite that easily re-equilibrates with aqueous solutions.

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  • Masayuki EHIRO
    Type: research-article
    2019 Volume 73 Issue 3 Pages 139-148
    Published: July 25, 2019
    Released: June 04, 2020
    JOURNALS FREE ACCESS

    Two Wuchiapingian (late Permian) ammonoids, Vedioceras sp. and Urartoceras sp., both belonging to the family Araxoceratidae, are described from the South Kitakami Belt, Northeast Japan. Araxoceratid ammonoids were widespread in the low-latitude regions of the Tethys, Tethys-Panthalassa border and eastern Panthalassa regions. The Wuchiapingian araxoceratid fauna of the South Kitakami Belt, including with the known araxoceratid ammonoids such as Araxoceras cf. rotoides, A. sp., Dzhulfoceras cf. furnishi, D. sp, Vescotoceras japonicum, V. sp. and Eusanyangites cf. bandoi, has a close similarity to that of the western Tethys Province in the generic composition.

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  • Bedding slip and water film generation in an unconsolidated layered system
    Koichi FUTAMURA
    Type: research-article
    2019 Volume 73 Issue 3 Pages 149-161
    Published: July 25, 2019
    Released: June 04, 2020
    JOURNALS FREE ACCESS

    Fluidization structures are clearly observed at base of the upper unit of the Lower Miocene Himaka Formation on Saku Island, Aichi Prefecture, central Japan. Characteristic ones are 1) the truncation-like structure of lamina at the top of a convolute sandstone layer, 2) the injection of small sandstone dikes into the siltstone layer above, and 3) the "top-to-NNE" shear deformation indicated by the attitudes of the axial surfaces of minor folds and the wedge-shaped small dikes. These fluidization structures suggest that dehydration occurred in response to the bedding slip to NNE, thereby forming a water film beneath the impermeable silt layer.

    Such a sequential fluidization seems to be understood as the phenomenon that excess interstitial water upwelling through the sand/mud interlayers has been trapped under an impermeable layer, then dewatered through the lateral sliding and fracturing of the overlying layers. Water films are likely to cause the bedding slip in un- and semi-consolidated interlayers. They temporarily form parallel with bedding planes, leaving only subtlest traces. Such traces will become important clues to understand the deformational mechanism of soft sediments.

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  • Masaki YUHARA, Fumiya MIZUTA, Eriko NISHI, Kairi KIYOURA, Atsushi KAME ...
    Type: research-article
    2019 Volume 73 Issue 3 Pages 163-178
    Published: July 25, 2019
    Released: June 04, 2020
    JOURNALS FREE ACCESS

    Major and trace elements analysis, U-Pb zircon dating, rare earth elements analysis and Sr-Nd isotope analysis were conducted on the Masaki Granite in the eastern part of northern Kyushu, Southwest Japan. This granite is mainly composed of coarse-grained porphyritic biotite granodiorite to granite. Wholerock chemical compositions of this granite indicate that the chemical variations can be mainly explained by fractional crystallization of plagioclase and biotite. The U-Pb zircon dating separated from the Masaki Granite gives an age of 103.73 ± 0.79 Ma. This age is thought to be a timing of activity of this granite. Initial Sr and Nd isotopic ratios corrected to 103.73 Ma of the two samples of the granite are the almost same and plotted within the range of the North Zone defined by Kagami (2005).

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  • Kikuji MATSUOKA, Masakazu RIKITA, Kiichi OBATA, Yuichi OKANO
    Type: short report
    2019 Volume 73 Issue 3 Pages 179-184
    Published: July 25, 2019
    Released: June 04, 2020
    JOURNALS FREE ACCESS

    The characteristic blocks of massive conglomerate including granitic cobbles and pebbles were discovered from the Yokozemachi Group of uppermost Neogene in the Chichibu Basin. These blocks are seen as pebble to boulder in the Neogene unconsolidated conglomerate, and are sub-angular to sub-round gravel. The gravel composition of massive conglomerates is mostly sandstone and mudstone, containing granitic rock, granite porphyry, andesite, and hornfels, while no sandstone or mudstone of the Neogene origin. Its sorting is bad, and its matrix is consolidated. Biotite in the matrix of massive conglomerates accounts for about 7.3% in mode composition. Such features are very similar to conglomerate of the Cretaceous Sanyama Formation in the Sanchu Graben. Therefore, it is deemed that during the sedimentation of the Yokozemachi Group, the Cretaceous system of the Sanchu Graben was exposed to the upper part of the Chichibu Belt adjacent to the east, from which massive conglomerates including the granite and other rocks were supplied.

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