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 126, Issue 2
Displaying 1-4 of 4 articles from this issue
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Articles
  • Sr isotope ratios and diatom ages of the Neogene carbonate concretions in Akita and northern Yamagata areas, Northeast Japan
    Osamu Nishikawa, Daiki Ando, Chieko Shimada, Daizo Ishiyama, Masatsugu ...
    2020 Volume 126 Issue 2 Pages 53-69
    Published: February 15, 2020
    Released on J-STAGE: June 16, 2020
    DOIhttps://doi.org/10.5575/geosoc.2019.0037
    JOURNAL FREE ACCESS
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    Strontium isotope ratios and diatom fossil ages were determined for carbonate concretions developed in Neogene sedimentary rocks in the Akita and northern Yamagata areas, northeastern Japan. The diatom biozones of five carbonate concretions were assigned to Denticulopsis dimorpha (North Pacific Diatom biohorizon 5D), Rouxia californica (7A), and lower Neodenticula kamtschatica (7Ba) zones. <sup>87</sup>Sr/<sup>86</sup>Sr isotope ratios of carbonate concretions were distributed between the values of paleo-seawater and those of igneous rocks in the study area, ranging from 0.709013 to 0.706749. The younger the age of a concretion, the smaller the deviation of its 87Sr/86Sr ratio from that of paleo-seawater. This indicates that the carbonate concretions were generated from pore water that had been contaminated by igneous material associated with volcanic activity in the area during the middle to late Miocene．

    Fossil shells occurring with concretions in the Tentokuji and Sasaoka Formations have Sr isotope ratios similar to that of paleo-seawater, leading to a hypothesis that the measured Sr isotope ratios of both fossil shells and concretions are primary and have remained unchanged throughout the burial process. This is supported by a significant difference in Sr isotope ratios between a given concretion and an included molluscan fossil with a preserved aragonite shell. Similarly, carbonate concretions in the horizons lower than the Funakawa Formation may also have inherited relatively low Sr isotope ratios from pore water during the early stages of diagenesis.

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  • New age constraints and tectonic significance of the late Oligocene marine biosiliceous mudstone in the Hidaka Belt, northeastern Hokkaido, Japan
    Futoshi Nanayama, Hiroshi Kurita, Jun Tajika, Toru Yamasaki, Hideki Iw ...
    2020 Volume 126 Issue 2 Pages 71-84
    Published: February 15, 2020
    Released on J-STAGE: June 16, 2020
    DOIhttps://doi.org/10.5575/geosoc.2019.0040
    JOURNAL FREE ACCESS
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    The Tatsuushi Formation in the Kitami-Monbetsu area of northeastern Hokkaido, northern Japan, consists mainly of siliceous mudstone and has previously been considered part of the Hidaka Supergroup along with the adjacent Uenkotan and Rurochi formations. In this study, the depositional age of the Tatsuushi Formation was constrained using two methods. A laser ablation-inductively coupled plasma-mass spectrometry U-Pb age of 26.9 ± 0.2 Ma was obtained for the maximum depositional age of the formation based on the minimum age of detrital zircon grains from turbiditic sandstone. In addition, the presence of dinoflagellate fossils in the formation indicates a late Oligocene (Williamsidinium sp. C and Spinidinium? sp. A zones) depositional age for the formation. Given that the depositional age of the Hidaka Supergroup is Paleocene to early Eocene, the Tatsuushi Formation should be excluded from the Hidaka Supergroup. Instead, it can be correlated with the lower units of the Tsubetsu and Kawakami groups on the basis of their depositional age and lithology. Similar siliceous mudstone crops out in the southeastern part of the Tatsuushi Formation across the Abashiri tectonic line, which has been an active right-lateral strike-slip fault since the late Oligocene. These three correlated bodies of rock may owe their origin to upwelling of nutrient-rich deep water from the Sea of Okhotsk after the opening of the Kuril basin.

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  • Phengite K-Ar ages of the Yamagami Metamorphic Rocks in the Motai-Matsugataira Belt
    －Probability of the extension of the Renge Metamorphic Belt to the Northeastern Japan Arc－
    Atsushi Miyashita, Koshi Yagi, Tetsumaru Itaya
    2020 Volume 126 Issue 2 Pages 85-93
    Published: February 15, 2020
    Released on J-STAGE: June 16, 2020
    DOIhttps://doi.org/10.5575/geosoc.2019.0042
    JOURNAL FREE ACCESS
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    The Motai-Matsugataira Belt (MMB) in the eastern margin of the northeastern Japan arc comprises Paleozoic metamorphic complexes. The high-P Yamagami metamorphic rocks in the belt are subdivided into the high-grade Yamagami I and low-grade Yamagami II units. The Yamagami I metamorphic rocks consist mainly of pelitic schist, epidote amphibolite, and garnet amphibolite. The occurrence of paragonite in rutile and the chemistry of amphiboles indicate that the Yamagami I metamorphic rocks underwent the highest-grade metamorphism in the MMB. Phengite K-Ar ages of the pelitic schists and amphibolitesindicateexhumation during 322 to 287 Ma. These petrological and geochronological results for the Yamagami metamorphic rocks indicate that part of the MMB is probably an extension of the Renge Belt of southwestern Japan.

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