Earth Science (Chikyu Kagaku)
Online ISSN : 2189-7212
Print ISSN : 0366-6611
Volume 74 , Issue 4
Showing 1-9 articles out of 9 articles from the selected issue
Research Article
  • Hiroaki ISHIGA
    Type: research-article
    2020 Volume 74 Issue 4 Pages 109-118
    Published: October 25, 2020
    Released: February 25, 2021
    JOURNALS FREE ACCESS

    In brackish Lake Shinji, southwest Japan, submerged macrophyte Potamogeton anguillanus Koidz has expanded from 2009 especially at the southern coast. Such plants have recently been known to adsorb and/or absorb to their organism a large quantity of heavy metal ions in various aquatic environments. In order to check this relation, heavy metal contents in the samples of P. anguillanus from several coastal localities during 2010 to 2012, were determined by XRF. Their leaves show very high manganese concentrations. And, their distinct increase in summer was found for the samples collected in 2012. On the other hand, SEM observations revealed many attached algae, diatom and crustacean on the surface of the leaves. At present, it is not clear whether the Mn concentration into P. anguillanus is due to its biological process or microbial activities of the surface micro-organism. The Mn behavior in Lake Shinji is possibly interpreted as follows. Mn2+ dissolves from the surface of the bottom sediments under reducing conditions into the bottom water in summer. The bottom water containing a large quantity of Mn2+ with suspended solids and organic colloids may move to the shallow coastal areas by wind driven currents. Thus, the aggregate patches of P. anguillanus eventually have high Mn contents.

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  • Kotaro UCHIMURA, Isao MOTOYAMA, Tomohiro NISHIMURA, Yojiro TAKETANI
    Type: research-article
    2020 Volume 74 Issue 4 Pages 119-136
    Published: October 25, 2020
    Released: February 25, 2021
    JOURNALS FREE ACCESS

    The Hobetsu area, central Hokkaido, lies within the Sorachi-Yezo Belt where Cretaceous–Cenozoic sedimentary sequences are severely folded and faulted. In this area, the Yezo Group representing Upper Cretaceous rocks is richly fossiliferous containing marine mollusks (mainly inoceramids and ammonoids) and reptiles (mosasaurs, plesiosaurs and turtles) and, therefore, many geological and paleontological works have been carried out so far. The Cretaceous unit extends to the Biratori District, but there are fewer studies from here. We have carried out mapping to provide a revised geological map in the Tounai Creek drainage area and analyzed molluscan and radiolarian fossils to estimate depositional ages.

    The Yezo Group in this area is subdivided into the Kashima and Hakobuchi Formations in ascending order. The Kashima Formation comprises fairly monotonous, hemipelagic mudstone intercalating with many felsic tuff beds. The Hakobuchi Formation, conformably overlying the Kashima Formation, is composed mainly of sandstone, conglomerate and sandy mudstone with cross-bedded sandstone and minor coal beds. Our detailed mapping revealed that the Kashima Formation is distributed in the western part of the studied area and probably extends to the north where the distribution of the Kashima Formation was recently discovered (Negishi et al. 2015). It was presumed that the Kashima Formation forms an anticline or a syncline in the northern part of the Tounai Creek area. However, our investigation on the facies and tuff bed structures supports a homoclinal structure rather than a fold.

    Fossils occur throughout the Cretaceous sequence; radiolarian assemblages include Dictyomitra formosa, Dictyomitra koslovae, Lithocampe manifesta and Spongotripus morenoensis, suggesting ages of Coniacian to Maastrichtian for the Kashima Formation, while molluscan fossils, including Inoceramus amakusensis, Inoceramus japonicus and Eupachydiscus haradai, collected from the Kashima Formation of the studied section indicate ages from Coniacian to Campanian. Consequently, the Kashima Formation in the studied region is correlated to the Coniacian to Campanian.

    A fault separates the Cretaceous rocks from the Miocene sequences of the Takinoue and Kawabata Formations, distributed at the western margin of the studied area. Radiolarians were rare and poorly preserved in these Miocene formations.

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  • Masahiro SHIBA, Kenji YOKOYAMA, Taichi OHASHI, Makoto MORIZUMI, Kouki ...
    Type: research-article
    2020 Volume 74 Issue 4 Pages 137-155
    Published: October 25, 2020
    Released: February 25, 2021
    JOURNALS FREE ACCESS

    The Lower to Middle Miocene series distributed in the northern part of Kakegawa City, Shizuoka Prefecture, Japan, is divided by unconformity into the Kurami Group and the Saigo Group in ascending order. The Kurami Group is composed of, in ascending order, the Todo, Amagata, Towata, Matsuba and Masago Formations. The Saigo Group consists of, in ascending order, the Tozawa and Gomyo Formations. The Kurami and Saigo Groups are assigned to the N6 to N8 and N8 zones of Blow (1969), respectively. Therefore, the Kurami Group is late Early Miocene and the Saigo Group is early Middle Miocene in age.

    In the late Early Miocene, subsidence started in the Kurami syncline, where the first channel-levee complex developed, and the Todo Formation was deposited. As the relative sea level subsequently rose, the sedimentary areas spread to the southeast and northwest sides, covering the basement unconformably, with the Amagata Formation deposited on the inner shelf and the Towata Formation deposited on the outer shelf and the slope. After that, the second channel-levee complex developed at the basin along the Kurami syncline axis, and the submarine fan deposit of the Matsuba Formation was formed. Following this, the Masago Formation was deposited from the outer shelf to the slope environment. During the sedimentation period of the Saigo Group, the southeast and northwest wings of the Kurami syncline were uplifted to form a steep slope. The Saigo Group deposited on the steep slopes of the Kurami Group unconformably. The Tozawa Formation consisting of debris flow deposits and volcanic debris formed by submarine landslides, and then, the Gomyo Formation consisting of muddy deposits on the slope were deposited there.

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  • Masaki YUHARA, Keiji UMEZAKI, Takanori MORI, Yoshinobu KAWANO
    Type: research-article
    2020 Volume 74 Issue 4 Pages 157-170
    Published: October 25, 2020
    Released: February 25, 2021
    JOURNALS FREE ACCESS

    Major and trace elements analysis, rare earth element (REE) analysis were conducted on tuffites from the Wakino Subgroup, Kanmon Group in the eastern part of northern Kyushu. Tuffites are composed of interbeds of dark layers and light layers in thickness from several millimeters to several centimeters. The dark layers consist of mudstone to very fine sandstone. The light layers consist of mudstone to fine sandstone. Tuffites indicate high CaO and Sr contents than that of mudstone, sandstone and conglomerate in the Wakino Subgroup. Tuffite samples from the Murasaki and Kuro Rivers have similar whole-rock chemical compositions. Samples from the Yakiyama River, however, indicate lower TiO2 and Al2O3 contents and some lower Cr, Ga, Nb, Th and Zr contents than that of tuffites from the Murasaki and Kuro Rivers. There is no clear difference in REE patterns. Hence, these trace elements are effective for presumption of a place of production of tuffites.

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Special Issue: Quaternary uplift in central Honshu, Japan (1)
Research Article
  • Yoshihiro KUBOTA, Team Niigata Plain, and Collaborative Research Group ...
    Type: research-article
    2020 Volume 74 Issue 4 Pages 173-186
    Published: October 25, 2020
    Released: February 25, 2021
    JOURNALS FREE ACCESS

    This paper focuses on the formation process of the arrangement of mountain ranges and isolated hills around the Echigo Plain and discusses its geological factors.

    The isolated hills are distributed on both the backslope and fault scarp sides of tilted blocks. This suggests that the uplift style around hinterlands changed from tilting in the Middle Pleistocene to vertical uplifting in the Late Pleistocene.

    The uplift area of hinterlands around the Echigo Plain has enlarged from south to north across the TsugawaAizu Geologic Province during the Early to Middle Pleistocene, forming tilted blocks. In the Middle Pleistocene, the first isolated hills were formed at the eastern foots of tilted blocks. In the Late Pleistocene, vertical uplifting became dominant, and the second isolated hills were formed at the western foots of tilted blocks.

    The present landscape of mountain ranges has a basic structure of tilted blocks during the Middle Pleistocene, and has been formed by the vertical block uplifting of mountains due to the expanding of the low velocity layer under the mountain areas during the Late Pleistocene to the Recent.

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  • Hisao ADACHI, Kazuhiro KOBAYASHI
    Type: research-article
    2020 Volume 74 Issue 4 Pages 187-199
    Published: October 25, 2020
    Released: February 25, 2021
    JOURNALS FREE ACCESS

    This paper examines the relationship between uplifted landforms and volcanic and seismic activity in the volcano-plutonic high thermal zone in the eastern part of central Honshu, and the deep structures interpreted from seismic tomography.

    As a result, (1) the low-velocity layers spread out horizontally at a depth of about 30-40 km under the VPHT zone (volcano-plutonic high thermal zone), (2) the distribution of earthquakes over the upper surface and the uplifted topography are generally concordant. (3) In addition, it is clear that the Quaternary volcanoes developed directly above the low-velocity zone separating from the low-velocity layer below. The concordance of three-factor unit: uplifted topography, hypocenter distribution, and low-velocity layer, strongly suggests that magmatic activity has been uplifting from the upper mantle.

    According to the geological and geodetic data in the VPHT zone and its surroundings, rapid uplift has been continuous from the Pleistocene to the present

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