The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 117, Issue 9
Displaying 1-6 of 6 articles from this issue
Articles
  • Atsushi Urabe, Yusuke Fujimoto, Kyoko S. Kataoka
    2011 Volume 117 Issue 9 Pages 483-494
    Published: September 15, 2011
    Released on J-STAGE: January 26, 2012
    JOURNAL FREE ACCESS
    The Echigo Plain, which faces the Sea of Japan in northeast Japan, is a typical coastal plain with numerous sand dunes. We examined the sedimentology of pumiceous sediments (lahar deposits) in the Agano River area of the Echigo Plain, derived mainly from a large-scale dam-break flood after the Numazawako eruption (about 5 ka), using an existing borehole dataset and facies analysis of sediment cores.
    The lahar deposits show different sedimentary facies in coastal areas versus the shallow marine environment. Coarse-grained pumiceous sediments (5—8 m thick) are interpreted to have been deposited under the delta front or in a delta plain environment in a coastal area. Sandy silt deposits (pro-delta facies), including pyroclastic material (horizons of concentrated volcanic glass shards and heavy minerals), are distributed in the shallow sea area.
    The large volumes of volcaniclastic sediment delivered during the flood event induced the rapid progradation of the delta system in the Agano River region. The deposition of volcaniclastic sediments in the shallow sea also caused progradation of the delta system and changed the coastal landforms along the margin of the Echigo Plain.
    The flood event resulted in an increase in the progradation rate of the depositional system from 0.9 to 2.5 km per 1000 years. An understanding of large-scale pyroclastic resedimentation can contribute to evaluating and predicting the volume of sediment discharge and the nature of hazards created by a volcanogenic flood and its aftermath in an alluvial plain setting.
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  • Case study of the Sakate tephra, western Japan
    Chisato Nakamura, Ken-ichi Yasue, Tsuneari Ishimaru, Koji Umeda, Akira ...
    2011 Volume 117 Issue 9 Pages 495-507
    Published: September 15, 2011
    Released on J-STAGE: January 26, 2012
    JOURNAL FREE ACCESS
    The Sakate tephra beds occur stratigraphically between late Pleistocene K-Ah and AT tephra beds in the Kinki and Hokuriku districts. The Sakate beds are characterized based on petrographic and mineralogical properties, such as the abundance of hornblende and the minor amount of cummingtonite. In this paper, two tephra beds (Kannabe 3-8 and OK-4-1, considered to be Sakate-type tephra) are correlated based on their petrographic properties and the major-element chemical compositions of volcanic glass shards. The Sambe—Ukinuno tephra is also considered to be the Sakate tephra, based on the similarity in petrographic properties and the compositions of glass shards. Accordingly, all three tephras are considered to be Sakate-type.
    However, it is difficult to correlate tephra beds if volcanic glass shards are absent. Therefore, we analyzed the chemical composition of glass inclusions in hornblende from the Sambe—Ukinuno, Kannabe 3-8 and OK-4-1 tephra beds. The chemical compositions of the inclusions are the same as or similar to those of the volcanic glass shards in the tephras studied here (i.e., Sambe—Ukinuno, Kannabe 3-8 and OK-4-1). Therefore, the chemical composition of glass inclusions in hornblende is an effective index for correlating thinned and weathered tephra beds.
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  • Molluscan fauna from central and northern parts of Akita Prefecture
    Kazutaka Amano, Arata Yoshida, Tokiyuki Sato
    2011 Volume 117 Issue 9 Pages 508-522
    Published: September 15, 2011
    Released on J-STAGE: January 26, 2012
    JOURNAL FREE ACCESS
    We examined Plio-Pleistocene mollusks from the Taihei and Minehama areas in Akita Prefecture with the aim of elucidating the change in faunal composition at Datum A (2.75 Ma), when glaciers expanded in the Northern Hemisphere. In both areas, the deposits below the Datum A consist of finer sediments than those above. In the Taihei area, 105 species of mollusks were recovered from 10 localities. On the other hand, 139 species were found from seven localities in the Minehama area. In terms of paleobathymetry, the sediments below the Datum A in both areas were deposited in the lower sublittoral zone, while those above the datum were deposited in the upper sublittoral zone. In both areas, the assemblages above the Datum A contain a higher proportion of individuals of cold-water species, despite the fact that they lived in shallower water depths. Moreover, some species now living in Hokkaido and northwards are confined to the upper horizons than the Datum A. In both areas, two Miocene relict species were collected from the deposits below the Datum A. These species might suffer from extinction by the cooling event at the Datum A. The results show that the cooling event at 2.75 Ma affected the molluscan fauna in Akita Prefecture.
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  • Takenori Yoshikawa, Hisao Ando, Takeshi Kozai, Yasuo Kondo
    2011 Volume 117 Issue 9 Pages 523-537
    Published: September 15, 2011
    Released on J-STAGE: January 26, 2012
    JOURNAL FREE ACCESS
    Six oyster-shell beds of exclusively Crassostrea sp. are exposed in a thick estuarine unit in the Campanian (Late Cretaceous) upper part of the Shiroyama Formation, Izumi Group, along the Doki River, Mannou area, Kagawa Prefecture, SW Japan. Five modes of fossil occurrence are observed in the oyster beds: (I) Relay type, (II)single-generation cluster type, (III) mixed type in upright and lying positions, (IV) closely packed side-lying type, and (V) scattered side-lying type. Five of the shell beds consist of more than one type of fossil occurrence, indicating a heterogeneous origin (i.e., both autochthonous and allochthonous), with the beds being of variable size, thickness, and shape. The sixth bed yields only allochthonous oysters (type IV). The former five shell beds were formed on a sandy tidal flat by the intermittent accumulation of autochthonous and allochthonous shells. The combined effects of (1) clustered oyster growth, (2) subsequent destruction by tidal currents or storms and short-distance transportation, and (3) renewed clustered growth over several generations resulted in the formation of thick shell beds. These modes of fossil occurrence, formed under high-energy conditions on a sandy tidal flat, are in contrast to the oyster reefs of modern Crassostrea, which dwells in a sheltered muddy estuary environment. The thick estuarine unit in the section along the Doki River suggests that in this part of the Izumi Basin, the rates of subsidence and sediment supply were in balance during the Campanian. Moreover, the heterogeneous oyster beds may have formed in an estuarine environment during occasional stagnation phases during a transgression related to syn-depositional subsidence of the Izumi Basin.
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