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 114, Issue 1
Displaying 1-6 of 6 articles from this issue
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Articles
  • Fission-track ages of the Pliocene strata in the Shinjo Basin, Yamagata Prefecture
    Shunji Moriya, Tohru Danhara, Hideki Iwano, Tohru Yamashita, Takeshi N ...
    2008 Volume 114 Issue 1 Pages 1-15
    Published: January 15, 2008
    Released on J-STAGE: February 18, 2009
    DOIhttps://doi.org/10.5575/geosoc.114.1
    JOURNAL FREE ACCESS
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    Fission-track dating was carried out for 18 tuff samples from the Pliocene strata in the Shinjo Basin, Yamagata Prefecture. After examination has been made to eliminate erroneous data by contamination of reworked zircon crystals, ages of the boundaries between the Nakawatari, Sakekawa, Yamuke and Motoaikai Formations are determined to be 4.3, 3.7 and 3 Ma, respectively.
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  • Fault geometry and paleo-movement of the central part of the Ushikubi fault, northern central Japan.
    Kiyokazu Oohashi, Kenta Kobayashi
    2008 Volume 114 Issue 1 Pages 16-30
    Published: January 15, 2008
    Released on J-STAGE: February 18, 2009
    DOIhttps://doi.org/10.5575/geosoc.114.16
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    The Ushikubi fault, located along the boundary between Toyama and Gifu prefectures, northern central Japan, is one of the longest active faults in Japan. We performed morphological and structural analyses based on geological survey of the central part of the Ushikubi fault. As a result, we identified the geometry and movement history of this fault. Along the Ushikubi fault, multiple fractures are branched, bent, paralleled, slanted intersection, and form a pretty complicated shear zone. Particularly in the central part of the study area, the multiple fractures form a strike-slip duplex. The geometry of the shear zone indicates it was originated from a sinistral strike-slip fault. In addition, the observation of polished sections and thin sections supports the result. In this study, the shear zone along the Ushikubi fault is called the Ushikubi shear zone. This shear zone was generated as a sinistral strike-slip fault in Late Cretaceous time, and has been inversely reactivated as the Ushikubi active fault.
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  • Tectonostratigraphy of the northern part of the Chichibu Composite Belt, western Shikoku, SW Japan
    Kazuo Kiminami, Keiichiro Saito, Tetsuya Mukai, Kenji Takeda
    2008 Volume 114 Issue 1 Pages 31-42
    Published: January 15, 2008
    Released on J-STAGE: February 18, 2009
    DOIhttps://doi.org/10.5575/geosoc.114.31
    JOURNAL FREE ACCESS
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    Tectonostratigraphic divisions of the Chichibu Composite Belt of Ozu-Akehama area, western Shikoku, have been controversial because the Kurosegawa Belt is not exposed in the area. We performed detailed geological survey, chemical analyses of greenstone and sandstone, and K-Ar dating of recrystallized white mica to make clear the tectonostratigraphy in the southwestern area of Ozu City.
    The study area can be divided into the Mikabu Greenstone, Ozu Unit I/II and Furuyabu Unit I/II on the basis of lithofacies. The Ozu Unit I is composed of sandstone and mudstone, and the Unit II is mainly of mudstone with minor chert and greenstone, and the both have been metamorphosd considerably. The Furuyabu Unit I consists of greenstone, chert and limestone, and the Unit I is made up mainly of chert and siliceous mudstone with minor sandstone. The Mikabu Greenstone is demarcated from the Ozu Unit by a high angle fault. Dips of the strata around the boundary between the Ozu and Furuyabu Units are very gentle. Field evidence suggests that the Ozu Unit is overlain structurally by the Furuyabu Unit.
    Whole-rock geochemistry of greenstones from the Furuyabu Unit I suggests an oceanic island for their origin. Sandstones from the Furuyabu Unit II have chemical affinities with those from the Togano Unit in Shikoku. The Furuyabu Unit I and II can be correlative to the Sambosan Unit and Togano Unit in the Southern Chichibu Belt, respectively. Sandstone geochemistry and K-Ar muscovite ages of metapelite in the Ozu Unit suggest that the unit is equivalent to the Cretaceous Shimanto accretionary complex. From these, it is concluded that metamorphosed Cretaceous Shimanto accretionary complex is exposed in the northern part of the Chichibu Composite Belt and has been overlain structurally by the Sambosan Unit and Togano Unit in ascending order.
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Short Articles
  • A fossil palatine of an alepisauroid fish from the Middle Miocene Iseyama Formation, Ueda City, Nagano Prefecture, central Japan
    Hideshi Suzuki
    2008 Volume 114 Issue 1 Pages 43-46
    Published: January 15, 2008
    Released on J-STAGE: February 18, 2009
    DOIhttps://doi.org/10.5575/geosoc.114.43
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    A fossil palatine of alepisauroid fish was found from the lower part of the Middle Miocene (15 Ma) Iseyama Formation in Ueda City, Nagano Prefecture, central Japan. In comparison with a Recent species, this is identified as a palatine of lancetfish which belongs to the genus Alepisaurus. This is the fourth record of alepisauroid fossils in Japan and also the third one in the Northern Fossa Magna Region of Nagano Prefecture and is the largest fossil palatine of lancetfish discovered from the Japanese Miocene strata. This is probably an undescribed species of the genus Alepisaurus.
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  • Significance of Neodenticula kamtschatica, an extinct planktonic diatom species from the middle Quaternary submarine deposit, IODP Exp. 303, North Atlantic Ocean
    Chieko Shimada, Tokiyuki Sato, Miyuki Kudo, Makoto Yamasaki
    2008 Volume 114 Issue 1 Pages 47-50
    Published: January 15, 2008
    Released on J-STAGE: February 18, 2009
    DOIhttps://doi.org/10.5575/geosoc.114.47
    JOURNAL FREE ACCESS
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    Neodenticula kamtschatica (Zabelina) Akiba and Yanagisawa, an extinct late Neogene planktonic diatom has been very first observed in the middle Quaternary submarine deposit in the northeastern North Atlantic. The species has been considered for a long time to be limited within the high-to-middle latitudinal North Pacific. In the North Pacific region, this species appeared in the geologic record approx. at 6.4 Ma as “first common occurrence”, and became extinct at 2.65 Ma occurrence. On the other hand, no record of the Atlantic specimens has been presented. Thus, its fossil imprint in the North Atlantic of our investigation implies that at least single episode of the trans-ocean migration (=floral interchange of North Pacific and North Atlantic via the Arctic) of the species occurred during the latest Miocene through Pliocene epoch, considering the tectonic and hydrographic backgrounds such as the initial opening of the Bering Strait (5.5 Ma~) and the final closure of the Panamanian Isthmus (~2.76 Ma). Our evidence is significant to give a further insight on the relationship between the process to establish biogeography of planktonic micro-organisms and the paleoenvironmental change in the Northtern Hemisphere.
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