The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 106, Issue 12
Displaying 1-10 of 10 articles from this issue
  • Shiro Chikashige
    2000 Volume 106 Issue 12 Pages 825-835
    Published: December 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Paleomagnetic studies have been made on the Lower Paleogene Yorii Formation, which is distributed on the Sambagawa belt in the northeastern margin of the Kanto Mountains and is considered to be a constituent of the Atokura Nappe originated from the Inner Zone of the Southwest Japan, based on the lithology of conglomerate.Fifty-seven paleomagnetic samples were collected from 6 sites of fine-grained sandstone. Through progressive thermal demagnetization tests, 6 reliable mean paleomagnetic directions with reversed polarity before folding were obtained. Overall mean directions are D=200.8°, I=-46.0°(α95=5.6°, κ=143.4). The paleomagnetic direction indicates 20.8°clockwise rotation.The restored paleomagnetic declination is different from the 94°clockwise rotation of the Kanto Mountains since 15 to 16 Ma. The difference suggests that the Yorii Formation is the Nappe from the Inner Zone of the Southwest Japan and is the allochthonous terrane having rotated counterclockwise by 73°during the nappe-forming process.
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  • Osamu Takahashi
    2000 Volume 106 Issue 12 Pages 836-852
    Published: December 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    On the basis of 120 radiolarian biochronologic data and detailed geologic mapping, fifteen tectonostratigraphic units have been recognized in the Southern Chichibu and Northern Shimanto Belts of the Kanto Mountains, central Japan. All units are in thrust contact with each other and are numbered from north to south as Units 1 to 15. The fifteen tectonostratigraphic units are composed of accretionary deposits which were successively accreted southward during Early Jurassic to Late Cretaceous time. Each unit in the study area primarily formed a stratigraphic succession from greenstone or limestone through chert, siliceous mudstone, and mudstone to sandstone (=oceanic plate stratigraphy). The combination of those rocks is recognized not only in coherent units but in melange units. Reconstruction of the "oceanic plate stratigraphy" of each unit helps our understanding of the pulse of the formation of accretionary complexes in the study area. There is a lack of Early Cretaceous accretionary complexes of the Hauterivian and the Aptian to middle Albian. The drastic changes from the formation of coherent units (Middle-Late Jurassic) through the formation of melange units (latest Jurassic-earliest Cretaceous) to the accretionary hiatus (Early Cretaceous) are recognized near the boundary between the Chichibu and Shimanto Belts. The formation process for the Jurassic to Cretaceous accretionary complexes in the Kanto Mountains is therefore divided into the following three tectonic phases : the Early to middle Late Jurassic (Phase I), late Late Jurassic to Early Cretaceous (Phase II), and Late Cretaceous (Phase III). This is possibly a record of the process and the historical changes in the ancient accretionary tectonics and the relative plate motions. Phase I (the accretion of complexes in the Chichibu Belt) and Phase III (the accretion of complexes in the Shimanto Belt) may be attributed to a difference in the two subducted oceanic plates : the former was old (Izanagi Plate) while the latter was young (Kula Plate) ; moreover, the conversion of the subduction of the different oceanic plates is likely to have caused the accretionary hiatus (Phase II) in the study area.
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  • Ayano Ota, Kametoshi Kanmera, Yukio Isozaki
    2000 Volume 106 Issue 12 Pages 853-864
    Published: December 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Litho- and bio-stratigraphy was analyzed for the Upper Permian shallow-water limestone derived from a mid-oceanic paleo-seamount. The limestone in the Kamura area in Kyushu occurs as an allochthonous block contained in the Jurassic accretionary complex of the Chichibu belt in Southwest Japan. A newly exposed limestone section consists of a black limestone of the Iwato Formation (17 m) and an overlying light gray dolomitic limestone of the Mitai Formation (17 m). Through microscopic observation of 304 thin sections, 6 genera (species indeterminate) and 6 species of 4 genera of fusulinaceans were identified. On the basis of fusulinacean biostratigraphy, the limestone in this section is clearly divided into four interval zones, i.e., Lepidolina Zone, barren interval (zone), Codonofusiella-Reichelina Zone, and Palaeofusulina Zone, in ascending order. The former two belong to the Iwato Formation, while the latter two to the Mitai Formation. The Yabeina-Lepidolina Zone is correlated to the Maokouan Stage (Middle Permian) in South China, while the Codonofusiella-Reichelina Zone and the Palaeofusulina Zone to the Wuchiapingian and Changhsingian Stages (Upper Permian), respectively. Thus this limestone of mid-oceanic seamount origin ranges for nearly 10 m.y. from the Maokouan (Yabeina-Lepidolina Zone) Stage to the Changhsingian Stage (Palaeo-fusulina Zone). This is the first report in the world on a fusulinacean-dated continuous pelagic shallow-water limestone sequence covering the entire Late Permian interval immediately before the Permian-Triassic boundary mass extinction. The present result is quite significant because this Upper Permian limestone may have recorded the global environmental changes in the surface of the superocean Panthalassa relevant to the Permo-Triassic boundary event.
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  • Takahiro Yamamoto, Keiichi Sakaguchi
    2000 Volume 106 Issue 12 Pages 865-882
    Published: December 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Adatara volcano is one of the active volcanoes at the volcanic front of the northeast Honshu arc. Tephrostratigraphic study and new 14C ages reveal the eruptive history of this volcano as follows. 1) pyroclastic deposits of this volcano are divided into the 250- to 200- ka Adatara-Mizuhara tephra group and the Adatara young stage the 120- to O- ka Adatara young stage tephra group. The volcano was quiescent for about 80 thousands years between the two tephra groups. 2) During the eruptions of the Adatara-Mizuhara tephra group, effusion of large-volume andesite lava took place and formed the main volcanic cones. 3) The young stage activity began with the eruption of the Adatara-Dake tephra. This tephra consists mainly of lower dacite Plinian fall deposits (about 1 km3 in dense rock equivalent, DRE, volume) and upper andesite pyroclastic flow deposits (about 1 km3 in DRE volume). The Numanotaira crater was formed in this eruption. 4) After the Dake eruption, Sub-Plinian or Vulcanian eruptions, forming andesite pyroclastic fall deposits of 10-2 to 10-1 km3 in DRE volume, occurred with 10 to 20 thousands years intervals. 5) During the recent 10 thousands years, six Vulcanian eruptions, forming pyroclastic fall deposits of 10-3 to 10-2 km3 in DRE volume, occurred at the Numanotaira crater with 0.5 to 2 thousands years intervals. The last magmatic eruption took place at 2.4 ka. 6) Since 2.4 ka, three major phreatic explosions, last one of which was the 1900 eruption, occurred depositing tephra layers around the Numanotaira crater. 7) If the magma plumbing system has been stable, Vulcanian eruptions or phreatic explosions will occur at the Numanotira crater again.
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  • Kazutaka Amano, Tokiyuki Sato, Takashi Koike
    2000 Volume 106 Issue 12 Pages 883-894_2
    Published: December 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Molluscan fauna of the Kuwae Formation in Niigata Prefecture was reexamined for elucidating the paleoceanographic conditions in the central part of Japan Sea Borderland. The Kuwae Formation is lithologically subdivided into the lower, middle and upper parts. The upper middle to upper parts of the formation yielding many molluscan fossils can be assigned to the lower to middle part of NN16 nannofossil zone (middle Pliocene). The fauna includes many characteristic species of the Plio-Pleistocene Omma-Manganji fauna. Besides many cold-water species, 35 warm-water ones were found. It is noted that much more warm-water species are recognized than the middle Pliocene Tentokuji fauna in Akita Prefecture, reflecting the geographical thermocline.Moreover, the Kuwae fauna includes some common species with the Plio-Pleistocene Kakegawa fauna in the southwestern Pacific side of Japan as well as some warm-water species which do not inhabit the Recent Japan Sea. This is believed to be the result of the shallow current which was warmer than today flowing over the cold-water mass.
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  • Kyoko Kataoka, Takeshi Nakajo
    2000 Volume 106 Issue 12 Pages 897-900
    Published: December 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Hyperconcentrated flow deposit, in the Ebisutoge-Fukuda tephra from Mie Prefecture, is thought to be deposited from dilution of the debris flow. Debris-flow deposit (facies A) is overlain by hyperconcentrated flow deposit (facies B). Typical hyperconcentrated flow deposit (facies B1 ; horizontal bedding part) changes upward into diluted hyperconcentrated flow deposit (facies B2 ; scour and fill structure part). These facies characteristics indicate that the debris flow changed through hyperconcentrated flow into normal streamflow by dilution during flow traveling.
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  • Yoichi Ezaki, Natsuko Adachi
    2000 Volume 106 Issue 12 Pages 901-904
    Published: December 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Well preserved calcimicrobes and microbialites are found from the Lower Devonian Fukuji Formation in the Hida "Gaien" Terrane. Calcimicrobes occur with skeletal metazoans and intergrow to form unique microfacies together with varied microbialite fabrics. Microbialites were ubiquitous and largely contributed to limestone formation. Research into calcimicrobial compositions and microbialites is indispensable to elucidate the biotic diversity and palaeoecological conditions at that time.
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  • Takahisa Ishihara, Hiroto Ohira, Masaaki Tateishi
    2000 Volume 106 Issue 12 Pages 905-908
    Published: December 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Fission-track age determination on the Cenozoic marine sedimentary sequences distributed in the Shinjo Basin, Yamagata Prefecture was carried out. Though the upper Sakekawa and Oriwatari Formations have been considered to be late Pliocene-Pleistocene in previous researches, the present result of FT dating on the volcanic tuffs intercalated in the basal part of the Oriwatari Formation showed 4.3-4.6 Ma the early Pliocene age.
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  • Tetsuro Hirono, Masamichi Shiono, Yujiro Ogawa, Tatsuhiko Sakamoto, Sa ...
    2000 Volume 106 Issue 12 Pages 909-912
    Published: December 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
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  • Simon Wallis, Mutsuki Aoya, Tetsuo Kawakami, Tatsuki Tsujimori
    2000 Volume 106 Issue 12 Pages XXV-XXVI
    Published: 2000
    Released on J-STAGE: November 26, 2010
    JOURNAL FREE ACCESS
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