The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 105, Issue 12
Displaying 1-9 of 9 articles from this issue
  • Masaki Matsukawa, Kosuke Nakada
    1999 Volume 105 Issue 12 Pages 817-835
    Published: December 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Nonmarine molluscan fossils are considerably abundant at many horizons of the Upper Jurassic to Lower Cretaceous Tetori Group. The typical section of the group in its central outcrop area (northern part of the Gifu Prefecture, central Japan) is composed mainly of nonmarine strata intercalated with marine beds. We, therefore, can recognize changes in sedimentary environments and molluscan assemblages of the Tetori Group through time, as influenced by transgression and regression phases.The Tetori Group is lithologically divided into the Ushimaru, Mitarai, Otaniyama, Okurodani, Amagodani, Okura and Bessandani formations, in ascending stratigraphic order. The oldest succession of the group, consisting of the Ushimaru to Mitarai formations, shows estuarine, deltaic and shallow marine environments. The shallow marine Mitarai Formation bears ammonites of Callovian age and reflects the climax of transgression. Subsequently, the two younger successions of the Otaniyama to Okurodani formations and the Amagodani to Bessandani through Okura formations, reveal distributary bay and braided and meandering river environments, respectively.Nonmarine molluscan assemblages occur in the Ushimaru Formation and in the succession of the Otaniyama and Okurodani formations, interbedded with shallow marine Mitarai Formation. Five assemblages of nonmarine molluscs are recognized : [1] Ostreidae gen. et sp. indet. - Myrene (Mesocorbicula) tetoriensis, [2] Myrene (Mesocorbicula) tetoriensis, [3] Tetoria (T.) yokoyamai, [4] Sphaerium sp. and [5] Plicatounio (P.) naktongensis-Trigonioides (Wakinoa) tetoriensis. These assemblages show various grades of brackish-to freshwater environments, and the species composition of each assemblage varies with salinity.Based on the occurrence of these five molluscan assemblages, the Ushimaru Formation reflects more repeated environmental change than the succession of the Otaniyama to Okurodani formations. This indicates that there is repetition of numerous minor cycles of transgression and regression in transgression-phase, but smooth environmental changes in the regression-phase. We can recognize more precise environmental changes by using non-marine molluscan assemblages.
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  • Hiroshi Ohkubo
    1999 Volume 105 Issue 12 Pages 836-851
    Published: December 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The Omma, Zukawa, Junicho and Yabuta Formations are typical of Plio-Pleistocene marine sediments in Hokuriku Province. The Omma and Junicho Formations have been previously dated to be early Pleistocene and late Pliocene to early Pleistocene respectively. The Takakubo Formation underlies the Omma Formation, and is dated from middle Miocene to early Pliocene.In order to establish tephrostratigraphy of the three formations (Omma, Zukawa and Junicho F.), and to make clear the stratigraphical relationship between Omma and Takakubo Formations, petrographical method, and analysis of the chemical components by energy dispersive X-ray spectrometry (EDS) were applied for the comparison of volcanic glass.As a result of this study, the following conclusions were obtained. (1) The uppermost part of the Takakubo Formation, and the lower parts of the Zukawa and Junicho Formations are in a relationship of contemporaneous heterotopic facies based on correlation of volcanic ash beds (TR 2-HK 2-SZp-T 3). (2) Based on tephrostratigraphy and biostratigraphy, the uppermost part of the Takakubo Formation is dated late Pliocene at eastern part of Kanazawa city and western part of Toyama Prefecture. (3) Between the type locality of the Omma Formation and that of the western part of Toyama Prefecture, correlation of horizons by previous biostratigraphy is inconsistent with the correlation by volcanic ash bed.
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  • Atsuo Tsushima, Yukimasa Oho
    1999 Volume 105 Issue 12 Pages 852-865
    Published: December 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    This paper describes the weathering of the Miocene mudstone in western Shimane Peninsula, from a standpoint of the outcrop occurrence and mineralogy. In this area, the mudstone is classified into three weathering zones (I, II and III zones from the bottom of the outcrops to the top), based on its color and hardness. This classification well agrees with the mineralogical characteristics in each zone : chlorite and pyrite have not been altered in the I zone ; chlorite has partly altered to chlorite-vermiculite interlayer (Ch-V) or vermiculite, and pyrite has partly altered to iron hydroxide (or oxide) in the II zone ; chlorite has mostly altered to Ch-V or vermiculite, and pyrite has mostly altered to iron hydroxide (or oxide) in the III zone. Alteration of chlorite and pyrite changes the color and hardness of mudstone, and plays an important role in forming the weathering zones in this area.
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  • Michio Takami, Risa Takemura, Yujiro Nishimura, Teruhiko Kojima
    1999 Volume 105 Issue 12 Pages 866-880
    Published: December 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The Jurassic-Early Cretaceous accretionary complexes in the Okinawa Islands, central Ryukyu Island Arc, are the southwestern extension of the Southern Chichibu Belt in the Outer Zone, Southwest Japan. They are chaotic sedimentary complexes which consist mainly of greenstone, chert, limestone, sandstone, and mudstone. Microfossil (radiolarian) biostratigraphic study was carried out for the Jurassic accretionary complex, newly called the Iheya unit, in Iheya and Izena Islands, which are in the innermost part of the Southern Chichibu Belt of the Okinawa Islands. A primary sedimentary sequence, that is, an oceanic plate stratigraphy, which consists of the Early to Middle Permian limestone, Late Carboniferous to Middle Triassic pelagic chert, Early Jurassic hemipelagic siliceous mudstone, and Early to Middle Jurassic terrigenous mudstone in ascending order, was reconstructed for the unit. Judging from the youngest age of the stratigraphy, an accretion age of the unit is interpreted to be in the Middle Jurassic.Present study has also resulted in the proposed of a new three-fold division of the Jurassic-Early Cretaceous accretionary complexes in the Southern Chichibu Belt of the Okinawa Islands. This was based on reconstructed oceanic plate stratigraphies compiled from lithologic features and fossil ages of the previous and present works. The three distinct geologic units are as follows : Iheya, Ie, and Motobu units from northwest to southeast or from upper structural horizon to lower one. An accretion age of the Ie unit is inferred to be in the earliest Cretaceous, and that of the Motobu unit to be in the Early Cretaceous, indicating a younging polarity in order of the Iheya unit, Ie unit, and Motobu unit, towards the lower structural units. This subdivision of the accretionary complexes almost coincides with that in the Southern Chichibu Belt of Southwest Japan.
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  • Tetsumaru Itaya
    1999 Volume 105 Issue 12 Pages 881-894
    Published: December 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    This is a review paper regarding the development of a mass spectorometric system for K-Ar dating using a small amount of samples and the prompt analyses of argon isotopes, and its application to geochronology of the Japanese Islands. The results, in particular, contributed to the volcanostratigraphy of late Cenozoic volcanoes, reconstructed unknown geologic units in accretinary complexes and analysed age distribution pattern of both high and low P/T metamorphic sequences. The application is based on the systematic sampling of lithologies from the geologic units to be examined and the K-Ar dating of mineral separates, making it possible to map geologic sequences by absolute ages. The geochronological mapping was also applied to major orogenic belts in the world and provided remarkable results to geologists.
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  • Satoshi Yamakita, Naoki Kadota, Takuya Kato, Ryuji Tada, Shigenori Ogi ...
    1999 Volume 105 Issue 12 Pages 895-898
    Published: December 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    We found earliest Triassic (Griesbachian) conodont, Hindeodus parvus, from the basal part of the black carbonaceous claystone of the Ubara Permian/Triassic boundary section in the Tamba Belt. We also obtained Late Permian (Changxingian) conodonts from the underlying gray siliceous claystone as already reported from other P/T boundary sections. The lithologic boundary between the gray siliceous claystone and the black carbonaceous claystone is confirmed to be the Permian/Triassic boundary in deep-sea sedimentary rocks.
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  • Noriko Matsuyama, Toshikazu Yoshioka, Akira Sangawa
    1999 Volume 105 Issue 12 Pages 899-901
    Published: December 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
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  • Aiming Lin, Takuya Wada, Tadashi Maruyama
    1999 Volume 105 Issue 12 Pages 902-905
    Published: December 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
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  • Kazuyuki Akaishi
    1999 Volume 105 Issue 12 Pages XXIII-XXIV
    Published: 1999
    Released on J-STAGE: November 26, 2010
    JOURNAL FREE ACCESS
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