The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 110, Issue 4
Displaying 1-7 of 7 articles from this issue
Articles
  • Hisatoshi Ito, Tomonori Taniguchi
    2004 Volume 110 Issue 4 Pages 197-203
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    Fission-track (FT) dating using both zircon and apatite was performed on sandstone of the Paleogene Kishima Group, Nagasaki Prefecture. The depositional age estimated by zircon FT using two statistical procedures was in good agreement with that estimated by other methods, which shows this method using sandstone is useful for evaluating the depositional age. Zircons ∼60 Ma in FT age were found in the sandstone and were assumed to have originated from Cretaceous granites around near-by Mt. Seburi because of their similarity in zircon FT ages. Because an apatite FT age of 23.8±5.2 Ma was obtained from the sandstone, the studied Paleogene sedimentary basin was assumed to have never experienced temperatures in excess of ∼100°C since 23.8±5.2 Ma.
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  • Masahiro Nakajima, Katsuhiro Nakayama, Arata Momohara, Minoru Tsukagos ...
    2004 Volume 110 Issue 4 Pages 204-221
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    Sedimentary environment of the Late Miocene Tokiguchi Porcelain Clay Formation at Ohbora, central Japan was studied together with occurrence and composition of plant macrofossils. Sandy braided river system distributed between surrounding hills and ponds in a sedimentary basin was reconstructed based on 10 types of sedimentary facies. The results indicated that fluvial systems dominated by sand and mud was a common characteristic to the sedimentary basins in the porcelain clay formation in the Tokai Group. Based on occurrence of plant macrofossils included in flood flow deposits, depositional processes of plants in the river system were reconstructed: 1) Two types of plant particles transported with bedload and suspension were recognized; 2) the difference was due to morphology and specific gravity of the particle which were influenced by permeability of water into the wooly surface and woody tissue; 3) as the result, plant macrofossils were hydrologically sorted and deposited; and 4) sedimentary facies of flood flow deposits which include plant macrofossils were classified into three types and different depositional processes were reconstructed based on vertical change of grain size distribution both of plants and the matrix.
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  • Tetsuji Onoue, Katsuya Nagai, Aya Kamishima, Mamoru Seno, Hiroyoshi Sa ...
    2004 Volume 110 Issue 4 Pages 222-236
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    Sambosan accretionary complex comprises a Late Jurassic to Early Cretaceous subduction-generated accretionary complex of southwest Japan. In order to understand the origin of basalts of the Sambosan accretionary complex, this paper describes their modes of occurrence, petrographic features, and geochemical properties.
    Sambosan basaltic rocks crop out as, (1) exotic blocks in a mudstone matrix of melange units; (2) displaced blocks in basaltic volcaniclastic rocks; (3) pillow fragment breccias intercalated in Middle Triassic ribbon chert. In the former two cases, the blocks of basaltic rocks, ranging in size from a few meters to several tens of meters, are mainly composed of vesiculated pillow lavas with interpillow limestone and subordinate massive lavas, pillow fragment breccias, and hyaloclastites. The basalts are described mostly as aphyric basalt, clinopyroxene basalt, and olivine basalt. Dolerite occurs locally.
    Major and trace elements of the Sambosan basalts were analyzed by using the X-ray fluorescence spectrometry. Interpretation of geochemical data focuses on the relatively alteration-insensitive elements (Ti, Zr, Nb, Y). On the basis of the discrimination diagrams and the N-MORB normalized patterns, it is revealed that most of the Sambosan basalts have a geochemical affinity with an oceanic island basalt (OIB) enriched in Ti and Nb.
    Field and petrographic properties indicate that the Sambosan basalts were formed at shallower depth than the carbonate compensation depth in mid-oceanic realm. The predominance of OIB-type basalt means that the Sambosan basalts were erupted to form seamounts by an intraplate volcanism that most presumably took place in a mid-oceanic realm.
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  • Koichi Okuzawa, Ken-ichiro Hisada
    2004 Volume 110 Issue 4 Pages 237-243
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    Detrital chromian spinels were newly found from the Early Miocene Haccho Formation of the Mineoka Group and the early Middle Miocene Sakuma Group in the southern part of the Boso Peninsula, central Japan. The detrital chromian spinels from those beds are similar in chemistry, and they are divided into two groups in terms of Ti content, namely low Ti (TiO2 < 1 wt%) and high Ti (TiO2 > 1 wt%) groups. Most of the low Ti group spinels chemically correspond to those of the Mineoka serpentinites, which seem to have been derived from the backarc peridotites. However, some of them have higher Cr content, and have been probably derived from the forearc peridotites or boninite. The high Ti group spinels chemically resemble those of alkaline and picritic basalts in the Mineoka, Hayama, Kobotoke and Setogawa Belts. The occurrence of these chromian spinels suggests that the ophiolitic rocks in the Mineoka Belt were emplaced in the Early Miocene or earlier times.
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