The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 105, Issue 6
Displaying 1-7 of 7 articles from this issue
  • Kazumi Akimoto, Mihiro Ureshino, Maki Sugiura, Toshiaki Irizuki, Atsus ...
    1999 Volume 105 Issue 6 Pages 391-409
    Published: June 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The paleoenvironment during the Miocene time in the Pohang basin is reconstructed on the basis of benthic foraminiferal assemblages. Q-mode principal components analysis of Miocene benthic foraminiferal assemblages reveals five important paleoenvironmental factors, namely CaCO3 contents, the deep and surface water masses and effects of coastal and cold water mass. Distributional patterns of fossil benthic foraminiferal species in the Pohang basin are deduced by integrating the result of this analysis, and assessing both the distribution of modern counterparts and water properties in the Northwest Pacific Ocean.In the northern area of the Pohang basin, the Heunghae Formation was deposited in the lower middle bathyal zone (800 to 2000 m water depth). In the central area of the basin, the paleodepth of the middle part of the Cheongogosa Formation and the Hagjeon Formation is estimated to be between the upper bathyal zone and the lower middle bathyal zone ranging from 200 to 800 m in water depth. The lower part of the Duho Formation was deposited in the lower middle bathyal zone. In the southern area of this basin, the Hagjeon Formation was deposited in the upper bathyal zone to upper middle bathyal zone. The paleodepth of the several horizons of this formation became as deep as the lower middle bathyal zone.The influence of the cold water mass is recognized in benthic foraminiferal assemblages in the middle to upper part of the Cheongogosa Formation, the Hagjeon and Duho Formations in the central area of this basin. This cold water mass flowed in this basin during the Zone N 8 to N 9.
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  • Sayuri Kita, Hirofumi Yamamoto, Akira Furusawa
    1999 Volume 105 Issue 6 Pages 410-420
    Published: June 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Weathered acidic volcanic glass shows features such as pitted textures. These microstructures were considered to be related to chemical weathering, but little have been studied about their formation. In order to clarify this process, we analyzed the Aira-Tn Ash which was erupted about 25, 000 years ago from the Aira Caldera in southern Kyusyu, and covers almost the whole area of the Japanese Islands.The glassy ash has been weathered under various subsurface environments, and many shallow, rounded pits are formed on the glass surfaces. The fresh glass, which is interbedded in clayey deep-sea sediments in the Sea of Japan, has no such pits but has smooth surface with elongated sharp holes like gashes. On the other hand, at Chigaki, Toyama Prefecture, we examined the ash interbedded in the terrace deposits, in which a successive change of the microstructure on the glass surface from well pitted to almost non-pitted was observed. Detailed analysis of this ash layer clarified that along with the progress of weathering, the size of the pits increases, the thickness of the glass decreases, and refractive index of the glass decreases.Etching by HF solutions shows that the small holes on the fresh glass have extended gradually and changed into shallow rounded pits similar to those on the weathered glass surface. The refractive index of heavily etched glass decreases to the same value as that of the weathered glass.From these observation, the volcanic glass in the Aira-Tn Ash is considered to be composed of the outer layer of 3 to 4μm thick where the refractive index is high and inner layer whose refractive index is low. Weathering dissolves the volcanic glass and removes the outer layers. This process may form the shallow rounded pits on the surface and decrease the refractive index.
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  • Satoru Kojima, Hideyuki Ando, Masahiro Kida, Shinjiro Mizutani, Yuko S ...
    1999 Volume 105 Issue 6 Pages 421-434
    Published: June 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    This paper describes petrographic properties and radiolarian ages of clastic rocks interbedded with Triassic bedded chert in the Ozaki-Amaike and Hisuikyo areas of the southern Mino terrane, central Japan.The clastic rocks in the Ozaki-Amaike area are 2.5 to 7 cm thick, and consist of angular, silt- to granule-sized clasts of chert, siliceous shale, basic or intermediate volcanic rocks, volcanic glass, polycrystalline quartz and radiolarians. Rarely found clasts include dolomite-bearing rocks, plagioclase, apatite and lutecite. Clastic rocks in the Hisuikyo area are contained in chert beds as laminae and scattered particles. Fourteen chert formations with the clastic materials occur in 4 fault-bounded zones, and apparently 25 clastic rocks-bearing chert beds can be observed in this area. Clast composition of the rocks is similar to that of the Ozaki-Amaike area, but they characteristically include glauconite-like minerals with or without siderite. Permian and Triassic conodonts are concentrated in the clastic rocks.Radiolarian fossils extracted from chert beds which occur above and below the clastic rocks indicate that the ages of clastic beds are early Carnian in Ozaki, middle Ladinian and early Carnian in Amaike, and late Anisian to early Ladinian in Hisuikyo ; although the ages are concentrated in the interval between late Anisian to early Carnian, they are not on the same horizon.Possible provenance of the clasts are oceanic island, island arc, continental shelf, and older accretionary complex.
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  • Ken-ichi Kano, Masashi Takeda
    1999 Volume 105 Issue 6 Pages 435-449
    Published: June 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Analysis of attitudes of slaty cleavage in the Mesozoic accretionary complex of the Mino-Tanba Belt in the Nosaka Mountains, north of Lake Biwa, Southwest Japan, has identified N-S trending mappable-scale chevron folds with subvertically-plunging axes. These folds were formed by horizontal buckling by cleavage-parallel compression after the formation of the regional steeply-dipping cleavages. The geometry of these folds indicates a E-W trending principal horizontal shortening of up to 25 percent or more over a distance of 6 km. They were probably formed under the same tectonic situation which formed the outcrop- and handspecimen-scale chevron folds and kinkbands with moderately- to vertically-plunging axes. This deformation mode with subvertically plunging rotation axes is believed to be one of fundamental processes in a shallow and brittle tectonic level of continental margin and/or island arc crusts which exhibit pre-existing steep planar fabrics.
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  • Masaaki Okuda, Yoshinori Yasuda, Takeshi Setoguchi
    1999 Volume 105 Issue 6 Pages 450-455
    Published: June 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Latest Pleistocene to Holocene pollen records with six AMS 14C ages from Lake Kopais, Southeast Greece are provided. While the Holocene zone (KP 11) is characterized by dense forest of Quercus (oak), the pre-Holocene zone (KP 10) is dominated by Chenopodiaceae (chenopod) and Artemisia (sagebrush) accompanied with Pinus (pine) and Ephedra (mahuang), showing that KP 10 represents a glacial phase in the Pleistocene. No grains of Olea (olive), Ericaceae (heath) nor Quercus ilex-type (evergreen oak) are found in this glacial phase. This denies that the Kopais region held the refugia for Mediterranean evergreen trees to survive the glacials.
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  • Yohei Suzuki, Tsutomu Otsuka, Akira Yao
    1999 Volume 105 Issue 6 Pages 456-459
    Published: June 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Late Triassic radiolarian fossils composed of genera Canoptum and Latium have been obtained from the western part of the Mino terrane, which is characterized by Mesozoic subduction complex, central Japan. The study area is underlain by a melange named the Otaki Unit which consists of basalt, limestone, chert and sandstone blocks and mudstone matrix. Although the sandstone yields Late Triassic bivalve Monotis, the age of the unit has remained unknown. The radiolarians obtained from the mudstone matrix of the melange unit show that the formation of the unit was Late Triassic and that the unit is one of the oldest in the Mino terrane.A Late Triassic unit has been reported to crop out only in the most northwestern most area of the Mino terrane and occupies the structurally highest portion of the subduction complex. The existence of the Late Triassic unit in the study area also gives important information for the discussion of the structure of the terrane.
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  • Aiming Lin, Tatsuro Fukuchi, Noriko Hasebe, Takayuki Higuchi, Kenta Ko ...
    1999 Volume 105 Issue 6 Pages XI-XII
    Published: 1999
    Released on J-STAGE: November 26, 2010
    JOURNAL FREE ACCESS
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