The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 110, Issue 8
Displaying 1-6 of 6 articles from this issue
Articles
  • Masakazu Niwa
    Article type: scientific monograph
    Subject area: Infomation Science
    2004 Volume 110 Issue 8 Pages 439-451
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    Middle Jurassic melange of the Hirayu Complex, which is an accretionary complex in the Mino Belt, crops out in the Takayama area, Gifu Prefecture. The melange is composed of a matrix of either black mudstone or gray siltstone enclosing clasts of massive sandstone, bedded units of sandstone and mudstone, felsic tuff, brownish siliceous mudstone, red mudstone, gray siliceous claystone, chert, limestone and mafic volcanic rocks. The Hirayu Complex structurally overlies the Yukawa Complex composed of alternating beds of sandstone and mudstone intercalated with chert, and underlies the Kohachigagawa Complex composed mainly of slabs of chert, limestone and mafic volcanic rocks. Shear zones of foliated cataclasite occur along the thrusts bounding these complexes. From the lithological and structural viewpoints, the Hirayu Complex can be correlated to the Middle Jurassic melange of the Kuze Unit in the Mino area, Gifu Prefecture.
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  • Toshio Arai, Yoshiaki Tainosho
    Article type: scientific monograph
    Subject area: Infomation Science
    2004 Volume 110 Issue 8 Pages 452-462
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    Late Cretaceous granitic rocks are widely distributed in the Rokko Mountains. They are divided into Nunobiki Granodiorite, Yahatadani Granite, Dobashi Quartz Diorite and Rokko Granite, according to their field occurrence and petrographical characteristics. The Nunobiki Granodiorite occurs at the southern foot of the Rokko Mountains. This granodiorite shows a weak gneissic fabric, trending east-west and dipping to the north. This rock is subdivided into porphyritic and equigranular facies. The Yahatadani Granite occurs as a small pluton in the southeastern Rokko Mountains. The Nunobiki Granodiorite and the Yahatadani Granite belong to the Younger Ryoke granitoids. The Dobashi Quartz Diorite occurs as aggregations of small xenoblocks within the Rokko Granite. The Rokko Granite is the most extensively developed pluton in the Rokko Mountains. This rock is subdivided into medium- to coarse-grained facies, fine-grained facies and micrographic facies according to the grain-size differences and petrographical characteristics. The Rokko Granite is a vertically zoned pluton. The fine-grained facies occupies the higher part, the medium-grained facies the middle part, and the coarse-grained facies the lower part in altitude of the Rokko Mountains. The upper part of the Rokko Granite is rich in SiO2 and K2O, whereas the lower part of the Rokko Granite is rich in TiO2, MgO, and CaO.
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  • Kozo Takahashi, Hitoshi Yamashita
    Article type: scientific monograph
    Subject area: Infomation Science
    2004 Volume 110 Issue 8 Pages 463-479
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    Temporal flux changes of radiolarians were investigated employing time-series sediment traps deployed in the western and central equatorial Pacific during January to November 1999. On board R/V Mirai the sediment traps were deployed at four sites located between 135°E and 175°E, i.e., across the western Pacific warm pool (WPWP) region and the Equatorial Upwelling region (EUR). Total radiolarian fluxes showed higher values at Sites MT1, 2, 5 than that at Site MT3. Radiolarian production reflected levels of biological productivity, i.e., the levels of nutrient supply. The radiolarian flux species compositions at each site did not vary much seasonaly whereas levels of total radiolarian flux varied significantly. Several useful environmental indicator species are presented in this study. For instance, Lithomelissa setosa and Pseudocubus obeliscus are good eutrophic indicators. They are important tracers of the present WPWP migration in the western and central equatorial Pacific. The flux pattern of Lophophaena cylindrica was nearly the same as that of total radiolarians, and this species could represent the whole radiolarian productivity in the western and central equatorial Pacific. Furthermore, in order to understand the vertical radiolarian transport, the sediment trap samples were compared with the samples from plankton tows as well as core tops obtained using a multiple corer. The changes of Nassellaria-Spumellaria ratios between the suspended and sinking radiolarian populations were not significant, but there were significant differences between the sinking and surface sediment populations. It suggested that in the western and central equatorial Pacific the significant dissolution of radiolarians took place in surface sediments. The radiolarian assemblages were affected by selective dissolution. It appears that every species receives different levels of dissolution during their vertical transportation. The results provide important information when radiolarian microfossils are used to reconstruct paleoceanographic conditions.
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  • Michiaki Fujioka, Koji Kameo
    Article type: scientific monograph
    Subject area: Infomation Science
    2004 Volume 110 Issue 8 Pages 480-496
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    New evidence is provided to correlate of eleven key tephra layers in the Lower Pleistocene strata between the Inubou Group in the Choshi district and the Kazusa Group in the Boso Peninsula. On the basis of petrographic and chemical characteristics of key tephra layers, the Ob1, Ob2, Ob4a, Ob4c, Ob5a, Ob5.1, Ob5.6, Ob5.7, Ob6a, Ob6c and Ob7b in the Obama Formation of the Inubou Group can be correlated with the Kd39, Kd38, Kd25, Kd24, Kd16 and Kd5A in the Kiwada Formation, the O7 and O3 in the Otadai Formation and the U8, U7 and U6A in the Umegase Formation of the Kazusa Group, respectively. Calcareous nannofossil datums detected in the Obama Formation also support this result. Tephrostratigraphic and biostratigraphic data indicate that the Obama Formation can be assigned to the lower part of the Kiwada to the middle part of the Umegase Formations. Chronological data demonstrate the sedimentation rates markedly decreased in the sandy interval, corresponding to the middle part of the Obama Formation. Our study suggests that further paleoecological and paleoceanographical studies in the Lower Pleistocene strata will be needed to clarify the cause of the extremely slower sedimentation in the middle part of the Obama Formation.
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Prompt Report
  • Yasuo Iijima, Yuji Kanaori, Hiroyoshi Sano
    Article type: scientific monograph
    Subject area: Infomation Science
    2004 Volume 110 Issue 8 Pages 497-500
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    This note presents mesoscopic and microscopic features of neptunian dykes of calcareous sediments hosted by Late Pliocene basaltic lavas of the Uegusukudake Formation of Kumejima Island, the Ryukyus. The dykes, 1 to 20cm wide, consist mainly of micritic limestone rich in varied skeletal debris of a shallow marine affinity, with a subhorizontal depositional surface. Two dominant strikes of the dykes, NE and NW, are parallel and perpendicular to the modern central axis of the Okinawa Trough, respectively. The youngest foraminifera in the dykes were identified as Globorotalia inflata, of which first appearance is in Late Pliocene. Available K-Ar age data of the basaltic lava and Sr age data of the overlying Ryukyu Limestones indicate that infilling rapidly occurred during a period from 2.1 Ma to 1.3 Ma. These date are synthesized as follows. Open fractures formed due to rifting in an extensional regime, and were filled with shallow-marine calcareous sediments to form neptunian dykes under the sea. This rifting is postulated to have been related to the reactivation of rifting at the Okinawa Trough.
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