The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 106, Issue 10
Displaying 1-9 of 9 articles from this issue
  • Sapri Hadiwisastra, Hisao Kumai
    2000 Volume 106 Issue 10 Pages 651-658
    Published: October 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Calcareous nannofossils of the Wungkal Formation in the Bayat area were studied in order to obtain new age-diagnostic data. Previously, the Paleogene sediments in the formation have been determined as Eocene based on the assemblage of larger foraminifera. The nannofossils observed in the Wungkal Formation range from the late Eocene (CP 14) to early Oligocene (CP 16 c). The Eocene/Oligocene boundary is recognized by the last occurrence of Discoaster saipanensis, Discoaster barbadiensis and Cribrocentrum reticulatum. The Subzone CP 16 c in the upper part of the section was identified by the co-occurrence of Reticulofenestra umbilicus, Cyclicargolithus floridanus and Reticulofenestra bisecta.
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  • Migiwa Sasaki, Shigeru Otoh
    2000 Volume 106 Issue 10 Pages 659-669
    Published: October 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    A NNE-trending, sinistral cataclastic shear zone-the Tsuwamono-zawa shear zone-was described in the Motai-Nagasaka area in the southwestern part of the Southern Kitakami Belt. The Tsuwamono-zawa shear zone bounds the Motai Metamorphic Rocks and the basement amphibolites of the Southern Kitakami Belt along the type route of the shear zone, where the transition of deformation structures across a 50-m-wide cataclastic shear zone was described in detail. The grain-size distribution of the cataclasite from the amphibolites along the above shear zone obeys a power-law equation with the fractal dimension Dd of 1.8. The Tsuwamono-zawa shear zone, which cuts a limb of a Valanginian fold and has been intruded by Early Cretaceous granite, must have been active in Early Cretaceous time almost concurrently with the Hizume-Kesennuma Fault. The southwestern part of the Southern Kitakami Belt must have also been subjected to the Early Cretaceous sinistral shear movement that formed the Hizume-Kesennuma Fault.
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  • Hiroshi Kawabata, Kenji Shuto
    2000 Volume 106 Issue 10 Pages 670-688
    Published: October 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Geological investigation was carried out on the Miocene volcanic strata (Setouchi volcanic rocks) named the Sanuki Group constituting three volcanic masses, Shichihosan, Iyadanisan and Gahaishiyama masses, in Kanonji and adjacent areas, northeast Shikoku.These volcanic masses have the following common lithological characteristics and stratigraphic sequences. Volcanic strata unconformably overlie the Creataceous Ryoke granitic rocks and the Miocene Tonosho Group. These volcanic masses are lithologically divided into the lower unit consisting mainly of rhyolitic volcaniclastic rocks and accosiated rhyolitic lavas, and the upper unit composed mainly of andesitic and dacitic lavas with subordinate amounts of their volcanic breccias. The upper unit includes the composite lava flows formed by the mingling of andesitic and dacitic magmas in Iyadanisan mass. Rhyolitic volcaniclastic rocks from the lower unit are characterized by parallel lamination and slump structute and rubbles with water-chilled joints, suggesting the volcanism in subaqueous environment. Based on their phenocryst mineral assemblages and groundmass texture, andesitic and dacitic rocks constituting the upper unit were divided into twelve rock types. Among them, rocks with hornblende and/or biotite phenocrysts seem to have been erupted at the early stage of andesitic-dacitic activity. It is suggested that andesites and dacites of the respective masses erupted from several vents within each mass, because of the difference in constituent rock types among the three volcanic masses. Rocks from basalt through andesite to dacite constitute composite lava flows and dikes in Iyadanisan mass and many other volcanic masses in northeast Shikoku, but rhyolitic rocks are not constituents of such lavas and dikes. This feature is consistent with stratigraphic relationship between the rhyolite and basaltic-dacitic rocks, suggesting the difference in genesis between the both rock types. The Iyadani composite lava flows could be formed by eruption from compositionally zoned magma chamber.
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  • Hiroki Hayashi, Masaki Takahashi
    2000 Volume 106 Issue 10 Pages 689-702
    Published: October 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Planktonic foraminiferal biostratigraphy has been used as one of the popular tools for correlating marine strata, especially in the tropical to subtropical region. However, it is difficult to correlate marine strata in and around Japan with those of the tropical to subtropical zones because of the sporadical occurrences of tropical to subtropical species. Only few biohorizons available for Japanese Miocene strata have been reported. A planktonic foraminiferal biostratigraphy for Japanese marine strata based on age-diagnostic species of middle latitudes is prerequisite to be established.The middle to upper Miocene Arakawa Group distributed in the Karasuyama area, central Japan yields abundant marine microfossils and is intercalated by many tuff layers. A planktonic foraminiferal biostratigraphy was examined from the upper part of the Kobana through the Ogane Formations of the Arakawa Group, which has been dated to yield K-Ar dates of 12.6 to 11.3 Ma.The studied sequence is correlated with the planktonic foraminiferal zones N. 12 to N. 14 of Blow (1969). Eleven planktonic foraminiferal biohorizons were recognized and a numerical age for each biohorizon is applied on the basis of those K-Ar dates. Diachroneities are suggested on some of these biohorizons based on a correlation between this section and other sections in Japan.
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  • Koen de Jong, Chikao Kurimoto, Phil Guise
    2000 Volume 106 Issue 10 Pages 703-712
    Published: October 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    ^<40>Ar/39Ar whole-rock age spectra have been obtained from three metapelite samples from the Mikabu and Sambagawa Belts in the western Kii Peninsula (Wakayama Prefecture) as a feasibility study. All samples yielded 2σ plateau ages and the integrated 40Ar/39Ar ages are concordant with the K-Ar ages reported by Kurimoto (1993).A phyllite with a shear band cleavage from the spotted zone of the Sambagawa Belt (Iimori Unit) yielded a plateau age of 73.0±0.8 Ma (82% 39Ar release). This result is interpreted as the age of the isotopic closure of white mica during retrograde recrystallization in this greenschist facies metamorphic rock.Two slate samples from the Mikabu Belt (Kebara Unit) yielded 96.4±1.0 Ma (54% 39Ar release) and 102.9±1.0 Ma (49% 39Ar release) plateau ages. The 7 Ma age difference between the Kebara samples may be interpreted by the smaller grain size of the white mica in the youngest sample. Alternatively, the age difference indicates that the main tectono-metamorphic recrystallization in both samples did not occur at the same time. Both interpretations imply that the main tectono-metamorphic phase in the Kebara Unit occurred significantly earlier than in the Iimori Unit.The age spectra of the Kebara samples show progressively increasing apparent ages during early incremental degassing climbing to age plateaux at intermediate- and high-temperature 39Ar release. Such staircase age spectra may be interpreted by partial loss of radiogenic 40Ar due to thermo-tectonic resetting related to movement along the Aridagawa Tectonic Line, that runs parallel to the Kebara Unit.
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  • Hiroyuki Hoshi, Nobuyoshi Ito, Isao Motoyama
    2000 Volume 106 Issue 10 Pages 713-726
    Published: October 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The depositional age of the Tertiary Hokusetsu Subgroup in the Shitara area, southeastern Aichi Prefecture, central Japan, has been controversial. To settle this, we investigated radiolarian zonal key species from the subgroup in the northwestern part of the area. Several hundreds of meters of clastic rocks in the mapped area are divided into the Taguchi (breccia and conglomerate), Kawakado (sandstone), Ohno (sandstone and mudstone), and Kadoya (tuffaceous sandstone, tuffaceous mudstone, and tuff) Formations in ascending order, characterized by an overall fining-upward sequence. We collected small spherical calcareous nodules from 14 outcrops of the Ohno and Kadoya Formations for a radiolarian study. Three samples yielded radiolarian assemblages containing Cenosphaera coronata, Cenosphaera cf. coronataformis, and Cyrtocapsella tetrapera. The presence of C. tetrapera demonstrates deposition in early to middle Miocene time. The occurrence of C. cf. coronataformis suggests that the two formations are correlated to the C. coronataformis Zone that spans Chron C 5 Cr to Chron C 6 n (about 17 to 20 Ma). Thus the upper part of the Hokusetsu Subgroup was deposited sometime between 17 and 20 Ma, and the almost entire part of the subgroup was probably formed in that period. Our results support the stratigraphic correlation of the Hokusetsu Subgroup to the lower Miocene Tomikusa Group in southern Nagano Prefecture that has been proposed by some researchers.
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  • Jun-ichi Tazawa, Wei-ping Yang, Yukio Miyake
    2000 Volume 106 Issue 10 Pages 727-735
    Published: October 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Late Devonian brachiopod Cyrtospirifer and lepidodendroid plant Leptophloeum are newly recorded from the Hida Gaien Belt, central Japan. The fossils, collected from light green acidic tuff of the upper part of the Rosse Formation in the Moribu area, Hida Gaien Belt, are named as and identified as Cyrtospirifer sp. A, Cyrtospirifer sp. B and Leptophloeum rhombicum Dawson. From the fossil evidence, the upper part of the Rosse Formation is clearly correlated with the Upper Devonian. In common with both the South Kitakami and Kurosegawa Belts, the Hida Gaien Belt is characterized by the presence of pyroclastic and clastic Upper Devonian units containing Cyrtospirifer and Leptophloeum. The above three belts were probably connected biogeographically and also geographically with the eastern Kazakhstan-northern Xinjiang-Inner Mongolia region in the Late Devonian time.
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  • Earthquake Disaster Research Group
    2000 Volume 106 Issue 10 Pages XIX-XX
    Published: 2000
    Released on J-STAGE: March 17, 2011
    JOURNAL FREE ACCESS
  • Harutaka Sakai, Rie Fujii
    2000 Volume 106 Issue 10 Pages XXI-XXII
    Published: 2000
    Released on J-STAGE: November 26, 2010
    JOURNAL FREE ACCESS
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