The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 111, Issue 12
Displaying 1-6 of 6 articles from this issue
Articles
  • Itoko Tamura, Haruo Yamazaki, Kiyohide Mizuno
    2005 Volume 111 Issue 12 Pages 727-736
    Published: 2005
    Released on J-STAGE: April 05, 2006
    JOURNAL FREE ACCESS
    A characteristic widespread tephra, the Sakai volcanic ash and its correlatives, is found in many major Pliocene sedimentary basins in central Japan. The Sakai volcanic ash and its correlatives consist of thin glass shards of bubble-wall type with very small amount of biotite phenocryst. The chemical composition of glass shards, rich in K2O and poor in CaO, Ba and Sr, differs markedly from other Pliocene tephra beds. Although this ash was given a local name in each sedimentary basin, namely the Sakai volcanic ash and Tarumi volcanic ash in the Tokai Group, the Ichibe volcanic ash in the Kobiwako Group, and the B22 volcanic ash in the Kakegawa Group, we rename this ash as the Sakai volcanic ash and its correlatives. The age of the Sakai volcanic ash and its correlatives is estimated at about 4.1 Ma based on the stratigraphical horizon situated at the upper part of N.19 zone by Blow (1969) and close to the upper boundary of the Cochiti Subchron in the Gilbert Chron. The source of this ash is thought to be situated to the west of the Kinki district due to the westward increasing of grain size.
    This tephra will play an important roll in reconstructing the environmental change and tectonic evolution since the Early Pliocene in Japan.
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  • Yasutaka Aizawa, Kenta Kobayashi, Kengo Umetsu, Ryo Yamamoto
    2005 Volume 111 Issue 12 Pages 737-750
    Published: 2005
    Released on J-STAGE: April 05, 2006
    JOURNAL FREE ACCESS
    This study has revealed some characters of the 2000 Tottori-ken Seibu earthquake by observing outcrops and thin sections.
    The earthquake occurred in a place where no active fault was identified. After the earthquake happened, airphoto-interpretations were performed again. Consequently, some lineaments in the direction of northwest were newly recognized. Although the fault planes generally strike the west-northwest and the northeast, they disagree to the lineaments obtained from the airphoto-interpretations. The strike of fault plane allowed a distinction to be made between the exposed ones and the seismic ones on the aftershock area of the 2000 Tottori-ken Seibu earthquake as well. The fault planes which we observed at outcrops were formed before it. And most fault planes with west-northwest strike indicated left-lateral slip and most of those with northeast strike showed right-lateral slip.
    The foliated cataclasite and fault gouge were discovered in study area. The foliated cataclasites were exposed only on the aftershock area. The evidence proved that fault movement has repeatedly been occurring at least more than one time. The width in cataclasite ranged from 1 mm to 15 cm.
    The fault gouges were distributed in whole area, specifically around the Lake Ryokusui. The earthquake surface fault in this time also occurred there, and the displacement was about 2 cm. The maximum fault density was more than 11 per 5 m at the outcrop of west of Lake Ryokusui and northeast part out of the aftershock area. The main thickness was 1 to 3 mm. The fault gouges showed various colors. A large percentage of color were white, pale yellow, pale red and black.
    Rhyolite, basalt-andesite and aplite dikes were formed in Miocene, which intruded the granite throughout study area. Their trend was similar to the fault planes. And the crush zones mainly formed around the granite/dikes boundary.
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  • Chizuru Takashima, Akihiro Kano
    2005 Volume 111 Issue 12 Pages 751-764
    Published: 2005
    Released on J-STAGE: April 05, 2006
    JOURNAL FREE ACCESS
    A large-scale travertine deposit is developed at Shionoha hot spring (Kawakami Village, Nara Prefecture). The water from the vent is relatively low in temperature (38~39ºC), high in CO2 and Ca2+, and DO-free. Chemical and stable isotopic compositions of the vent water indicate that water of a meteoric origin is circulated in deep, and that deep-originated CO2 is reacted with rock. The vent water, during flowing a 70 m-long valley, first deposits Fe-rich deposits and then calcitic travertines. Changes of dissolved Fe2+ and Ca2+ in the water along the valley are consistent with distributions of the Fe-rich deposits and the calcitic travertine.
    Near the vent, Fe-rich deposit is formed partly due to quick uptake of oxygen from the atmosphere. However, its texture showing upward-branching filaments of 10-20 μm thick indicates that metabolism of iron-oxidizing bacteria probably controls formation of the iron precipitate. Results of XRD, EPMA and TEM-EDS indicate that the Fe-mineral is amorphous ferrihydrite. Calcite precipitation becomes dominant in the lower valley, and forms travertine domes. The travertines exhibit various textures, including of submilimeter-milimeter order lamination, which consists of bundles of needle-shape calcite crystals intercalating with fine-grained layers. Observation result indicates that about 40 bands were formed during a period of 41 days. Therefore, the banded texture was defined as daily rings. Depositional rates were evaluated about 5 mm/year for the Fe-rich deposit, and 20 cm/year for a laminated travertine.
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  • Takeru Moriyama, Hiroshi Yamamoto
    2005 Volume 111 Issue 12 Pages 765-778
    Published: 2005
    Released on J-STAGE: April 05, 2006
    JOURNAL FREE ACCESS
    The Takahama metamorphic suite, which belongs to the Nagasaki metamorphic sequence, is exposed in Amakusa-Shimoshima Island, western Kyushu, Japan. The Takahama metamorphic suite consists of two subhorizontal lithological units. The upper unit is separated from the lower by a thrust or shear zone and is composed of epidote amphibolites, garnet amphibolites, and garnet-mica gneisses. The lower unit is composed of pelitic schists, siliceous schists, and basic schists. The Upper Cretaceous Himenoura Group overlies the Takahama metamorphic suite with a subhorizontal fault contact. Therefore, the upper unit of the Takahama metamorphic suite is tectonically intercalated between the underlying lower unit and the overlying Himenoura Group. The Takahama metamorphic suite underwent at least six deformational events (D0-D5); the most conspicuous ductile deformation took place during D2. Kinematic indicators for D2 suggest northeastward extrusion of the upper unit. Integrating previous studies, these results support the interpretation that the Takahama metamorphic suite is a western extension of the Sanbagawa metamorphic belt.
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  • Yuki Fujiwara, Toshiaki Irizuki, Hiroki Hayashi, Yuichiro Tanaka, Mayu ...
    2005 Volume 111 Issue 12 Pages 779-791
    Published: 2005
    Released on J-STAGE: April 05, 2006
    JOURNAL FREE ACCESS
    The Miocene Awa Group occurs in the eastern part of Iga City, Mie Prefecture, central Japan. We examined the geology and microfossils in detail. The Awa Group is composed of three formations (the Higashitanihata, Hiramatsu and Makino Formations). The Higashitanihata Formation unconformably overlies granite or gneiss of the Cretaceous Ryoke Complex and consists of basal conglomerate. The Hiramatsu Formation is composed of sandy conglomerate, sandstone and mudstone, and is divided into four lithofacies, suggesting two cycles of transgression-regression. The Makino Formation is composed of pebbly mudstone and unconformably overlies the Hiramatsu Formation. Relative sea-level lowering occurred at the boundary between the two formations. The Higashitanihata and Hiramatsu Formations in the central to northeastern areas strike E-W to NW-SE and dip about 10 to 20 ºS. Whereas those in the southern area vary by the development of a fault, a syncline and an anticline running NNE-SSW to NE-SW. Basin structure is developed only in the Makino Formation. Fossil foraminifers and calcareous nannofossils were examined to establish the biostratigraphy. The Hiramatsu Formation contains poorly preserved fossil planktonic foraminifers. The Makino Formation contains many index fossils showing the planktonic foraminiferal zones N.8 to N.9 of Blow (1969) and the calcareous nannofossil zone CN3 of Okada and Bukry (1980). Thus, the Makino Formation was certainly deposited between 16.4 and 15.6 Ma.
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