The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
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Volume 112, Issue 8
Displaying 1-7 of 7 articles from this issue
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Articles
  • Tufa in Oga Limestone Plateau, Okayama Prefecture: Its developmental conditions and annual lamination
    Masako Hori, Akihiro Kano
    2006 Volume 112 Issue 8 Pages 491-502
    Published: 2006
    Released on J-STAGE: December 02, 2006
    DOIhttps://doi.org/10.5575/geosoc.112.491
    JOURNAL FREE ACCESS
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    Tufas are largely developed in a ca. 1200-m-long stream at Nagaya in the northern Oga Limestone Plateau (Takahashi City, Okayama Prefecture, SW Japan). The water mainly issues from the spring located at the thrust boundary between the Nakamura Limestone and the lower lying shale and sandstone unit, and deposits tufa along the steep stream. These conditions allow the water being supersaturated with respect to calcite by increasing Ca2+ content (>60 mg/L) and decreasing PCO2 (<2.0 matm). Variation in chemical properties of the springwater, such as PCO2 and SIc, resembles to the seasonal change, which has been previously described in other tufa localities in SW Japan. However, the springwater of Nagaya is characteristic in low Ca2+ content in summer and large seasonal variation in temperature, which suggest a small water mass in the underground system. Some of the tufa specimens display very regular annual laminations, which display clear boundary between summer dense laminae and winter porous laminae. Cyanobacteria and diatoms densely occur in the porous laminae, whereas the dense laminae is poor in microbial remnants and characterized by large rhombic calcite crystals. The textural change of the annual lamination is inconsistent to inorganic precipitation rate (PWP-rate) calculated by chemical data, but is rather ascribed to change in water flow due to agricultural drainage and seasonal rainfall pattern.
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  • Warm wind holes at the top of the Yasukajigamori lava dome, Mikurasima volcano, Steaming only during the winter
    Akihiko Terada, Masayuki Hino, Keiji Takeiri
    2006 Volume 112 Issue 8 Pages 503-509
    Published: 2006
    Released on J-STAGE: December 02, 2006
    DOIhttps://doi.org/10.5575/geosoc.112.503
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    Warm wind hole swarms were found at the top of the Yasukajigamori lava dome of the Mikurasima volcano, Japan. A white steam column with height of less than 1-2 m was seen rising from the major hole Y-a only during the depth of winter. The results of temperature and relative humidity measurements in and around the Y-a hole indicate that temperature and density of vapor in the Y-a hole strongly depend on the temperature of the outer air. This suggests air circulation involving outer air percolating through porous lava dome. Thermal energy discharged from the Y-a hole is estimated to be about 22 GJ in each year, similar to the value reported from at the Nakayama wind holes in Shimogo, Fukushima. As a result of measurements of temperature, pH and electric conductivity of water around the lava domes, we have found no evidence to suggest volcanic thermal activities. Some of wind holes at other active volcanoes could be mistaken as volcanic fumaroles.
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  • Late Permian Boreal-Tethyan mixed brachiopod fauna from the Maizuru Belt, southwest Japan: Fossil evidence for the tectono-sedimentological setting of the Maizuru Group
    Jun-ichi Tazawa
    2006 Volume 112 Issue 8 Pages 510-518
    Published: 2006
    Released on J-STAGE: December 02, 2006
    DOIhttps://doi.org/10.5575/geosoc.112.510
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    The Kawahigashi fauna from the upper part of the Maizuru Group (Changhsingian) in the Kawahigashi and Yakuno areas, Kyoto Prefecture, Maizuru Belt, southwest Japan is a typical Boreal-Tethyan mixed brachiopod fauna. It contains the Boreal (antitropical) elements Lamnimargus and Megousia and the Tethyan element Eolyttonia. The Maizuru Belt, with some Late Permian (Lopingian) Boreal-Tethyan mixed brachiopod faunas such as the Kawahigashi fauna, was probably a foreland basin, which was located at the eastern margin of North China (Sino-Korea) and belonged to the transitional zone between the Boreal and Tethyan realms in the Late Permian.
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  • Selective generation surface of pseudotachylyte−example from the Asuke Shear Zone, SW Japan−
    Hideaki Sakamaki, Koji Shimada, Hideo Takagi
    2006 Volume 112 Issue 8 Pages 519-530
    Published: 2006
    Released on J-STAGE: December 02, 2006
    DOIhttps://doi.org/10.5575/geosoc.112.519
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    Ductile to brittle fault rocks including pseudotachylyte are narrowly distributed along the NE-SW trending Asuke Shear Zone in the Inagawa granite bodies of the Ryoke Belt, Chubu region, SW Japan. The Asuke Shear Zone is composed of many small-scale shear zones with left-stepping en echelon arrangement (P shear orientation for sinistral shear zone). Kinematic indicators and stretching lineations in mylonite and foliated pseudotachylyte of the small-scale shear zones indicate sinistral - extensional shear. These lines of evidence suggest that the Asuke Shear Zone has thicken under transtensional tectonic regime. The existence of mylonitized pseudotachylyte and mylonitized cataclasite, and fractured mylonite suggests that the major deformation along the shear zone took place in cataclastic-plastic transition regime. Pseudotachylyte fault veins tend to generate along the P shear whereas cataclasite along the Y and R1 shear surfaces. This result is explained by that frictional fusion preferentially occurs along mesoscopically transpressional fault surface where the rate of heat production per unit area is relatively high.
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Prompt Reports
  • Failed submarine landslides on the shelf break, off Fukiage-hama, Satsuma peninsula, Kyushu, Japan
    Yo Iwabuchi, Kenjiro Mukaiyama
    2006 Volume 112 Issue 8 Pages 531-534
    Published: 2006
    Released on J-STAGE: December 02, 2006
    DOIhttps://doi.org/10.5575/geosoc.112.531
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    Elongated trough-like depressions were discovered on the shelf break, southwest off the Kyushu. Each depression is 200−250 meters in width, hundreds meters to 2 kilometers in length and 10−20 meters in relative depth. The depressions are discontinuously aligned for 18 kilometers parallel to the strike of the shelf slope. The shelf and upper shelf slope consist of a deposit of the Pleistocene Ito pyroclastic flow in origin. The distribution of the depressions and geological information indicate that the depressions are not any of erosional features by tidal currents or gravitational flows, but “crown crack”, which is a kind of tensional cracks on the top of a submarine slide body during the initial stage of sliding.
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  • Early Permian radiolarians from mudstone at Hisage in the Dai area, Nagato Tectonic Zone, Southwest Japan
    Masao Kametaka
    2006 Volume 112 Issue 8 Pages 535-538
    Published: 2006
    Released on J-STAGE: December 02, 2006
    DOIhttps://doi.org/10.5575/geosoc.112.535
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    Early Permian radiolarian fossils were discovered from mudstone of the Nagato Tectonic Zone, at Hisage in the Dai area, Yamaguchi Prefecture, Southwest Japan. The mudstone consists of a broken formation of alternating beds of sandstone and mudstone. The radiolarian fauna, composed of Albaillella sinuata, Pseudotormentus kamigoriensis, Raciditor gracilis and others, indicates late Early Permian (Kungurian) age. This radiolaria-bearing broken formation does not correspond to the clastic rocks of the Akiyoshi terrane, but it corresponds to the formations of the Maizuru terrane or the Hida Gaien terrane.
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