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 111, Issue 3
Displaying 1-8 of 8 articles from this issue
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Articles
  • Cretaceous-Paleogene magmatism in the central San-in district, Southwest Japan: an examination based on Rb-Sr isochron ages
    Kazuhiro Nishida, Teruyoshi Imaoka, Shigeru Iizumi
    2005 Volume 111 Issue 3 Pages 123-140
    Published: 2005
    Released on J-STAGE: July 01, 2005
    DOIhttps://doi.org/10.5575/geosoc.111.123
    JOURNAL FREE ACCESS
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    Cretaceous to Paleogene intrusive rocks associated with small amounts of contemporaneous volcanic rocks cluster in the central San-in district, SW Japan. Geological and Rb-Sr isochron age data are presented for these intrusive and volcanic rocks in order to clarify the time and correlation of the Cretaceous-Paleogene magmatism in the San-in district. The intrusive rocks comprise a composite batholith, occupying an area of about 2,400 square kilometers. Greater part of the batholith was formed by intrusions of intermediate to felsic magmas during about 25 million years from 75 Ma to 50 Ma, and the batholith was completed by intrusions of late Paleogene small stocks. Based on presently and previously reported geochronological data, the Cretaceous-Paleogene magmatism in the San-in district is divided into three stages, i.e., Cretaceous volcanic rocks and the Mochigase-stage intrusive rocks (~78 Ma), the Inbi-stage intrusive rocks and contemporaneous volcanic rocks (75 to 50 Ma), and the late Paleogene Tamagawa-stage volcanic and intrusive rocks (44 to 30 Ma). A hiatus for six million years is recognized between the Inbi and Tamagawa stage magmatism.
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  • Formation processes of the fine-grained mafic rocks of the Ryoke metamorphic belt in the Asuka area, central Kinki district, SW Japan.
    Hitoshi Nishiwaki, Takamoto Okudaira
    2005 Volume 111 Issue 3 Pages 141-155
    Published: 2005
    Released on J-STAGE: July 01, 2005
    DOIhttps://doi.org/10.5575/geosoc.111.141
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    In the Ryoke metamorphic belt in the Asuka area, Kinki Province, SW Japan, many mafic rocks are found as sheet-shaped xenoliths in the Older Ryoke granitoids. They show a strong to weak foliation defined by preferential alignment of long axis of plagioclase and amphibole grains. The grains of plagioclase and amphibole exhibit hypidiomorphic to xenomorphic shapes, and show no chemical zonings. Almost all plagioclases and amphiboles have unique chemical compositions of An40-45 and magnesiohornblende-ferrotschermakite, respectively. Using a conventional thermometer, equilibrium temperatures of plagioclase and amphibole rims are estimated to be 500-600°C that is too low to obliterate the pre-existing zoning profiles due to low diffusivity in plagioclase and amphibole under such mid-crustal conditions. These observations suggest that the mafic rocks were amphibolite-facies metamorphic rocks. In some outcrops, there are obliquities between the trend of foliation of the mafic rocks and that of their contact planes to the Older granitoids. In many cases, the trend of foliation of the mafic rocks is oblique to that of the Older granitoids. The foliation of the Older granitoids is defined by preferential alignment of long axis of euhedral plagioclases and biotites, and may result from the magmatic-submagmatic deformation during their ascending and emplacement. Because there is a discordant relationship between foliation of the mafic rocks and that of the Older granitoids, the formation of foliation of the mafic rocks may predate the magmatic-submagmatic deformation of the Older granitoids. Consequently, the amphibolite-facies metamorphism of the mafic rocks occurred before their involving to the Older granitoids ascending/emplacing to the upper crust.
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  • Calculation methods for horizontal and two-dimensional ground air flow induced by atmospheric pressure fluctuations
    Tooru Itazu
    2005 Volume 111 Issue 3 Pages 156-169
    Published: 2005
    Released on J-STAGE: July 01, 2005
    DOIhttps://doi.org/10.5575/geosoc.111.156
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    The ground air flow induced by atmospheric pressure fluctuations is known as the barometric pumping. In most diluvial uplands of Japan, ground air flows mainly through permeable layer between upper surface aquiclude and lower ground water table. Consequently, vertical flow of ground air to atmosphere is impeded and ground air flows horizontally. However, any study about horizontal and two-dimensional ground air flow has not been proposed yet. In this paper, we regarded permeable layers in diluvial upland as rectangles and showed the analytical solution (the periodic steady state solution) for ground air pressure fluctuation in the rectangular homogeneous porous medium induced by the trigonometric atmospheric pressure fluctuation on condition that ground air flows two-dimensionally and horizontally.
    The calculations using the analytical solution show that the ground air flow in the rectangle approximately are regarded as one-dimensional flow (round-trip movement) toward the closest point on the boundary and the rectangle is divided into the regions in which ground air flows toward the long side and the short side. The max length of the ground air pressure propagation path in the rectangle is half-length of the short side. We showed the finite element method to calculate ground air pressure fluctuation in permeable layers of which shapes are complicated. To verify the possibility of decreasing necessary memory of computer in the calculation, we investigated applicability to use the finite element method in the small polygon that is cut from the large polygon. In case that the small polygon is cut at appropriate position, it is possible to calculate ground air pressure fluctuations by using the small polygon.
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  • Early Jurassic radiolarians from the Chichibu Composite Belt in the Sannokou area, central Kii Peninsula, Southwest Japan
    Kenji Kashiwagi, Masakazu Niwa, Tetsuya Tokiwa
    2005 Volume 111 Issue 3 Pages 170-181
    Published: 2005
    Released on J-STAGE: July 01, 2005
    DOIhttps://doi.org/10.5575/geosoc.111.170
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    Early Jurassic radiolarians were recovered from the Sannokou Complex, which is a part of the accretionary complex of the Chichibu Composite Belt in the central Kii Peninsula, Southwest Japan. The Sannokou Complex, which tectonically overlies the Middle Jurassic to middle Early Cretaceous complexes, predominantly consists of thickly bedded sandstone with conglomerate and mudstone intercalations; and minor amounts of siliceous mudstone, bedded chert, and sheared mudstone including various kinds of blocks. Radiolarian fossils were extracted from three and four samples of siliceous mudstone and chert, respectively. The siliceous mudstone samples bear well-preserved radiolarians, which are assigned to be early and middle Toarcian of late Early Jurassic in age. Age of cherts, ranging from Sinemurian to Pliensbachian, is slightly older than that of siliceous mudstone.
    It has been argued that the Chichibu Composite Belt in the central Kii Peninsula has been divided into seven complexes. Furthermore the six complexes except the Sannokou Complex are tectonically stacked with a downward-younging polarity from early Middle Jurassic to middle Early Cretaceous due to radiolarian biochronology. On the basis of occurrences of Early Jurassic radiolarians, the Sannokou Complex, which occupies the uppermost tectonic horizon in the Chichibu Composite Belt, shows the older age than these complexes. Thus the tectonic stacking with a downward-younging polarity can be recognized overall in the Chichibu Composite Belt of the central Kii Peninsula.
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  • Ripples with secondary crests as a possible indicator of palaeo-wave direction: a laboratory experiment
    Tomohiro Sekiguchi
    2005 Volume 111 Issue 3 Pages 182-186
    Published: 2005
    Released on J-STAGE: July 01, 2005
    DOIhttps://doi.org/10.5575/geosoc.111.182
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    The geometry of ripples having single or double secondary crests in each trough was examined through a wave flume experiment. The results showed that ripples with a secondary crest (s) exhibit asymmetrical profiles. In the case of ripples with a single secondary crest, the crest appears on the onshore side of each trough. In the case of double secondary crests, the distance from an onshore main ripple crest to one secondary crest on the offshore-dipping slope is larger than the corresponding distance between the other secondary crest and an offshore-adjacent main crest, and also the former secondary crest is located higher than the latter. Such geometric asymmetry becomes significant depending on the degree of velocity asymmetry, which is inherent in wave-induced oscillatory flow. The present experiment suggests that the geometrical characteristics of this type of ripples in the geological record enable us to infer the direction of ancient wave propagation.
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Prompt Report
  • Early Devonian radiolarians from the Ohno Formation in the Hikoroichi area of the South Kitakami Belt, Northeast Japan
    Toshiyuki Kurihara, Yoshitaka Sato, Jun-ichi Tazawa
    2005 Volume 111 Issue 3 Pages 187-190
    Published: 2005
    Released on J-STAGE: July 01, 2005
    DOIhttps://doi.org/10.5575/geosoc.111.187
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    The following four species of Early Devonian radiolarians have been discovered from felsic tuff of the lower part of the Ohno Formation in the Hikoroichi area of the South Kitakami Belt, Northeast Japan: Protoholoeciscus hindea Aitchison, Protoholoeciscus triangularis (Wakamatsu, Sugiyama and Furutani) , Glanta sp. cf. G. fragilis Wakamatsu, Sugiyama and Furutani, and Tlecerina horrida Furutani. The radiolarian fauna is correlated with the Tlecerina-Glanta Assemblage recognized in the Devonian of the Kurosegawa Belt, Southwest Japan. This fauna is no older than Lochkovian or Pragian and is no younger than Emsian, considering comparisons to other Devonian radiolarian assemblages. This age assignment is consistent with macroinvertebrate age constraints of the Ohno Formation and the stratigraphic relationship to the overlying Eifelian Nakazato Formation. Based on the occurrence of these radiolarians, the Ohno Formation can be correlated with the Nakahata Formation of the Kurosegawa Belt.
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