The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 119 , Issue 10
Showing 1-5 articles out of 5 articles from the selected issue
  • Ryosuke Sato, Shintaro Murai, Akiko M. Kitase, Hideo Yamazaki, Shusaku ...
    2013 Volume 119 Issue 10 Pages 655-664
    Published: October 15, 2013
    Released: March 26, 2014
    We examined temporal and spatial trends in the concentrations of spheroidal carbonaceous particles (SCPs) and the chemical compositions of SCPs in core samples of bottom sediments collected from ponds and lakes in China, Korea and Japan. Fresh snow samples were also collected for analysis from the top of Mt. Fuji, Japan.
    Temporal trends in the concentrations of SCPs at Yasha-ga-ike pond (YSH), in the Hokuriku region, are similar to those in urban areas in Japan before the 1970s. However, the trends differ for the period since the 1980s. Since the 1980s, the chemical compositions of SCPs at YSH are similar to those in China. Furthermore, several SCPs in the snow samples are similar in composition to SCPs in China. Therefore, some SCPs are transported from China to Japan via passive drifting in the free troposphere.
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  • Naoki Miyagi, Sotaro Baba, Ryuichi Shinjo
    2013 Volume 119 Issue 10 Pages 665-678
    Published: October 15, 2013
    Released: March 26, 2014
    Based on whole-rock chemical compositions of sandstone and greenstone, we estimate the geologic provenance and tectonic setting of pre-Neogene basement rocks of Okinawa Island and neighboring islands. Our results suggest that sandstone clasts in the Iheya Unit were derived from a continental arc and dissected arc, whereas sandstone clasts in the Motobu Unit were formed through the mixing of various rocks derived from different provenances. Sandstone clasts in the Nago and Kayo formations were derived from similar geologic provenances (continental island arc, active continental margin, continental arc, and dissected arc), and sandstones in both formations can be divided into two groups using abundances of Nb. The chemistry of detritus garnet in sandstones from the Nago Formation indicates the existence of high-grade metamorphic rock in the provenance. The Kerama Formation has traditionally been regarded as a continuation of the Nago Formation, but the whole-rock chemical compositions of sandstones of the two formations are different. Our results, when compared with those for sandstone in the Shimanto belt, present significant differences in the contents of Sr, Cr, and Ni. These differences were presumably caused by mafic rock sources of clastic materials and by the paleoweathering conditions of the provenance.
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  • Hiroyuki Hoshi, Kenji Hattori, Satoshi Tanaka, Toru Usami, Ryohei Naka ...
    2013 Volume 119 Issue 10 Pages 679-692
    Published: October 15, 2013
    Released: March 26, 2014
    Newly determined remanent magnetization directions of both tephras and fine-grained clastic sediments are used to redefine the Gauss-Matuyama boundary (GMB) in the Tokai Group of central Japan. Samples collected from 19 sites within a ~70-m-thick sedimentary sequence were subjected to progressive thermal or alternatingfield demagnetization, and the demagnetization data were statistically analyzed to determine site-mean remanent magnetization directions. Demagnetization results and rock magnetic experiments show that most samples contain magnetite as the dominant magnetic carrier and that most samples also contain hematite. In the northern part of the Kameyama area in Mie Prefecture, the GMB lies about 30 m below the Reiho volcanic ash bed. This finding is clearly different from a previous magnetostratigraphic study that determined the Reiho ash bed to be of normal polarity and that placed the GMB above the bed. A recent tephrostratigraphic study proposed that the Reiho ash bed could be correlated with a tephra bed below the GMB in a sedimentary sequence on the Boso Peninsula, more than 300 km to the east of the present study area, but this idea needs to be reconsidered. The 22 site-mean directions, which include those reported from above the Reiho ash bed, pass a reversals test and yield an overall mean direction that provides a reliable paleomagnetic direction for the latest Pliocene to earliest Pleistocene (around 2.6 Ma).
    The mean direction is marked by a slight easterly deflection of declination from the north, and this raises the possibility that during the Quaternary, a small but significant clockwise rotation (7.4 ± 4.6°) occurred in the study area with respect to Earth’s rotational axis.
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  • Go-Ichiro Uramoto, Kazuma Seike
    2013 Volume 119 Issue 10 Pages 693-698
    Published: October 15, 2013
    Released: March 26, 2014
    We report a quantitative examination of stacking patterns of sheet-like turbidite beds in a submarine-fan succession using the Straub method. The Straub method is a quantitative metric for the detection of compensational stacking of sedimentary successions in which the standard deviation of the sedimentation rate of the deposits is described by a power-law function within a given measurement time window. The method can be applied to successions in which it is possible to trace both the spatial distribution of sediment and datum levels. Bed-by-bed correlation was used to reveal the spatial distribution of sheet-like turbidite beds within a submarine-fan succession in the middle Kiyosumi Formation of the Boso Peninsula, central Japan. The compensation index of the studied interval represents the exponent of the power-law trend in the decay of the standard deviation and can serve as a quantitative indicator of the degree of compensational stacking. The values of compensation index are consistent with sheet-like turbidite stacking patterns determined from stratigraphic cross-sections of the studied succession. The Straub method is useful for exploring sediment stacking patterns in sedimentary environments characterized by feedback from basin morphology.
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