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 115, Issue 11
Displaying 1-7 of 7 articles from this issue
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Articles
  • Ancient submarine-canyon system developed within fore-arc basin fill, Mio-Pliocene “Miyazaki facies,&rdquo: Miyazaki Group, western Japan
    Yasuhiro Takashimizu
    2009 Volume 115 Issue 11 Pages 559-577
    Published: 2009
    Released on J-STAGE: May 29, 2010
    DOIhttps://doi.org/10.5575/geosoc.115.559
    JOURNAL FREE ACCESS
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    The late Miocene to Pliocene “Miyazaki Facies&rdquo: (we use the term “Miyazaki Facies&rdquo: because it is established in the literature, although it represents the central part of the Miyazaki Group rather than a facies in the sedimentary sense) in the central part of the Miyazaki Group, consists of infill material within a fore-arc basin that morphologically consists of an active hinterland, narrow shelf, steep slope, and continuous gentle slope. The Kamurano Formation, which contains coarse-grained clastic sediments on the slope of the fan delta, is well preserved in the “Miyazaki Facies.&rdquo: Fifteen depositional facies are recognized in the “Miyazaki Facies&rdquo: and have been integrated into four facies associations according to the type and nature of flow energy: facies association CR consists of limestone, FL was transported and deposited by fluvial energy, WS by wave and storm energy, and GF by sediment gravity flow. Based on the results of sedimentologic and geologic analyses, a single submarine canyon system (Kamurano Submarine Canyon) was recognized on the continental slope.
    During a relative rise in sea level, the fan delta was unable to prograde to the shelf edge, and the shelf was covered with fine sediment. During a relative fall in sea level, however, the narrow width of the shelf (about 2 km) meant that the fan delta was easily able to prograde to the shelf edge and thereby directly provide coarse-grained sediment to the submarine canyon.
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  • Marked change in sandstone composition during the Middle Jurassic in Jurassic accretionary complexes of SW Japan, and geologic significance
    Kazuo Kiminami, Seiichi Kishita, Teruyoshi Imaoka
    2009 Volume 115 Issue 11 Pages 578-596
    Published: 2009
    Released on J-STAGE: May 29, 2010
    DOIhttps://doi.org/10.5575/geosoc.115.578
    JOURNAL FREE ACCESS
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    To clarify the signatures and provenance evolution of Jurassic accretionary complexes, we studied the petrographic features and bulk chemistry of sandstones in the Southern Chichibu Belt and northern margin of the Shimanto Belt, eastern Kyushu, and in the Tamba Belt (Kuga Group), eastern Yamaguchi Prefecture. The sandstones are classified as high-Ti and high-Zr types based on a Zr/Nb-Ti/Nb diagram. The high-Ti sandstones are characterized by relatively low SiO2 contents, high contents of TiO2, MgO, Na2O, and V, and high concentrations of volcanic rock fragments. In contrast, the high-Zr sandstones are characterized by high SiO2 contents, low contents of TiO2, MgO, Na2O, and V, and low concentrations of volcanic rock fragments. The high-Ti and high-Zr sandstones were derived mainly from a magmatic arc and an area dominated by crystalline rocks without volcanic rocks, respectively. The compositional change from high-Ti to high-Zr sandstones occurred during the Middle Jurassic. The crystallization ages of granitic rocks of the Korean, Liaodong, and Jiaodong peninsulas reveal a northwestward migration of the magmatic arc during the Early-Middle Jurassic and a magmatic hiatus during the Late Jurassic and early Early Cretaceous. The extinction of the magmatic arc and rapid unroofing of granitic and continental basement rocks upon the Korean Peninsula during the Late Jurassic and early Early Cretaceous could have produced the high-Zr sandstones. The inland migration of the magmatic arc and subsequent magmatic lull could be explained by a rapid shallowing of the subduction angle and the occurrence of flat-slab subduction, respectively. Flat-slab subduction developed in response to the subduction of a large, buoyant oceanic plateau during the Jurassic.
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  • Influence of a warm-water current on the northern Fossa Magna Region during the Pliocene, based on analyses of molluscan fauna from the Shitoka Formation and the lowest part of the Uonuma Group along the Kamakurazawa River in Minami-Uonuma City, Niigata Prefecture
    Kazutaka Amano, Kaku Nagata, Tokiyuki Sato, Yukio Yanagisawa, Yoshitak ...
    2009 Volume 115 Issue 11 Pages 597-609
    Published: 2009
    Released on J-STAGE: May 29, 2010
    DOIhttps://doi.org/10.5575/geosoc.115.597
    JOURNAL FREE ACCESS
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    The molluscan faunas from the Shitoka Formation and the lowest part of the Uonuma Group were examined along the Kamakurazawa River, central Niigata Prefecture, to assess the influence of a warm-water current on the northern Fossa Magna Region during the Pliocene. The Shitoka Formation consists of pebbly mudstone and siltstone, and unconformably overlies the Nishitajiri Formation, which is assigned an upper Miocene age based on diatom fossils, representing the lower to middle part of the NN16 nannofossil zone. The lowest part of the Uonuma Group is composed mainly of conglomerate and sandstone, and conformably overlies the Shitoka Formation. The Kamakurazawa fauna within both formations includes seven characteristic species of the Plio-Pleistocene Omma-Manganji fauna as well as one Miocene relict species. In addition to 14 cold-water species, 14 warm-water species were found in almost all sample horizons. During the late Pliocene in the northern Fossa Magna Region, the proportion of warm-water species decreased from the northernmost Shibata area (18.3%) to the southernmost Nagano area (2.0%). The studied area has a proportion of warm-water species intermediate between these values, and lies halfway between the Shibata and Nagano areas. During the Pliocene, a large northeast-facing bay developed at the north Fossa Magna, and a shallow warm current flowed northeastward from around the Tsushima Strait. The Shibata area, facing the open sea, was directly influenced by the warm-water current, whereas the Nagano area, located in the innermost part of the bay, was not.
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Short Article
  • Early Jurassic radiolarian fossils from mudstone within an accretionary complex south of Hirosaki City, Aomori Prefecture, Japan
    Hayato Ueda, Miwako Mori, Izumi Sato
    2009 Volume 115 Issue 11 Pages 610-613
    Published: 2009
    Released on J-STAGE: May 29, 2010
    DOIhttps://doi.org/10.5575/geosoc.115.610
    JOURNAL FREE ACCESS
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    Radiolarian assemblages including Canoptum spp. in part with Natoba minuta or ?Parahsuum spp. were extracted from terrigenous mudstone within an accretionary complex located south of Hirosaki City, near the western margin of the North Kitakami Belt. These assemblages suggest sedimentation during the Early Jurassic, older than any known fossil ages (Middle Jurassic to latest Jurassic-earliest Cretaceous) obtained for mudstone in the North Kitakami Belt. This result indicates the occurrence of near-continuous accretionary tectonics in NE Japan from the Early Jurassic to earliest Cretaceous, comparable with age data obtained for SW Japan. Although the accretionary complex located south of Hirosaki occurs as an isolated inlier among younger cover sediments, its age and location suggest that it corresponds to one of the structurally uppermost units in the North Kitakami Belt.
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Note
  • Temporal reduction in radiocarbon counting efficiency evaluated by a Benzene-liquid scintillation method
    Masako Hori, Hisao Kondo, Koji Okumura
    2009 Volume 115 Issue 11 Pages 614-619
    Published: 2009
    Released on J-STAGE: May 29, 2010
    DOIhttps://doi.org/10.5575/geosoc.115.614
    JOURNAL FREE ACCESS
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    To assess the reliability of the Liquid Scintillation Counting method, we monitored the weight and counting ratio of a benzene sample (STD059) prepared in 2000. Both benzene weight and the counting ratio generally decreased during the 2 years of analysis, at rates of 7 mg/yr and 0.073 cpm/gC/yr, respectively. The total decrease in the counting ratio was 97% of the initial value, as calculated using a linear approximation. The counting efficiency of STD059 was 71–73%, less than that for IAEA standards prepared in 2006–2007 (74.3–78.5%). Spectral quenching parameters (SQP) and Channel ratios (R) were similar between the STD059 and IAEA standards (C-1 to C-5), suggesting insignificant quenching of STD059. The temporal reduction in counting efficiency was possibly caused by density quenching, although the scintillator concentration of the solvent was lower than 15.9 g/L during the study period. According to the counting efficiency of STD059, we determined that the 14C activities of the IAEA standards were within 0.10-1.81 pMC of the values recommended by Rozanski et al. (1992), while the C-1 and C-4 samples showed~2% of modern carbon contamination, which resulted in a +1.0 pMC excess.
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