The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 116, Issue 10
Displaying 1-6 of 6 articles from this issue
Review
  • Tomoaki Morishita, Tatsuhiko Yamaguchi, Hisakazu Mashiba, Takahiro Kam ...
    2010 Volume 116 Issue 10 Pages 523-543
    Published: 2010
    Released on J-STAGE: February 01, 2011
    JOURNAL FREE ACCESS
    Ostracoda, a crustacean group, have valves composed of low-magnesium calcite. The magnesium/calcium (Mg/Ca) and strontium/calcium (Sr/Ca) ratios of fossil ostracodes have been used as indicators of the paleoenvironment. The Mg/Ca ratios and partition coefficients of marine ostracodes generally correlate with the water temperature of the ostracode habitat, and the Mg/Ca and Sr/Ca ratios of freshwater ostracodes have been used as a proxy for water chemistry. We reviewed previous studies on the Mg/Ca and Sr/Ca ratios of ostracode valves and obtained the following findings. (1) Diagenesis is likely to affect the Mg/Ca and Sr/Ca ratios of subfossil and fossil valves ; however, it is difficult to assess the degree of diagenetic alteration using methods developed in previous studies, such as that based on the visual preservation index (VPI) . Our statistic analysis, using published data, revealed no significant correlation between VPI and the Mg/Ca or Sr/Ca ratios of artificial dissolved specimens. (2) The partition coefficients of Mg/Ca and Sr/Ca ratios vary with the taxa, habitat, and phyletic group. Cypridoidea have statistically higher Mg/Ca and lower Sr/Ca partition coefficients than do Bairdioidea and Cytheroidea. Compared with marine taxa, freshwater taxa have more variable partition coefficients. The Mg/Ca partition coefficient of marine ostracodes is clearly dependent on the phyletic group at the family level. (3) The Mg/Ca and Sr/Ca partition coefficients are independent of sex but may depend on ontogenic stage. (4) The variable nature of partition coefficients in marine species indicates that Mg/Ca thermometers have an error of ±2–4°C. (5) Spatial variations in Mg concentration within an individual valve have an effect on the Mg/Ca ratio of the whole valve. The accuracy of Mg/Ca and Sr/Ca ratios as proxies for temperature and water chemistry would be improved if the effects of diagenesis and spatial heterogeneity are evaluated quantitatively and if the partition coefficients of various taxa are available.
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Articles
  • Hisatoshi Ito, Akihiro Tamura, Tomoaki Morishita, Shoji Arai
    2010 Volume 116 Issue 10 Pages 544-551
    Published: 2010
    Released on J-STAGE: February 01, 2011
    JOURNAL FREE ACCESS
    Zircon U-Pb dating by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and zircon fission-track (FT) dating were performed on granitic rocks from the Nojima Fault and its vicinity on northern Awaji Island, Hyogo Prefecture, Japan. The analyses yielded zircon FT ages of 52-78 Ma, which are in agreement with previously reported zircon FT ages. Zircon U-Pb (238U-206Pb) ages of 83-88 Ma were also determined. By combining these results with existing radiometric age data, we estimated the entire cooling history of the granitic basement on northern Awaji Island, spanning the period from the time of intrusion to the present. U-Pb ages were determined from zircons that were prepared for FT dating. Moreover, U-Pb ages were corrected based on analyses of Fish Canyon Tuff zircons as a standard material. As a result, a precise and accurate age of 16.3±0.1 Ma was obtained for the Buluk Member Tuff. This result demonstrates that the effect of common Pb is minimal even when analyzing zircons that are chemically etched, and that the Fish Canyon Tuff, which is a well-known age standard for FT dating, can be used as a standard for U-Pb dating. Many previous studies have reported on zircon FT analyses performed throughout the world. The method applied in the present study should pave the way for adding information on the crystallization age of the studied rocks by analyzing the zircons prepared previously for FT dating.
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  • Norihiko Kobayashi, Kazuhiko Kano, Takeshi Ohguchi
    2010 Volume 116 Issue 10 Pages 552-562
    Published: 2010
    Released on J-STAGE: February 01, 2011
    JOURNAL FREE ACCESS
    Rhyolitic to basaltic dikes crop out along the west coast of the Oga Peninsula, NE Japan, and have a coherent N39° E trend in the volcanic piles of the Late Eocene Monzen Formation and the Early Miocene Nomuragawa Formation. The dikes are similar in petrographic features and isotopic ages to the volcanic successions, and some dikes are connected to lava flows in the host successions. Map-scale normal faults within the Late Eocene and Early Miocene successions have similar trends to the dikes, suggesting NW-SE extensional stress at the time of dike emplacement. The crust in this area was extended by~9% due solely to dike intrusion, as deduced from the total thickness of the dikes. The total thickness of Late Eocene dikes is ca. 1.5 times greater than that of Early Miocene dikes. Crustal extension was initially achieved mainly by igneous intrusions, but later by waxing normal faulting that resulted in the opening of the Japan Sea.
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  • Koji Kameo, Ryota Shindo, Toshiaki Takayama
    2010 Volume 116 Issue 10 Pages 563-574
    Published: 2010
    Released on J-STAGE: February 01, 2011
    JOURNAL FREE ACCESS
    We examined calcareous nannofossil assemblages to determine the geologic age of the Kiyosumi Formation of the Awa Group, central Boso Peninsula, Japan. Prior to analyzing the Kiyosumi Formation, we examined size variations of Reticulofenestra specimens in lower Pliocene sediments in Hole 806B, Western Equatorial Pacific, revealing a number of distinct bioevents defined by changes in the sizes of Reticulofenestra specimens.
    As a result, we identified a nannofossil datum (top of Amaurolithus spp.) in the Kiyosumi Formation, and the boundary between Zones CN10 and CN11 was identified in the uppermost part of the formation. Moreover, the Kiyosumi Formation records a similar stratigraphic size variation of Reticulofenestra specimens and bioevents to those seen in Hole 806B. Geological ages estimated from the datum and bioevents appear to coincide with previous interpretations based on magnetic reversals recorded in the formation. The results suggest that changes in the size of Reticulofenestra are useful in terms of biostratigraphy.
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Short Article
  • Honami Sato, Tetsuji Onoue
    2010 Volume 116 Issue 10 Pages 575-578
    Published: 2010
    Released on J-STAGE: February 01, 2011
    JOURNAL FREE ACCESS
    The magnetic fraction of a lower Norian claystone layer (ca. 5 cm thick) in a radiolarian chert succession within the Mino Terrane, Inuyama area, Central Japan, contains a large number of small euhedral to subhedral crystals of oxidized Ni-rich spinels. The stratigraphic position of this concentration of Ni-rich spinels is clearly indicated by a sharp increase in magnetic susceptibility at the claystone layer. These spinels are distinguished from typical igneous spinels by their high Ni and Fe3+ contents, and show large variations in composition. The textures and chemical compositions of the spinel crystals are similar to those of Ni-rich spinels at the Cretaceous-Paleogene boundary. The discovery of Ni-rich spinels in a lower Norian claystone suggests an important sedimentary record of an extraterrestrial impact in the Late Triassic.
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