The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 120, Issue 2
Displaying 1-4 of 4 articles from this issue
Article
  • Nobutaka Tsuchiya, Tomoyo Takeda, Kenichiro Tani, Tatsuro Adachi, Nobu ...
    2014 Volume 120 Issue 2 Pages 37-51
    Published: February 15, 2014
    Released on J-STAGE: June 24, 2014
    JOURNAL FREE ACCESS
    Supplementary material
    The ‘Wariyama Sheared Granodiorite’ occurs along the east side of the Futaba Fault with the Wariyama uplift zone, Miyagi, Japan. Rocks from the ‘Wariyama Sheared Granodiorite’ can be geochemically classified as typical adakite, and further subdivided into medium-grained and K2O- and Sr-poorer rocks, and coarse-grained and K2O- and Sr-richer rocks. Zircons from the former yield U–Pb ages of 308 ± 3 and 302 ± 4 Ma, whereas zircons from the latter have ages of 118 ± 2 and 117 ± 1 Ma. On the basis of these results, the ‘Wariyama Sheared Granodiorite’ can be redefined. The late Carboniferous granite is renamed the Wariyama Granite as in the past, but we use the term ‘granite’ in a broad sense as describing the rock type. The Early Cretaceous granite is newly defined as the Takase Granite. The Takase Granite crops out on the eastern side of the central region of the Wariyama Granite with faulted contact. The Takase Granite is divided into the SiO2-poor Takase Granite A and the SiO2-rich Takase Granite B. The U–Pb zircon age of 300 Ma obtained here corresponds to an interval of five major granitic events, and the timing of the onset of Permian granitic magmatism (280 Ma) may actually be ca. 300 Ma. Typical adakites are considered to be the product of slab melting, which occurred in response to unusual tectonic conditions, such as subduction of young lithosphere and/or an active ridge at ca. 300 Ma.
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  • Chie Kusu, Atsushi Nozaki, Makoto Okada, Hideki Wada, Ryuichi Majima
    2014 Volume 120 Issue 2 Pages 53-70
    Published: February 15, 2014
    Released on J-STAGE: June 24, 2014
    JOURNAL FREE ACCESS
    This study presents the results of detailed lithological, paleomagnetic, and rock-magnetic analysis of a 105.0 m core (Core I) recovered from the northern Miura Peninsula, on the Pacific side of central Japan. The core covers the lowermost part of the Ofuna Formation (0.8-6.0 m core depth; massive mudstone) and the upper part of the Nojima Formation (6.0-105.0 m; mudstone to sandstone) of the Lower Pleistocene forearc basin fill of the middle Kazusa Group. The section of the Nojima Formation in Core I is divided from bottom to top into three segments on the basis of lithological, sedimentological, and magnetic characteristics. Horizon A (73.2-105.0 m) consists of massive mudstone and sandy mudstone. Horizon B (38.5-73.2 m) consists of a coarsening-upward sequence from massive mudstone to conglomeratic sandstone. Horizon C (6.0-38.5 m) consists of a fining-upward sequence from medium-grained sandstone to alternating beds of sandy mudstone and massive mudstone. Horizon C contains the YH02 and Sg3 tuff beds that correlate with the Kd39 (1.76 Ma) and Kd38 (1.75 Ma) tuffs, respectively, intercalated in the Kazusa Group on the Boso Peninsula. We interpret Horizon B as slump deposits for the following reasons: the beds have greatly variable dips (16-62°); the anisotropy of magnetic susceptibility and paleomagnetic inclination data indicate that the sediments suffered deformation; and the base of this horizon has a magnetic discontinuity interpreted as a slump scar. The upper boundary of the Olduvai subchronozone was identified between 84.64 and 86.77 m within Horizon A using paleomagnetic inclination data. The duration of this polarity reversal is estimated to be 2800 or 3200 years, based on two alternative age models that focus on fluctuations in the oxygen isotope ratios of the planktonic foraminifer Globorotalia inflata.
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  • Takashi Goto, Shinji Nasuno, Toshiaki Irizuki, Hiroto Ohira, Hiroki Ha ...
    2014 Volume 120 Issue 2 Pages 71-86
    Published: February 15, 2014
    Released on J-STAGE: June 24, 2014
    JOURNAL FREE ACCESS
    The Pliocene Mita Formation outcrops in the Yatsuo-machi area of Toyama City, Toyama Prefecture, Japan. This formation containsthree key tuff beds (OT3, MT1, and MT2); two of these beds, OT3 and MT2, previously yielded ages of 5.2 ± 0.5 Ma and 2.2-2.3 Ma, respectively. This study focuses on the geology of the area surrounding the Akae-gawa River in the Hirabayashi region, and provides new fission track ages for the MT1 tuff exposed at two study sites in this area. These new data are combined with new fossil ostracod and planktonic foraminiferal faunal data to reconstruct the paleoenvironment during deposition of the MT1 tuff. This study presents new ages of 3.5 ± 0.2 and 3.4 ± 0.2 Ma for the MT1 tuff at the two sites that form the focus of this study. Integrating these data with the results of previous studies indicates that the first warm-water molluscs within the Mita Formation lived between 3.66 and ca. 3.4 Ma. Microfossil assemblage data from the study area also indicate that the paleoenvironment in the study site changed from a cool-water bay or shallow sea to an open temperate shallow sea setting, with warm-water ostracod species present in the uppermost horizon of the formation.
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