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 121, Issue 1
Displaying 1-2 of 2 articles from this issue
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Articles
  • Redefinition of the Cretaceous terrigenous strata in the northeastern Toyama Prefecture based on U-Pb ages of zircon
    Makoto Takeuchi, Masahiro Ohkawa, Kentaro Kawahara, Satoru Tomita, Hid ...
    2015 Volume 121 Issue 1 Pages 1-17
    Published: January 15, 2015
    Released on J-STAGE: May 19, 2015
    DOIhttps://doi.org/10.5575/geosoc.2014.0046
    JOURNAL FREE ACCESS
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    Undated terrigenous strata distributed in northeastern Toyama Prefecture, central Japan, have been correlated with the Middle Jurassic to Lower Cretaceous Tetori Group, based solely on lithological criteria. Within the sequence in Toyama Prefecture, the Kurobishiyama Formation conformably overlies the Mizukamidani Formation, and the Uchiyama Formation unconformably overlies the Kurobishiyama Formation. The Shiritakayama and Uchiyama Formations, which are composed of different facies of the same age, are also conformably overlain by the Oyashirazu Formation.
    The U-Pb ages of zircons in these strata and in dacite dikes intruding the strata constrain the depositional ages of the formations. Based on the youngest U-Pb age of detrital zircons from the Mizukamidani Formation of ca. 123 Ma, and the U-Pb ages of zircons from a dacite dike intruding the Mizukamidani and Kurobishiyama formations of ca. 109 Ma, the Mizukamidani and Kurobishiyama formations are considered to be of Aptian age (126.3-113.0 Ma). The Mizukamidani and Kurobishiyama Formations are correlated with the Nochino and Bessandani Formations, which are the uppermost horizons of the Tetori Group, western Hokuriku District. The Shiritakayama and Uchiyama Formations are younger than the Tetori Group, as the youngest detrital zircons in these strata yield U-Pb ages of ca. 110 Ma.
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  • Depositional environment of a Late Jurassic reefal limestone and mode of occurrence of rudist individuals from Youra Peninsula, eastern Oita Prefecture (Kyushu Island, southwest Japan)
    Yoshihiro Kakizaki, Seishiro Furuyama, Akihiro Kano
    2015 Volume 121 Issue 1 Pages 19-34
    Published: January 15, 2015
    Released on J-STAGE: May 19, 2015
    DOIhttps://doi.org/10.5575/geosoc.2014.0047
    JOURNAL FREE ACCESS
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    Lithofacies and biofacies of a rudist-bearing reefal limestone from the uppermost Jurassic Tsui Formation on the Youra Peninsula, eastern Oita Prefecture, southwest Japan, were quantitatively examined. The 35-m-thick limestone section displays a vertical transition from a reefal fauna in the lower-middle part (dominated by hermatypic corals and stromatoporoids) to the upper part (characterized by abundant rudists, corals, and chaetetids). Lithological features and faunal compositions indicate an oligotrophic shallow-water depositional environment in a calm setting on a small carbonate mound. In the upper part of the section, requieniidine rudists (Epidiceras speciosum) occur densely in layers 0.5‒1.5 m thick, forming biostromes with corals. Reconstructed ecological features of Late Jurassic Epidiceras are similar to those of Triassic megalotondntid and Cretaceous requieniid rudists, but differ greatly from those of primitive uncoiled hippuritidine rudists (‘Valletia’ cf. auris) reported from another Torinosu-type limestone in the Shirokawa area, Shikoku Island, southwest Japan (Upper Jurassic-lowermost Cretaceous). These results suggest that rudists had already ecologically diverged in the late Late Jurassic, during an early stage of their evolutionary history.
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