The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 118, Issue 12
Displaying 1-5 of 5 articles from this issue
Articles
  • Soichiro Kawabe, Takashi Okamoto
    2012 Volume 118 Issue 12 Pages 769-781
    Published: December 15, 2012
    Released on J-STAGE: April 26, 2013
    JOURNAL FREE ACCESS
    This study outlines an attempt to establish the Cretaceous megafossil biostratigraphy of the Migi-no-sawa area (Haboro Dome), a tributary of the Haboro River in northwestern Hokkaido. The Cretaceous sediments in this area consist of the following two formations of the Yezo Group: (1) the lowermost Upper Haborogawa Formation, an upward-coarsening cycle of sandy siltstone or laminated siltstone to coarse-grained sandstone, and (2) the uppermost Nagareya Formation, consisting of a lower coarse-grained section of silty sandstone and an upper finer-grained section of siltstone. These Cretaceous sediments contain abundant megafossils, especially the Haborogawa Formation and the lower part of the Nagareya Formation. The stratigraphic distribution of ammonoids and inoceramids indicates that the Haborowaga and Nagareya formations are Santonian and Campanian in age, respectively. The study area has some faunal differences, primarily in heteromorphic ammonite abundances, when compared with the Sakasa-gawa area; these differences suggest that sediments in the Sakasa-gawa area formed in deeper water environments than those within the study area. Hamitoid ammonites dominate the study area, indicating that these ammonites inhabited shallower areas than other heteromorphic ammonites.
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  • Teruyoshi Imaoka, Hiroaki Komuro, Erika Yamawaki, Yuji Kanaori, Yuri O ...
    2012 Volume 118 Issue 12 Pages 782-800
    Published: December 15, 2012
    Released on J-STAGE: April 26, 2013
    JOURNAL FREE ACCESS
    Geology, petrography, and gravity data have been used to determine the 3D geometry and emplacement mechanisms involved in the formation of the Late Cretaceous Hofu batholith. Grain size distributions and textures have been used to classify granites of the batholith into five lithofacies: coarse-, medium-, and fine-grained granites, porphyritic granite, and granodiorite. These granites are vertically stacked sheet-like bodies that were emplaced at a depth of < 2 km, as determined from their intrusive relationships with surrounding volcanic rocks.
    The batholith is associated with a negative gravity anomaly, particularly the northern part of the intrusion that is bounded by the Fushinogawa–Oharako and Sabagawa faults. The batholith has an areal extent of 33 × 48 km, with the base of the batholith estimated using a gravity data inversion to be at a depth of ca. −500 to −3500 m, suggesting that the batholith is tabular. The fact that the deep parts of the batholith occur at depths of −3000 m near both faults suggests that the granite magma may have been transported along near-vertical fault-controlled conduits.
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  • Ippei Kitano, Takeshi Ikeda
    2012 Volume 118 Issue 12 Pages 801-809
    Published: December 15, 2012
    Released on J-STAGE: April 26, 2013
    JOURNAL FREE ACCESS
    The Suo metamorphic rocks of the Asakura area of Fukuoka Prefecture underwent contact metamorphism during the intrusion of granitic magmas in the Cretaceous, forming a contact aureole. This aureole is divided from south to north with ascending metamorphic grade into chlorite, chlorite–biotite, biotite, and andalusite zones, based on pelitic mineral assemblages. The andalusite isograd is defined by a muscovite + quartz = andalusite + K-feldspar + water reaction, based on the coexistence of andalusite and K-feldspar. Garnet–biotite thermometry and garnet–biotite–plagioclase–quartz barometry indicate that the andalusite zone formed at temperatures of 560 ± 50°C and at pressures of 2.3 ± 0.90 kbar. The Suo metamorphic rocks were exhumed to ca. 9 km depth by around 95 Ma, assuming that heat was supplied from the Asakura granodiorite to the north, and reached the surface and were overlain by sediments of the Nogata Group sometime before 44 Ma.
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  • Kazutaka Amano, Masui Hamuro, Toshikazu Hamuro, Tokiyuki Sato, Rio Ogi ...
    2012 Volume 118 Issue 12 Pages 810-822
    Published: December 15, 2012
    Released on J-STAGE: April 26, 2013
    JOURNAL FREE ACCESS
    Molluscan fossils from the Zukawa Formation in Toyama Prefecture, central Honshu, were examined to investigate faunal changes across Datum A (2.75 Ma), the timing of the expansion of ice sheets in the Northern Hemisphere. Calcareous nannofossils indicate that the lowest part of the formation can be assigned to the Lower Pliocene NN15 Zone. Datum A was recognized in the upper part of the Zukawa Formation. A total of 125 species of mollusks were recovered from 11 localities in the Zukawa Formation. Among them, Tridonta-Limatula and Mizuhopecten-Chlamys associations are recognized. Tridonta-Limatula association might live in a lower sublittoral zone and occurring in horizons below Datum A. On the other hand, Mizuhopecten-Chlamys association inhabiting upper sublittoral zones was found from horizons above Datum A. In terms of paleobathymetry, the sediments below Datum A were deposited in a lower sublittoral zone, whereas the sediments above Datum A were deposited in an upper sublittoral zone. The assemblages immediately above Datum A contain higher ratio of number of individuals of cold-water species than the assemblages immediately beneath Datum A, despite the former might live in shallower water depths than the latter. In addition, and barring Megacrenalla columbiana, two species now living in Hokkaido and northwards are confined to horizons above Datum A. This suggests that the 2.75 Ma cooling event affected the molluscan fauna in the western part of Toyama Prefecture.
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