The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 107, Issue 6
Displaying 1-5 of 5 articles from this issue
  • Reishi Takashima, Takeyoshi Yoshida, Hiroshi Nishi
    2001 Volume 107 Issue 6 Pages 359-378
    Published: June 15, 2001
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The Upper Mesozoic Sorachi and Yezo Groups are widely distributed in the Yubari-Ashibetsu region, located in the axial zone of central Hokkaido, the northernmost island of Japan. The Sorachi Group in this area consists mainly of volcanic rocks and volcaniclastic turbiditic rocks, and is divided into the Gokurakudaira, Ashibetsugawa, Nakatengu and Shirikishimanaigawa Formations in ascending order. The Gokurakudaira Formation constitutes mostly picrites, accompanied by intercalation of dacitic-andesitic volcaniclastic rocks and arkosic terrigenous turbiditic sandstones derived from the East Asian Continent. The Ashibetsugawa Formation is composed of medium-bedded red mudstone with several andesitic volcanic sandstone beds. The other two formations are characterized by andesitic volcaniclastic rocks. The rocks of the Nakatengu Formation are thick breccia beds and very thin-bedded mudstones, while those of the Shirikishimanaigawa Formation are thick- to medium- bedded sandstones, mudstones and tuff. The Yezo Group conformably overlying the Sorachi Group comprises alternating beds of terrigenous turbiditic sandstone and hemipelagic mudstone, presumably resulted from the erosion of the uplifted Jurassic accretionary prisms (Oshima Belt) located in southwestern Hokkaido. The two stratigraphic marker units of olistostrome unit containing "Orbitolina Limestone" blocks (the Upper Aptian Kirigishiyama Olisto- strome Member) and felsic tuffaceous unit (the Upper Albian Maruyama Formation), are recognized in the Yezo Group. Based on the biostrati-graphic study using radiolarians and planktonic foraminifers, the Sorachi Group and the lower part of the Yezo Group range in age from Tithonian to early Albian. Many previous works have been considered that the Sorachi Group consisted of basaltic rocks and cherts. However, we found intercalation of thick sedimentary sequence composed of arkosic sandstones within the basaltic rocks in the investigated area. This suggest that the basaltic eruptions, in part, occurred in an adjacent area close to the Asian continent. Moreover, some volcaniclastic rocks of the Sorachi Group also contain many ooids including angular andesite fragments. The occurrence of these rocks also indicates shallow marine environment on the volcanic mound.
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  • Masayuki Torii, Motoyoshi Oda
    2001 Volume 107 Issue 6 Pages 379-391
    Published: June 15, 2001
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The Izaku Pyroclastic-Flow Deposit in Kagoshima Prefecture was found to be correlated with the Hisamine Tuff(HST-4 tuff bed) in the upper part of the Miyazaki Group, on the basis of their petrographic features and paleomagnetism. The HST-4 tuff bed is a co-ignimbrite ash-fall deposit along with the Izaku pyroclastic flow eruption because it mainly consists of bubble wall type glass shards. Therefore, we propose to call these deposits the Izaku-Hisamine Tephra. Magneto- and planktic foraminiferal bio-stratigraphy of the upper part of the Miyazaki Group revealed that the HST-4 tuff bed occurs at the lowest part of a reversed MMR-B magnetic zone corresponding to Chron 2An.2 r of geomagnetic polarity time scale. Therefore, the eruption age of the Izaku Pyroclastic Flow is estimated to be 3.3 Ma.
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  • Yoshihiro Sawada, Takao Tokuoka, Seiki Yamauchi, Yoshiaki Sampei, Kiyo ...
    2001 Volume 107 Issue 6 Pages 392-405
    Published: June 15, 2001
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The Shinji Rift System forms a half graben more than 160 km long at least reaching Tottori offshore. A hitherto unknown Pleistocene andesitic volcano has been discovered recently in Miho Bay, along the axial zone of the Shinji Rift System. The volcanic edifice has a dome-like shape approximately 500 m in diameter, and the summit lies 26 m below the present sea level. K-Ar whole rock and zircon fission track ages are 0.9±0.3 Ma and 0.16±0.04 Ma, respectively.The andesite is extremely peraluminous with Al2O3=22.8-18.6 wt% and SiO2=60-61 wt%, and up to 13 wt% normative corundum. It has a diverse mineral assemblage. Phenocrysts are Cr-bronzite-hypersthene, Cr-diopside-augite and plagioclase with a wide compositional range. Orthopyroxene, clinopyroxene, pigeonite, hornblende, plagioclase, quartz, alkali feldspar, Cr-spinel, magnetite, ilmenite, hemoilmenite, zircon, apatite, silica minerals, an unknown Al-Mg material and glass occur as groundmass constituents and inclusions in phenocrysts. There are two kinds of opacite phenocrysts consisting of F-phlogopite (up to 6.6 wt% F), hypersthene, augite, plagioclase, quartz, spinel, Fe-Ti oxides, cummingtonite and actinolite.We infer that the volcano originally formed on land in the glacial stage, and was then eroded and drowned by transgression. This new edifice and several other Pliocene and Pleistocene eruptive centers are aligned along the axial zone of the Shinji Rift System, indicating presence of a deep fracture zone originally formed by rifting related to opening of the Japan Sea.
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  • Jun Takiguchi, Hisao Tanaka
    2001 Volume 107 Issue 6 Pages 406-410
    Published: June 15, 2001
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    We found a mylonite zone up to 1300 m in width in a granodioritic pluton in Nanyo City, Yamagata Prefecture, and named it the Ringou Mylonite Zone. Geological and petrographical features of the mylonite zone, situated between the Tanagura Tectonic Line in the south and the Oisawa Tectonic Zone in the north, are described. The mylonite zone shows sinistral shear sense, which is in harmony with that of the Tanagura Tectonic Line in the southern Abukuma Mountains. A K-Ar hornblende age of granodiorite (114 Ma) distributed about 5 km east of the mylonite zone is similar to the isotopic ages obtained from Abukuma Granites. On the other hand, granodiorites on the western side of the mylonite zone are petrographically identical to some of the granitic rocks in the Asahi Mountains. From these facts, we infer that the Ringou Mylonite Zone corresponds to the northern extension of the Tanagura Tectonic Line.
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  • Tsuyoshi Sakurai, Akira Takasu
    2001 Volume 107 Issue 6 Pages XIII-XIV
    Published: 2001
    Released on J-STAGE: November 26, 2010
    JOURNAL FREE ACCESS
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