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 114, Issue 2
Displaying 1-6 of 6 articles from this issue
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Articles
  • Formation of the Hiei Granite pluton and its geological implications for the Cretaceous felsic magmatism in southwest Japan
    Michio Kiji, Toshio Kutsukake, Satoshi Nakano, Sadahiro Nishimura, Kaz ...
    2008 Volume 114 Issue 2 Pages 53-69
    Published: February 15, 2008
    Released on J-STAGE: February 21, 2009
    DOIhttps://doi.org/10.5575/geosoc.114.53
    JOURNAL FREE ACCESS
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    The Hiei Granite pluton, 6 km (E-W) × 7 km (N-S) in extent, is one of the Cretaceous granite plutons around Lake Biwa, southwest Japan. It was emplaced within the Jurassic accretionary complex of the Tanba, and is situated at the western margin of the Cretaceous Biwa-ko Cauldron (60 × 40 km). The Hiei Granites can be petrographically divided into the following two rock-types: medium-grained equigranular granite and porphyritic granite. The latter occupies the core of the pluton. They are essentially biotite granites, and sometimes hornblende is contained. The Hiei Granites of about 100 Ma differ as regards the age, lithology and geochemistry from other Cretaceous granites of about 70 Ma around the south of Lake Biwa, and its designation to the ring bodies associated with the Cauldron is ruled out. In the eastern part of the pluton, granite porphyry and granodiorite porphyry dykes, several hundred meters in width, are intruded into the granite. These dykes would be the members of the ring-dykes encircling the Biwako Cauldron. Numerous thin dykes of lamprophyre, basalt, rhyolite and other rock-types, are also intruded into the granites.
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  • Depositional age of the Middle Pleistocene Atsumi Group in Atsumi Peninsula, central Japan, based on tephra correlation
    Rei Nakashima, Kiyohide Mizuno, Akira Furusawa
    2008 Volume 114 Issue 2 Pages 70-79
    Published: February 15, 2008
    Released on J-STAGE: February 21, 2009
    DOIhttps://doi.org/10.5575/geosoc.114.70
    JOURNAL FREE ACCESS
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    The Middle Pleistocene Atsumi Group, distributed in Atsumi Peninsula, central Japan is composed of the Futagawa, Tahara and Toyohashi Formations, in ascending order. Each formation shows a depositional sequence intensely affected by glacio-eustatic sea-level change. We investigated the Atsumi Group exposed around Ikobe Town, Toyohashi City, and revealed the depositional age of the Tahara and Toyohashi Formations based on the tephrostratigraphy.
    We analyzed two tephras, At-3up in the Akasawa Mud Member of the Tahara Formation and Ikb-1 in the Terasawa Mud Member of the Toyohashi Formation. At-3up contains volcanic glass shards (T type (pumice shape) and O type (no bubbles): refractive index (n) =1.505−1.508) and green hornblende (refractive index (n2)=1.674−1.683). Based on these refractive indices and the chemical composition of the glass shards by the EDX analysis, At-3up is considered to be correlative with the Takatsukayama tephra distributed around Rokko Mountains, Kinki District. Ikb-1 is composed exclusively of volcanic glass shards (H type (bubble-wall shape): refractive index (n) =1.501−1.503). The refractive index and chemical compostion by the EDX analysis of the glass shards of Ikb-1 are comparable with those of the Kakuto tephra (Kkt) which is a well-known widespread tephra in Japan. The Takatsukayama and Kakuto tephras are believed to have fallen at transgressive stages of MIS 12-11 and 10-9, respectively. Therefore, the depositional sequences of the Tahara and Toyohashi Formations are considered to have formed at MISs 11 and 9, respectively.
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  • Early Anisian (Triassic) foraminifers from the Hiraki Formation of the Maizuru Terrane in the Tatsuno-Aioi area, Hyogo Prefecture, Japan
    Fumio Kobayashi
    2008 Volume 114 Issue 2 Pages 80-87
    Published: February 15, 2008
    Released on J-STAGE: February 21, 2009
    DOIhttps://doi.org/10.5575/geosoc.114.80
    JOURNAL FREE ACCESS
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    The Maizuru Terrane in the Tatsuno-Aioi area, Hyogo Prefecture is composed of the Upper Permian Tatsuno Formation and the Middle Triassic Hiraki Formation, besides the Yakuno Complex. Both formations are redefined based on fossil evidence from the limestone. They are correlatable to the Middle and Upper formations of the Maizuru Group and the Waruishi Formation of the Yakuno Group in the type area, respectively. The Hiraki Formation consists mostly of mudstone and sandstone and contains limestone blocks less than 20 m in longer diameter in three horizons. Limestones consist of bioclastic oolid grainstone/packstone with many detrital quartz grains, and arrange irregularly regardless the general stratification of the formation. Mode of occurrence and lithologic characters of limestone blocks of the Hiraki Formation are closely similar to those of the Kaizawa Formation and Tanoura Formation. Eight species of foraminifers including Meandrospira dinarica and M. pusilla are identified from limestone blocks of the Hiraki Formation. Taxonomic diversity of the Hiraki foraminiferal fauna is exceedingly low in comparison with the Kaizawa and the Tanoura faunas. Along with low taxonomic diversity, absence of Pilammina densa and involutinid foraminifers, which are characteristic in Kaizawa, Tanura and other late Anisian (Pelsonian and Illyrian) faunas, suggests early Anisian age (Aegean) of the Hiraki fauna.
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