The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 110, Issue 7
Displaying 1-5 of 5 articles from this issue
Articles
  • Takeyuki Ueki, Takehiko Suzuki
    2004 Volume 110 Issue 7 Pages 389-394
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    The paleomagnetic directions of four pyroclastic flow deposits from the Hakkoda Caldera, Northeast Japan, were determined using both progressive thermal and alternating field demagnetization. The Hakkoda second-and first-stage pyroclastic flow deposits acquired remanent magnetization during the Brunhes Chron, while the Hakkoda zero-stage and Osegawa pyroclastic flow deposits acquired it during the Matuyama Chron. Our results support a recent tephrochronologic view that the Hakkoda first-stage pyroclastic flow deposit is the proximal source pyroclastic flow of the widespread, early Middle Pleistocene, Hakkoda-Kokumoto tephra bed.
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  • Akira Ono
    2004 Volume 110 Issue 7 Pages 395-402
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    Occurrences, lithofacies and K-Ar ages of clasts from Miocene conglomerates in the northeastern part of the Kanto Mountains are studied to reveal their provenance. Lithofacies and K-Ar ages of the most of clasts are similar to those of metamorphic and granitic rocks of the Ryoke Belt of central Japan. Provenance of the studied clasts are considered to be allochthonous geological bodies of the Ryoke Nappe which were previously located in the Kanto Mountains where Sanbagawa metamorphic rocks, the Atokura Nappe and Jurassic accretionary complex of the Chichibu Belt are presently exposed. The Ryoke Nappe is considered to be the uppermost structural unit in the Middle Miocene time. The Atokura Nappe near the Chichibu Belt was structurally covered by the Ryoke Nappe, and thus clasts derived from the Atokura Nappe were not found in most of Miocene conglomerates exposed in the Chichibu Belt.
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  • Atsushi Oyaizu, Kazuo Kiminami
    2004 Volume 110 Issue 7 Pages 403-416
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    The Shimanto Supergroup in the Inomisaki- Nakamura area, western Shikoku, is favorable for the examination of geologic development around the boundary between the Cretaceous and the Paleogene, because the Maastrichtian- Paleocene strata, which are not exposed in most of the Shimanto Belt, occur as good outcrops in this area. We have studied the petrographical features and bulk chemistry of sandstones in the area, in order to clarify the geologic developments of the Shimanto accretionary complex during the late Cretaceous and early Paleogene. These data, together with chronological and geologic data of the Sanbagawa Metamorphic Belt, led to the following conclusions.
    1. The Shimanto Supergroup in the study area can be classified into the following three intervals on the basis of petrographic and geochemical features of sandstones: Interval 1 (Santonian-Early Paleocene), Interval 2 (Late Paleocene) and Interval 3 (Early and Middle Eocene).
    2. The sandstones from Interval 1 were mainly derived from felsic volcanic rocks in the Inner Belt of Southwest Japan.
    3. The sandstones from Interval 2 are characterized by lithic fragments of pelitic schist, siliceous schist, and weakly- to non-metamorphosed chert. An emergence and erosion of the Sanbagawa Metamorphic Belt took place in this interval along with the Mikabu Greenstones and Northern Chichibu Belt.
    4. Unroofing and resulting emergence of many granitic rocks in the provenance of the Inner Belt have occurred in the depositional age of the Inomisaki Formation of Interval 3.
    5. Voluminous and rapid underplating induced by a large amount of supply of clastic sediments to the trench area during the Santonian-Campanian in Shikoku might have resulted in an exhumation of the Sanbagawa Belt.
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  • Itoko Tamura, Haruo Yamazaki
    2004 Volume 110 Issue 7 Pages 417-436
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    The Plio-Pleistocene Hokuriku Group includes the Otogawa and the Omma Formations distributed along the Japan Sea in Ishikawa and Toyama Prefectures, central Japan. In order to characterize the tephra layers within the Hokuriku Group, authors investigated the stratigraphy, thickness, color, shape of volcanic glass, mineral composition, refractive indices of volcanic glass and orthopyroxene, and the major and trace element composition of volcanic glass. Thirty tephra layers were detected from the Hokuriku Group in the study area. Some tephra layers are traceable over several kilometers, and are important as key beds. They were named as the Tsuribe 1, Sunakodani1, Sunakodani2, Teramachi, MT2·Chk, O1, O2, O3 tephra layers in an ascending order. These tephra layers in the Hokuriku Group are correlated with the tephra layers of the other Plio-Pleistocene Groups in central Japan by lithologic and petrographic properties. Ten wide-spread tephra layers : Ya-4, Ya-5 and Sakai, Kosugaya :4 Ma, Ohta-Znp : 3.7Ma, T2, Tng: 2.2-2.3 Ma, Bando2 : 2.1 Ma, Eb-Fukuda : 1.75 Ma, Om-SK110 : 1.65 Ma, KMT : 0.6 Ma were found from the Hokuriku Group. Therefore, the age of the Takakubo Formation is estimated to be 3 Ma at the period of the Gauss chron, and the age of lower part of the Omma Formation is about 2.3 Ma. The unconformity between the Takakubo and the Omma Formations, in which it was previously thought that a long-term period of non-sedimentation existed, does not indicate a long-term interval of time. The age of the Kurehayama gravel bed at the eastern edge of Toyama Plain is correlated with the lowest member of the Omma Formation which interbeds Taniguchi tephra (2.2-2.3Ma). Therefore, it is clear that the Hida Mountain began to be upheaved before 2.2-2.3 Ma.
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