The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 114, Issue 8
Displaying 1-4 of 4 articles from this issue
Articles
  • Shunji Moriya, Kiyotaka Chinzei, Takeshi Nakajima, Tohru Danhara
    2008 Volume 114 Issue 8 Pages 389-404
    Published: August 15, 2008
    Released on J-STAGE: March 25, 2009
    JOURNAL FREE ACCESS
    Pliocene paleogeography of the western Shinjo Basin, Yamagata Prefecture was reconstructed by analyzing sedimentary facies, paleocurrent directions, and thicknesses of the depositional sequences. It revealed the regression history of the Shinjo Basin, which was originally formed as a part of a NW-deepening rift basin crossing the area now raised as the Dewa Hills. A rapid regression was recognized in the southwest of the basin in the Early Pliocene, ca. 5 Ma, then in the north, and finally in the middle part of the basin. The paleogeographic change is attributed to the inception of uplift of the Dewa Hills at 5 Ma. The uplift started in the south, then in the north prior to the central part of the Dewa Hills. This differential uplifting history has determined the location of the Mogami River which incises the central Dewa Hills.
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  • Masayuki Ikeda, Atsushi Yamaji
    2008 Volume 114 Issue 8 Pages 405-414
    Published: August 15, 2008
    Released on J-STAGE: March 25, 2009
    JOURNAL FREE ACCESS
    Tectonic inversion commenced in the NE Japan arc sometime after the late Middle Miocene. This paper concerns the controversial timing of the inversion by dealing with the geology of the Higashiyama Hill in the southern part of the inner arc. The overturned Plio-Pleistocene formations make up the western limb of the north-south trending Higashiyama Anticline, which has a horizontal hinge line and a gently dipping eastern limb. Based on detailed geological mapping, paleocurrent and fault-slip analyses, we show that the anticline began uplifting no earlier than the deposition of the middle part of the Ushigakubi Formation, i.e., <3 Ma. Specifically, it was found that the hinge line largely coincided with one of the main stream of the Pliocene paleocurrent system of this area. The stream branched off to the both sides of the present hinge line. This pattern was persistent over a few million years until∼3 Ma, during which sandstone layers of the Araya Formation were deposited in the Higashiyama area including the hinge zone. This long-standing pattern evidences flat paleo topography or even a shallow paleo trough along the anticline. Our geological mapping and fault-slip analysis suggest that the broader exposure of the Ushigaskubi and older formations to the east of the hinge line than the western limb is the surface expression of the north-south extensional deformation of the strata, resulted from northward gravitational spreading of the culmination of the anticlinorium that is bounded at its western margin by the Higashiyama Anticline.
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  • Norihito Kawamura, Toshinori Shinohara
    2008 Volume 114 Issue 8 Pages 415-425
    Published: August 15, 2008
    Released on J-STAGE: March 25, 2009
    JOURNAL FREE ACCESS
    The stratigraphy of the Quaternary formation in the subsurface of the Uwa Basin in the western part of Ehime Prefecture is clarified by the investigation of its lithology and intercalated wide-spread tephra with drilling core samples. Uwa Formation is stratigraphically divided into the lowermost, lower, middle and upper parts. In Uwa Formation, the volcanic ashes named Uwa-1, Uwa-20, Uwa-23, Uwa-24, Uwa-26 volcanic ash can be correlated with Shikito or Yellow I, Seiganji-Toga, Hiwaki, Kobayashi-Kasamori, and Kakuto tephras, respectively. The volcanic ash in the middle terrace deposit at the southern end of the Uwa basin, named Inou volcanic ash, can be correlated with Aso-2(TL) tephra. Formation of the lowermost part of Uwa Formation started before 1.3Ma and ceased around 1.0Ma, in the Early Pleistocene. The sedimentation of the lower part began in 1.0Ma and finished by 0.33Ma. The middle part formed after 0.33 Ma. The formation age of the upper part is around 0.026 Ma, the Late Pleistocene. The most of the middle terrace deposit emerged between 0.2 Ma and 0.1Ma, at the end of Middle Pleistocene.
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Note
  • Kiyokazu Oohashi, Kenta Kobayashi, Hiroki Mashima
    2008 Volume 114 Issue 8 Pages 426-431
    Published: August 15, 2008
    Released on J-STAGE: March 25, 2009
    JOURNAL FREE ACCESS
    To make thin sections or polished slubs of the brittle fault rocks like fault gouge and fault breccia without volume loss, is pretty difficult because of its fragility and softness. In general, the reinforcement by resin is performed in such specimens, but it's not so effective for the brittle fault rocks that contain swelling clay minerals. Additionally, detailed and through instructions about how to make thin sections of these rocks have not been published. Therefore, we introduce the grinding method which does not use the water for brittle fault rocks and altered rocks containing swelling clay minerals. With this method, we can make the thin sections, polished slubs, even sections specially prepared for SEM, TEM, EPMA analyses of noncohesive fault rocks containing swelling clay minerals without volume loss.
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