The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 107, Issue 4
Displaying 1-6 of 6 articles from this issue
  • Toshiyuki Ikawa, Teruyoshi Imaoka
    2001 Volume 107 Issue 4 Pages 243-257
    Published: April 15, 2001
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The Cretaceous Abu Group and related intrusive rocks are widely distributed in the central part of Yamaguchi Prefecture, and form the Chomonkyo igneous complex. A restudy of the volcanostratigraphy of this complex revealed that it is divided into the following three formations and nine members ; Ato Formation (Kane Welded Tuff Member and Tokusagamine Sandstone-Mudstone Member), Ikumo Formation (Yunose Welded Tuff Member and Usutaniyama Rhyolitic Lava), and Sasanami Formation (Somakidani Welded Tuff Member, Mitsuke Siltstone Member, Kitakoma Welded Tuff Member, Kobukidao Felsic Intrusive Rocks, and Komatsugatani Dacitic Lava) in ascending order. A new cauldron, named Sasanami cauldron composed of the latter five members, was discovered. The cauldron has an elliptical outline, 17 km in diameter from east to west and 14 km from north to south. The Somakidani Welded Tuff Member and Kitakoma Welded Tuff Member are distinct from the Kane Welded Tuff Member and Yunose Welded Tuff Member with respect to phenocryst mode, and particularly the Nb content of the bulk rock chemistry.The Sasanami cauldron was formed by two subsidences, synchronous with the eruptions of the Somakidani Welded Tuff Member and Kitakoma Welded Tuff Member. The collapsed block was destroyed over a distance of several kilometers and bounded by a discontinuous ring fracture, and these features approximately correspond to a piecemeal collapse. This collapsed block is intruded by a ring dike (Kobukidao Felsic Intrusive Rocks) and by a dacite lava dome (Komatsugatani Dacitic Lava) in the central part of the cauldron. The cauldron has a basin structure with dip of up to 30°, and the caldera collapse appears asymmetric with a maximum subsidence of about 500 meters along the ring fracture judging from the thickness of the intra-caldera tuffs.
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  • Masanori Shimamoto, Satoshi Ota, Hiroki Hayashi, Osamu Sasaki, Tsunema ...
    2001 Volume 107 Issue 4 Pages 258-268_1
    Published: April 15, 2001
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The Hatatate Formation overlying conformably the Moniwa Formation is distributed in the southwestern part of Sendai City, Miyagi Prefecture and its foraminiferal biostratigraphy is presented. The Hatatate Formation is composed mainly of well-sorted fine-grained sandstone intercalating some fine-grained tuff, pumice tuff and scoria beds. This formation exhibits a coarsening-upward sequence, and grades upward into biotite-bearing medium-grained sandstone and coarse- to very coarse-grained sandstone which contains fragments of lapilli and pumice.Planktonic foraminifers were successively detected from the Hatatate Formation. The lowermost to lower part of the formation yields planktonic foraminifers, including Orbulina suturalis, O. universa, Globorotalia peripheroronda, and Grt. ichinosekiensis, and is assigned to the N. 10 planktonic foraminiferal zone of Blow (1969). The middle to upper part of the formation yields abundant foraminifers, including Grt. rikuchuensis, Grt. iwaiensis, and Neogloboquadrina mayeri, and is correlative with the N. 11 to N. 14 planktonic foraminiferal zones of Blow (1969), possibly not ranging up to the N. 15 Zone.
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  • Takanori Anegawa, Yuichiro Miyata
    2001 Volume 107 Issue 4 Pages 270-282
    Published: April 15, 2001
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    A series of fluid injection structures (FIS) occur in the turbidite sequence of the Tertiary Nichinan Group in the Izakibana area, Nichinan City, Miyazaki Prefecture. They are funnel structures, vent structures (newly defined) and small sandstone dikes. Reverse cone-type funnel structures are formed stepwise in a single sandstone bed located from Middle Unit to lower Upper Unit. Vent structures lager than funnel structures penetrate through multiple beds of mainly Middle Unit. Mudstone beds in all sedimentary units contain small dikes tapering upwards or downwards. Tank experiments were carried out on multiple layered system to investigate the deformation processes induced by the upward migration of the fluid. As a result, a lens-shaped water-filled void was formed temporarily beneath a clay layer, which expands with increasing fluid percolating upward through the underlying sand layer. The failure process of the overlying layers varies according to the migration process of the water-filled void. The most distinctive characteristic is a stepwise migration of void with local fluidization within single sand layer. The cracks in clay layer are filled with sand avalanching due to strength reduction induced by incipient fluidization. The fluidized part in multiple layers is vertically expanded with increasing apparent flow velocity.The formation of the FIS at the Izakibana area can be explained as follows ; (1) concentration of fluid with higher flow velocity through mud layer formed vent and some funnel structures, (2) flow velocity decreased due to lower pressure in divided voids, (3) stepwise funnel structures and small scale sand dikes were formed by upward migration of smaller voids, (4) cracks in a split mud layer are filled by avalanched sands into void forming sand dike tapering downward. These FIS are suggestive of genuine of fluid-filled void formed in a multiple layered system.
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  • Takeshi Makinouchi, Shinobu Mori, Tohru Danhara, Keiji Takemura, Geolo ...
    2001 Volume 107 Issue 4 Pages 283-295
    Published: April 15, 2001
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Several widespread tephras have been identified in core samples recovered from the coastal area of Nohbi Plain. They are K-Ah (Kikai-Ah tephra) and U-Oki (Ulreung-Oki tephra) in the Holocene Nan'yo Formation ; BT8 (a tephra layer intercalated in bottom sediments of Lake Biwa) and AT (Aira-Tn tephra) in the latest Pleistocene Nohbi Formation ; and BT34 and BT36 (tephra layers intercalated in bottom sediments of Lake Biwa) in the lower marine clay member of late Pleistocene Atsuta Formation. The following conclusions were reached, based on ages and horizons of the identified widespread tephras.(1) Many researchers have agreed that the basal gravel of Alluvium (BG) coincides with the Last Glacial Maximum (LGM, about 18 ka), middle stage of the Marine Oxygen Isotope Stage 2 (MIS-2). However, it was found that BG in the Nohbi Plain (First Gravel Formation) does not coincide with LGM but precedes it, because BG in the Nohbi Plain underlies the AT (about 25 ka) horizon of the Nohbi Formation. The age of BG is 35± to 30± ka (late MIS-3).(2) BG in the coast area of Nohbi Plain formed when fluvial gravel advanced downstream during the sea-level falling stage before LGM.(3) The deposition of BG is divided into two stages in central and western Japan. The earlier one precedes LGM, and the later one is concurrent with LGM. It is inferred that the distance from the coastline at LGM determined the formational stage of BG ; BG was formed later in places where the distance from the coastline was shorter.(4) Based on the widespread tephra layers intercalated in the lower marine clay member of the Atsuta Formation, it can be concluded that the deposition of the lower marine clay member started and ceased at approximately 130 ka and 110 ka, respectively.
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  • Akira Miyake
    2001 Volume 107 Issue 4 Pages 296-298
    Published: April 15, 2001
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
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  • Akira Miyake
    2001 Volume 107 Issue 4 Pages VII-VIII
    Published: 2001
    Released on J-STAGE: November 26, 2010
    JOURNAL FREE ACCESS
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