The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 106, Issue 3
Displaying 1-7 of 7 articles from this issue
  • Ryoji Wani, Hiromichi Hirano
    2000 Volume 106 Issue 3 Pages 171-188
    Published: March 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    This paper describes the biostratigraphy of the Middle and Upper Yezo Groups exposed in the Kotanbetsu area of northwestern Hokkaido. The Cretaceous deposits exposed in the upper reaches of the Kotanbetsu River consist of the following five formations in ascending order : (1) the Takimibashi Formation of the Middle Yezo Group (over 600 m thick), which is composed of conglomerate and siltstone ; (2) the Tenkaritoge Formation of the Middle Yezo Group (about 2, 200 m thick), which is composed of sandstone and silstone ; (3) the Shirochi Formation of the Middle Yezo Group (about 500 m thick), which is composed of alternating beds of sandstone and siltstone, and generally shows coarsening-upward sequence ; (4) the Middle Haborogawa Formation of the Upper Yezo Group (1, 080-2, 200 m thick), which is composed predominantly of siltstone with many interbeds of sandstone ; and (5) the Upper Haborogawa Formation of the Upper Yezo Group (about 790 m thick), which has at least three cycles of upward-coarsening sequence (U2-U4) ranging from sandy siltstone to sandstone.The Upper Cretaceous deposits yield abundant molluscan fossils in the Kotanbetsu area. As the results of the detailed examination of the assemblages and stratigraphic ranges of the contained species, the stage boundaries between the Cenomanian and the Campanian arc allocated to the upper part of M4 - lowermost part of M5 (Cenomanian/Turonian), the lower part of U1 (Turonian/Coniacian), the middle part of U1 (Coniacian/Santonian) and the upper part of U2 - lower part of U4 (Santonian/Campanian) respectively.
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  • Yasufumi Satoguchi, Mahito Watanabe, Takeshi Nakajo, Kyoko Kataoka
    2000 Volume 106 Issue 3 Pages 189-204
    Published: March 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Kd 38 pyroclastic key bed which is intercalated near the boundary of Pliocene-Pleistocene series is a widespread volcanic ash bed and important key bed for the stratigraphy of the Kazusa Group. Redefined Kd 38 pyroclastic key bed as the Kd 38 volcanic ash bed were reexamined for correlation in thirteen localities in the Boso Peninsula based on their lithofacies and petrographic properties. Thirteen localities are divided three areas, which are namely Sano-Kaisho, Yokoze-Oppara and Orikizawa-Koshikoma areas based on lithofacies of Kd 38 volcanic ash. Lithofacies and petrographic properties of Kd 38 volcanic ash are different at each area. However, Kd 38 volcanic ash bed in different localities can be correlated by correlation of the Kd 38.2 volcanic ash bed below the Kd 38 volcanic ash bed and relationship of lithofacies and petrographic properties. Their differences of lithofacies and petrographic properties of the Kd 38 volcanic ash bed at each localities reflects the difference in depositional processes of this volcanic ash after fallout.
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  • Masami Otake
    2000 Volume 106 Issue 3 Pages 205-222
    Published: March 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The South Kurikoma geothermal area has a caldera cluster within 25 by 15 km area. They are the Mukaimachi caldera, Akakura caldera, Nakayamadaira collapse basin, and are covered with Pliocene to Pleistocene felsic pyroclastic rocks including welded tuff. The caldera cluster coincides with a broad negative gravity anomaly outlined by steep marginal gradients and it surrounds the Lower to Middle Miocene basements, showing a regional tumescence.The Akakura caldera is about 10 km north-south by 9 km east-west of topographic wall and has a well-preserved resurgent dome, corresponding to a local positive gravity anomaly zone. This zone is characterized by radially outward dips of the caldera fill and a half-circular reverse fault. The Akakura caldera fill, >1, 400 m thick, has been divided into earlier "caldera-forming" and later "post-caldera" deposits. The caldera-forming deposits consist of two pyroclastic facies ; (1) felsic lapilli tuff facies interpreted as caldera-forming ash flows whose eruption caused the collapse of caldera. Analysis of accidental fragments within the felsic lapilli tuff facies indicated that the voluminous ash flows ejected from vents along structural boundary. K-Ar age data indicate that the caldera-forming eruption took place during Pliocene or Pleistocene. (2) tuff breccia facies, intecalated with the felsic lapilli tuff facies, is interpreted as debris-avalanche deposits derived from oversteepened caldera wall scarp during and after cruption of ash flows. The post-caldera deposits which ponded with the collapse basin are composed of breccia, volcanic sandstone, pumice tuff, fine tuff, and mudstone. These were mainly deposited by sediment gravity flows in subaqueous environments with dacitic volcanism.The Akakura caldera is similar to the Valles-type caldera because of pre-caldera tumescence, the pattern of gravity anomaly, caldera fill facies, vents along structural boundary inferred from lithic analysis, and resurgent domc.
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  • Tomoaki Morishita
    2000 Volume 106 Issue 3 Pages 223-233
    Published: March 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    A serpentinized cataclasite characterized by many angular harzburgite clasts of various size in serpentine matrix was found as an alluvial boulder in the Horoman peridotite complex, Hokkaido, northern Japan. A computer-aided image analysis of the serpentinized cataclasite indicates that clast shapes do not change with size variations, and that clast-size distributions obey a modified power-law distribution of Nagahama ct al. (1994). The serpentine matrix of the serpentinized cataclasite shows no distinct structures such as foliation and lineation, and has few fine-grained peridotite clasts (>10μm) and their pseudomorphs, whereas cataclasite matrix at an outcrop of the Horoman complex is almost all occupied by pseudomorphs of fine-grained peridotite clasts. The size distribution of mesoscopic clasts (visible to the naked eye) in the serpentinized cataclasite is similar to that of microscopic peridotite clasts (visible under microscope) in the serpentine matrix, but is different from that of microscopic fine grains bounded by serpentine mesh in clasts of the serpentinized cataclasite, These lines of evidence suggest that the serpentine matrix of the serpentinized cataclasite (more than 50% of the rock) had been once occupied by very fine-grained peridotite clasts (<10μm) formed by fragmentation of the peridotite during faulting before the serpentinization. Deficiency of smaller clasts on clast-size distributions is, therefore, interpreted as resulting from selective serpentinization of very fine-grained peridotite clasts, but not from normal serpentinization of the peridotite under static conditions. Fluid must have been locally supplied in the fault zone for a short period during or after the brittle deformation to cause the serpentinization.
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  • Olivier Fabbri
    2000 Volume 106 Issue 3 Pages 234-243
    Published: March 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The deformational structures (outcrop-scale normal faults and extensional joints) observed in the upper Osaki Formation of the middle Miocene (16-10 Ma) Kukinaga Group exposed on Tanegashima Island, northern Ryukyu arc, result from the superposition of two stages of extension : (1) a NE-SW (N43°E on average) extension, well documented by fault-slip data from eight localities and by extensional joints or non-striated normal faults from eight localities ; (2) a NW-SE (N134°E on average) extension, inferred from fault-slip data at one locality and by joints or non-striated normal faults from five localities. These two orthogonal extensions probably correspond to a unique stress field with, in most localities, σV1 and permutating σ2 and σ3 axes. Like elsewhere in the Ryukyu arc (Miyako Island region), the permutation between the intermediate principal axis and the least principal axis can be accounted for by arc-parallel stretching in response to the increasing curvature of the oceanward-migrating arc.The tensional stress field has been active since the time of deposition of the Osaki Formation (ca. 11-10 Ma). At present, it is expressed by NW-SE normal faulting (apparent N55°E extension) of Quaternary coastal terraces.
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  • Masayuki Sakakibara, Takeshi Hori
    2000 Volume 106 Issue 3 Pages 244-247
    Published: March 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The Omogiyama hornfels is one of the hornfelses sporadically occurring in the Sanbagawa metamorphic belt, central and western Shikoku, southwest Japan. They are originated from the Sanbagawa basic schists and distributed parallel to its schistosity. Large igneous bodies have not been found around the hornfels. The hornfelses are characterized by the assemblage of hornblende + actinolite + biotite + plagioclase + quartz. Based on the lack of large igneous rocks and the mode of occurrence of the Omogiyama hornfels, it seems that the hornfels was formed by high-temperature fluid flow along the schistosity of the Sanbagawa basic schists.
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  • Shin'ichi Sato
    2000 Volume 106 Issue 3 Pages V-VI
    Published: 2000
    Released on J-STAGE: November 26, 2010
    JOURNAL FREE ACCESS
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