The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 111, Issue 4
Displaying 1-7 of 7 articles from this issue
Review
  • Kazuhisa Goto
    2005 Volume 111 Issue 4 Pages 193-205
    Published: 2005
    Released on J-STAGE: August 01, 2005
    JOURNAL FREE ACCESS
    The Chicxulub crater in the Yucatán Peninsula is now well accepted as an impact crater formed at the Cretaceous/Tertiary (K/T) boundary. Thick K/T boundary breccia and sandstone beds around the impact site are interpreted as K/T boundary tsunami and/or gravity flow deposits. It has been generally believed that the Chicxulub impact was the major cause of the K/T boundary mass extinction. However, G. Keller and others have suspected the synchronicity of the Chicxulub impact and the K/T boundary, and presence of the K/T boundary tsunami and/or gravity flow deposits. In particular, they concluded that the Chicxulub impact predated the K/T boundary by 300 ka based on studies of the K/T boundary deposits in northeastern Mexico, Haiti, and inside the crater, and that the Chicxulub impact had no relation with the K/T boundary mass extinction. The debate on this issue was heated, especially on the web site of Geological Society of London by G. Keller and J. Smit in 2003, and it was called "the Great Chicxulub Debate". In this paper, I review all the arguments on this issue and discuss the timing of the Chicxulub impact. I further take up the recent debate on the sedimentary process of the uppermost part of the impactite in ICDP (International Continental Scientific Drilling Program) Chicxulub drill core (YAX-1 core), which is considered as a conclusive evidence that the Chicxulub impact predates the K/T boundary by G. Keller and others, although other many researchers interpreted that this part was formed by reworking due to the invasion of ocean water into the crater immediately after the impact.
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Articles
  • Minoru Utada, Tomihiro Sawada
    2005 Volume 111 Issue 4 Pages 206-216
    Published: 2005
    Released on J-STAGE: August 01, 2005
    JOURNAL FREE ACCESS
    The Early Cretaceous Kanmon Group is distributed in western coastal areas of Honshu and northern Kyushu, facing the Sea of Japan. It consists of non-marine sediments including clastics, volcanics, and volcaniclastics. It is frequently intruded by intrusive bodies.
    Alteration proccesses in the Kanmon Group are classified into regional, hornfelsic, and hydrothermal, and weathering alteration. Among them, regional alteration is characterized by quartz-albitized plagioclase-chlorite-(illite)-(Fe oxide or sulfide mineral) assemblage. This mineral assemblage occurs in all kinds of rocks and all horizons through the Kanmon Group. However, there is no evidence for successive formation of authigenic minerals during diagenesis. Hornfelsic alteration is superimposed on regional alteration with the addition of amphibole, biotite or cordierite. It may have been formed under higher temperatures than regional alteration. Hydrothermal alteration occurs locally, and clay minerals, carbonates, zeolites, and others were formed along probable fissures, fractures and faults that may have been formed at the latest stage. Weathering alteration is relatively rare, but often overlaps other types of alteration, especially hydrothermal alteration.
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  • Hidetoshi Hara, Ken-ichiro Hisada
    2005 Volume 111 Issue 4 Pages 217-223
    Published: 2005
    Released on J-STAGE: August 01, 2005
    JOURNAL FREE ACCESS
    K-Ar ages of illite-rich fraction have been determined for phyllite from the Southern Chichibu and Shimanto accretionary complexes in the Mitsumine district of the Kanto Mountains, central Japan. The K-Ar ages indicate 71. 74 and 76 Ma from the Nippara Formation of the Southern Chichibu accretionary complex. In the Shimanto accretionary complex, the K-Ar ages present 65 and 76 Ma from the Futase Unit, 49Ma from the Kawamata Unit. In addition, phyllite deformed by pressure solution cleavage with shear is collected from Futase Unit, which indicates the K-Ar age of 50 Ma. It is conclueded that the Southern Chichibu and Shimanto accretionary complexes underwent the metamorphism simultaneously during 65-76 Ma in the latest Cretaceous time.
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  • Kei Odawara, Yasufumi Iryu, Hiroki Matsuda, Tokiyuki Sato, Shun Chiyon ...
    2005 Volume 111 Issue 4 Pages 224-233
    Published: 2005
    Released on J-STAGE: August 01, 2005
    JOURNAL FREE ACCESS
    The Chinen Formation in southern Okinawa-jima, southwestern Japan, is composed of calcareous sandstone and sandy limestone correlative with the transitional lithofacies between massive siltstone of the Upper Miocene to Pliocene Shimajiri Group and detrital to reefal limestone of the Pleistocene Ryukyu Group. The "Reddish Limestone" is the lowest lithostratigraphic unit in the Ryukyu Group in southern Okinawa-jima and consists mainly of highly altered detrital and rhodolith limestones. The calcareous nannofossil biostratigraphy of the Chinen Formation and the "Reddish Limestone" in two boring cores from Komesu and Giza areas were studied. Abundant specimens of the calcareous nannofossil, Gephyrocapsa spp. (small), whose base occurred at 2.09 Ma, were recognized in the uppermost Shimajiri Group to large part of the Chinen Formations in the two cores. Although the latest Pliocene datum plane, the last occurrence of Discoaster brouweri. (1.97 Ma), was recovered from the lower Chinen Formation in Komesu core, it was not detected in Giza core due to relatively rare and sporadic occurrence of this genus. It was likely that two early Pleistocene datum planes, the first occurrence of Gephyrocapsa caribbeanica (1.73 Ma) and the first occurrence of G. oceanica (1.65 Ma), were concentrated at the base the "Reddish Limestone". The early Pleistocene datum plane, the first occurrence of Gephyrocapsa spp. (large) (1.45 Ma), is situated in the "Reddish Limestone" in Giza core and possibly at the top of the Limestone in Komesu core. These indicate that the Chinen Formation is correlated with the latest Pliocene (>1.73 Ma) and that the "Reddish Limestone" formed in Early Pleistocene time between 1.65 Ma and 1.21 Ma.
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  • Katsuhiro Fujisaki, Makoto Shinohara, Kenji Nanba, Hisashi Nirei
    2005 Volume 111 Issue 4 Pages 234-248
    Published: 2005
    Released on J-STAGE: August 01, 2005
    JOURNAL FREE ACCESS
    To reclaim the lakes to their pristine conditions, it is essential to examine the environment where the lakes occur and rejuvenate natural water circulation. As an attempt to achieve this target, a program is planned for the western shore area of Kitaura utilizing groundwater with the knowledge of its flow system. In the program, a water amenity park/lakeside bathing resort is planned with the flowing wells as source of water. To measure aquifer coefficients, flow tests of wells, which have been done rarely in Japan, were carried out in the area. The flow tests using flowing wells do not need lifting pumps in contrast to the tests of ordinary wells, and because of that aquifer coefficients can be obtained by just measuring the amount of discharge. A method of calculation, for how many mutually interfering flowing wells are to be required to secure necessary amount of water for the water amenity park/lakeside bathing resort, is presented based on the result. The amount of total discharge for the group of flowing wells is examined using the calculated result.
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