The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 113, Issue 10
Displaying 1-6 of 6 articles from this issue
Articles
  • Hajime Katayama, Takuya Itaki
    2007 Volume 113 Issue 10 Pages 505-518
    Published: 2007
    Released on J-STAGE: October 30, 2008
    JOURNAL FREE ACCESS
    Spatial distribution of sedimentation rates was estimated in the large area from the shelf to the Yamato Basin in the eastern Japan Sea off Akita, using depths of reflectors on 3.5 kHz sub-bottom profiler (SBP) records and estimated ages of the reflectors. Sedimentation rates remarkably change between the Tobishima-Oga Basins and the Mogami Trough, and between the Mogami Trough and the Sado Ridge areas. They are estimated more than 60 cm/ky on the shelf and the Tobishima-Oga Basins, 15-50 cm/ky on the Mogami Trough, 5-10 cm/ky on the Sado Ridge and the Yamato Basin on the average. These differences are thought to reflect the contribution of sediment transport through the bottom layer controlled by the topographic barriers as well as the distance from the sediment sources. Sedimentation rates also vary in each region according to the small-scale topography, folds or faults. It is expected that the spatial distribution of sedimentation rates is estimated more widely because muddy sediments are distributed and acoustic reflectors can be traced widely in the Japan Sea.
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  • Masaki Yuhara, Chie Uto
    2007 Volume 113 Issue 10 Pages 519-531
    Published: 2007
    Released on J-STAGE: October 30, 2008
    JOURNAL FREE ACCESS
    The Shikanoshima Granodiorite and the Shikanoshima basic rocks that have been emplaced during the Cretaceous age are exposed on the Shikanoshima Island, northern Kyushu. We found new field relationship between these two rocks. The Shikanoshima basic rocks are intruded by the Shikanoshima Granodiorite with an irregular boundary and have a chilled margin on its boundary. This occurrence suggests that the Shikanoshima basic rocks were not formed by hybridization of postdating the Shikanoshima Granodiorite magma, but generated by rapid cooling of mafic magma caused by intrusion of the coeval felsic magma. The chemical compositions of the Shikanoshima basic rocks resemble those of the Cretaceous high-Mg diorite in northern Kyushu. It means that the regional activity of high-Mg intermediate to mafic magmatism occurred in northern Kyushu during the Early Cretaceous.
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  • Kazuo Kiminami, Yukiko Sugiura, Mariko Miyamoto
    2007 Volume 113 Issue 10 Pages 532-541
    Published: 2007
    Released on J-STAGE: October 30, 2008
    JOURNAL FREE ACCESS
    The Sanbagawa Belt is a typical subduction-related high-P/T metamorphic terrane that is composed mainly of crystalline schists that originated as clastic and basic rocks, and it is exposed along the south side of the Median Tectonic Line in SW Japan. This belt has radiometric ages of Cretaceous time. The Northern Chichibu Belt, which occurs to the south of the Sanbagawa Belt, is a Jurassic accretionary complex, and mainly made up of chert, mudstone and sandstone. We have performed systematic measurements of vitrinite reflectance on coalified phytoclasts in pelitic rock from metamorphosed to weakly metamorphosed accretionary complexes, the Sanbagawa Metamorphic and the Northern Chichibu belts, eastern Shikoku. We have measured apparent maximum and minimum reflectances on polished particulate specimens. The measurements have revealed that the vitrinites are optically biaxial. We approximated the values of maximum (Rmax), intermediate (Rint) and minimum (Rmin) reflectances. The maximum reflectance value increases toward the lower structural level in the Northern Chichibu Belt, although, it shows no systematic variation in the Sanbagawa Belt. The average of maximum reflectance values from the Northern Chichibu Belt (5.21%) is higher than that from the Sanbagawa Belt (4.79%). The flattening for maximum and minimum reflectances ((Rmax-Rmin) /Rmax) increases generally to the north, that is, to the lower structural level. This would imply an increase of burial depth or depth of accretion.
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Short Articles
  • Haruyuki Ina, Takeshi Saito, Motohiro Kawase, Wei-Ming Wang
    2007 Volume 113 Issue 10 Pages 542-545
    Published: 2007
    Released on J-STAGE: October 30, 2008
    JOURNAL FREE ACCESS
    The genus Liquidambar (Hamamelidaceae) is one of a number of taxa associated with the Daijima-type floras of late Early to early Middle Miocene age, which also include many warm-temperate to subtropical elements. However, Liquidambar is very rare or absent in the Aniai-type floras of the early Early Miocene. We describe fossil leaves, fruits and pollen of this genus found in the Lower Miocene Nakamura Formation in the southern part of Gifu Prefecture. The flora of the Nakamura Formation (Hiyoshi flora) shows features common to those of the conformably underlying Hachiya Formation (earliest Early Miocene) but differs from the unconformably overlying Hiramaki Formation (late Early Miocene) in the same region. This fact combined with the occurrence of Liquidambar in the Hiyoshi flora, associated with warm-temperate elements such as Pseudolarix, Castanea, Celtis and Lindera, clearly supports one of two former views: the flora is of a southern type within the Aniai-type floras.
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