The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
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Volume 115, Issue 2
Displaying 1-5 of 5 articles from this issue
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Articles
  • Boundary between the Kazusa and Shimosa groups beneath the central Kanto Plain: Recognition of MIS 12 in the GS-KS-1 core (Koshigaya, Saitama Prefecture, central Japan)
    Tsutomu Nakazawa, Hiroomi Nakazato, Hideaki Oshima, Seiji Horiuchi
    2008 Volume 115 Issue 2 Pages 49-63
    Published: February 15, 2009
    Released on J-STAGE: August 07, 2009
    DOIhttps://doi.org/10.5575/geosoc.115.49
    JOURNAL FREE ACCESS
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    We examined a sediment core recovered from Koshigaya, central Kanto Plain, central Japan, with the aim of understanding the stratigraphic location of MIS 12, which corresponds to the boundary between the Kazusa and Shimosa groups of the Boso Peninsula. Detailed analyses of sedimentary facies, pollen and diatom assemblages, and tephrochronology indicate that MIS 12 could be assigned to a bay ravinement surface at which stratigraphic level coarse sediments (e.g., river-channel deposits) are absent. Because the equivalent of the upper part of the Kazusa Group immediately below the bay ravinement surface consists of alternating beds of marine and non-marine sediments similar to those of the Shimosa Group, MIS 12 in the central region of the Kanto Plain is less significant in terms of lithostratigraphic subdivisions than that in the stratotype of the Boso Peninsula.
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  • Middle Pleistocene pollen biostratigraphy in the Osaka sedimentary basin, southwest Japan, with special reference to paleoenvironmental change
    Misao Hongo
    2008 Volume 115 Issue 2 Pages 64-79
    Published: February 15, 2009
    Released on J-STAGE: August 07, 2009
    DOIhttps://doi.org/10.5575/geosoc.115.64
    JOURNAL FREE ACCESS
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    Pollen data from the upper part of the Tanaka Formation in the Higashinada 1700-m drill core, obtained from Kobe City, southwest Japan, enable the construction of a pollen biostratigraphy and detailed record of paleoenvironmental change during the Middle Pleistocene.
    In this study, 174 sediment samples were collected at ca. 2-m intervals from the upper part of the Tanaka Formation, which approximately corresponds to the upper Osaka Group. This formation, which is the hypostratotype for the Pleistocene stratigraphy in the Osaka sedimentary basin, consists mainly of fluvial and lacustrine sediments intercalated with 19 marine clay beds.
    The fossil pollen assemblages in the marine clay beds are dominated by tree and shrub pollen belonging mainly to temperate and cool-temperate deciduous broad-leaved and temperate evergreen coniferous tree taxa, accompanied by pollen of warm-temperate deciduous coniferous and evergreen broad-leaved tree taxa. The pollen assemblages within non-marine sediments that immediately underlie each of the marine clay beds are dominated by herbaceous taxa and tree taxa such as Alnus; they also contain Pteridophyte spores.
    The pollen biostratigraphy from Ma 2 to Ma 12 marine clay beds in the Higashinada core comprises 20 pollen assemblage zones and 5 superzones: the Quercus-Metasequoia, Fagus-Quercus, Cryptomeria-Fagus, Cyclobalanopsis-Sciadopitys, and Pinaceae-Cryptomeria, in ascending stratigraphic order.
    At the time of the Quercus-Metasequoia Superzone, the paleovegetation consisted of warm-temperate deciduous broad-leaved forest. Subsequently, temperate evergreen conifers show a gradual increase in abundance. At the time of the Fagus-Quercus Superzone, the paleovegetation consisted of mixed warm-temperate deciduous broad-leaved and temperate evergreen conifer forest. The Cryptomeria-Fagus Superzone is characterized by alternating periods of temperate evergreen conifers and temperate deciduous broad-leaved trees. The Cyclobalanopsis-Sciadopitys Superzone is characterized by warm-temperate evergreen broad-leaved forest accompanied by temperate evergreen conifers in the hinterland, and the Pinaceae-Cryptomeria Superzone is dominated by temperate evergreen coniferous forest.
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  • Ultra-Tamba Terrane in the Kawanishi-Inagawa area, southeastern part of Hyogo Prefecture, Southwest Japan
    Yoshiaki Sugamori
    2008 Volume 115 Issue 2 Pages 80-95
    Published: February 15, 2009
    Released on J-STAGE: August 07, 2009
    DOIhttps://doi.org/10.5575/geosoc.115.80
    JOURNAL FREE ACCESS
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    The Ultra-Tamba Terrane in the Kawanishi-Inagawa area, southeast Hyogo Prefecture, tectonically overlies the Tamba Terrane and is subdivided into the Kunisaki and Inagawa complexes (redefined) in descending structural order. The Kunisaki Complex consists of coherent clastic rocks and equivalent broken and mixed facies. The mixed facies contains variably sized blocks of clastic rock, chert, and mafic rock within a muddy matrix. Earliest Late Permian and early Late Permian radiolarians were recovered from the felsic tuff and mudstone of this complex, respectively. The Inagawa Complex consists of coherent clastic rocks and equivalent broken facies. Felsic tuff within this complex contains radiolarians of late Middle to middle Late Permian age, and the mudstone yields middle Late Permian radiolarians that indicate the youngest age of clastic rocks of the Ultra-Tamba Terrane in the study area. The lithological characteristics of the Inagawa Complex are similar to those of the Takatsuki Formation exposed to the east of the study area, which includes clastic rocks younger than those in the Inagawa Complex. The Kunisaki and Inagawa complexes represent a pile of nappes, as the latter includes clastic rocks younger than those in the former. The age, lithological features, and structural characteristics of these complexes and the Takatsuki Formation suggest an origin via progressive growth of accretionary prism.
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Short Article
  • Early Miocene parallel dike swarms in the Tsuruga Bay area, back-arc side of central Japan
    Hiroyuki Hoshi, Miho Takagawa
    2008 Volume 115 Issue 2 Pages 96-99
    Published: February 15, 2009
    Released on J-STAGE: August 07, 2009
    DOIhttps://doi.org/10.5575/geosoc.115.96
    JOURNAL FREE ACCESS
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    We analyzed Early Miocene dike swarms in the Tsuruga Bay area, Fukui Prefecture, Japan, with the aim of understanding the stress field that accompanied the formation of the Japan Sea back-arc basin. The dikes have coherent strikes of N50−60° E and are about 20 Ma in age, based on new K/Ar data and previous radiometric ages. The dominant NE−SW strike is consistent with the trends of other 20-Ma parallel dike swarms found on the back-arc side of central Honshu, indicating a regional tectonic stress with a NE−SW maximum horizontal compressive axis and a NW−SE minimum principal stress axis. The back-arc region of central Honshu was probably subject to a NW−SE extension at ∼20 Ma.
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