The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 116, Issue 5
Displaying 1-6 of 6 articles from this issue
Articles
  • Katsunori Iizumi, Yoshiki Koda, Wataru Koike, Toyohiro Nishimoto, Hisa ...
    2010 Volume 116 Issue 5 Pages 243-251
    Published: 2010
    Released on J-STAGE: October 13, 2010
    JOURNAL FREE ACCESS
    A limb bone of a marine mammal, recently recovered from the riverbed of the Hanamurogawa River, Tsukuba City, Ibaraki Prefecture, central Japan, is identified as the left humerus of an adult male Japanese Sea Lion, Zalophus japonicus (pinniped). A metapodial specimen reported by Nakashima et al. (2002) was re-analyzed and identified as an astragalus of an adult female of Z. japonicus. The humerus of the present study is inferred to have been derived from the Sakuragawa Terrace Deposits or their equivalents, based on radiocarbon dating (27,900±120 14C BP and 31,950-31,300 cal BP) of bone collagen and the mode of fossil occurrence. This evidence indicates that the deposits bearing mammalian fossils such as Palaeoloxodon naumanni and associated cervids were deposited in the latest Pleistocene. The taphonomical processes by which the humerus and astragalus of Z. japonicus ended up in a fossil assemblage dominated by land mammals should be further investigated in view of human activity during the Late Paleolithic period.
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  • Susumu Tanabe, Toshimichi Nakanishi, Rei Nakashima, Yoshiro Ishihara, ...
    2010 Volume 116 Issue 5 Pages 252-269
    Published: 2010
    Released on J-STAGE: October 13, 2010
    JOURNAL FREE ACCESS
    The latest Pleistocene to Holocene incised-valley fill (Alluvium) beneath the Nakagawa Lowland upon the Kanto Plain consists of thick, muddy marine sediments. It has previously been proposed that the marine sediments were deposited as a result of delta progradation from the bayhead of the paleo-Okutokyo Bay to the bay mouth, following the mid-Holocene sea-level highstand. Based on an analysis of sedimentary facies and radiocarbon dates obtained for three sediment cores recovered from the Nakagawa Lowland, we determined the accumulation pattern of incised-valley fill along a transect oriented across to the valley. The period of sedimentation can be divided into three stages based on the obtained patterns of accumulation: Stage I (~10 cal kyr BP): aggradation of a meandering river system; Stage II (10-6 cal kyr BP): large aggradation of the estuary system at the axis of the valley and minor aggradation at the valley margin; and Stage III (6-0 cal kyr BP): progradation of the delta system from the bayhead to bay mouth and from the western margin of the valley to the eastern margin. During Stages II and III, a strong tidal current was active along the valley axis, possibly explaining the eastward migration of the delta.
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  • Yuki Kusano, Sumio Miyashita, Kazuya Ikeda
    2010 Volume 116 Issue 5 Pages 270-282
    Published: 2010
    Released on J-STAGE: October 13, 2010
    JOURNAL FREE ACCESS
    The Jurassic accretionary complex of the Ashio belt, Japan, consists mainly of sandstone and mudstone, although metabasalt, chert, and limestone are commonly observed in the Hayade-gawa area, Gosen region, Niigata prefecture. The metabasalt bodies, which are considered to occur as blocks in the accretionary complex, consist of massive lava, pillow lava, and hyaloclastite of generally low vesicularity (<5 vol.%), although highly vesicular rocks are locally observed. The metabasalts are classified into five petrographic types based on phenocryst assemblage: Pl, Pl+Ol, Pl+Ol+Cpx, Pl+Cpx, and aphyric. The whole-rock compositions of 10 metabasalt samples show an oceanic island basalt (OIB) affinity. Metabasalts from the southern part of the study area show higher Zr concentrations, higher Nb/Zr and La/Zr ratios, and lower TiO2/Zr and Y/Zr ratios than those in the northern part. Chondrite-normalized REE patterns show an enrichment in LREEs (similar to OIB) that is most pronounced in the Zr-rich metabasalts from the southern area. Clinopyroxene from 16 samples shows a wide range of Mg# (0.67-0.85), and clinopyroxene chemistry indicates that the magma had affinities with non-orogenic tholeiite and alkali basalt. Clinopyroxenes with low Mg# are divided into two groups: a high-Ti group from the southern area and low-Ti group from the northern area. In contrast, clinopyroxenes with high Mg# have similar Ti contents in both areas. Chromian spinels from 11 samples also have intraplate basalt signatures, similar to those of Hawaiian tholeiitic and alkali basalts. In summary, the whole-rock and mineral chemistry data indicate that the metabasalts in the Hayade-gawa area originated from oceanic island volcanism, with oceanic alkali basalt in the southern area and oceanic tholeiitic basalt in the northern area.
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Short Articles
  • Keiichi Hayashi, Masaki Matsukawa, Hiroto Ohira, Peiji Chen, Jinshen Z ...
    2010 Volume 116 Issue 5 Pages 283-286
    Published: 2010
    Released on J-STAGE: October 13, 2010
    JOURNAL FREE ACCESS
    The age of the non-marine Cretaceous Sasayama Group, distributed in western Honshu, Japan, was reassessed based on ostracod and conchostracan bio-stratigraphy, and zircon fission-track dating. The ostracod genera Mongolocypris and Eucypris from the lower part of the Sasayama Group, combined with the conchostracan genera Yanjiestheria and Nemetheria from the upper part of the group, suggest an age range from Albian to Cenomanian. Zircon fission-track dating of the lower and upper parts of the group yields ages of 106±9, 100.9±8.4, and 96.1±7.4 Ma, also indicating a range from Albian to Cenomanian. These data indicate that deposition of the Sasayama Group began about 30 Ma later than previously thought.
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  • Yoshihiro Kase, Wataru Maejima
    2010 Volume 116 Issue 5 Pages 287-290
    Published: 2010
    Released on J-STAGE: October 13, 2010
    JOURNAL FREE ACCESS
    The Lower Cretaceous Nishihiro Formation in the Yuasa-Aridagawa Basin is characterized by quartzose and arkosic sandstones, as well as minor conglomerate and mudstone. Sedimentary facies analysis of the Lower Sandstone Member of the formation reveals the dominance of braided fluvial deposits. Paleocurrent analysis indicates the consistent southerly dispersal of coarse clastics from the source terrane located to the north of the basin. The provenance includes granitic rocks and Siluro-Devonian felsic tuffs, which are characteristic of the Kurosegawa Terrane, which presently occurs in the axial zone of the Chichibu Terrane; however, during deposition of the Nishihiro Formation, this terrane also occurred north of the Yuasa-Aridagawa Basin, possibly as a nappe that lay upon the Chichibu Terrane.
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