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 111, Issue 7
Displaying 1-6 of 6 articles from this issue
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Articles
  • Correlation of the key tephra bed 'Kd38' occurring near the boundary between the Tertiary and Quaternary in the southern part of the Kanto Province, central Japan−Correlation in the Kazusa Group and the Chikura Group, Boso Peninsula−
    Naoki Takahashi, Takashi Mitsuoka, Arata Katoh, Kazumi Yokoyama
    2005 Volume 111 Issue 7 Pages 371-388
    Published: 2005
    Released on J-STAGE: November 01, 2005
    DOIhttps://doi.org/10.5575/geosoc.111.371
    JOURNAL FREE ACCESS
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    Key tephra bed 'Kd38' occurs near the boundary between Tertiary and Quaternary. The tephra, characterized by the presence of Fe-rich orthopyroxene, had been confirmed at many localities in central Boso Peninsula and Choshi area. In this study, we confirmed the tephra at one or two study routes in the Chikura Group, southern part of the Boso Peninsula mainly based on chemical composition of orthopyroxene in the tephras, referring to the results of microfossil and paleomagnetism studies.
    In the Hata path, 'HT08B' and 'HT07' tephras are correlated to the key tephra beds 'Kd38' and 'Kd39' in the central Boso Peninsula, respectively. In the Yamakura path, 'YK03' tephra corresponds to the key tephra bed 'Kd39', and it seems that 'YK04C' tephra can be correlated to the key tephra bed 'Kd38'.
    Both the key tephra beds 'Kd38' and 'Kd39' were confirmed to occur at the Yokohama area from Miura Peninsula. 'YH10' and 'YH02' tephras in the Hitorizawa route are correlated to 'Kd38' and 'Kd39', respectively. 'Tzc' tephra from Niigata Prefecture is one of famous tephra beds corresponding to the key tephra bed 'Kd38'. It was reconfirmed by the compositional variations of orthopyroxenes in this study.
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  • Revision of the stratigraphy of the Toyora and Toyonishi Groups in the Ouchi-Kikugawa area, Yamaguchi Prefecture, west Japan
    Toshihiro Yamada, Terufumi Ohno
    2005 Volume 111 Issue 7 Pages 389-403
    Published: 2005
    Released on J-STAGE: November 01, 2005
    DOIhttps://doi.org/10.5575/geosoc.111.389
    JOURNAL FREE ACCESS
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    The stratigraphy of the Toyora (Lower to Middle Jurassic) and Toyonishi (Upper Jurassic to Lower Cretaceous) Groups in the Ouchi-Kikugawa area, south of the Tabe Fault, in the western part of Yamaguchi Prefecture is revised. The revised succession is, in ascending order, the Higashinagano, Nishinakayama and Utano Formations in the Toyora Group, the Kiyosue and Yoshimo Formation in the Toyonishi Group. The Kiyosue Formation is further subdivided into the Nakao Siltstone and Nanami Sandstone Members. The Utano Formation has been thought to be marine in the type Toyora area, while non-marine in the studied area. However, marine facies, characteristic in the Utano Formation, is ascertained in the formation hitherto classified as the "Nishinakayama Formation" in the studied Ouchi-Kikugawa area. The deposits referred to the non-marine sequence of the "Utano Formation" by previous authors are re-classified into the Nakao Siltstone Member of the Kiyosue Formation.
    Newly established stratigraphy suggests that the age of the so-called "Utano flora", reported from the Kiyosue Formation as well as the "Kiyosue flora", of the studied area is younger than Bathonian, contrarily to the previous age assignment older than Bathonian. The relationship of these two florae should be explored in the future.
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  • Seasonal changes in fluxes and assemblages of radiolarians collected by sediment trap experiments in the northwestern Pacific: a family-level analysis
    Isao Motoyama, Miyuki Ota, Tomotsugu Kokushou, Yuichiro Tanaka
    2005 Volume 111 Issue 7 Pages 404-416
    Published: 2005
    Released on J-STAGE: November 01, 2005
    DOIhttps://doi.org/10.5575/geosoc.111.404
    JOURNAL FREE ACCESS
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    Time-series sediment traps were deployed in the subtropical (Station WCT-1) and transitional (Station WCT-2) areas in the northwestern Pacific for one year from 1997 to 1998. Radiolarians were counted and categorized into families to study seasonal and regional changes in radiolarian productivity and faunal composition. The polycystine annual mean flux was 10,406 shells m−2day−1 at WCT-2, which was higher than that at WCT-1 (3,901 shells m−2day−1). The phaeodarian annual mean flux was low (1,583 shells m−2day−1) at WCT-2 and extremely low (7 shells m−2day−1) at WCT-1. These indicate that radiolarian productivity is generally higher in the transitional area than in the subtropical area. Temporal fluxes of polycystines at the transitional site showed variations associated with high flux during the spring and low flux during the summer, while polycystine fluxes at the subtropical site did not show apparent seasonal variations. Family-level percentage composition and statistical testing did not show significant seasonal changes at either site. This seasonal stability in faunal composition was a quite unexpected result, since there were seasonal changes in the sea surface temperature in the studied areas. This stability indicates lack of response of polycystine radiolarians to oceanic seasonality in the subtropical and transitional regions. There was a difference in the annual polycystine composition between the two sites, probably corresponding to differences in oceanic water masses. Comparing the polycystine annual composition and flux between the two sites, three families, Collosphaeridae, Coccodiscidae and Pterocorythidae, are more abundant elements in the subtropical region, whereas dominance of Plagiacanthidae characterizes the transitional region.
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  • Correlation of the tephra beds in the Middle Pleistocene Older Ontake Volcano with those in the Kazusa Group in Boso Peninsula, central Japan
    Yoshihiro Takeshita, Yasuyuki Miyake, Jun'ichi Sakai
    2005 Volume 111 Issue 7 Pages 417-433
    Published: 2005
    Released on J-STAGE: November 01, 2005
    DOIhttps://doi.org/10.5575/geosoc.111.417
    JOURNAL FREE ACCESS
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    Heavy mineral assemblage and chemical compositions of hornblende of the nine Middle Pleistocene tephra beds (Ku6E, Ku5C, Byakubi tephra, Ka2.4A, Ka2.4B, Ch3, Ch1.5, Ks18, Ks12) from the Kazusa Group, in Boso Peninsula were examined in order to correlate them with the tephra from the Older Ontake Volcano in central Japan. Hornblende composition from the nine tephra beds can be distinguished and two tephra beds, the Byakubi tephra (BYK) and the Ks12 tephra, were correlated with the tephras from the Older Ontake Volcano, the YUT4 or 5 of the H Substage and KZT of the Lava Stage, respectively. The age of these tephra beds of the Kazusa Group can be inferred from the stratigraphic relationships with the dated lavas on the foot of the Older Ontake Volcano, and from the well-known widespread tephras and magnetostratigraphy in Boso Peninsula. These two correlated tephra beds became valuable marker tephras for geochronological studies of both inland and marine sediments from central Japan. It should be also emphasized that the BYK and YUT4 or 5 can provide a datum plane of the Early-Middle Pleistocene boundary in both regions. The present study indicates that hornblende chemistry can be a useful tool for correlation of highly weathered tephra, in which volcanic glass is strongly altered.
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  • Rock densities for the geologic units in the Japanese Islands: an estimate from the database PROCK (Physical Properties of Rocks of Japan)
    Toshimitsu Iwaya, Kazuhiko Kano
    2005 Volume 111 Issue 7 Pages 434-437
    Published: 2005
    Released on J-STAGE: November 01, 2005
    DOIhttps://doi.org/10.5575/geosoc.111.434
    JOURNAL FREE ACCESS
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    Mean dry and wet bulk densities of the rocks of various ages and occurrences in Japan were estimated by sorting a set of reliable data retrieved from the database PROCK. Within the limited data, dry bulk density of clastic sedimentary rocks obviously increases up to 2.7×103 kg·m−3 with increasing age; largely from recent to early Miocene time and slightly in earlier times. Wet bulk density is larger than dry bulk density for younger clastic rocks, especially for younger sandstone. The difference, however, becomes negligible with increasing age, perhaps due to increasing porosity reduction by compaction and chemical precipitation. No obvious difference in variation of density is found between sandstone and mudstone and between their occurrences. Felsic and mafic volcanic rocks slightly increase in dry and wet bulk densities from 2.4 to 2.6×103 kg·m−3 and from 2.5 to 2.8×103 kg·m−3 with increasing age. Dry and wet densities of felsic plutonic rocks are between 2.6 and 2.7 ×103 kg·m−3. Only few data is available for mafic plutonic rocks and ultramafic rocks. The dry or wet bulk densities of crystalline schist and gneiss are close to 2.7 and 2.8×103 kg·m−3. Few density data are available for other types of metamorphic rocks.
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