The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 112 , Issue 9
Showing 1-4 articles out of 4 articles from the selected issue
  • Isao Motoyama, Hideaki Nagamori
    2006 Volume 112 Issue 9 Pages 541-548
    Published: 2006
    Released: January 01, 2007
    Radiolarians were studied from the typical section of the Neogene of the Hokushin district, northern part of Nagano Prefecture. The studied section covers the Aoki, Ogawa and Shigarami Formations, which have been dated mainly by radiometric dating methods applied for volcanic rocks and ash layers. Radiolarians were found from the Arakurayama Pyroclastic Member and the Ogikubo Sandstone and Mudstone Member of the Shigarami Formation but were not from the other formations. The radiolarian assemblage from the Arakurayama Pyroclastic Member is that of Spongurus pylomaticus Zone (5.2-4.5 Ma). The Ogikubo Sandstone and Mudstone Member yielded radiolarians characteristic of the Pliocene (S. pylomaticus Zone-lower Cycladophora sakaii Zone; 5.2-2.6 Ma). These estimated ages show good agreement with the radiometric ages reported by the previous study.
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  • Kou Takahashi, Kuniaki Nishiki
    2006 Volume 112 Issue 9 Pages 549-567
    Published: 2006
    Released: January 01, 2007
    The Northern Yatsugatake volcanoes occupy the northern part of the Yatsugatake volcanic chain, central Japan. According to the geological and the petrographical studies, the volcanic products of the Northern Yatsugatake volcanoes are grouped into the four volcanic units such as Kasuga, Nijinotaira, Fukinotaira and Tateshina volcanics in ascending order. The previously reported age and paleomagnetic data are re-evaluated, and it became clear that the lavas of Nijinotaira and Fukinotaira volcanics were formed in the Early Pleistocene age. The whole rock chemistry for sixty samples from the study area was newly analyzed. The results show that the rocks of the Kasuga volcanics are mainly basalt to basaltic andesite. On the other hand, the rocks of the Nijinotaira and Fukinotaira volcanics are andesite to dacite. The volcanic activity of the northern Yatsugatake started about 1.2Ma. The basalt to basaltic andesite magma erupted between 1.1 to 1.0Ma. The volcanism was characterized by eruption of thin lava flows, which formed stratovolcanoes. Then the andesite to dacite magma erupted between 1.0 to 0.9 Ma. The volcanism was characterized by eruption of thick lava flows. At the last stage, andesite to dacite lavas erupted during the Middle to Late Pleistocene. The petrochemical features of the magma of the northern Yatsugatake volcanoes are similar to those of the Enrei volcanic rocks, which located on the west of the northern Yatsugatake volcanoes. In the Northern Yatsugatake and Enrei areas, about 200 km3 of magma erupted during the Early Pleistocene. Such a voluminous magma eruption may have been performed under the tensional stress field, which formed by the horizontal movement of the Itoigawa-Shizuoka Tectonic Line.
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  • Takaharu Tawara, Fujio Kumon, Yoshitaka Nagahashi, Naoko Kakuta, Yasuh ...
    2006 Volume 112 Issue 9 Pages 568-579
    Published: 2006
    Released: January 01, 2007
    The Takano Formation in Nagano City, central Japan consists of Late Pleistocene lacustrine sediments. An undisturbed core of 53.88 m in length was collected by double tube core barrel system in June 2004. The sediments are almost clayey silt, and intercalated a large number of tephra layers, some of which are widespread marker tephras. Ages were estimated from the radiometric determinations of five wide-spread marker tephras such as DKP, Aso-4, K-Tz, Ata, and Aso-2, although it was presumed that sedimentation rate was constant.
    The TOC variations from around 169 ka to 37 ka, are concordant with those of the pollen compositions reported in previous studies for the outcrops of the Takano Formation. Paleoclimate changes from around 169 ka to 37 ka have been reconstructed by TOC in sediments of the Takano Formation in about 28 year interval, which shows long periodicities as a few tens thousands years. It is found that the TOC variations can be correlate with the marine δ18O curve of SPECMAP from MIS 6 to early MIS 3. The values of TOC are high in MIS 3 and 5, while low in MIS 4 and 6. In addition, the TOC variations during MIS 5 can be correlated with the substages such as MIS 5a, 5b, 5c, 5d and 5e.
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