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 113, Issue 2
Displaying 1-5 of 5 articles from this issue
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Articles
  • Change in groundwater pH by infiltration of meteoric water into shallow part of marine deposits
    Eiichi Ishii, Katsuhiro Hama, Takanori Kunimaru, Haruo Sato
    2007 Volume 113 Issue 2 Pages 41-52
    Published: 2007
    Released on J-STAGE: June 20, 2007
    DOIhttps://doi.org/10.5575/geosoc.113.41
    JOURNAL FREE ACCESS
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    The relationship between petrological and hydrogeochemical characteristics of Neogene marine deposits in the Horonobe area, northern Hokkaido, Japan, was investigated in order to clarify the water-rock interaction associated with changes in groundwater pH. Whole-rocks chemical compositions, mineral compositions, cation exchange capacity, the exchangeable cation content of the rocks, and the chemical compositions of ground and surface waters were determined at various borehole sites at depths of up to 700 m. Leaching of Na from the marine deposits occurred in the shallow meteoric water infiltration zone, caused mainly by cation exchange between Na+ in smectite and cations in the groundwater. Groundwater pH in the shallow zone is inferred to be controlled primarily by cation exchange between Na+ in smectite and H+ in groundwater, where the pH is elevated by the infiltration of meteoric water. This interpretation is in close agreement with other hydrogeochemical models of similar deposits.
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  • Stratigraphy of Early to Middle Pleistocene pyroclastic deposits around Akan caldera, eastern Hokkaido, Japan
    Takeshi Hasegawa, Mitsuhiro Nakagawa
    2007 Volume 113 Issue 2 Pages 53-72
    Published: 2007
    Released on J-STAGE: June 20, 2007
    DOIhttps://doi.org/10.5575/geosoc.113.53
    JOURNAL FREE ACCESS
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    Stratigraphy of pyroclastic deposits around Akan and Kutcharo calderas in eastern Hokkaido was investigated in order to reconstruct the eruptive history of the Akan volcano, and to clarify the stratigraphic relationships among pyroclastic deposits of Akan and other volcanoes. In addition to the geological data, source volcano of each deposit was identified using petrological features of juvenile materials, such as their phenocryst assemblages, whole-rock and glass chemistry. On the basis of the presence of significant time intervals, indicated by paleosol and erosional gap, we recognized at least 40 eruptive units of pyroclastic deposits (Akan pyroclastic deposits). Many of them are successions of pyroclastic fall deposits followed by pyroclastic flow deposits. Based on petrological features of juvenile materials of each unit, we grouped these units into 17 eruptive groups (Ak1 to Ak17 in descending order). Each group is composed of a single or sequential series of eruptive units. Whole-rock chemistry of juvenile materials is different from one group to another. This suggests that activities of each eruptive group were derived from distinct magma system.
    We found several exotic pyroclastic flow deposits and distal air-fall ash layers within the Akan pyroclastic deposits. Pyroclastic flow deposits from adjacent Kutcharo caldera are interbedded between Ak2 and Ak3. One of them was dated to be 0.34 Ma. Distal rhyolitic ash layers from central Hokkaido (1.3−1.46 Ma) are recognized within Ak14. These dates suggest that the large-scale explosive volcanism at Akan volcano started in the Early Pleistocene and repeated many times for more than one million years. During the activity, the magma system was repeatedly renewed to produce eruptive group characteristics. This complex and long-term eruptive history of Akan volcano is probably reflected in the rectangle shape of Akan caldera. The shape of the possible composite caldera is different from the circular shapes of many other calderas.
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