International Geologiical Congress - Oslo 2008


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HYH-01 General contributions to hydrogeology


Hydrogeological structure of unconsolidated layered sediments deposited in the lift zone of the Boso peninsula, Japan


Masayuki Ishibashi, Nagoya University (Japan)
hideyo Takahata, Technoearth Co. (Japan)
Tomoyo Hiyama, Technoearth Co. (Japan)
Hiroshi Fujita, (NPO)The Society of Geo-pollution Control (Japan)
Hidekazu Yoshida, Nagoya University Museum (Japan)
Hisashi Nirei, (NPO)The Society of Geo-pollution Control (Japan)


Understanding the feature controlling groundwater flow of unconsolidated sediments is one of the most important issues for the remediation of polluted sediments. Although there are many investigations of behavior of groundwater flow in consolidated rocks, it is little known about the groundwater flow in unconsolidated sedimentary layer. Therefore, the study focuses on the relation between the feature of groundwater flow and geological structures in less consolidated sediments. This study has been carried out on Anno Formation (middle Pliocene) of the Awa Group, Kurotaki Formation (upper Pliocene), Tomiya Formation (upper Pliocene) and Kiwada Formation (lower Pleistocene) of Kazusa Group in Otsukayama area of Boso peninsula, Japan. Anno Formation is represented by the series of marine environments ranging from outer shelf to foreshore. And Kurotaki, Tomiya and Kiwada Formations are represented by the series of marine environments ranging from foreshore to outer shelf. These formations are composed of well sorted sandstones and conglomerates thought turbidite sedimentations and pyroclastic fall deposits, mudstones and poorly sorted sandstones or conglomerates. Groundwater discharge and occurrence of Fe-oxyhydorxides precipitation was observed in the turbidite deposits and pyroclastic fall deposits. That wasn?ft almost observed in Faults and fractures zone. Permeability test showed that well sorted granule stones considerated pyroclastic fall deposit was 10-3 cm/sec and the other sedimentary rocks were about 10-7 cm/sec. Thin section observation and permeability tests conducted on the three directions according the strike, dip direction and vertical direction of layered bed, showed the clear existence of anisotropy formed by the micro-scale sedimentary layer. These detailed geological observations and hydraulic measurement show that permeability is controlled by difference of sedimentation facies, because of well sorted granule stones due to turbidite sedimentations and pyroclastic fall deposits have high permeability in hypabyssal unconsolidated sediments. And permeability test of three direction suggest that groundwater flow is controlled by the micro-scale sedimentary structure in particular sediments.


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