泥火山周辺の地質構造と地下水の地化学特性

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書誌事項

タイトル別名
  • Geological Structure and Groundwater Geochemistry around Mud Volcanoes in the Kamou Area, Tokamachi City, Niigata Prefecture
  • デイカザン シュウヘン ノ チシツ コウゾウ ト チカスイ ノ チ カガク トクセイ ニイガタケン トオカマチシ カモウ ニ オケル ケントウ
  • ―新潟県十日町市蒲生における検討―

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抄録

 Mud volcanoes are defined as conical geometric high mounds composed of erupted mud that originates deep underground, and are induced by abnormal pore water pressure. It is well known that a serious problem occurred due to swelling mudstone during tunnel construction work in the Tertiary sedimentary basin in Japan. The swelling mudstone is partly caused by the activity of mud volcanoes. However, it has not been demonstrated in the field yet. Boreholes with a depth of 120 m were drilled at an altitude of 329 m inside a depression in the Kamou area, Tokamachi City, Niigata Prefecture where two active mud volcanoes and an inactive mud volcano are distributed and the Nabetachiyama Tunnel with swelling mudstone is located. Geologic structure and geochemical properties of groundwater around the mud volcanoes were studied by core logging and geochemical analysis of pore water squeezed from cores. Humus soil occurs from ground surface to a depth of 2.1 m. Erupted deposits composed of mudstone fragments and clay are located in the interval from 2.1 m to 5.8 m in depth. Intact massive mudstone is located from 5.8 m to about 50 m in depth. Mud breccia composed of mudstone fragments and scaly soft clay is located from fifty meters in depth to the bottom of a borehole. Mud breccia is classified into two types based on its textural and structural characteristics. The distribution of each type of mud breccia suggests that mud breccia was formed by hydro-fracturing caused by the activity of a mud volcano mainly at depths greater than fifty meters. As a result of an XRD analysis of clay minerals and XRF analysis of rock, Ca-type smectite is found to be dominant and CaO content is rich from the ground surface to 55 m in depth, whereas Na-type smectite is dominant and Na2O content is rich at depths greater than 50 m. Groundwater squeezed from cores greater than fifty meters in depth is highly saline. Therefore, the cation exchange from Ca2+ to Na+ occurred due to the highly saline groundwater. In summary, saline pressurized groundwater ascended with hydro-fracturing to a depth of fifty meters and replaced the fresh groundwater. Variation of stable isotope ratios of pore water with depth supports the assumption above.

収録刊行物

  • 地学雑誌

    地学雑誌 118 (3), 350-372, 2009

    公益社団法人 東京地学協会

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