火山ガラス化学組成の層内垂直変化--大阪層群ピンク・アズキ火山灰層を例にして

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Vertical variations of volcanic glass major element composition were examined on two representative volcanic ash layers of the Pleistocene Osaka Group, namely the Pink and Azuki Volcanic Ash Layers. Both layers are widely distributed in the Kinki district, well exposed in its surrounding hilly area, and are most reliable marker beds in their stratigraphy.The Pink Volcanic Ash Layer lies on the widespread Marine Clay 1 (Ma 1) horizon and is estimated at ca. 0.92 Ma in FT-age. The Azuki Volcanic Ash Layer is interbedded also in the transgressive Marine Clay 3 (Ma 3) and is measured at ca. 0.87 Ma in FT-age. At both sides of the Azuki we can recognize drastic climatic and vegitational changes in the fossil evidence.In this study major element composition of volcanic glass shards were analyzed with an energy-dispersive X-ray micro-spectrometry by grain-by-grain method. Analytical results were processed statistically and offered for further discussion.On the chemical stratigraphy, the Pink Volcanic Ash Layer shows a stable compositional succession in major elements through a whole layer, in spite of its clear lithological facies change, especially in grain size distribution and matrix clay color.The Azuki Volcanic Ash Layer shows two types of sedimentary facies. One is a direct fall and water-laid-type deposit, and the other is a water-transported-type deposit. Binocular observation shows that Azuki volcanic glasses are composed of two type of glass, colorless and colored. Color variation in the latter ranges from light brown to deep brown and sometimes opaque black. In this study black and opaque glass shards are rejected and restricted in transparent glass only.In Azuki Volcanic Ash glass, major element composition is stable in the colorless layer and changes drastically in the colored glass, about 10cm above the base. In addition, the colored Azuki Volcanic Ash Layer shows a gradual change in chemical composition within the layer, and chemical composition shows little difference in the two types of glass because of admixture and sorting during transportation.
日本第四紀学会の論文

著者

西田史朗
奈良教育大・地

火山ガラス化学組成の層内垂直変化--大阪層群ピンク・アズキ火山灰層を例にして

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