Stratigraphy and Geologic Structure of the Osaka Group(Pliocene and Pleistocene) in Keihanna Hills, Kinki District, Japan.

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  • Mitamura Muneki
    Department of Geosciences, Faculty of Science, Osaka City University

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  • 京阪奈丘陵の大阪層群の層序と地質構造
  • ケイハンナ キュウリョウ ノ オオサカソウグン ノ ソウジョ ト チシツ コウ

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The stratigraphy of the Pliocene-Pleistocene Osaka Group in Keihanna Hills is summarized as follows, <br>1. The Osaka Group in this area, about 300 meters thick, is lithologically subdivided into four formations: the Tomigaoka, Tanabe, Seika and Shoudai Formations in ascending order. More than 14 volcanic ash layers and 5 marine clay beds are intercalated in these strata. Among them, 10 volcanic ash layers and 5 marine clay beds are useful as key beds. They are the Kitadani, Higashibata, Fugenji, Takagi, Susudani I, Susudani II, Pink, Azuki, Hacchoike and Kasuri volcanic ash layers, and the Ma1, Ma2, Ma5, Ma6 and Ma 8 beds, in ascending order.<br>2. On the basis of lithologic data on the volcanic ash layers and magnetostratigraphic data, the Osaka Group in this area can be correlated with the standard stratigraphy from the lower horizon of Mitsumatsu volcanic ash layer to the upper horizon of the Ma 8 bed.<br>3. This area is subdivided into three areas: the south area to the Fugenji Flexure (characterized by north-south-trending flexures and faults), the north area to the Katano Fault- Nagao Flexure (northeast-southwest-trending flexures and faults) and the area between the Katano Fault-Nagao Flexure and the Sonenji Flexure (the northern part of the Ikoma Range). These flexure and fault structures in the Osaka Group have resulted from the tilting of the basement blocks by foundation folding in the east-west compressional stress.

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