The Kobiwako Group in the Western Part of Minakuchi Hills, Shiga Prefecture, Japan

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  • 水口丘陵西部の古琵琶湖層群
  • ミズグチ キュウリョウ セイブ ノ コ ビワコソウグン

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The Research Group made a stratigraphic investigation of the Kobiwako Group distributed in the western part of Minakuchi Hills lying in the eastward of the Lake Biwa. The results are summarized in the following. 1. The Kobiwako Group in this area, having thickness of about 385 m, is divided into five members, namely Hazama clay, Kiyota alternations, Yokigaoka clay, Toyota alternations and Gamodo gravels Members, in ascending order. Among 29 volcanic ash layers found in these members, 13 layers are available as key beds. They are named Hazama, Komazuki, Mushono, Aotani, Manakodani, Yama, Shimizudani, Bessho, Yokigaoka II, Tokutani, Toyota, Nakayama and Gamodo volcanic ash layers, in ascending order. 2. Some plant megafossils collected from this area are the elements of the Metasequoia flora. As going to the upper horizon, these elements gradually diminish and the elements of another flora showing the Quaternary cold climate are found in the uppermost horizons of the Kobiwako Group. 3. The Kobiwako Group in this area gently dips to the northwest in general. The main geological structures are Nakahata syncline, Takamatsu block-like fold, Shimoda flexure and Shofukuji fault. 4. Based on the study of plant megafossils, analyses of volcanic ash and palaeomagnitic reseach, the Kobiwako Group in this area is supposed to be correlated to the lowermost and the lower parts of the Osaka Group.

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