上野盆地西部,花ノ木丘陵の古琵琶湖層群

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The Pliocene to Pleistocene Kobiwako Group in the Iga and Omi basins is composed of fluviolacustrine loose sediments with more than 100 layers of volcanic ash. It is lithologically divided into five formations, i. e. the Iga, the Koka, the Gamo, the Kusatsu and the Katata Formations in ascending order. The Iga Formation in Hananoki Hills, west of Ueno City, is composed mainly of gravels and silts. It is lithologically divided into three members, namely the Yono, the Ishiuchi and the Hokke Members in ascending order. The Yono Member is composed of alternating beds of sand, silt and clay, and intercalates gravels. The Ishiuchi Member is composed mainly of gravels with silt lens, and the Hokke Member is composed of alternating beds of silt, sand and gravels. Strata ranging from the base of the Ishiuchi Member up to the top of the Hokke Member show an fining upward sequence of large scale. The Kobiwako Group in this area generally abuts unconformablly on the basement rocks on the south margin of the basin, while it is in fault contact with them on the north margin. The strike of unconformity plain has NNW-SSE and ENE-WSW directions, and that of fault plains in the basement rocks underlying the group also shows NNW-SSE and ENE-WSW directions. These facts suggest that the basin-forming movement of Kobiwako involved the development of fault systems in the basement. Gravels in the Yono Member are of acidic tuff of Miocene Muroo Group south of this area, and of chert, sandstone, and shale which were derived from Paleozoic to Mesozoic Tamba Group, and granite and gneiss of Ryoke Complex. On the contrary, gravels of the Ishiuchi and Hokke Members are of acidic pyroclastic rocks probably of Mesozoic Koto Rhyolitic Rocks around Lake Biwa, and of Tamba Group and Ryoke Complex origins. Therefore it is assumed that at the time of beginning of the sedimentation of the Ishiuchi Member, the direction of source area of these gravels was changed from south to north, causing of upheaval of the surrounding area of Lake Biwa.
1986-11-25

上野盆地西部,花ノ木丘陵の古琵琶湖層群

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