The process of the generation and evolution in the Neogene sedimentary basin at the western margin of Chichibu Basin, central Japan

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  • 堆積盆地の発生・発達過程の研究 ―秩父盆地西縁における解析―
  • タイセキ ボンチ ノ ハッセイ ハッタツ カテイ ノ ケンキュウ チチブ ボンチ セイエン ニ オケル カイセキ

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Abstract

The Neogene strata are distributed in the Chichibu Basin in the northwestern part of the Kanto Mountains. The western region of the Chichibu Basin where the lowermost part of the Neogene is distributed has been studied to elucidate the genetic process of the Chichibu sedimentary basin. As a result, following was confirmed; the several tuff layers occur in the mudstone layers which form the lowermost strata in the western region of the Chichibu Basin, and one of these layers extends to the Nenokami Formation (Chichibu Basin Collaborative Research Group 1999) which is distributed in the northern region in the Chichibu Basin. Judging from this fact, it is possible to divide the Neogene sediments in this area, as well as the Neogene distributed in the northern to northwestern region, into the Ushikubi, Tomita, Nenokami, Miyado and Yoshida Formations, in ascending order. The contact relations between the Neogene sediments and basement rocks are observed at 25 outcrops, an unconformarble relations between both are observed at 18 outcrops of them. Moreover, it is confirmed in nine district that the Neogene sediments abuts against a surface of unconformity with a high angle dip. Formation of the sedimentary basin began with the development of fractures, followed by collapsing of the basement, which were taken place at four stages and accompanied with volcanism. Uplifting of the surface due to the ascent of magma may have occurred prior to fracturing. Judging from these data, the Chichibu sedimentary basin also is estimated to be formed through the same process as a volcanic collapsed basin. In this area, the center of subsidence during the period of Neogene sedimentation progressed from north to south, creating the so-called Shogidaoshi structure.

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