設楽盆地北部にみられる大峠陥没盆地
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概要
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The Shidara group of Miocene age is widely distributed in the northern part of the Shidara district situated on north side of the Median tectonic line. On the other hand so-called Shidara volcanics are widely distributed in the southern part of the district. Stratigraphical sequence of the Shidara group is as follows; Toei, Tsubosawa, Mitsuse, Oro, Otoge, Oshimagawa, Myojinyama, Kada and Tsugu formations in ascending order (Table 1). Partial unconformity between the Tsubosawa and Kuroze formations pointed by Kato (1962) was not found, but partial unconformity between the Oro and Mitsuse formations was newly found. Stratigraphical relations of these formations show inclined unconformity near Toei and south of Hikita, but are comformable near Mitsuse situated between Toei and Hikita. On the basis of such relation, division of the Hokusetsu and Nansetsu subgroups were redifined. Upheaval forming this partial unconformity is considered to be related to movement of the Median tectonic line in the Toei area and doming up in the Hikita area. The Otoge formation of the Nansetsu subgroup is distributed in a restrict area, which is about 4km in diameter and is surrounded by straight boundary lines, abuts on steeply dipping surface of unconformity. Poorly sorted taluslike conglomerates which include angular boulder gravel attaining 5 m in diameter occur along the margin of the distributed area of the Otoge formation. Many of reconsolidated fractures and dykes are also distributed near the margin. From the above-mentioned field evidences, the following process will be presented. Firstly, doming, which may be due to ascending of magma, took place and was succeeded by collapse along the fault-fracture planes. The surfaces of fractures can be interpreted as the steep slope or cliff of the mountain-land around the collapse basin. The collapse basin was filled up by rhyolitic pyroclastic rocks erupted after collapse. It is considered that these processes coincide with those of generation of sedimentary basin of the Green Tuff region proposed by Fujita (1973, etc).
- 日本地質学会の論文
- 1979-03-30