Tsui, Shakumasan and Yukagi Formations of the Chichibu Terrain in eastern Kyushu

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  • 九州東部秩父累帯の津井・尺間山・床木3層について
  • キュウシュウ トウブ チチブ ルイタイ ノ ツイ シャクマサン ユカギ

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The greater part of the mapped area geotectonically belongs to the Chichibu Terrain and is composed of the Yukagi, Shakumasan and Tsui Formations. The Yukagi Formation occupies the southernmost part of the Chichibu Terrain, and is bounded against the Azamui Formation of the Shimanto Terrain by a NW-dipping thrust named the Tsui-Kiura tectonic line. The Tsui, Shakumasan and Yukagi Formations are also bounded by strike faults with one another. The Tsui Formation, about 250m thick, consists mainly of shale and mudstone, being accompanied with limestone of the Torinosu type and some beds of sandstone and alternating sandstone and shale (Fig. 2). The Shakumasan Formation is composed mainly of chert, sandstone and shale~slate, and is accompanied with alternation of sandstone and slate. Some beds of conglomerate and thin carbonaceous sandstone and shale, and a bed of siliceous tuff are also intercalated. The Yukagi Formation, about 500 m thick, is composed mainly of chert and limestone, and is accompanied with basic tuff and lava, slate, sandstone and alternation of sandstone and slate (Fig. 1). According to a preliminary investigation by Prof. T. MATSUMOTO, the ammonite faunule collected from the Tsui Formation, although the specimens are ill-preserved, shows a possibility of Middle Jurassic. On the other hand, the bivalves from the same formation have been identified by Dr. I. HAYAMI with the species known from the Upper Jurassic Torinosu Group. Thus the age of the Tsui Formation is referred to Jurassic, but the detailed age is still uncertain. Some more collections and paleontological studies of fossils are necessary. The Yukagi and Shakumasan Formations are of unknown age, but can be correlated with a part of the Konose Group in Kumamoto Prefecture and the Sambosan Group in Kochi Prefecture, by their resemblance in rock fades and occupying the same tectonic position.

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