飯能層研究の背景と展望

概要

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The Hanno Group is divided into three formations, which are the lower or Yaoroshi formation, the upper or Hanno formation, and the Bushi formation. The lower formation consists of silt and mud with angular to sub-angular gravels, which is often intercalated by a pyroclastic tuff with hornblende crystals. The upper formation consists of well rounded gravels in large size. The Bushi formation consists of silt and mud with marine shells, peat layers, and rounded gravels. The lower formation lies unconformablly on the Chichibu zone of the Kanto Mountains. It is found that a part of the Chichibu zone is reddishly weathered and overlain by the lower formation. The thickness of the formation is 35 m thick. The base of the upper formation is the red conglomerate facies which is composed mostly of weathered granitic rocks and soft sandstones derived from the Miocene granitic zone and its vicinity of the Kanto Mountains. The thickness of the formation is 200 m thick. The upper formation is conformably overlain by the Bushi formation, 113m thick. The environmental condition for the lower formation was supposed to be backmarsh behind the main stream. That for the upper formation was piedmont fan at the time when the Kanto Mountains started to uplift. The time of uplifting might be before or around 1.20-1.36 Ma, assuming that the A 9 tuff in the Bushi formation would correspond to the HU 2 tuff at the Tama Hill.