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Iriyamazawa Formation (Kitamura et al., 1968) Yamamoto, 1992

Invalid synonym: Iriyamazawa Tuff, Shimonakatsukawa Formation, Onarisawa Formation

• Type locality: Iriyama creek, Otani river, Mishima city, Onuma county, Fukushima prefecture
• Volcanic rocks
• Collapse caldera
• Miocene
• Intruded by (Post-caldera stage): Hornblende dacite, Biotite hornblende dacite, Orthopyroxene clinopyroxene hornblende dacite, Biotite orthopyroxene clinopyroxene hornblende dacite, Orthopyroxene clinopyroxene andesite

• Sedimentary facies (Post-caldera stage): Lacustrine
• Sedimentary facies (Debris avalanche deposit): Caldera-collapse deposit
• Lithofacies (Lacustrine deposit): Sandstone, Conglomerate, Siltstone
• Lithofacies (Lacustrine deposit, Minor): Pyroclastic turbidite
• Lithofacies (Debris avalanche deposit): Breccia
• Sedimentary structures (Lacustrine deposit): Graded bedding, Reversed graded bedding

• Thickness (Lacustrine): 400 m or thicker
• Thickness (Pyroclastic flow deposit): 1,000 m
• Upper limit: Unconformity (Hotokezawa Pyroclastic Flow Deposit, Niitsuru Pyroclastic Flow Deposit, Sunagohara Formation)
• Lower limit: Abut unconformity (Higashiomata Formation, Takizawagawa Formation)

------ Conformity, ~~~~~~ Unconformity, ~~~--- Local unconformity, '''/,, Interfingered, :::::: Fault, /--/-- Thrust, +__+__ Ground surface, ?--?-- Unknown

• Distribution: Iriyama creek, Otani river, Mishima city, Onuma county, Fukushima pref.

• Yamamoto and Komazawa (2004) Geology of the Miyashita District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, AIST, 71p. (in Japanese with English abstract 5p.)
• Yamamoto and Yoshioka (1992) Geology of the Wakamatsu District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, 73p. (in Japanese with English abstract 5p.)