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Oto Formation Suzuki, 1964
- Type locality: Aga river, Aizuwakamatsu city, Fukushima prefecture
- Accretionary complex
- Correlation: Ashio Belt
- Intruded by: Late Cretaceous biotite hornblende granodiorite, Biotite adamellite, Miocene dykes
- Sedimentary facies: Marine
- Lithofacies: Hornfels
- Lithofacies (Protolith): Sandstone, Mudstone, Interbedded sandstone and mudstone
- Lithofacies (Protolith, Minor): Chert
- Sedimentary structures (Protolith): Graded bedding, Planar cross-stratification, Low angle cross-stratification
- Distribution: Mount Oto - Okawa dam, Aizuwakamatsu city, Fukushima pref.; Ouchi dam - Jonoiri creek - Ono river, Shimogo city, Minamiaizu county, Fukushima pref.; Kasshi, Nishigo city, Nishishirakawa county, Fukushima pref.
- Fossils (Radiolarians): Acanthocircus sp., Archaeospongoprunum sp.
- Yamamoto (1999) Geology of the Tajima District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, 85p. (in Japanese with English abstract 8p.)
- 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.)