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Obonai Formation Omura, 1936

Invalid synonym: Rentaki Formation

• Type locality: Obonai river, Senboku city, Akita prefecture
• Volcanic rocks
• Miocene (22.0, 11.9 Ma)
• Correlation: Kumazawagawa Formation, Omata Formation

• Lithofacies: Clinopyroxene andesite - Orthopyroxene-bearing clinopyroxene basalt lava, Hyaloclastite
• Lithofacies (Minor): Pillow lava, Acidic volcanic rocks, Sandstone, Siltstone

• Thickness: ca. 600 m
• Upper limit: Unconformity (Kunimitoge Formation, Ainaizawa Formation)
• Lower limit: Unconformity (Obonai Granodiorite)

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

• Distribution: Obonai river - Mount Aizawa - Kumanodai - Mount Kaibuki, Senboku city, Akita pref.

• Suto and Ishii (1987) Geology of the Shizukuishi District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, 142p. (in Japanese with English abstract 5p.)