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Yamamoto Formation (Yoshimura and Ishibashi, 1979) Nagamori et al., 2010

Invalid synonym: Iwatoyama Formation, Hirakurayama Formation, Yokokura Formation, Todo Formation

• Type locality: Yamamoto bridge - Nakayama bridge, Himekawa river, Itoigawa city, Niigata prefecture
• Volcanic rocks
• early Miocene: 16.0 - 15.8 Ma, K-Ar (Andesite); 16.6 - 14.3 Ma, K-Ar (Basaltic andesite)
• Intruded by: Basalt sills, Dykes

• Sedimentary facies: Subaqueous
• Lithofacies: Clinopyroxene orthopyroxene andesite
• Lithofacies (Minor): Olivine clinopyroxene orthopyroxene andesite
• Lithofacies: Andesite lava, Lapilli tuff, Lapillistone, Volcanic breccia, Tuff breccia
• Lithofacies: Basalt pillow lava, Tuff breccia, Lapilli tuff
• Lithofacies (Minor): Tuffaceous sandstone, Tuffaceous mudstone

• Thickness: ca. 200 - 400 m
• Upper limit: Unconformity (Imai Formation, Sennozawa Formation)
• Lower limit: Unconformity (Tokurayama Formation)

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

• Distribution: Yamamoto bridge - Nakayama bridge - Oyachi - Oami pass - Negoya - Mount Tokurayama - Yokokawa, Itoigawa city, Niigata pref.; Komyo, Otari city Kitaazumi county, Nagano pref.; Aonizawa creek - Sugesawa creek, Hakuba city, Kitaazumi county, Nagano pref.; Aoni - Sugisawa creek - Mount Takatoyama - Minamigata - Warabidaira - Kazono - Mount Hirakurayama, Hakuba city, Kitaazumi county, Nagano pref.

• Nagamori et al. (2018) Geology of the Itoigawa District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, AIST, 75p. (in Japanese with English abstract 3p.)
• Nagamori et al. (2010) Geology of the Kotaki District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, AIST, 130p. (in Japanese with English abstract 4p.)
• Takeuchi et al. (2010) Jour. Geol. Soc. Japan, vol.116, Supplement, pp.123-142 DOI
• Nakano et al. (2002) Geology of the Shiroumadake District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, AIST, 105p. (in Japanese with English abstract 5p.)