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Koetoi Formation (Watanabe, 1914) Fukuzawa, 1985

• Coherent formation
• Foreland basins by collision between northeast Japan island arc and Kuril island arc
• late Miocene - Pliocene

• Sedimentary facies: Marine
• Lithofacies: Siliceous mudstone, Diatomaceous mudstone
• Lithofacies (Upper): Sandy
• Lithofacies (Minor): Tuff
• Sedimentary structures: Unclear bedding planes

• Upper limit: Bottom of muddy sandstone bed
• Upper limit: Conformity (Yuchi Formation)
• Lower limit: Bottom of the bed with unclear bedding planes
• Lower limit (previous definition): boundary of Opal-A/Opal-CT
• Lower limit: Transitional (Wakkanai Formation)

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

• Distribution: Soya region, Hokkaido
• Distribution: Horonobe city, Teshio county, Hokkaido
• Fossils: Shellfishes, Diatom (Neodenticula kamtschatica)

• Kase et al., (2018) Jour. Geol. Soc. Japan, vol.124, no.8, pp.627-642 DOI
• Ishii et al. (2008) Jour. Geol. Soc. Japan, vol.114, no.6, pp.286-299 DOI
• Ishii et al. (2007) Jour. Geol. Soc. Japan, vol.113, no.2, pp.41-52 DOI
• Niizato et al. (2007) Jour. Geol. Soc. Japan, vol.113, Supplement, pp.119-135 DOI