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Seguchi Formation Fujimoto et al., 1946

Invalid synonym: Byobuiwa Formation

• Type locality: Matsuda river, Seguchi, Iiyama city, Nagano prefecture
• Coherent formation
• late Pliocene
• Correlation: Shiroiwa Formation, upper Nishiyama Formation

• Sedimentary facies: Shallow marine
• Sedimentary facies: Inner bay
• Depth of water: Upper - lower sublittoral
• Lithofacies: Sandstone
• Sedimentary structures: Bioturbation
• Sedimentary structures (Minor): Hummocky cross-stratification

• Thickness: 15 - 250 m
• Upper limit: Bottom of the horizon intercalating gravelly sandstone and non-marine mudstone beds
• Upper limit: Conformity (Ogawa Formation)
• Lower limit: Conformity (Nigoriike Formation), Unconformity (Tomikura Formation)

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

• Distribution: Matsuda river, Seguchi, Iiyama city, Nagano pref.
• Fossils (Omma-Manganji fauna): Anadara (Anadara) amicula, Anadara aff. subcrenata, Crassostrea gigas, Dosinia (Dosinella) penicillata, Felaniella usta, Glycymeris (Glycymeris) albolineata, Glycymeris (Glycymeris) yessoensis, Mizuhopecten yokoyamae, Myadora fluctuosa, Peronidia venulosa, Spisula (Pseudocardium) sachalinensis, Turritella saishuensis saishuensis, Yoldia notabilis
• Tephra: Hrp

• Yanagisawa et al. (2000) Geology of the Iiyama District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, 144p. (in Japanese with English abstract 6p.)