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Oya Formation (Ohta, 1949) Takahashi and Yoshikawa, 2008

Invalid synonym (Upper): Yamamoto Formation

Invalid synonym (Lower): Nagaoka Formation

• Type locality: Oya town, Utsunomiya city, Tochigi prefecture
• Type locality: Tagawa river, Kawaraya town, Utsunomiya city, Tochigi prefecture
• Coherent formation
• middle Miocene (14.7, 13.7, 12.9 Ma, FT; 14.2 Ma, K-Ar)
• Correlation: Lower Kobana Formation
• Intruded by: Rhyolite, Dacite, Hornblende pyroxene andesite

• Sedimentary facies: Rhyolitic volcanic fan deposit
• Sedimentary facies: Submarine pyroclastic rocks, Volcaniclastic turbidite
• Lithofacies: Rhyolitic lapilli tuff, Tuff breccia, Tuff, Tuffaceous sandstone, Siltstone
• Lithofacies (Upper): Pumice lapilli tuff
• Lithofacies (Middle): Tuffaceous sandstone
• Lithofacies (Lower): Interbedded rhyolitic lapilli tuff, tuff and tuffaceous sandstone
• Lithofacies (Lower, Minor): Conglomerate, Tuff breccia
• Sedimentary structures (Upper, Minor): Channel structure, Slump structure
• Sedimentary structures (Lower, Minor): Parallel stratification, Trough cross-stratification, Planar cross-stratification

• Source: Mount Kuroto - Mount Kabuto - Mount Takadate
• Source: Tagawa river, Yokoyama town - Kawaraya town, Utsunomiya city, Tochigi pref.
• Thickness: ca. 370 - 600 m
• Upper limit: Conformity (Ozo Formation)
• Lower limit: Conformity (Yokoyama Formation), Abut unconformity (Myogazawa Formation), Unconformity (Kazamiyamada Formation)

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

• Distribution: Oya town - Tokujiro town, Utsunomiya city, Tochigi pref.; Tagawa river, Kawaraya town, Utsunomiya city, Tochigi pref.; Mount Hibarigatoya - Mount Tagesan, Utsunomiya city, Tochigi pref.
• Fossils: Fishes, Sharks (Teeth), Echinoids, Solitary corals, Plants, Silicified wood, Foraminifera, Radiolarians
• Fossils (Moniwa fauna): Chlamys kaneharai, Nanaoclamys notoensis notoensis, Turritella tanaguraensis, Vasticardium shiobaraense

• Matsui et al. (2011) Jour. Geol. Soc. Japan, vol.117, Supplement, pp.89-102 DOI
• Yoshikawa et al. (2010) Geology of the Utsunomiya District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, AIST, 79p. (in Japanese with English abstract 6p.)
• Hayashi et al. (2008) Jour. Geol. Soc. Japan, vol.114, no.3, pp.141-144 DOI