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Otsuka Formation Ishii et al., 1982

Invalid synonym: Yagami Formation, Shukuura Formation, Takagi Formation, Rikuzenotsuka Formation, Urato Formation

• Type locality: Otsuka - Shukuura, Higashimatsushima city, Miyagi prefecture
• Coherent formation
• early - middle Miocene

• Sedimentary facies: Marine
• Lithofacies: Siltstone
• Lithofacies (Minor): Tuffaceous sandstone, Sandy siltstone, Breccia

• Thickness: 300 m
• Upper limit: Unconformity (Neko Formation, Lower Member, Hataya Formation, Nemari Tuffaceous Sandstone Member, Shiomidai Formation, Kameoka Formation, Tatsunokuchi Formation)
• Lower limit: The boundary between siltstone-dominant horizon and underlying pumice tuff and tuffaceous sandstone-dominant horizon
• Lower limit: Transitional (Upper Pumice Tuff Member), Conformity, partly interfingered (Toguhama Formation)

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

• Distribution: Otsuka - Shukuura - Takagi river - Naruse river - Miyato island, Higashimatsushima city, Miyagi pref.; Katsura island - Sabusawa island, Shiogama city, Miyagi pref.
• Fossils (Diatoms): Achnanthes cfr. groenlandica, Actinocyclus ehrenbergii, Actinocyclus ingens, Actinocyclus ingens var. nodes, Actinocyclus octonaris, Actinoptychus senarius, Actinoptychus splendens, Actinoptychus undulatus, Amphora costata, Asteromphalus sp., Aulacosira granulata, Bacillaria paradoxa, Bacteriastrum varians, Biddulphia aurita, Biddulphia calamus, Biddulphia tuomyi, Clavicula polymorpha, Cocconeis californica, Cocconeis costata, Cocconeis scutelium, Cocconeis vitrea, Coscinodiscus cfr. curvatulus, Coscinodiscus endoi, Coscinodiscus lewsianus, Coscinodiscus marginatus, Coscinodiscus aff. nodulifer, Coscinodiscus oculus-iridis, Coscinodiscus symbolophorus, Coscinodiscus vetutissimus, Craspedodiscus rhombicus, Cymatogonia amblyoceras, Cymatosira debyi, Cymatosira aff. lorentziana, Diatoma? sp., Delphineis penellliptica, Denticulopsis lauta, Denticulopsis nicobarica, Denticulopsis norwegica, Diploneis smithii, Eunotia sp., Glyphodesmis sp., Goniothecium rogersii, Grammatophora sp., Hemiaulus bipons, Hemiaulus aff. polymorphus, Hyalodiscus obsoletus, Hyalodiscus ukaiensis, Goniothecium tenue (<= Ikebea spp.), Isthmia sp., Kisseleviella carna, Liradiscus bipolaris, Medialia splendida, Melosira sol, Melosira sulcata, Navicula spp., Neodelphineis? sp., Nitzschia challengeri, Paradia sulcata, Plagiogramma staurophorum, Pleurosigma sp., Porosira cfr. gracialis, Pseudodimerogramma elegans, Pseudopodosira aff. hyalina, Pterotheca subulata, Raphidodiscus marylandiscus, Rhabdonema japonicum, Rhabdonema minutum, Rhaphoneis elegans, Rhaphoneis miocenica, Rhizosolenia alata, Rhizosolenia miocenica, Rhizosolenia cfr. praebarboi, Rhizosolenia styliformis, Rhizosolenia aff. praebarboi, Rouxia naviculoides, Sawamuraia biseriata, Stephanogonia aff. hanzawae, Stephanogonia polyacantha, Stephanopyxis spp., Stictodiscus kittonianus, Synedra jouseana, Synedra miocenica, Thalassionema hirosakiensis, Thalassionema nitzschioides, Thalassiosira leptopus, Thalassiothrix longissima, Trachyneis aspera, Triceratium condecorum, Trochosira spinosa, Zygoceros quadricornis

• Ishii et al. (1983) Geology of the Shiogama District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, 112p. (in Japanese with English abstract 6p.)
• Ishii et al. (1982) Geology of the Matsushima District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, 121p. (in Japanese with English abstract 9p.)