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Nakaobara Formation Otsuka, 1947
- Type locality: Nakaobara, Nakasakuma, Kyonan city, Awa county, Chiba prefecture
- Coherent formation
- middle Miocene
- Interfingered (Lower): Okuyama Formation, Okuzure Formation
- Sedimentary facies: Shallow marine
- Water temperature: Warm ocean current
- Lithofacies: Interbedded sandstone and mudstone
- Lithofacies (Minor): Sandstone
- Sedimentary structures (Minor): Slump structure
------ Conformity, ~~~~~~ Unconformity, ~~~--- Local unconformity, '''/,, Interfingered, :::::: Fault, /--/-- Thrust, +__+__ Ground surface, ?--?-- Unknown
- Distribution: Nakaobara - Myohonji - Hara - Fukuyoshi - Mobara - Okune - Hinatabatake - Dogoshi - Yoshihama, Kyonan city, Awa county, Chiba pref.; Hoteigo, Kamogawa city, Chiba pref.; Kanbata, Futtsu city, Chiba pref.
- Fossils: Silicoflagellates
- Fossils (Foraminifera): Amphistegina radiata, Cassidulina pacifica, Cibicides refulgens, Eponides haidengeri, Lepidocyclina makiyamai, Miogypsina kotoi, Quinqueloculina seminulum, Quinqueloculina vulgaris, Robulus limbous, Rotalia ikebei
- Takahashi et al. (2016) Jour. Geol. Soc. Japan, vol.122, no.8, pp.375-395, DOI
- Suzuki et al. (1990) Geology of the Nago District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, 48p. (in Japanese with English abstract 3p.)