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Ohara Formation Mitsuchi, 1939
- Type locality: Ohara, Isumi city, Chiba prefecture
- Coherent formation
- Fore-arc basin sediments (Kazusa trough)
- Contemporaneous heterotopic facies: Kurotaki Formation
- Sedimentary facies: Abyssal plain, lower Submarine fan, slope base of Submarine fan
- Lithofacies: Sandstone-dominant interbedded sandstone and mudstone, Mudstone-dominant interbedded sandstone and mudstone, Mudstone
- Sedimentary structures (Middle): Slump structures
- Thickness: ca. 250 m
- Upper limit: Top of Oh1
- Lower limit: Bottom of sandstone-dominant interbedded sandstone and mudstone
- Lower limit: Conformity (Namihana Formation)
------ Conformity, ~~~~~~ Unconformity, ~~~--- Local unconformity, '''/,, Interfingered, :::::: Fault, /--/-- Thrust, +__+__ Ground surface, ?--?-- Unknown
- Distribution: Ohara, Isumi city, Chiba pref.; Kamifuse, Onjuku city, Isumi county, Chiba pref.; Shukudo - Haga - Kobato - Matsuno, Katsuura city, Chiba pref.
- Fossils (Shellfishes): Cavolinia tridentata, Clio pyramidata, Diacriatrispinosa sp., Nuculana
- Fossils (Foraminifera): Grobolitaria truncatulinoidesDiscoaster brouweri
- Fossils (Calcareous nannofossils): Discoaster brouweri
- Fossils (Trace fossils): Chondrites isp., Skolithos isp., Zoophycos isp.
- Tephra: Oh1, Oh2, Oh3-1, Oh3-2, Oh4, HS A, HS B, HS C, Oh8, Oh9, Oh10, Oh11, Oh12, Oh13, Oh14, Oh15, Ooi 2, Ooi 1, Oh17, KB, Oh19
- Utsunomiya and Ooi (2019) Geology of the Kazusa-Ohara District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, AIST, 127p. (in Japanese with English abstract 4p.)
- Tamura et al. (2019) Jour. Geol. Soc. Japan, vol.125, no.1, pp.23-39 DOI
- GSSP proposal group (2019) Jour. Geol. Soc. Japan, vol.125, no.1, pp.5-22 DOI
- Suzuki and Murata (2011) Jour. Geol. Soc. Japan, vol.117, no.7, pp.379-397 DOI