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Seki Unit (Matsushima, 1952 ex Sugimoto, 1974)

Invalid synonym: Seki Formation

• Type locality: Tobetsu river, Seki, Yamagata village, Kuji city, Iwate prefecture
• Accretionary complex
• Deposition (Chert): Permina, Middle Triassic Anisian, Late Triassic Carnian (Conodonts)
• Deposition (Mudstone): Middle Jurassic, Bathonian, Late Jurassic, Kimmeridgian (Radiolarians)
• Accretion: Late Jurassic, Kimmeridgian (Radiolarians)

• Sedimentary facies: Marine
• Lithofacies: Coherent facies (3 Chert-clastic sequences)
• Lithofacies (Top): Sandstone (430 - 500 m)
• Lithofacies (Upper): Mudstone (115 - 1,500 m)
• Lithofacies (Middle): Siliceous mudstone (13 - 100 m)
• Lithofacies (Lower): Chert (100 - 500 m)

• Thickness: 2,000 - 2,800 m
• Upper limit: Takinosawa-Yomogigamori thrust fault (Kassenba Formation)
• Lower limit: Iwaizumi Tectonic Line, Seki - Odaira Fault

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

• Distribution: Kawai - Seki, Yamagata village, Kuji city, Iwate pref.; Tayashiki, Akka, Iwaizumi city, Shimohei county, Iwate pref.; Hikusa - Maruko, Karumai city, Kunohe county, Iwate pref.; Yukiya, Kunohe city, Kunohe county, Iwate pref.
• Fossils (Siliceous mudstone, Radiolarians): Archaeodictyomitra sp., Eucyrtidiellum sp., Spongocapsula sp., Striatojaponocapsa sp., Tethysetta sp., Transhsuum sp., Williriedellum sp., Zhamoidellum sp.
• Fossils (Chert, Conodonts): Epigondolella abneptis, Epigondolella bidentata, Neogondolella sp.
• Deposit: Bedded manganese deposit

• Tuzino et al. (2018) Geology of the Ichinohe District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, AIST, 161p. (in Japanese with English abstract 4p.)
• Takahashi et al. (2016) Jour. Geol. Soc. Japan, vol.122, no.1, pp.1-22, DOI
• Ehiro et al. (2008) Jour. Geol. Soc. Japan, vol.114, Supplement, pp.121-139 DOI
• Yoshida et al. (1987) Geology of the Rikuchu-Ono District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, 70p. (in Japanese with English abstract 6p.)