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Tonoyama Formation Saito, 1956

• Coherent formation
• middle Miocene

• Sedimentary facies: Marine
• Lithofacies (Upper): Sandy mudstone
• Lithofacies (Middle): Sandstone, Interbedded sandstone and conglomerate
• Lithofacies (Lower): Mudstone, Interbedded sandstone and mudstone

• Thickness: ca. 190 m
• Upper limit: Unconformity (Uwaichi Terrace Gravels)
• Lower limit: Angular unconformity (Chikko Formation)

• Distribution: Nakaminato - Hiraiso town, Hitachinaka city, Ibaraki pref.
• Fossils: Whales, Echinoids (Astricripeus sp.), Trace fossils
• Fossils (Shellfishes): Acila yanagawaensis, Anadara sp., Calliostoma namuchakuensis, Calyptoraea cf. tabura, Cancellaria pristina, Cardilia cf. yudaensis, Clinocardium cf. shinjiense, Crassostrea cf. gigas, Crepidula cf. symmetrica, Crepidula zyobanica, Dosinia sp., Katherinella iizukai, Lucinoma acutilineatum, Megasurcula kurodai, Modiolus sp., Nassarius simizui, Nucula (Ennucula) akitana, Paphia sp., Ringicula ninohensis, Teredinids, Turritella s-hatai, Venericardia siogamensis

• Sakamoto et al. (1972) Geology of the Nakaminato District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, 94p. (in Japanese with English abstract 5p.)