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Ketto Formation Shimazu et al., 1983

• Type locality: Nakatsu river, Koakasawa, Sakae city, Shimominochi county, Nagano prefecture - Ketto - Sakamaki, Tsunan city, Nakauonuma county, Niigata prefecture
• Volcanic rocks
• Miocene
• Correlation: Nanatani Formation (Foraminifera)

• Sedimentary facies: Marine
• Lithofacies: Pyroxene andesite lava, Lapilli tuff, Tuff breccia, Volcanic breccia
• Lithofacies (Minor): Andesite welded tuff, Basalt lava (Pillow lava), Hyaloclastite, Dacite welded tuff, Mudstone, Sandstone

• Thickness: 1,500 m or thicker
• Upper limit: Unconformity (Akiyamago Formation, Nishitajiri Formation, Uonuma Formation, Takayamazawa-Onizawa Pyroclastic Flow Deposit, Uenohara Welded Tuff, Shirakurayama Lavas, Maekura Lavas and Pyroclastics, Kayanotaira Pyroclastic Flow Deposit, 1st stage products, 2nd stage products, 3rd stage products)

• Distribution: Ketto - Sakamaki, Tsunan city, Nakauonuma county, Niigata pref.; Nakatsu river - Io river, Tsunan city, Nakauonuma county, Niigata pref.; Akiyamago - Koakasawa - Kamakawa river, Sakae city, Shimominochi county, Nagano pref.; Kamakawa river - Chikura creek, Nakasato, Tokamachi city, Niigata pref.
• Fossils: Foraminifera

• Igarashi et al. (2007) Jour. Geol. Soc. Japan, vol.113, no.11, pp.565-584 DOI
• Shimazu and Tateishi (1993) Geology of the Naebasan District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, 90p. (in Japanese with English abstract 5p.)