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Main member

Invalid synonym (Top): Kacchi Andesite Member

Invalid synonym (Middle): Iwanamesawa Mudstone Member

Invalid synonym (Lower): Onabezawa Andesite Member

• Type locality: Waga river, Oarasawa, Nishiwaga city, Iwate prefecture (Submerged)
• Type locality: Oishi creek and Iwaname creek, Nishiwaga city, Iwate prefecture (Redefined, 2013)
• Coherent formation
• Half-graben associated with rifting of the back-arc basin (Japan sea)
• early - middle Miocene (16.0 - 14.9 Ma, Foraminifera, Calcareous nannofossils; 17.9 Ma, 11.8 Ma, FT; 16.0 Ma, U-Pb)

• Sedimentary facies: Marine
• Sedimentary facies (Lower): Submarine volcano
• Depth of water: Shallow - Upper bathyal
• Lithofacies (Upper): Mudstone, Sandstone, Dacite, Dacitic pyroclastic rocks
• Lithofacies (Middle): Mudstone, Sandstone
• Lithofacies (Lower): Tuff, Lapilli tuff

• Thickness: 700 - 900 m
• Upper limit: Conformity (Kawashiri Tuff Member)
• Upper limit: Unconformity (Oarasawa Formation), previous interpretation has been conformity or interfingered.

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

• Distribution: Waga river, Oishi, Nishiwaga city, Iwate pref.; Iwaname creek - Oishi creek - Minashi creek - Mujinai creek - Washinosu river - Ashike creek, Assabi creek, Nishiwaga city, Iwate pref.
• Fossils: Sponges (Makiyama chitanii), Echinoids (Spines), Bryozoans
• Fossils (Bivalves): Astarte sp., Chlamys nisataiensis, Ennucula sp., Lucinoma cf. otukai, Natica sp., Nemocardium cf. samarangae, Patinopecten kimurai, Patinopecten cf. kimurai ugoensis, Placopecten akihoensis, Panomya simotomensis, Peronidia lutea
• Fossils (Foraminifera): Cyclammina japonica, Globigerinoides bisphericus, Globorotalia birnagea, Globorotalia fohssi barisannensis, Globorotalia opima continuosa, Globorotalia scitula praescitula, Haplophlagmoides renzi, Martinottiella communis
• Fossils (Calcareous nannofossils): Cyclicargolithus floridanes, Discoaster deflandrei, Sphenolithus heteromorphus

• Hosoi et al. (2018) Jour. Geol. Soc. Japan, vol.124, no.10, pp.819-835 DOI
• Hosoi and Amano (2013) Jour. Geol. Soc. Japan, vol.119, no.9, pp.630-646 DOI
• Hosoi and Amano (2012) Jour. Geol. Soc. Japan, vol.118, no.8, pp.493-498 DOI
• Ozawa et al. (1971) Geology of the Kawashiri District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, 40p. (in Japanese with English abstract 5p.)