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Shonai Group Kujiraoka, 1953

• Type locality: Terauchi - Oishi, Sakata city, Yamagata prefecture
• Coherent formation
• late Pleistocene
• Correlation: Shibikawa Formation
• Correlation (Lower): Tengumori Volcaniclastic Rocks

• Lithofacies: Sands, Gravels, Mud
• Lithofacies (Minor): Andesite tuff breccia, Volcanic breccia, Lapilli tuff, Lava, Agglomerate, Lignite

• Thickness: 150 - 800 m
• Upper limit: Unconformity (Chokai volcano products)
• Lower limit: Unconformity (Kannonji Formation, Jozenji Formation, Maruyama Formation)

• Distribution: Terauchi - Oishi - Nikko river - Kannonji - Narahashi, Sakata city, Yamagata pref.
• Fossils: Bivalves (Corbicula? sp.), Plant fragments

• Nakano and Tsuchiya (1992) Geology of the Chokaisan and Fukura District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, 138p. (in Japanese with English abstract 7p.)
• Tsuchiya et al. (1984) Geology of the Tsuruoka District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, 77p. (in Japanese with English abstract 5p.)
• Ikebe et al. (1979) Geology of the Sakata District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, 42p. (in Japanese with English abstract 3p.)