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Isoai Formation

• Coherent formation
• Late Cretaceous, Maastrichtian

• Sedimentary facies: Marine
• Lithofacies: Turbidite
• Lithofacies: Sandstone, Interbedded sandstone and siltstone, Siltstone
• Lithofacies (Minor): Conglomerate
• Sedimentary structures: Parallel stratification, Cross stratification, Graded bedding, Slump structure, Flute cast, Groove cast, Mudstone dikes
• Paleocurrent: Northwest to Southeast

• Thickness: ca. 930 m
• Lower limit: Conformity (Hiraiso Formation)

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

• Distribution: Ibaraki pref.
• Fossils: Echinoids (Diadema sp. Nipponaster nakaminatoensis), Sharks (Isurus nakaminatoensis), Trace fossils
• Fossils (Ammonoids) Baculites inornatus, Baculites cf. rex, Polyptychoceras sp.
• Fossils (Shellfishes): Acila (Truncacila) cf. hokkaidoensis, Brunonia cassidaria, Ezonuculana mactraeformis, Glycymeris (Hancia) Hokkaidoensis multicostata, Inoceramus cf. shikotanensis, Lucina (Myrtea) ezoensis, Lucina nakaminatoensis, Ostrea sp., Tellina sp.
• Fossils (Foraminifera): Bathsiphon alexanderi, Bathsiphon sakuensis, Bramletteia ezoensis, Dentalina gracilis, Globotruncana paraventricosa, Haplophragmoides horridus, Lenticulina obirashibensis, Robulus cf. matsumotoi, Rzehakina epigona, Saracenaria sp., Silicosigmoilina futabaensis

• 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.)