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Hiraiso Formation

Invalid synonym: Hiraisomachi Member

• Coherent formation
• Late Cretaceous, Campanian

• Sedimentary facies: Marine
• Lithofacies: Turbidite
• Lithofacies (Upper): Sandstone, Interbedded sandstone and siltstone, Siltstone
• Lithofacies (Upper, Minor): Gravel-bearing siltstone
• Lithofacies (Lower): Siltstone
• Lithofacies (Lower, Minor): Sandstone, Gravel-bearing siltstone
• Sedimentary structures: Slump structure
• Paleocurrent: Northwest to Southeast

• Thickness: ca. 580 m
• Upper limit: Conformity (Isoai Formation)

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

• Distribution: Ibaraki pref.
• Fossils: Echinoids (Niponaster nakaminatoensis), Shellfishes (Inoceramus (Endocostea) cf. balticus), Foraminifera (Dentalina sp., Textularia sp.)
• Fossils (Ammonoids): Didymoceras awajiense, Didymoceras nakaminatoense, Margarites cf. inornatus, Pravitoceras sp.
• Fossils (Spores): Anemia exiloides, Moliria mutabura, Polypodiaceoisporites, Granulatisporites (or Cyatheaceoisporites)
• Fossils (Pollen): Ginkgo cf. labia-pollenites, Pityosporites sp., Populoipollenites sp., Taxoceoipollenites sp., Tricolpites cf. triangulatus

• 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.)