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Sekimon Formation (Ujiie and Matsumaru, 1977) Umino et al., 2016
Invalid synonym: Yusan Formation, Okimura Formation
- Type locality: Sekimon, Hahajima, Ogasawara city, Tokyo
- Coherent formation
- Upper: Eocene (33.8 Ma, Foraminifera)
- Lower: Eocene (40.6 - 38.5 Ma, Foraminifera; 40.2 Ma, Ar-Ar)
- Sedimentary facies: Shallow marine
- Sedimentary facies: Calm sea
- Sedimentary facies (Top): Coral reef, Continental shelf
- Sedimentary facies (Upper): Continental slope
- Sedimentary facies (Lower): Shallow continental shelf, Lagoon
- Depth of water: 100 - 200 m
- Lithofacies (Upper): Hyaloclastite, Limestone
- Lithofacies (Lower): Calcareous sandstone, Sandstone, Conglomerate, Pumiceous tuff breccia, Pyroclastic fall deposit, Base-surge deposit
- Distribution: Sekimon - Nankinhama beach - Yusan coast - Higashidai - Gagyukaku - Mount Sakaigatake - Mount Chibusayama - Komoridani - Motochi, Hahajima, Ogasawara city, Tokyo
- Fossils (Top): Corals, Calcareous algae
- Fossils: Shellfishes, Echinoids, Calcareous algae
- Fossils (Top, Foraminifera): Amphistegina radiata, Asterocyclina pentagonalis, Asterocyclina stella, Biplanispira absurda, Biplanispira mirabilis, Discocyclina dispansa, Eorupertia boniensis, Fabiania cassis, Pellatispira orbitoidea, Pellatispira provalei
- Fossils (Lower, Foraminifera): Alveolina elliptica, Asterocyclina asterodisca, Asterocyclina hahajimensis, Asterocyclina incisuricamerata, Asterocyclina pentagonalis, Asterocyclina stella, Daviesina boninensis, Discocyclina augustae, Discocyclina dispansa, Eorupertia boninensis, Nummulites aturicus, Nummulites gizehensis, Nummulites millecaput, Nummulites pengaronensis, Nummulites perforatus, Operculina schwageri, Orbitoclypeus kimurai
- Fossils (Upper, Trace fossils): Polychaeta
- Umino et al. (2016) Geology of the Hahajima Retto District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, AIST, 46p. (in Japanese with English abstract 2p.)