Chichibumachi Formation

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Chichibumachi Formation (Watabe et al., 1950) Makimoto and Takeuchi, 1992

Invalid synonym: Chichibumachi Group

Invalid synonym (Lower): Nagura Formation

Type locality: Akahira river - Nagaru river, Nagura - Nagaru, Ogano city, Chichibu county, Saitama prefecture
Coherent formation
Half graben-fill deposit
middle Miocene
Correlation: Ichinokawa Formation

Sedimentary facies: Marine, Lower submarine fan - Bottom of sedimentary basin
Depth of water: Upper bathyal - Shallow
Lithofacies (Upper): Sandy siltstone, Siltstone, Conglomerate
Lithofacies (Lower): Sandstone, Sandy siltstone, Interbedded sandy siltstone and sandstone
Sedimentary structures (Lower): Flute cast

Thickness (Upper): ca. 500 - 600 m
Thickness (Lower): 1,700 m
Lower limit: Unconformity (Oganomachi Formation)
Lower limit: Top of the turbidite facies horizon

Akahira Group

[ Upper ]
?--?--?--?--?-
Chichibumachi Formation
--------------
Oganomachi Formation
--------------
Nenokami Formation
--------------
Tomita Formation
--------------
Shirasu Formation
~~~~~~~~~~~~~~
[ Lower ]

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

Distribution: Akahira river - Nagaru river, Nagura - Nagaru, Ogano city, Chichibu county, Saitama pref.; Shimotsuyagi - Hannya, Ogano city, Chichibu county, Saitama pref.; Onohara - Tadenuma - Terao - Tochiya - Yokoze river - Kuroya, Chichibu city, Saitama pref.
Fossils: Desmostylus sp., Paleoparadoxia tabatai, Mysticeti, Crocodiles, Sea turtles (Kurobechelys tricarinata), Gannets (Sula sp.), Fishes (Scomberomorus sp.), Crabs, Calcareous algae, Trace fossils (Double cylinders)
Fossils (Shellfishes): Chlamys sp., Lucinoma? sp., Mitylus? sp., Yoldia sp.
Fossils (Foraminifera): Bolivina robusta Bray, Bolivina sp., Reophax sp., Nonionella sp., Lepidocyclina sp., Miogypsina sp., Nodosaria sp., Cibicides sp.

Matsui et al. (2016) Jour. Geol. Soc. Japan, vol.122, no.6, pp.261-266, DOI
Takahashi and Yanagisawa (2004) Jour. Geol. Soc. Japan, vol.122, no.12, pp.625-635.
Makimoto and Takeuchi (1992) Geology of the Yorii District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, 136p. (in Japanese with English abstract 6p.)