Shirahama Group

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Shirahama Group (Tayama and Shinno, 1931) (Photo)

Type locality: Shirahama seashore, Shimoda city, Shizuoka prefecture
Volcanic rocks
Late Miocene - Pliocene (ca. 9.0 - 3.0 Ma)
Correlation: Hayakawa Tuff Breccia
Intruded by: Diorite porphyry, Quartz diorite porphyry, Dacite body

Sedimentary facies: Shallow marine
Sedimentary facies: Submarine volcanoes
Depth of water: Sublittoral (50 - 250 m)
Lithofacies: Lava, Tuff breccia, Lapilli tuff, Hyaloclastite, Submarine pyroclastic rocks
Lithofacies (Upper): Pyroxene andesite
Lithofacies (Upper, Minor): Volcanic conglomerate, Tuff breccia
Lithofacies (Middle): Fossils shell-bearing tuffaceous sandstone
Lithofacies (Lower): Dacite lava, Clastic rocks
Lithofacies (Lower, Minor): Basalt, Andesite, Rhyolite
Lithofacies (Bottom): Limestone
Sedimentary structures: Channel structure, Coss-stratification, Hummocky cross-stratification, Parallel-stratification

Thickness: 700 - 1,000 m or thinner
Upper limit: Unconformity (Chokuro Andesites, Jaishi volcano)
Lower limit: Unconformity (Yugashima Group, Shirakawa Basalts, Shiradagawa Volcanics)

Dogashima
[ Upper ]
?--?--?--?--?-
Dogashima Formation
'''''''/,,,,,,
Matsuzaki Formation
~~~~~~~~~~~~~~
[ Lower ]

Kawazu
[ Upper ]
--------------
Kamikawazu Andesite
--------------
Shimokawazu Andesite
--------------
Mitaka Dacite
--------------
Mitaka Sandstone
--------------
Shobuhama Basalt
~~~~~~~~~~~~~~
[ Lower ]

Shirahama
[ Upper ]
--------------
Harada Formation
--------------
Itami Formation
~~~~~~~~~~~~~~
[ Lower ]

Shimoda
[ Upper ]
?--?--?--?--?-
Itado Andesite
~~~~~~~-------
Harada Formation
--------------
Asahi Formation / Suzaki Andesite
~~~~~~~~~~~~~~
[ Lower ]

Southern Izu
[ Upper ]
?--?--?--?--?-
Yoshida Andesite
~~~~~~~-------
Ishiki Formation / Nijo Dacite
'''''''/,,,,,,
Irozaki Andesite
~~~~~~~-------
Nakagi Formation
~~~~~~~~~~~~~~
[ Lower ]

Cape Irozaki
[ Upper ]
?--?--?--?--?-
Ogamo Rhyolite
--------------
Aoichi Basalts
--------------
Ichijo Andesite
--------------
Ishiki Formation / Nijo Dacite
'''''''/,,,,,,
Irozaki Andesite
~~~~~~~~~~~~~~
[ Lower ]

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

Distribution: Shirahama - Shimoda - Toji - Cape Irozaki - Koura, Shimoda city, Shizuoka pref., Kumomi - Matsuzaki, Matsuzaki city, Kamo county, Shizuoka pref., Nishina - Dogashima - Tako, Nishiizu city, Kamo county, Shizuoka pref.
Distribution (Basalt): Shobuhama, Kawazu city, Kamo county, Shizuoka pref.; Mount Takanesan, Shimoda city, Shizuoka pref.; Mount Chokuroyama, Matsuzaki city, Kamo county, Shizuoka pref.
Distribution (Andesite): Matsuzaki - Yagiyama - Nango - Nishina, Matsuzaki city, Kamo county, Shizuoka pref.
Distribution (Pyroxene andesite): Kora - Urui island, Minamiizu city, Kamo county, Shizuoka pref.; Yugano, Kawazu city, Kamo county, Shizuoka pref.
Distribution (Dacite): Yatsu - Nawaji mine - Mount Tenreisan, Kawazu city, Kamo county, Shizuoka pref.; Kosugihara - Yagiyama, Matsuzaki city, Kamo county, Shizuoka pref.; Kako - Dogashima, Nishiizu city, Kamo county, Shizuoka pref.; Magome, Minamiizu city, Kamo county, Shizuoka pref.; Mount Manzoyama - Mount Uenoyama, Shimoda city, Shizuoka pref.
Distribution (Rhyolite): Mount Manzoyama, Shimoda city, Shizuoka pref.
Fossils: Barnacles
Fossils (Shellfishes): Acila schencki, Aloidis cf. taiwanensis, Amussiopecten cf. praesignis, Arca? sp., Barbatia sp., Bentharca sp., Callista sp., Cerithidea sp., Chlamys kakisakiensis, Chlamys miurensis, Chlamys satoi, Chlamys shirahamaensis, Cryptopecten javanus?, Cryptopecten vesiculosus, Decatopecten izuensis, Decatopecten tayamai, Dentalium sp., Diraricella sp., Dosinia sp., Fulvia sp., Glycymeris rotunda, Glycymeris totomiensis, Inquisitor sp., Keenaea cf. samarangae, Lima zushiensis, Lucinoma sp., Mytilus sp., Nassarius caelatus, Onustus exutus, Ostrea sp., Paphia exilis takaokaensis, Paphia cf. schnelliana, Patinopecten planicostulatus, Pholadomya pacifica, Pillucina? sp., Portlandia cf. lischkei, Pterygia sp., Septifer sp., Spondylus anacanthus, Strombus sp., Tonna sp., Turbo sp., Turcicula crumpii yokoyamai, Vasticardium arenicola, Venericardia crenulicostata, Veneridae, Venus foveolata, Vollsella? sp.,
Fossils (Brachiopods): Asprella sieboldianus, Terebratulina sirahamaensis
Fossils (Limestone, Foraminifera): Amphistegina radiata, Carpenteria sp., Elphidium crispus, Lageridae, Globigerina sp., Globorotalia sp., Lepidocyclina (Nephrolepidina) angulosa, Lepidocyclina (Nephrolepidina) nipponica, Lepidocyclina (Nephrolepidina) nipponica var. izuensis, Miogypsina kotoi
Distribution (Limestone): Nashimoto, Kawazu city, Kamo county, Shizuoka pref.; Matsuzaki - Ikeshiro - Kosugihara - Mount Basarayama, Matsuzaki city, Kamo county, Shizuoka pref.

Kano and Ito (2016) Jour. Geol. Soc. Japan, vol.122, no.8, pp.413-432, DOI
Sawamura et al. (1970) Geology of the Shimoda District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, 41p. (in Japanese with English abstract 4p.)
Ono and Sumi (1959) Inatori, Explanatory Text of the Geological Map of Japan, Scale 1:50,000, Geological Survey of Japan, 23p. (in Japanese with English abstract 3p.)
Sumi (1958) Mikomotojima, Explanatory Text of the Geological Map of Japan, Scale 1:50,000, Geological Survey of Japan, 33p. (in Japanese with English abstract 6p.)