Oshima sedimentary-volcanic basin 

(late Paleogene to Quaternary) 

(Hokkaido Island, Japan, unit Czvi)

Consists of following sedimentary units: (1) Futoro, Matahachizawa, Aonaegawa and lower Kayamuma 
Formations (late Oligocene to early Miocene). These units consist chiefly basalt lava, rhyolite-andesite lava 
and tuff, sandstone and mudstone. K-Ar age of andesite lava is 24 Ma, and fission track ages of rhyolite 
range from 21.8 to 31.9+1.3 Ma. (2) Kunnnui Formation (early to middle Miocene) which consists chiefly 
of sandstone, mudstone, tuff and andesite lava. Fission track age of tuff is 14.5 and 17.1 Ma. Contains 
diatoms. (3) Hiyama Group (middle Miocene to Pliocene), which consists chiefly of marine sandstone, 
siltstone, mudstone, tuff, and andesite lava and pyroclastic rocks. Includes mollusks, foraminifera and 
diatoms. Fission track age of tuff ranges from 11.2 to 4.0. And (4) Setana Formation (Pliocene) which 
consists chiefly of sandstone and conglomerate. Includes mollusks and foraminifers. Quaternary of this 
basin consists mainly of lacustrine, fluvial, and terrace deposits. Basin overlies Oshima terrane. 
REFERENCES: Kato and others, 1990; Kano and others, 1991.