Ishikari-Tenpoku-Hidaka sedimentary-volcanic basin 

(Cretaceous and Late Paleogene to Holocene) 

(Hokkaido Island, Japan, unit CzKs)

Consists of following sedimentary formations at Ishikari coal field: (1) Kumaneshiri Group (Cretaceous). 
Consists chiefly of andesite and basalt volcaniclastic sedimentary rocks, sandstone and mudstone. Total 
thickness greater than 3000 m. Unit contains Early Cretaceous radiolarians. Ar-Ar age of andesite and 
basalt is 101.3+3.6 Ma. (2) Rebun Group (Cretaceous). Chiefly andesite and basalt lava and tuff, and 
volcaniclastic sedimentary rocks. Thickness greater than 2500 m. Contains Early Cretaceous ammonites, 
corals, and other fossils. Ar-Ar age of volcanic rocks is 100.6 Ma. (3) Ishikari Group (middle to late 
Eocene). Chiefly nonmarine sandstone and siltstone, lesser brackish to shallow marine sandstone and 
mudstone. Includes plant fossils and mollusks. Contains Ishikari and Rumoi coal fields. Total thickness 
about 2400 m. (4) Uryu Group (middle to late Eocene). Chiefly nonmarine sandstone and mudstone, and 
lesser brackish sedimentary rocks with coal beds. Divided into Shiraki, Uryu, Tachibetsu, Migioomata, 
Showa, Icchanai, and Opirafuneppuannai Formations in ascending order. Includes plant fossils and 
mollusks. Coeval with Ishikari Group. (5) Poronai Group (late Eocene to Oligocene). Chiefly marine 
sedimentary rocks. Divided into Poronai and Momijiyama Formations in ascending order. Contains 
foraminiferas, mollusks. K-Ar age of mafic volcanic rocks of Momijiyama Formation is 32.6+1.7. Total 
thickness about 1600 m. (6) Takinoue Formation (early to middle Miocene). Chiefly basal conglomerate, 
glauconite sandstone in the lower part and mudstone in the upper part. Includes middle Miocene mollusks. 
Unconformably underlain by Yezo Group of Sorachi-Yezo terrane. Thickness ranges from 150-450 m. (7) 
Asahi Formation (Miocene). Chiefly nonmarine sedimentary rocks with coal beds. Includes Daishima-type 
plant fossils. (8) Kawabata Formation (middle Miocene). Chiefly turbidite deposits composed of sandstone, 
mudstone, and conglomerate. Thickness greater than 4000 m. (9) Iwamizawa Formation (latest middle to 
late Miocene). Chiefly diatomaceous mudstone locally with intercalated sandstone and conglomerate. 
Thickness ranges from 100-500 m. And (10) Minenobu Formation (Pliocene). Chiefly sandstone with local 
pumiceous tuff layers. Contains mollusks and diatoms. Thickness is 200 m. Overlying Quaternary deposits 
of region consist mainly of lacustrine, fluvial and terrace deposits. The Ishikari-Tenpoku-Hidaka 
sedimentary-volcanic basin overlies Oshima, Sorachi-Yezo, Kamuikotan and Hidaka terranes. 
REFERENCES: Kato and others, 1990; Kano and others, 1991.