Depositional age of the Kuromatsunai Formation was examined on the basis of biostratigraphic study of Neogene radiolarians in the Imakane area, southwestern Hokkaido, Japan. Three radiolarian biozones, the
Lithelius barbatus Zone (6.8-5.6 Ma), the
Stylacontarium acquilonium Zone (5.6-5.2 Ma), and the
Spongurus pylomaticus Zone (5.2-4.2 Ma), were recognized from the Sumiyoshi Siltstone Member of the formation. The lowest and the upper most parts of the member were correlated to the middle part of the
L. barbatus Zone (ca. 6 Ma) and the base of the
S. pylomaticus Zone (5.2 Ma), respectively.
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