This paper discusses the following subjects concerning the Cretaceous system belonging to the Hetonaian series in the border region of Tokachi and Kushiro districts, Hokkaido.
1. This Cretaceous system is divided into three formations as follows, in descending order. Tokomuro sandy silt member Tomikawa formation Katsuhira mudstone member Tomikawa sandstone and conglomerate member Upper mudstone member Kawaruppu formation Lower sandstone member Kawakami formation
2. According to Iwamoto and Komura (1958), foraminiferal assemblages of this Cretraceous system are divided into the following two groups.
1) The assemblage of Kawakami and Kawaruppu formations: Dominant forms are Haplophragmoides horridus, H. obesus, H. spp. Bathysiphon alexanderi and Bramletteia ezoensis. Characteristic species are Involutina cretacea, I. gaultina, I. hashimotoi, Dorothia bulletta, D, hokkaidoana and Spiroplectammina grzybowskii.
2) The assemblage of Tomikawa formation: Numbers of species and individuals are few. It is characteristic that only the dominant forms of the lower group continue and other species are not found.
3. The author presumes the correlation between the Cretaceous system in this region and that in the Kushiro-Atsukeshi coastal region as follows, based on lithofacies and foraminiferal assemblages.
