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This study outlines an attempt to establish the Cretaceous megafossil biostratigraphy of the Migi-no-sawa area (Haboro Dome), a tributary of the Haboro River in northwestern Hokkaido. The Cretaceous sediments in this area consist of the following two formations of the Yezo Group: (1) the lowermost Upper Haborogawa Formation, an upward-coarsening cycle of sandy siltstone or laminated siltstone to coarse-grained sandstone, and (2) the uppermost Nagareya Formation, consisting of a lower coarse-grained section of silty sandstone and an upper finer-grained section of siltstone. These Cretaceous sediments contain abundant megafossils, especially the Haborogawa Formation and the lower part of the Nagareya Formation. The stratigraphic distribution of ammonoids and inoceramids indicates that the Haborowaga and Nagareya formations are Santonian and Campanian in age, respectively. The study area has some faunal differences, primarily in heteromorphic ammonite abundances, when compared with the Sakasa-gawa area; these differences suggest that sediments in the Sakasa-gawa area formed in deeper water environments than those within the study area. Hamitoid ammonites dominate the study area, indicating that these ammonites inhabited shallower areas than other heteromorphic ammonites.

Restudy of the Upper Cretaceous megafossil biostratigraphy in the Migi-no-sawa area, a tributary of the Haboro River, northwestern Hokkaido