Abstract
This paper describes the biostratigraphy of the Middle and Upper Yezo Groups exposed in the Kotanbetsu area of northwestern Hokkaido. The Cretaceous deposits exposed in the upper reaches of the Kotanbetsu River consist of the following five formations in ascending order : (1) the Takimibashi Formation of the Middle Yezo Group (over 600 m thick), which is composed of conglomerate and siltstone ; (2) the Tenkaritoge Formation of the Middle Yezo Group (about 2, 200 m thick), which is composed of sandstone and silstone ; (3) the Shirochi Formation of the Middle Yezo Group (about 500 m thick), which is composed of alternating beds of sandstone and siltstone, and generally shows coarsening-upward sequence ; (4) the Middle Haborogawa Formation of the Upper Yezo Group (1, 080-2, 200 m thick), which is composed predominantly of siltstone with many interbeds of sandstone ; and (5) the Upper Haborogawa Formation of the Upper Yezo Group (about 790 m thick), which has at least three cycles of upward-coarsening sequence (U2-U4) ranging from sandy siltstone to sandstone.The Upper Cretaceous deposits yield abundant molluscan fossils in the Kotanbetsu area. As the results of the detailed examination of the assemblages and stratigraphic ranges of the contained species, the stage boundaries between the Cenomanian and the Campanian arc allocated to the upper part of M4 - lowermost part of M5 (Cenomanian/Turonian), the lower part of U1 (Turonian/Coniacian), the middle part of U1 (Coniacian/Santonian) and the upper part of U2 - lower part of U4 (Santonian/Campanian) respectively.
