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The Chikubetsu area occupies the northwestern part of the Cretaceous outcrops in Hokkaido (Fig. 1). The Cretaceous sequence in this area consists primarily of massive siltstone with fairly frequently ...interbedded sandstone and belongs to the upper part of the Upper Yezo group. It is locally divisible into five members, A to E, in ascending order (Figs. 2, 3). From the evidence of fossils, listed on Table 2, members A to D are referred to the Upper Urakawan (K5β, Lower to Upper Santonian in terms of the international scale, and member E may range up to the Uppermost Urakawan〔K5β, Lower Campanian. The whole sequence of the Chikubetsu Cretaceous, which reaches 1200 m in thickness, is much thicker and has more sandstone than the contemporaneous deposits of the other already studied areas in Hokkaido and South Sakhalin. The fauna, consisting of ammonites, inocerami and other shells, is, however, essentially similar to that of the typical part of the Upper Urakawan, without any elements of littoral or brackish water environments. The Cretaceous・and the unconformably overlying Tertiary strata form an anticiline of NNW-SSE trend (see the geological map and profiles). The pre-Tertiary structure in the Cretaceous rocks seems to have been a dome of irregular outline.show more
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