Cretaceous stratigraphy in the Chikubetsu area, Hokkaido

Abstract

The Cretaceous deposits distributed in the Chikubetsu area, northwestern Hokkaido, are lithostratigraphically divided into the underlying Upper Yezo and the Hakobuchi Groups. They form an asymmetrical anticlinal structure, whose axis has a NNW-SSE trend and plunges to the NNW with an axial-plane fault. The Upper Yezo Group is subdivided into two formations, the underlying Middle Haborogawa Formation (over 450 m thick) and the Upper Haborogawa Formation (about 800 m). The former consists of mudstone, sandy mudstone and muddy sandstone with frequent intercalations of thin sandstone, while the latter of a coarsening upward sequence is composed of dark gray mudstone, bioturbated sandy mudstone and light gray muddy sandstone, and cross-bedded gray or bluish gray glauconitic sandstone in ascending order. The Hakobuchi Group is represented by the Pankezawa Formation (135-170 m thick), which is redefined in this study. It consists of dark gray mudstone, gray or bluish gray sandstone and parallel or cross laminated tuff in ascending order. The Upper Yezo Group is unconformably overlain by the Hakobuchi Group. Based on the stratigraphic distribution of ammonoids and inoceramids, the Middle Haborogawa Formation and the lower part of the Upper Haborogawa Formation are correlated to the Santonian, and the upper part of the Upper Haborogawa Formation and the Pankezawa Formation to the Campanian to the Maastrichtian. A time gap between the Upper Yezo Group and the Hakobuchi Group seems not to be large.