2009 Volume 115 Issue 3 Pages 122-129
The uppermost part of the Cretaceous Yezo Group exposed in the Kotanbetsu area of Hokkaido is defined as the Hakobuchi Formation. The formation consists of alternating beds of fine- to medium-grained sandstone with hummocky cross-stratification and thin mudstone, and conformably overlies muddy deposits of the Haborogawa Formation of the Yezo Group. The present investigation found several ammonoids and inoceramids within the formation, but no reliable age-diagnostic mega-fossils. In the Kotanbetsu and Haboro areas, however, Sphenoceramus orientalis and S. schmidti, which are indicative of the middle-upper part of the lower Campanian, have been reported from the uppermost Haborogawa Formation. Therefore, the biostratigraphic position of the Hakobuchi Formation in the Kotanbetsu area is thought to correspond to the lowest part of the S. orientalis-S. schmidti Zone. Based on the distribution of the Hakobuchi Formation in the Kotanbetsu area and the coarsening-upward sequence in the uppermost part of the Haborogawa Formation of the Haboro and Tappu areas, it is possible that the Hakobuchi Formation was deposited over a wide part of the Haboro-Kotanbetsu-Tappu area.
