Sequence stratigraphy and biofacies of the Cretaceous and Tertiary (K/T) boundary and its nearby horizons in Hokkaido, northern Japan

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During four years of this project, 1998 to 2001, the Upper Cretaceous Hokobuchi Group and its representatives of the Yezo Supergroup in Hokkaido, have been surveyed from viewpoints of sedimentary facies, megafossil biostratigraphy, biofacies, pollen stratigraphy and others. Besides, the Nemuro Group in east Hokkaido and the Nakaminato Group in Ibaraki Prefecture both correlative with the Hakobuchi Group, are also surveyed briefly for comparison. These results are included in a separated volume of the project report as a list of papers printed and aural presentations in conferences, and their some reprints.In the Nakatonbetsu area, northern Hokkaido, our field survey for a few years, revealed the most high-resolution biostratigraphic framwork of the Campanian to Maastrichtian around Japan, by using ammonoid and inoceramid successions in surface outcrop sections (Ando et al., 2001). Though the upper part of the Hakobuchi Group there include the upper Paleocene according to micro fossil b … More iostratigraphy, we found the unconformity eroding the K/T boundary and its nearby horizons. The uppermost Maastrichtian and lower Paleocene seem to have been eroded away. This means the Yezo forearc basin had continued until late Paleocene after a major event during late Maastrichtian and early Paleocene.The Hokobuchi Group in the Nakagawa area to the west of Nakatonbetsu, shows the western marginal sedimentary facies of the basin, It is composed of the five shallow-marine upward-coarsening facies successions (PS: parasequences) which are several tens to a few hundreds meters in thickness. In the Wakkanai area to the north of Nakagawa, we also discriminated five PSs, On the basis of macrpfossil biostratigraphy established in Nakatonbetsu, the group consists mainly of the Campanian without the Maastichtian in Nakagawa, but of the Campanian and the upper Maastichtian in Wakkanai.These stratigraphic data will provide the important information for reconstruction of dynamic geohistory of the Yezo basin. Less