Cretaceous-Paleogene paleoenvironmental changes in the Yezo basin, Northeast Japan.

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During 2005 to 2007 the Lower Cretaceous to Paleocene Yezo Group in Hokkaido and the Lower Cretaceous Miyako and Upper Cretaceous Kuji groups in the Pacific side of Tohoku, have been surveyed from viewpoints of sedimentary facies, megafossil and pollen biostratigraphy, biofacies and others.Our survey revealed that six depositional sequences showing shallow-marine coarsening-and showing show-upward facies successions, are traceable in the Campanian Hakobuchi Formation over the Nakagawa area, northern Hokkaido. In the uppermost Albian to Turonian Mikasa Formation distributed in central Hokkaido, a total of 12 fourth-order depositional sequences can be observed in the eastern limb of the Ikushunbetsu- Sorachi anticline area. Each of sequences 20 to 100 m thick constitutes inner to outer shelf bioturbated sandy mudstone in the lower part and shoreface fine/medium massive/hummocky to trough cross-stratified sandstone in the upper. Furthermore, three third-order depositional sequence sets fo … More r both of the Hakobuchi and Mikasa formations, can be identified through stacking patterns of the sequences. Those sequences and sequence sets seem to have formed by repetitive relative sea-level changes influenced by global eustacy, as typically exemplified in the Western Interior Cretaceous in North America. The high-resolution sequence stratigraphic framework enables reconstructing sedimentary history and paleogeography in the western margin of the Yezo basin. Molluscan fossil occurrences and their composition such as ammonoids, inoceramids and other bivalves within each sequence indicate the taphonomic relation between fossil habitats and depositional environments.In the Santonian to Campanian Kuji Group exposed along the north Sanriku Coast, the stratigraphic variation in pollen flora shows a hinterland paleovegetational change represented by increase of bisaccate pollen. The lateral variation suggests that distribution of plant communities would have been analogous to present-day vegetational variation in several tens kilometer scale. The integrated approach of palynology and sedimentary facies analysis illustrates detailed paleovegetation and fine-scale paleoclimatic fluctuations during Late Cretaceous. Less