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

2005 ~ 2007年，对北海道下白垩统—古新统Yezo群和东北太平洋侧下白垩统宫古、上白垩统Kuji群进行了沉积相、巨化石、花粉生物地层学、生物相等方面的研究。调查结果表明，北海道北部中川地区坎帕系博渊组有6个沉积层序，均表现为浅海粗化和向上的相序列。分布在北海道中部的上阿连统—土伦统三加萨组，在Ikushunbetsu—Sorachi背斜区东翼共可观察到12个4级沉积层序。每层序厚度为20 ~ 100 m，下部为陆架内-外陆架生物扰动砂质泥岩，上部为滨面细/中块状/丘状-槽状交叉层状砂岩。此外，通过层序的叠置模式，还可识别出白渊组和三笠组的3个三级层序集。这些层序和层序集似乎是由全球海平面上升影响下的相对海平面重复变化形成的，北美西部内白垩纪就是典型的例子。高分辨率层序地层格架有助于重建野左盆地西缘的沉积历史和古地理。各层序中甲壳类、双壳类等软体动物的化石产状及其组成表明了化石生境与沉积环境之间的地理关系。北三陆海岸三东系—坎帕系库吉群花粉区系的地层变化表现为以双盐花粉增加为代表的腹地古植被变化。这种横向变化表明，植物群落的分布可能类似于今天几十公里尺度上的植被变化。孢粉学和沉积相分析的综合方法揭示了晚白垩世详细的古植被和精细尺度的古气候波动。少

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