Transition of clepositional sequences and fossil facies in the Crefaceous Yezo Supergroup

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The uppermost Albian to upper Turonian Mikasa Formation and the Lower Campanian to Lower Maastrichtian Hakobuchi Group, both of which represent the shallow-marine depositional facies of the Yezo Supergroup in the meridional zone of Hokkaido, were investigated from viewpoint of sequence stratigraphy and molluscan biofacies transition. The Mikasa Formation is thought to be composed of three third-order depositional sequences (DS) formed by relative sea-level cycles. Although the lateral and vertical distribution of DSs and constituting systems tract are depending on horizons and geographic position, the basin center seems to have shifted from north to south. Shallow-marine shell beds tend to be contained in the lower shoreface to inner shelf facies of highstand systems tracts.The Hakobuchi Group conspicuously consists of two third-order DSs in the Ashibetsu area, but more than seven DSs of somewhat smaller size in the southern Oyubari to Hobetsu areas. Therefore, relative sea-level chang … More es might have been locally different more or less among areas within the Yezo basin. Since the group in the Nakatonbetsu area of northern Hokkaido is dominated by very thick offshore mudstone facies, the comparison with the Ashibetsu to Hobetsu areas is difficult. But two or three DSs are presumed to exist there. The uppermost one may be Paleocene in age, judging from foraminiferal micropaleontology and paleomagnetic data.Sequence stratigraphic schemes synthesized in this study are compared with other Upper Cretaceous strata in Northeast Japan. The general distribution patterns of DSs for those strata are well synchronous with the Haq curve (global sea-level curves), suggesting an influence of global eustacy, though not all, in addition to local tectonic movement.Shallow-marine molluscan faunal changes of the Mikasa Formation at the Cenomanian-Turonian boundary suggest a remarkable extinction and drastic change of elements in benthic ecosystems. Though fossil beds are scarce in the Hakobuchi Group, such a remarkable change can not be observed during Campanian to early Maastrichtian at least. Less