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

从层序地层学和软体动物生物相转换的角度研究了代表北海道岛弧带叶佐超群浅海沉积相的Albian-上Turonian米卡萨组和Lower Campanian-Lower Maastrichtian Hakobuchi群。米卡萨组由三个三级沉积层序组成，它们是由相对海平面旋回形成的。虽然DS及其组成体系域的横向和纵向分布取决于层位和地理位置，但盆地中心似乎已由北向南转移。浅海贝壳层往往包含在高位体系域的下滨面到内陆棚相中。箱渊群在芦别地区明显地由两个三级DS组成，但在南部的Oyubari到Hobetsu地区则有七个以上规模较小的DS。因此，相对海平面变化 关于我们 在Yezo盆地内的地区之间，ES可能或多或少存在局部差异。由于北方北海道的Nakatonbetsu地区的岩群主要为非常厚的近海泥岩相，因此很难与芦别至Hobetsu地区进行比较。但据推测那里有两三个DS。根据有孔虫微体古生物学和古地磁资料判断，最上层的可能为古新世。这些地层的DS的一般分布模式与Haq曲线（全球海平面曲线），这表明全球海平面的影响，但不是所有的，除了局部构造运动。在Cenomanian-Turonian边界的米卡萨组的浅海软体动物群的变化表明，一个显着的灭绝和底栖生态系统中的元素急剧变化。尽管箱渊群中的化石层很少，但至少在Campanian至Maastrichtian早期没有观察到如此显著的变化。少

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