2005 年 70 巻 1 号 p. 24-36
The Cretaceous to Paleocene Yezo forearc basin was developed between the western Early Cretaceous volcanic arc and the eastern Early Cretaceous to Early Eocene accretionary complexes in the northeastern margin of Eurasia Plate. Its sediments are distributed across a 150 km-wide and 1400 km-long belt from Northeast Japan to Sakhalin including off the Pacific coast in north-south direction. It is subdivided into three subbasins on the basis of geologic setting: Hokkaido, Kitakami and Joban subbasins. In the first half of this paper, the tectonic setting is reviewed not only for the better understanding of the basin, but also in order to evaluate the distribution of carbonaceous rocks as source rocks and reservoir sandstone.
In the latter half, twenty-seven stratigraphic sections including 4 onshore and 8 offshore boreholes and 15 surface sections are correlated to delineate the sedimentary history and basin evolution through the temporal and spatial distributions of facies successions and depositional sequences. Two second-order shallowing-upward cycles for the Cretaceous are recognized in Hokkaido but are obscure in northern Honshu. Shallow-marine to paralic sediments of the first cycle (early Albian to Turonian) are represented by the Mikasa Formation (late Albian to Turonian) as the western marginal facies in Hokkaido. The Campanian to Maastrichtian shallow-marine and non-marine facies of the second cycle (Coniacian to Maastrichtian) are characterized by the Hakobuchi Formation in the Hokkaido subbasin and its correlatives in the Kitakami and Joban subbasins. The two formations include several third- and fourth-order depositional sequences with coarsening-upward facies successions. The uppermost Maastrichtian to Upper Paleocene interval is missing everywhere in the Yezo basin. Furthermore, two third-order depositional sequences of the late Paleocene (uppermost Hakobuchi Formation) and the uppermost Paleocene to lower Eocene (Haboro Formation) have developed with unconformities in a few areas.