There is a sedimentary basin filled with thick Tertiary sediments lying from the northern to southern end in Central Hokkaido. The southern part of the basin is covered by younger sediments except for the eastern marginal part. Therefore, the lower Neogene in this region has been remained unsolved in definite stratigraphic relationship. Fortunately, many wells for oil exploration have been recently drilled in this region and some of them penetrated the Neogene. These wells provided valuable informations on the subsurface distribution of the lower Neogene and the original feature of the sedimentary basin at that time.
The Takinoue Formation, Early-Middle Miocene, encountered in the Nishiumaoi and Abira wells is more than 2000m in thickness and contains a warm pollen flora in the lower part, while in the eastern surface area it is usually 300-600m thick and the lower part containing the warm pollen flora is lacking or very thin. From these facts it is presumed that the formation accumulated with on-lap structure. The center of the basin has been presumed to be situated in the neighbour of the Nishiumaoi and Abira wells. However, it is clarified by the present pollen analytical study that the formation becomes thicker estwards from the wells and the thickest part is concealed under the block which was pushed up from the east by a great thrust (Fig. 8). This means that the basin was far wider than that estimated from the present distribution of the formation in the surface. Considering the distribution of the magnetic field together with gross structure of Hokkaido (Fig. 1), the author concludes the center of the sedimentary basin stretches northwards to the Uryu region, although concealed under the blocks which were pushed up by thrusts.
Although many arguments have been done on the existence of the Lowest Miocene sediments in the central part of Hokkaido, it is most probable that only the Fukuroji Formation in the Uryu region falls to it (SATO, 1976). The formation is presumed from the investigation of foraminiferal fossils to have been accumulated in a bay. As the lowest Miocene is not found northwards in the Haboro and Tempoku regions the bay should have had a connection with the Pacific Ocean southwards through the Ishikari-Hidaka region; the sea connecting to the Pacific Ocean was just situated at the above-noted central part of the basin, which is now concealed deeply under the ground.
Holes 438 and 439 of DSDP are situated at the southern marginal part of the sedimentary basin discussed here. Considering the result of the present study, that is, the lowest Miocene in this basin is expected to exist only in the central part of the basin, the lowest Miocene established in these Holes is presumed to be younger in geologic age.
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