Abstract
The Teshio plain is one of the largest Cenozoic sedimentary basin in Hokkaido, Japan. Shallowing-upward thick successions ranging from shelf to terrestrial environments are distributed in and around this plain. The Yuchi and Sarabetsu Formations comprise Plio-Pleistocene sediments deposited in shallow sea to terrestrial environments in the latest stage of sedimentary basin formation. Geological ages of the Yuchi and Sarabetsu Formations are determined on the basis of biostratigraphic and fission track ages, and both formations have contemporaneous heterotopic facies. However, the geological age of the strata in the Teshio plain is unclear because of the thick alluvium covers. We conducted a deep drilling survey, to a depth of 1,000 m, in the coastal zone of the Teshio plain and subsequent laboratory analysis (microfossil and tephra analysis) of the core. From the results of this analysis, it is clear that the geological succession is as follows: alluvium to a depth of 87.8 m; the Sarabetsu Formation at depths ranging between 87.8 m and 471.5 m; and the Yuchi Formation at depths ranging between 471.5 m and 1,004 m. Palynological successions of the Yuchi and Sarabetsu formations were divided into three pollen zones. Further, the Larix zone, which had formed after 1 Ma, has been confirmed at depths between 90 m and 220 m. Tephra, which is approximately 1.5 Ma, has been discovered at a depth of 930 m. The diatom zone has not been recognized because most diatom fossils are redeposited species. However, the presence of confirmed species is consistent with other analytical results. The geological age of both the Yuchi and Sarabetsu Formations, in the study area at depths between 90 m and 1,000 m, is estimated to be in the range of 0.8 to 1.5 Ma. In addition, the fact that both these formations are contemporaneous heterotopic facies until they reach Teshio plain is widely accepted. Moreover, both the strata in the Teshio plain are at least 0.5 million years younger than the strata in hilly areas to the east.
