(4) Oblique shoreline progradation during the Quaternary regressive stage: case study from Yufutsu Plain, Hokkaido, Northern Japan
Geological Survey of Japan, AIST
We conducted the seismic survey to reveal subsurface structures of the coastal zone aroud Yufutsu Plain. This plain has two hinterlands, Hidaka Mountains in NE and Shikotsu-Touya Volcanoes in NW. And the dranage area of Hidaka Mountains are overwhelmingly large compared to the area of Shikotsu-Touya Volcanoes. The present shelf edge off the plain is about 150 m in water depth and faces SSW, as same as the shoreline. As a result of seismic survey, the sediment of the area can be subdivided into three sequences based on erosional surface. Each sequence shows onlap patterns to a lower unit in the lower part and down lap patterns in the upper part. These internal structures indicate that these sequences were formed under the influence of relative sea-level rise and fall. The n sequences were considered to be formed under the glacial sea-level fluctuations after Marine Isotope Stage (MIS) 7. The shelf is a type of sedimentary shelf and covered by the regressive systems tract in MIS 5e to MIS 2. Paleo-shorelines are reconstructed based on the internal structures which show down lap pattern to reconstruct the development history of the shelf. As a result, shoreline faced SW during every regressive stages (MIS 7 to 6, MIS 5e to MIS 2) although present shelf and shoreline face SSW. The difference between the trend can be explained by the influence from the two hinterlands in NE and NW. The shoreline during regressive stage is directly influenced by the direction of the sediment supply. Then shoreline was normal to the river trend of Hidaka Mountains with large dranage area. On the other hand, the influence from Shikotsu-Touya Volcanoes relatively increased during other stages. Then shelf edge and shoreline in present deformed from SW, the original trend, to SSW.