三陸海岸南部・気仙沼大川平野の地形発達過程

抄録

This paper discussed the formation process of small alluvial plain at an embayment of ria coast as an example of the Kesennuma Okawa Plain, northeast Japan, where existing boring logs are easily obtainable because of many drilling surveys related to earthquake disaster reconstruction after 2011 Tohoku-oki earthquake, based on analysis results of one borehole core and utilization of information from existing boring logs. Total over fifty radiocarbon ages, most of which is from soil samples of existing boring logs related to 2011 Tohoku-oki earthquake disaster reconstruction, enable us to describe sediment-accumulation curves corresponding to delta system. Depositional rate is high (10 ～ 20 mm/yr) in the lower part (tidal flat to estuary), low (0.4 ～ 3 mm/yr) in the middle part (inner bay), and high (3.5 ～ 20 mm/yr) in the upper part (deltafront). Geomorphic/geologic cross section with 1000 year isochrones shows that study area is characterized by delta prgradation since 5,000 cal BP at the latest with progradation rate of less than 1 m/yr, which is consistent with smallness of river and size of alluvial plain. Depositional rate change and sediment stacking pattern corresponding to Holocene delta sequence probably reflect narrow incised valley and closed inner bay environment. This study is first example in the alluvial plain in the Sanriku coast which shows accumulation rate changes and sediment stacking pattern similar to those identified in relatively large typical coastal plain.