Abstract
Sedimentary environment and sedimentation rate of the Ishikari floodplain were investigated by analyzing borehole core sediments. The sedimentary environment changed from meandering river channel to floodbasin before 10,000calBP, and sedimentation rate increased around 8,000 calBP in response to the landward shift of the depocenter caused by sea-level rise. Sedimentation rate decreased largely after approximately 7,500 calBP in response to the deceleration of sea-level rise. Between 7,500 and 5,000calBP, the sedimentary environment was mainly occupied by floodbasin. Relatively successive peat had accumulated for 3,500 years after 5,000calBP under sea-level standstill. The timing of the decrease in sedimentation rate in the meandering fluvial system nearly coincides with that of the initiation of bayhead delta progradation in the barrier-lagoon system located just downstream of the meandering fluvial system. It is important to note that the evolution of meandering fluvial systems as well as of delta systems is influenced by the rate of sea-level change.
