石狩低地内陸部で採取されたIK1コアが示す完新世の堆積環境·堆積速度の変化

摘要

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 10,000cal BP, and sedimentation rate increased around 8,000 cal BP in response to the landward shift of the depocenter caused by sea-level rise. Sedimentation rate decreased largely after approximately 7,500 cal BP in response to the deceleration of sea-level rise. Between 7,500 and 5,000 cal BP, the sedimentary environment was mainly occupied by floodbasin. Relatively successive peat had accumulated for 3,500 years after 5,000 cal BP 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.%大規模な蛇行河川システムが発達する石狩低地内陸部を対象に,地形·堆積学的手法を用いて,その堆積環境と堆積速度の変化を検討した,調査地の堆積環境は,10,000年前以前に網状河川システムから蛇行河川システムへと変化した.8,000年前頃に海水準の上昇にともなう堆積中心の陸側への移動により，堆積速度が増加した.7,500年前頃には海水準上昇速度の低下にともない.堆積速度の大幅な低下が認められた.5,000～1,500年前には比較的連続して泥炭が堆積した.7,500年前頃の蛇行河川システムにおける堆積速度の低下は.下流域に発達した湾頭デルタの前進開始時期とほぼ一致する.蛇行河川システムの発達も.デルタの発逹と同樣に海水準変動の影響を受けている可能性がある.