Abstract
Landforms from around the prodelta to the deltafront were reconstructed from grain size distributions of two sediment cores (Kaizu and Ooyamada) obtained from the Kiso River delta. Relations between the landform and the accumulation rate based on 14C ages are also discussed. Both cores show the typical deltaic succession of prodelta (MM-I or -II), deltafront slope (US-I), and deltafront platform (US-II). However, the characteristics of grain size distributions from each core are varied, corresponding to the core location. At the Ooyamada core, which is located at the edge of the plain, bimodal sediment (MM-II) was deposited under the influence of small tributary influx of the western Yoro Mountains, with the prodelta environment. At the lobate deltafront slope (Unit US-I), sediments were mainly supplied along the main axis of the river during flooding. Typical deltafront successions are preserved in the Kaizu core, which was estimated to be located along the main axis of the lobe. In contrast, the Ooyamada core was estimated to be located at the outer edges of the lobe, with smaller sediment supply, coarse and wellsorted sediments(around 2∅); it was relocated from the upper deltafront platform (Unit US-II) with waves of coastal process, and its sediments were preserved in a higher ratio. Accumulation rates of the two cores reflect the landform reconstructed above. A high accumulation rate was recorded in the Kaizu core (-56.8mm/yr) with its higher sediment flux from the river, and a low accumulation rate was recorded in the Ooyamada (-12.6mm/yr) core with its smaller sediment flux from the river and the preservation of relocated sediments.
