木曽川デルタにおける沖積層の堆積過程

抄録

The Kiso river delta locates in the southern part of the Nobi plain, central Japan. Evolution of the delta has been strongly influenced by post-glacial sea-level changes, and landforms of the delta changed remarkably during the Holocene. Holocene sediments of the delta can be classified into the lower sandy, middle muddy, upper sandy and -uppermost terrestrial units.
Silty or sandy transitional zones can be recognized in the upper and lower horizones of the middle muddy units by means of a particle size analysis. Sediments of the transitional zones are considered to be deposited between the foreset and bottom set beds of the delta as delta front sediments.
Based on the ¹⁴C ages and characteristics of the sedimentary facies, Holocene sequence and evolution of the Kiso river delta are considered as follows.
The post glacial transgression in the early Holocene (10000-8500 years BP) was slow, and in this slow transgressive stage, the lower sandy unit deposited as the flood plain sediments. During the rapid transgressive stage in the period between 8500-6500 years BP, the lower Kiso river delta submerged and the coast line retreated towards the central part of the present Nobi plain. Lower sandy unit deposited as a foreset bed of the Kiso river delta in the rapid transgressive stage.
In the middle Holocene, muddy sediments of the middle unit deposited as a bottom set bed, and the sandy sediments deposited aggradationally in the innermost part of the embayment as a foreset bed of the Kiso river delta. The delta was in the central part of the present Nobi plain. This aggradational stage was in the period between 6500-5500 years BP. Following the culmination of the post glacial transgression since 5500 years BP, the Kiso river delta has advanced towards the southern direction. In this progradational stage, the upper sandy unit deposited progradationally as a foreset bed of the Kiso river delta. Surface of the delta has been covered with flood plain sediments as the uppermost terrestrial unit, and the delta front has advanced towards the present coastline. During the periods of ca. 5000-4500 years BP and ca. 3000-2000 years BP, slight regressions of the sea-level can be seen in the Kiso river delta region.