Geomorphic evolution of the coastal lowlands and changes in the sedimentary environment in the Lower Kumozu River, Mie Prefecture

Abstract

The author researched the Holocene sediments and geomorphic evolution of the Kumozu River lowlands using diatom analysis, electric conductivity, and 14C dating, and discusses how the sedimentary environment changed during the late Holocene. The results are summarized as follows. 1) After the Holocene transgression, sedimentation of upper marine sand in the southern lowlands proceeded to the northern lowlands and began to accumulate around 4,800∼4,300 cal. BP. It is possible that the earlier river channel was in the southern lowlands and a shift of the river channel toward the north caused the difference in the depositional age. 2) Three beach ridges finished forming before 2,600 cal. BP, 1,600 cal. BP, and the present, respectively. Beach ridge I, which formed before 2,600 cal. BP, developed straight along the coast, while beach ridge II, which was formed before 1,500 cal: BP, extended toward the sea around the river mouths. This suggests that the coastal lowlands was first dominated by waves, although it was dominated by the river after 2,600 cal. BP and before 1,500 cal. BP. 3) Two flood sand layers are found in fluvial deposits. Flood sand I is relatively coarse and was deposited before 3,000 cal. BP. The terrestrial environment was stable for a short period during 3,000∼2,200 cal. BP. Flood sand II began to accumulate from 2,200 cal, BP and it is finer and better sorted than flood sand I. 4) Based on the increase in the thickness of the upper marine sand, the distribution of beach ridges, and sedimentation of the flood sand layers, it is assumed that the sediment supply increased after ca. 2,000 cal. BP. This environmental change appears to be common to the Pacific coasts in central Japan.