Fluvial terrace and geomorphology in the Tokigawa (Shonaigawa) river basin since the middle pleistocene

Abstract

Many papers reported that Quaternary climate and sea-level fluctuations controlled riverbed elevation in the river basins in northeastern Japan. However, in the southern part of the Chubu region of Japan, such climatic-controlled riverbed fluctuations have not been reported, except in a few papers based on uncertain chronological data. In this research, we investigated fluvial terraces along the Tokigawa (Shonaigawa) River that flows through the low-relief mountainous areas in the northernmost part of the Mikawa Highlands, in the southern part of the Chubu region of Japan, and examined whether riverbed fluctuations similar to those in rivers in northeastern Japan occurred in the Tokigawa River basin. We mapped fluvial terraces based on air photo analysis and inferred the age and climate at the time of formation of these terraces based on 14C dating, tephra analysis, and pollen analysis. Fluvial terraces in the Tokigawa River basin were classified into 10 treads (H1-4 surfaces, M1-3 surfaces, L1-3 surfaces). We estimated that the L2 terrace was a depositional terrace formed during marine oxygen isotope stage (MIS) 2, based on 14C age, marker tephra (Kikai Akahoya tephra) in the soil layer covering terrace deposits, distribution pattern and morphology of terrace surfaces, and thickness of terrace deposits. The M2 terrace was correlated to MIS 6 based on the relationship between the marker tephra (Aso-4 tephra, Kikai Tozurahara tephra) and terrace deposits, existence of red weathered soil, and distribution pattern of the M2 terrace. Based on these results, we concluded that the fluvial terraces in the Tokigawa River basin were formed as a consequence of riverbed fluctuations linked to climate change. We estimated uplift rates in the Tokigawa River basin at 0.11-0.16 mm/year based on the elevation difference between M2 and L1 terrace surfaces.