Holocene sea-level changes and evolution of the lower Tokachi River plain, Hokkaido, Japan

Abstract

Litho- and biostratigraphic data obtained from analyses of borehole logs, 14C dates by accelerator mass spectrometry, and diatom assemblages record Holocene relative sea-level (RSL) changes and the evolution of the lower Tokachi River plain in Hokkaido, Japan. The Holocene sedimentary fill consists of a lower sandy unit, a middle marine clayey unit, an upper sandy unit, and an uppermost peaty unit. This stratigraphy is typical for deltaic settings in relation to Holocene sea-level changes. RSL rose from about -18 m to about -2 m above present mean sea level between c. 9300 (9753–9008) and c. 6700 (6844–6574) cal. yr BP, at an average long-term rate of c. 6.2 (5.1–7.4) mm/yr. RSL in the mid-Holocene appears to have not risen above 0 m. RSL at c. 5000 (5042–4840) and at c. 4300 (4420–4154) cal. yr BP were both about -1 m. RSL rose slightly to almost 0 m at c. 3800 (3962–3695) cal. yr BP. By c. 8500–8000 cal. yr BP, transgression had occurred because the rising sea flooded the valley. Rapid progradation of the delta occurred from c. 7500 cal. yr BP, when the RSL rise rate probably decreased, to c. 6500 cal. yr BP, and marsh expanded over the delta plain. Sand barriers had already begun to form before c. 4000 cal. yr BP. Vertical aggradation of the floodplain began after c. 4200 cal. yr BP. The main course of the Tokachi River in the delta plain appears to have stabilized during the late Holocene, on the basis of the gravel-sized sediment distribution and lithostratigraphy of the subsurface deposits. Obvious sand sheets recognized in the uppermost peat layer in the coastal zone imply the occurrence of large tsunamis.