Magnitudes of sea-level falls at lowstands of the past 900,000 years inferred from gravels underlying the Nobi Plain, central Japan

Abstract

The Nobi Plain alluvial lowland is drained by the Kiso River system, which discharges large volumes of water and sediment into Ise Bay, Japan. The basin is bounded to the west by the Yoro fault and has been tilted down to the west by repetitive movement on the fault. The basin stratigraphy and its stacking patterns suggest uniform and rapid rates of subsidence and tilting in response to movement on the Yoro fault during the middle and late Quaternary. We used lithostratigraphy, tephrochronological and paleomagnetic data, facies analysis, and diatom assemblage analysis of a 601 m sediment core to identify ten marine transgression–regression sequences, characterized by basal fluvial gravel intervals formed by the Kiso River drainage system, and correlated them with glacial–interglacial sea-level changes during the past 900 ky. We inferred that the mean maximum size of clasts in the basal gravels may be proportional to the magnitude of the fall of sea level inferred from MIS curves since MIS 16; that is, clasts inferred to be deposited during MIS 12 and 16 are largest, and those deposited during MIS 14 are the smallest since MIS 16. These observations suggest that the Kiso River system has adjusted its longitudinal profile to accommodate sea-level changes and that a steeper profile was formed during periods of lower sea level. We also found that the size of gravel clasts in the present-day river is proportional to tractive force on the riverbed, which is controlled mainly by the slope of the river profile. Conversely, the facts that larger gravel clasts were deposited during MIS 20 and that gravel intervals deposited during MIS 22 were thicker than those during MIS 2, suggest that a glacio-eustatic sea-level fall comparable in magnitude to that of the Last Glacial Maximum occurred during MIS 22.