Abstract
This study discusses landform development in the Central Echigo Plain in relation to change in depositional areas of fluvial sediment.
The plain is located on the Japan Sea side of the central Honshu Island, and it extends about 60 km from north to south and 50 km from east to west. The plain was formed by depositions of the Shinano River and its tributaries. The micro-landform of the plain can be divided into five components ; natural levees, back marsh, recent dunes, alluvial fans, and abandoned channels, based on the interpretations of aerial-photographs.
The authors obtained core samples from about 40 sites in the study area, measured their 14C ages, analyzed their electric conductivity, and referred to the existing boring data and archaeological data. These analyses indicate the following history of the sedimentary environment during the late Holocene.
A remarkable Holocene transgression (the Jomon transgression) occurred and formed a large lagoon around 6,000 yrsBP in the study area. After the Holocene transgression, fluvial deposits buried the lagoon gradually and center deposition moved the coast forward. After 4,000 yrsBP, sediment from the Shinano River rapidly formed the foreset bed and sand dunes on the coast side. Meanwhile the sedimentary environment of the floodplain became stable, and sediment from Shinano River gradually formed a topset bed. During 1,400-1,000 yrsBP, peaty soil was formed in a wide area of the floodplain. After 1,000 yrsBP, floodplain sedimentation became rapid, and natural levees on the present plain's surface were mainly formed during 1,000-800 yrsBP.
