Key Words :

alluvial plain, geomorphic development, FeS2 analysis, geomorphic environment, Nuta River

Abstract

Alluvial formations of the plain are divided into four stratigraphic units; Lower Sand Bed, Middle Mud Bed, Upper Sand Bed and Uppermost Bed (Fig. 3). Middle Mud Bed contains ash layer of the K-Ah tephra which erupted about 6,300 y.B.P. FeS2-S (pyritic sulfur) contents analysis of the sediments (Fig. 4) and the structure of alluvium indicate that shoreline during Hypsithermal transgression retreated at least 10 km inland from the present shoreline. Then the shoreline turned to progress gradually following slow sedimentation at the sea level around the present position. The upper limit of marine sediments defined by FeS2 analysis is 0.6 m above present mean sea level, which lies within the present tidal range. Sedimentation has not been so active and most of the plain was emerged by reclamation during the medieval period and thereafter.

The longitudinal profile along the Nuta River (Fig. 8) and locations of archaeological relics and distribution of reclaimed lands (Fig. 9) suggest that development of the plain is divided by several stages. That is, by the Kofun era, Hongo Lowland completely emerged, while Nuta Lowland reclaimed during the medieval period. Natural levees along the Nuta River are presumed to have developed in the latter half of the medieval period. Reclamation in Mihara Delta began in the 17th century from the north-west end, and most of this area emerged by reclamation during the 19th and 20th centuries.