On the Topographies and Soils of the Ishikari Coastal Plain

Abstract

The purpose of this study is to discuss the geomorphic development and soil genesis mainly of the sandy area of the Ishikari coastal plain, Hokkaido, northern Japan. To serve as an aid to this purpose, many archaeological, tephrochronological, granulometrical data are used.The results are summerized as follows:1) The sandy area is classified into three morphologic provinces, namely Momijiyama sand dunes, Bannaguro emerged sand bars, Ishikari sand dunes, from the inland to the coast respectively. This arrangement of each topography, such as an older dune, a sand bar and a younger dune, are generally observed in large alluvial plains in Japan. This pattern is differentiated from that of narrow coastal plains where the younger dune is piled upon the older one.2) The soils are classified into two by the stage of formation, namely the lower dark sandy loam and the upper black humus.3) Momijiyama sand dunes cover the pleistocene lower terrace deposits or upper tertiary formation, probably being at the depth near the present sea level.These dunes developed on a gravel or sand bar which was formed during the maximum transgression stage of Early Jomon. The sea level of this stage reached 4m or more meters above the present sea level. Before the early stage of Middle Jomon, these dunes were already transformed into the longitudinal dunes and fixed by the lower dark sandy loam. These dunes are correlated to the older sand dune definited by ENDO (1969).4) Bannaguro emerged sand bars are the assemblage of more than twenty parallel sand bars, which consist of shallow marine deposits built during and after the maximum transgression stage. Then a large number of these sand bars were emerged and immediately fixed by the lower dark sandy loam before the later stage of Middle Jomon. Thus, wind-blown sands were deposited only on a small scale on the emerged sand bars.5) The lower dark sandy loam is correlated to the older humus sand definited by ENDO (1969), and its formation continued at least from Middle to Later Jomon in this area. During Middle Jomon, the soil genesis was probably performed under rather warm climate, but it suffered podosolization under cool and humid climate during Latest Jomon. Thus, the lower dark sandy loam is a kind of polygenetic soils. This type of older humus sand is only observed north of this area in Japan.6) In the early stage of Latest Jomon (3000-3500 years B.P.), peat bogs appeared at depressions between several emerged sand bars, although in the inland alluvial plain inside of Momijiyama sand dunes, the deposition of peat had already begun since 5000-6000 years B.P..7) The lower part of Ishikari sand dunes were formed on the marine sand beds deposited during the small transgression of possibly Satsumon age (1500-1000 years B.P.), and fixed by the upper black humus.The upper black humus is correlated to the younger humus sand definited by ENDO (1969). The upper part of Ishikari sand dunes are the present dunes.