Loess Buried in the Paleo-sand Dunes in San' in andHokuriku along the Coast of Japan Sea

Abstract

The eolian sand members along the coast of Japan represent at least five major periods of accumulation in the last Glacial age and Holocene. These deposits include well-defined paleosols which divide individual eolian sand members into five parts. The five major periods of eolian sand accumulation can be classified as follows : 4, 000-5, 000 y.B.P. (the young est); 18, 000-20, 000 y.B.P. ; 30, 000 y.B.P. ; 50, 000 y.B.P. ; 70, 000 y.B.P. (the oldest).In Izumo, Kumihama, Amino and Fukui along the coast of Japan Sea, Paleo-sand dunes developed noticeably and five paleosols were buried in these eolian dune sand.The purpose of this paper is to point out that eolian dust deposited much in paleosol horizons and that silt and clay fractions in paleosols originated in eolian dust which came from China in the last Glacial age.The seven samples were collected from the outcrops of paleo-sand dunes in Izumo, Kumihama, Amino and Fukui. Three Chinese loess samples were collected in Chuhua and Manchouli in China. Two desert soils from Wuwei in China and one tephric loess sample from New Zealand were collected.The results from our research are summarized as follows : 1) The mean diameter of paleosols in these areas in Japan is ranging from 0.001 to0.011 mm. Texture of paleosols differentiates from light and heavy clay. The clay fractions of paleosols are characterized by predominance of 14A minerals, illite, kaolinite and quartz. Contents of quartz in the silt fraction (5-20μ) of paleosols are 32-74%. According to the amount of SiO2, Al2O3and -H2O·IL in the <0.02 mm fraction of paleosols, Japanese samples belong to the eolian dust group. These results are very much consistent with those of the author's previous analysis of loess from Northern Kyushu and Yonaguni Island in Japan, Chinese loess and desert soils, and New Zealand loess.2) Based on the analyses of size distribution, chemical properties and X ray diffraction, we can conclude that silt and clay fraction in paleosols had originated in eolian dust from China, and paleosols had been formed from loess which compose major part of them in the last Glacial age.3) Eolian dust had been not only much deposited on the paleosol horizon, but also had less mixed with the eolian sands. This indicates that eolian dust had come continuously from China in the last Glacial age. Following the interruption of dune formation, formation of paleosols had taken place. Simultaneously eolian dust had began to deposit thickly on the dune surface.