COASTAL DUNE OF FUKIAGEHAMA IN KAGOSHIMA PREFECTURE : On the Property of Dune Sand

Abstract

This paper is to present the result of the analysis of dune sand and the interpretation of the properties of sand in the Fukiagehama dune field. Along the Fukiagehama coast facing to the East China Sea, the extensive dune field ranges 19km in length, with the width of 0. 6km at Yoshitoshi and of 2. 5km at Takahashi, from North to South. The horizon of these dunes may be divided into five layers ; A1, A2, A3, A4, and D. A1: Fore dune sand, active. A2 : Secondary dune sand, stabilized by vegetation. A3 : Consolidated sand of the more mature inland dune. A4 : Beneath the humus soil layer. D : Lower Pleistocene. Each sand has its own properties. Attention was focussed to clarify the environment during dune formation and the geomorphic history of the region through the analyses of sand : grain size distribution, mineral content and grain morphology. The results are summerized as follows: 1 ) A certain synthetic analysis composed of grain size distribution, mineral content and grain morphology may be necessary to distinguish dune sand and beach sand. A single analysis does not indicate the clear differerence between them. 2 ) Dune sands show the following properties: their mean diameter ranges from 0. 75 to 2. 2, sorting from 0. 45 to 0. 8, kurtosis from-0. 5 to 0. 7. And they have higher values of roundness, of proportion of heavy mineral content and of EM, RM type than beach sands. 3 ) As the dune sands become older, the value of Mdφ decreases; the value of roundness increases; grain morphology transforms from NU. EL type to EM type, and further to RM type. The value of mineral content differs considerably according to the location, but quartz contain of A4 sand increases compared with A1, A2 and A3. 4 ) As a whole, the value of Mdφ and mineral content of those dune sands changes gradually from North to South. This tendency may be caused by the littoral current. But near river mouths, what is mentioned above is not always appliable: contain of volcanic glass and pumice from Shirasu bed increases in dune sands. This was also favourably tested in the three analyses.