山陰海岸西部のポケットビーチの特徴と元素組成からみた成熟度

　The characteristics of beach sand in the western San’in coast are examined in relation to beach shape, grain size distribution, and geochemical compositions. The area examined covers the 82 km long coastline from Ohda to Masuda in western Shimane Prefecture, Japan. Thirty-two identical beaches occur along the coast. The longest of these is Mochiishi beach (12.5 km), situated in the western part, and is characterized by seven composite sub-beaches. The average length of all beaches examined is 1.24 km. Seventeen beaches are less than 1 km in length, indicating that relatively small pocket beaches dominate in the western area. Radian values (arc length of the beach / the radius; ?/r) of <1 characterize short and concave pocket beaches. Thirteen beaches have ?/r values ?/r>1. Sand samples were collected from the shorelines in the foreshores of 32 beaches in March and April, 2010. Median grain size (d50) ranges from ?0.31 to 2.15(φ), and sorting is generally less than 0.35. SiO2 contents range from 70.66 to 89.50 wt％, with most samples containing more than 80wt %. SiO2 contents show negative correlation with sorting values. Al2O3 contents vary from 4.54 to 16.46 wt% and exhibit strong negative correlation with SiO2. Zr contents range from 55 to 100 ppm. Beaches less than 1km in length (L) have relatively higher ?/r values of over 1.0. Their ?/r ratios show weak negative correlation with both SiO2 content and SiO2 / Al2O3 ratios, suggesting significant sorting of quartz grains in small pocket beaches.