Major and trace element abundances in the <180μm fractions of stream sediments from the Kando River, Shimane Prefecture, Japan

Sieved fractions（＜180μm）of 86 stream sediment samples from active channels of the Kando River and its tributaries in east Shimane Prefecture were analyzed by X-ray fluorescence for major elements and 14 trace elements．Histograms of all data show that distributions are generally normal or are right-skewed to isolated higher values，as is typical for river sediments．Several elements（e.g．K_2O，Rb）have clearly bimodal distributions that are related to source．There is no evidence of anthropogenic impacts for elements which could be affected，such as P_2O_5，Cr，Ni or Pb．The results show clear contrasts in the composition of sediments collected upstream and downstream of the site of the Shitsumi dam．Those upstream are more aluminous，and tend to have greater abundances of K_2O，P_2O_5，MnO，Na_2O，Ba，Ce，Nb，Rb，Th，Y and Zr than those downstream．Conversely，Fe_2O_3T，TiO_2，Sc and V are generally more abundant in samples from below the dam site．These contrasts are related to source geology，with the catchment above the Shitsumi dam site being dominated by granitoids and felsic volcanic rocks，and that below by mixed volcanic and sedimentary rocks of the Hata，Kawai-Kuri and Omori Formations．The data clearly suggest that isolation of the upstream granitoid and felsic volcanic source terrane by closure of the Shitsumi dam will ultimately lead to more mafic compositions in stream sediments in the downstream reaches of the Kando River．