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

Hand-sieved sand fractions of 83 stream sediment samples from active channels of the Kando River system in the northem San-in district were analyzed by X-ray fluorescence for major elements and 14 trace elements．Histograms of individual elements generally display nornal or positively skewed distributions，as is typical of stream sediments．Several elements（e.g．MgO，Fe_2O_3T，K_2O，Rb）have bimodal and polymodal distributions due to contrasting source proportions．There are clear differences in the compositions of sediments collected from secondary drainages upstream and downstream of the site of the Shitsumi dam．Those upstream are less aluminous，and tend to have greater abundances of K_2O，SiO_2，Ba，Ce，Nb，Pb，Rb，and Th than those downstream．In contrast，TiO_2，Fe_2O_3T，MgO，P_2O_5，Cr，Sc，Ni，V，and Y are generally more abundant in samples from below the dam site．These elemental associations and contrasts are directly related to source geology，as the catchment above the Shitsumi dam site is dominated by granitoids and felsic volcanic rocks，whereas that below consists of more mafic volcanic and sedimentary rocks of the Hata，Kawai-Kuri and Omori Formations．Intermediate compositions in the main channel in the lower reaches reflect mixing and obscuration of these two source signatures，although a geochemical fingerprint of a subordinate adakitic volcanic source（Mt．Sanbe）is also evident in this section of the river．