Geochemical characteristics of river mouth sediments around Lake Biwa, southwest Japan: Implications for provenance, weathering and sorting of sediments

摘要

Lake Biwa, the largest lake in Japan, first formed some four million years ago. The main lithotypes in the catchment of Lake Biwa are Paleozoic-Mesozoic Tamba terrane rocks, late Cretaceous to Paleogene granites and associated rhyolites, and the Plio-Pleistocene Kobiwako Group sediments and younger alluvium. Thirty-three river sediments from the mouths of 17 rivers flowing into Lake Biwa were analyzed by X-ray fluorescence to determine their bulk geochemical compositions. These were then compared with published data for the main source rocks. Samples were divided into three categories according to their main sources, i.e. Group 1:alluvium-derived, Group 2: granitoid-derived and Group 3: Tamba terrane-derived. Group 2 sediments are marked by depletion in MgO, CaO, Na_2O, Cr, Ni, Sr, V and enrichment in Rb relative to average upper continental crust (UCC). Group 3 sediments show lesser depletion in CaO, Na_2O and Sr, but are richer in ferromagnesian elements, with contents closer to UCC. Group 1 samples, the source sediments of which were themselves derived from Groups 2 and 3 sources, show intermediate and more diverse compositions. Established geochemical diagrams show that while the sediments generally retain the geochemical fingerprints of their source rocks, significant shifts in composition have occurred due to source weathering and recycling. Sorting and heavy mineral accumulation have less effect, although some Group 1 samples show zircon concentration, probably due to reworking of alluvium which was mainly derived from granitoids. Group 3 samples also show V enrichment, possibly due to presence of fine-grained magnetite and clays. The present study suggests that the chemical composition of river mouth sediments is useful not only for its provenance research but also interpretation of the weathering and sorting.