LAUSR

Abstract In recent 30 years, Th/U ratio has been using widely in evaluating the paleoredox conditions during the depositions of source rocks. However, there are few studies on the preconditions of the use of Th/U ratio for evaluating the paleoredox conditions. In many areas, the evaluation results of redox conditions in paleoenvironment obtained by Th/U ratio is different from that reflected by other parameters. The accuracy of judgment of paleoredox conditions by Th/U ratio is questioned. In this paper, the weathering degree of source rocks during the deposition are determined by chemical index of alteration (CIA) and Al2O3-CaO, NaO-K2O (A-CN-K) ternary diagram, whereas the lithologies of parent rocks during source rocks depositions are determined by A-CN-K ternary diagram, Al2O3/TiO2 ratio and REEs. Furthermore, the effects of the weathering degree, the properties of parent rocks and sedimentation rate on Th/U ratio to evaluate the accuracy of paleoredox conditions are analyzed. The results show that in lacustrine basins, although the content of uranium (U) gradually increased with the decrease of oxygen content in the water column, the obvious enrichment of thorium (Th) in sediments because of parent rock undergoing strong weathering and relative fast sedimentation rate influences the accuracy of Th/U ratio for evaluating the paleoredox conditions. Additionally, the different initial content of U, Th and Th/U ratio in different lithologies also influence the accuracy of Th/U ratio for evaluating the paleoredox conditions. The distribution characteristics of Th/U ratio are the result of the comprehensive influence of various conditions, not just paleoredox condition. When evaluating the redox condition of paleolake water by the enrichment factors of redox sensitive elements (RSEs) and bimetal ratio, the effect of weathering, lithologies of parent rocks and sedimentation rate should be taken into consideration on distributions of parameters.