Geochemical Characteristics and Source Area Analysis of Quaternary Mudstones in the Sanhu Depression of the Qaidam Basin

Abstract: In order to deeply understand the source properties of Quaternary mudstones in Sebei area of the Sanhu Depression in the Qaidam Basin, this paper focuses on sedimentary geochemistry and conducts analysis and testing of major, trace, and rare earth elements on core samples from different depths of the Quaternary in the Sebei area's Setan 1 and Setan2 wells (ST1 and ST2 wells). In general, the samples has low silicon (SiO2=48.17%), high aluminum (Al2O3=15.23%, aluminum index A / NK=2.54, aluminum saturation index A / NK = 1.59), rich magnesium and calcium (MgO=3.81%, CaO=7.91%), relatively rich iron (Fe2O3=4.99), low total alkali content (K2O + Na2O=5.94%), significant enrichment of Ba and Cu compared to UCC, Nb depletion, and other trace and rare earth elements comparable to the upper crust. Quaternary mudstones in Sebu area are less affected by sedimentary sorting, rewinding, weathering and diagenesis and its elemental features can effectively indicate the source and tectonic background. The results show that the characteristics of the two well samples are relatively consistent. The F1-F2, Co / Th-La / Sc, La / Th-Hf, La / Yb-Σ REE discrimination diagrams, rare earth right-dip flat distribution pattern and related parameters all show that the sedimentary source of Quaternary mudstones in the study area is mainly derived from the upper crust felsic sedimentary rocks and igneous rock sources. (SiO2/ Al2O3) - (K2O / Na2O), (K2O / Na2O) -SiO2, [K2O / (Na2O + CaO)] - (SiO2 / Al2O3), Ti / Zr-La / Sc, La-Th-Sc, Th-Co-Zr / 10 and Th-Zr / 10 ternary phase diagrams and related parameters determine that the source area is an active continental margin and continental island arc environment. The potential source areas of Quaternary sediments in the Sebei area of Sanhu Depression mainly include the eastern Kunlun orogenic belt on the south side of the depression and the south Qilian orogenic belt on the north side of the depression.