Geochemical characteristics, provenance and paleodepostional environment of the Lower Jurassic Huxishan Formation in the Lenghu area, northwestern Qaidam Basin, North West China: Implications for organic matter origin

Abstract

The organic-rich mudstone of Lower Jurassic Huxishan Formation (J1h) in the Lenghu area is the main oil prone source rocks in the northwestern Qaidam Basin. However, the evolution of dispositional environment and the provenance of dispositional rocks and organic matter are still unclear. Based on the data of element contents, total organic carbon and biomarkers of 143 samples, this study aims to investigate the provenance, paleoclimate, paleowater environment and origin of organic matter. The samples show high contents of terrigenous elements Ti, Sc, La and Co. Moreover, the Ti/Zr–La/Sc, La–Th–Sc, SiO2–K2O/Na2O and K2O/Na2O–SiO2/Al2O3 discrimination diagrams indicate that the tectonic settings of the provenance are oceanic island arc and continental island arc. Weight ratios of Ti/Zr, La/Sc and Th/Co of the J1h samples show values of 6.86–204.08, 0.19–2.16 and 0.015–0.123, which demonstrate a common source of clasts from mafic rocks. The Co/Th–La/Sc and La/Th–Hf discrimination diagrams also indicate that the provenance is mafic rocks with background of oceanic island arc, suggests that the provenance came from different stages in the subduction of the regional ocean and land in the Early Paleozoic. The weathering indices suggest that the samples faced moderate to intense weathering, combining with climate indices, illustrate that the paleoclimate was warm and humid, and the precipitation showed an increased trend from bottom to top. The values of CaO/MgO·Al2O3 show a decreased trend, implying that the paleotemperature had a decreased trend and the average annual temperature was less than 14.2 °C. The values of Sr/Ba, Rb/K and discrimination diagram of Pr/nC17–Ph/nC18–Pr/Ph indicate that the paleowater was predominantly fresh water with occasional brackish and salt water, which also provide evidence of lower salinity of water within the lower strata and increased salinity (fresh water to salt water) within the middle and upper of the strata. The values of V/(V + Ni), V/Cr, U/Th and Pr/Ph indicate that the samples mainly deposits in weak to moderate reducing environment. The reduction degree decreased first and then increased from bottom to top with a few oxidation sections are distributed in the middle of the formation, which indicate a process of lake regression and transgression. The paleoproductivity during organic-rich mudstone sedimentation was high in terms of P/Ti, Cu + Ni + Zn and TOC (1.99%–22.59%), which have a positive trend with paleosalinity. This indicates that the water with higher salinity is more active in nutrient supply. The productivity from large to small is the middle section, the upper section and the lower section, respectively. The homologous series of C27, C28 and C29 regular steranes, main carbon number perks, maximum carbon number, Ph/nC18–Pr/nC17 and ∑nC21-/∑nC22+ all indicate that the source of the organic matter during the J1h organic-rich mudstone was mainly terrestrial higher plants, the proportion of algae and phytoplankton increased in the upper section. The results show that the organic-rich mudstones of the Huxishan Formation developed under the background of lake regression and lake transgression. The dispositional environment and organic types of huxishan formation are obviously different in different sections.