Geochemical characteristics of Upper Paleozoic mudstone in southern North China Basin and their geological significances

Marine and continental alternative shale of Upper Paleozoic is important as a source rock in southern North China Basin. In this paper, twenty-eight mudstone and silty mudstone samples collected from Upper Paleozoic Benxi, Taiyuan, Shanxi and Xiashihezi Formations on Taikang Uplift, southern North China Basin were analyzed for REEs and trace elements, by means of ICP-MS and XRF to investigate the characteristics of paleoenvironment and their effect on the enrichment of organic matters. The result shows that mudstone in Taiyuan and Shanxi Formations, especially the latter, has similar features of obvious Eu-anomalies and weak Ce-anomalies, higher total REE contents and higher fractionation between LREE and HREE compared with samples from the Benxi and Xiashihezi Formations. The Ce abnormity, which can be used to reveal redox of lake water, shows that the studied samples are mainly formed in a hypoxic reducing environment. Vertical distribution of ΣREE and TOC suggests a change of climate from cold-arid to warm-humid, and a variation of sedimentation rate of mudstone from high to low to high. Among the four studied Formations, the Taiyuan Formation is the one with a stable sedimentary environment of deep water and low sedimentation rate, which benefits the accumulation and preservation of organic matters.