Geochemical characteristics of the 3rd Member of Paleogene Shahejie Formation in Dongpu Depression and their geological implications

Abstract：Abundant geological information can be obtained from trace elements and REE in sedimentary rocks, which may reveal the provenance attribute and tectonic setting and help to reconstruct the paleo-sedimentary environment. Geochemical characteristics of mudstones and rock salt from the 3rd Member of Paleogene Shahejie Formation in Dongpu depression of Bohai Basin were studied in detail in this paper. The results show that the samples have relatively high Sr and Ba and low Rb, V, Ni, Mn, Fe, Cr and Br. Concentrations of most of the trace elements are higher than their Clark values in the same sorts of rocks. The total content ranges from 47.1×10-6 to 268.02×10-6, and the average content is 181.55×10-6，which is similar to the average value of PAAS. The ratios of LREE/HREE range from 7.96 to 11.46, and the average content is 9.71. These data show that LREE are obviously richer than HREE, and the distribution patterns of the samples are characterized by “V”-shaped curve with obvious negative Eu (from 0.48 to 0.70) anomaly and indistinct negative Ce anomaly (from 0.89 to 0.97). In addition, the content of rock salts is much lower than their content in other samples. According to an analysis of the characteristic parameters (Sr/Cu, V/(V+Ni),（La/Yb）N, Sr/Ba,δCe and Ceanom), it is revealed that the study area belonged to continental sedimentation, and its paleo-environment was under the oxygen-free, anaerobic and saltwater condition. The characteristic parameters such as Eu, δEu, ωREE—ω(La)/ω(Yb), La/Th-Hf and Co/Th—La/Sc in the discrimination diagram can indicate that the source rocks were mainly intermediate-acidic felsic and mixed felsic/basic rocks as well as very insignificant sedimentary rocks, derived from detrital materials of Neihuang uplift and Luxi uplift.