Deposition environment and provenance of the Palaeogene Shahejie Formation in Nanpu Sag: Evidences from trace and rare earth element geochemistry

Abstract

In order to reveal the formation mechanism of the organic-rich shales of the Shahejie Formation in Nanpu Sag, the depositional environment and provenance were investigated in detail through analyzing trace and rare earth elements(REE) geochemical characteristics. The results show that the trace elements Li, Cs and Bi are enriched, Cr and Sn are relatively depleted, and other trace elements are close to those in the upper continental crust(UCC).The total amount of REE vary widely, which is close to or higher than the average value in the UCC, and the supply of terrestrial source debris is adequate. The REE allocation pattern shows light rare earth element(LREE) enrichment with a high degree of divergence, and a relative deficit of heavy rare earth element(HREE) with a low degree of divergence. Eu negative anomaly is obvious, Ce is basically normal. The variations of Sr abundance and Sr/Ba ratio reflect that, in Nanpu Sag, the lake water was separated to a certain extent, and the fresh water, brackish water and saline water coexisted during sedimentary period of the third member of Shahejie Formation(Es3), and during these dimentary period of the first member of Shahejie Formation(Es1), the connectivity of lake water was enhanced, which was dominated by brackish water.The values of the V/(V+Ni), Th/U, δCe, and Ceanom comprehensively delineate the water body presented suboxic environment with moderate stratification. Respectively, the source Ba content indicated the water body was dominated by high paleoproductivity. The REE assemblage characteristics suggests that the provenance of Shahejie Formation were primarily derived from Yanshanian granites developed in Yanshanian fold belt and a small amount of sedimentary rocks. The development of organic-rich shales in the Shahejie Formation in Nanpu Sag is closely related to paleoenvironment and paleogeography, which can be summarized as the followings: ①suitable paleosalinity as well as sufficient nutrients at water body contributed to the booming of various planktonic algae, improving the production efficiency of the aquatic organic matter; ②suboxic water column slowed down the degradation of oxygen-sensitive material during the burial process, enhancing the preservation efficiency of sedimentary organic matter.

Keywords

• nanpu sag
• shahejie formation
• trace element
• rare earth element
• deposition environment
• provenance