Abstract:

Based on the coring samples of mudstones in the southeastern Ordos Basin, this article focuses on the analysis of trace elements and rare earth elements in different horizons and studies the evolution of sedimentary environment and provenance from Benxi Formation to Lower Shihezi Formation. Core data show that from Benxi Formation to Lower Shihezi Formation the color of mudstones varies from deep to shallow and the reducibility of sedimentary environment changes from strong to weak. According to the analysis of environmental sensitive trace elements (such as V, Ni, Sr, Cu, etc.) and Ce_anom, from the Benxi Formation to the Lower Shihezi Formation, the sedimentary environment is generally the weak reducing-reducing environment, and the reducibility tends to decrease gradually; the climate is generally warm and humid with the wettability of climate gradually decreasing from bottom to top. The Benxi Formation is marine saline deposits, and the Shanxi Formation and Lower Shihezi Formation are dominated by continental fresh water deposits. The depth of water is shallow in Benxi Formation, more deeper in Shanxi Formation, which is more deeper than in Lower Shihezi Formation. The REE indexes analyses, such as δEu,(La/Yb)_N, Gd/Yb and so on, show that the sediments in the study area are mainly from upper crust rocks and there are two sources respectively from south and north. The northern provenance is from Alanshan-Yinshan ancient continent in the northern basin and the southern provenance is from the northern Qinling orogenic belt. Under the control of the south and north sedimentary origins, the value of Al₂O₃/CaO of the proximal sediments in the southern area is generally higher than that of the far-source sediment in the northern area.

Key words: Ordos Basin, mudstone, trace element, sedimentary environment, provenance