Geochemical Characteristics of Mudstone in the Huanhe Formation in the Southwestern Ordos Basin and Their Paleoenvironmental Significance

Abstract

The trace elements and rare earth elements in sedimentary rocks are closely related to the sedimentary environment, especially mudstone. Conducting trace element analysis on mudstone is an effective means of studying palaeosedimentary environments. The Cretaceous period in the southwestern part of the Ordos Basin is an important period in geological history; however, research on its geochemical characteristics and palaeoenvironment is still relatively weak. The Huanhe Formation in the Zhenyuan area was selected as the research object. Based on geochemical analysis methods, such as major elements, trace elements, and rare earth elements, the indicative significance of geochemical characteristics for its sedimentary palaeoenvironment was explored. The research results show that CIA and C contents, Ga/Rb, and K2O/Al2O3 indicated a warm and humid environment. CaO and MgO contents also support that mudstone experienced dry climatic conditions with strong evaporation. Cr and V/(V + Ni) indicate dysoxic to anoxic conditions. Ni/Co vs V/Cr relationship indicates the oxic condition of the depositional environment. The Huanhe Formation was deposited in an environment ranging from dysoxic to anoxic conditions, and during certain periods, there were also oxic environments present. The ratios of Sr/Ba and Th/U indicate deposition in saline water.