# Eocene Sedimentary-Diagnetic Environment Analysis of the Pingtai Area of the Qaidam Basin
## 6 Discussion  
Both sedimentary processes and diagenesis may cause the migration and loss of elements. Therefore, when the geochemical properties of elements are used to reconstruct the sedimentary environment and paleoclimate conditions, the sources of elements should be analyzed. Only trace elements that are mainly autogenous and maintain their initial contents can be used to accurately evaluate paleoenvironmental conditions [70]. The chemical components in sedimentary rocks generally come from three main sources: terrigenous clastic materials, biological processes and authigenic sources [71]. To better determine the enrichment pattern of redox-sensitive elements, it is first necessary to determine whether there are terrigenous debris-derived components in the sediments. Among them, Th, as an important index to measure terrigenous clastic composition, is less affected by the grain size of sediments [72], and the diagenesis of sedimentary rocks has little influence on Zr and Y/Ho. Therefore, the ratios of Th to Zr and Y/Ho can effectively indicate the contribution of terrigenous clastics to diagenesis [73]. The relationships between Th and Zr and between Th and Y/Ho in Figure 8 show that there were no obvious correlations between these elements, indicating that the sources of the elements in the clastic rock were not solely terrigenous clastic materials, but that some elements were authigenic. In nature, Y and Ho have very similar geochemical properties; therefore, they migrate or precipitate together in many geological processes. [PERSON] and [PERSON] (1996) [74] found that the Y/Ho ratio in clastic sediments is about 28, and the closer to this value, the smaller the contribution of terrigenous clastics in sedimentary rocks which are mainly authigenic [75]. The ratio of Y/Ho in Paleogene samples in the Pingtai area is between 22.5 and 27.7, with an average value of 26, which is close to the Y/Ho ratio in the clastic sediments, indicating that the chemical components of sandstone in the study area are mainly authigenic and that the geochemical characteristics can be used to reconstruct the sedimentary environment and paleoclimatic conditions [75]. Figure 7: Chondrite-normalized REE patterns (**a**) and NASC-normalized REE patterns (**b**) of sandstones from the Eocene in the Pingtai area. Previous researchers have considered both Cu and Zn to be transition metal elements. According to the Nernst equation, the pH of the medium will affect the oxidation-reduction process, and Cu and Zn are separated in the environmental medium as the oxygen fugacity decreases. Therefore, the oxidation-reduction environment during deposition can be divided according to the Cu/Zn ratio. A ratio of less than 0.21 indicates a reducing environment, 0.21-0.38 represents a weakly reducing environment, 0.38-0.5 denotes a reduction-oxidation transitional environment, 0.5-0.63 indicates a weakly oxidizing environment and greater than 0.63 specifies an oxidizing environment [76]. The Cu/Zn ratio of the Lulehe Formation in the Pingtai area ranges from 0.28 to 0.59 with an average of 0.43, and the Cu/Zn ratio of the Xiaganchaigou Formation ranges from 0.28 to 0.47 with an average of 0.35. These results indicate that the Pingtai area was dominated by a weakly reducing to reducing-weakly oxidizing transitional environment during the Paleogene sedimentary period, and that the Xiaganchaigou Formation was more hypoxic and reducing. The geochemical properties of Th and U are very different in oxidizing environments but similar in reducing environments, which can be used to reconstruct the sedimentary environment. Based on the formula &U = U/[0.5 \(\times\) (Th/3 + U)], a value that is greater than 1 represents a reducing environment, and a value less than 1 denotes an oxidizing environment [77]. A Th/U ratio between 0 and 2 indicates a reducing environment, and this ratio can reach 8 under strongly oxidizing conditions [78].