# Eocene Sedimentary-Diagnetic Environment Analysis of the Pingtai Area of the Qaidam Basin
## 6 Discussion
In the Pingtai area, the &U value of the Lulehe Formation is between 1.13 and 4.15 with an average of 2.24, and the Th/U ratio is between 3.01 and 4.75 with an average of 3.91. In the Xiaganchaigou Formation, &U is between 2.68 and 3.71 with an average of 3.12, and Th/U ratio is between 2.34 and 4.23 with an average of 3.36. The lower &U ratio and higher Th/U ratio of the Xiaganchaigou Formation also represent stronger reducing conditions. Among the REEs, Ce is the only element with redox characteristics; it is depleted in oxidizing environments and enriched in oxygen-deficient reducing environments. [PERSON] et al. (1987) [79] proposed that positive and negative values of Ce\({}_{\text{anom}}\) = lg[3 Ce\({}_{\text{n}}\)/(2 La\({}_{\text{n}}\) + Nd\({}_{\text{n}}\))] can be used to determine whether Ce is enriched. A positive value of Ce\({}_{\text{anom}}\) indicates enrichment, whereas a negative value indicates depletion. In this formula, \"n\" signifies shale-normalized concentrations (using the convention established by [PERSON] et al., 1984 [49]). The value of Ce/La can also reflect the redox environment. A value greater than 2.0 indicates that Ce is enriched and the environment is reducing; a value less than 1.5 indicates a negative Ce anomaly and an oxidizing environment [80]. The Ce/La values of all Paleogene samples in the Pingtai area are greater than 1.5, and the Ce/La values of the Lulehe Formation are between 1.73 and 2.07 with an average of 1.86; the Ce/La values of the Xiaganchaigou Formation are between 1.83 and 2.07 with an average of 1.97. These results indicate that, on the whole, the samples are enriched in Ce representing a weakly oxidizing to reducing sedimentary environment, and that the Xiaganchaigou Formation was more reducing overall. The element V is easily adsorbed and enriched in an oxidizing environment, and most V in sediments is authoristic  
Figure 8: Diagrams of Th–Zr and Th–Y/Ho relationships among samples of the Lulehe Formation and Xiaganchaigou Formation. and migrates little during the diagenetic process. It retains the original state of the sedimentary period and is an ideal indicator of discrimination. In contrast, Ni is relatively depleted in oxidizing environments and relatively enriched in reducing environments. The value of V/(V + Ni) can be used to indicate the redox environment; a ratio greater than 0.7 indicates an anoxic environment [81]. The V/(V + Ni) values of the samples vary little, ranging from 0.69 to 0.80 with an average value of 0.75, which indicates that the Paleogene sedimentary period was dominated by an anoxic environment. The most important climate warming event since the Cenozoic occurred during the middle Paleocene to early Eocene. During 55-45 Ma (the deposition period of the Lulehe Formation), the climate gradually became colder, global sea level slowly declined and the Antarctic continent formed a permanent ice sheet, forming a long-term gradual cooling process. It was not until the climate warmed again in the late Oligocene that the ice sheet began to melt [82; 83; 84]. Studies on sedimentation of the North American continent [85], vegetation change [86] and animal evolution [87] have shown that from the early Eocene (the Lulehe Formation deposition period) to the late Eocene (the Xiaganchaigou Formation deposition period) the climate underwent significant changes; the climate changed from cold and dry to alternately dry and wet. Studies on the European continent [88], Asia and Oceania [89] have also suggested that the climate in the middle to late Eocene was relatively humid. The global climate entered a dry and cold period again in the early Oligocene. Both clay mineral composition and geochemical components in the Pingtai area indicate that, from the early Eocene to the middle and late Eocene, the climate gradually changed from cold and dry to cold and alternately dry and wet, and the depositional environment was dominated by anoxic reducing conditions.