## 5 Discussions
### Analysis of Tectonic Background in Provenance Area  
Because the tectonic environment can not only control the material source of terrigenous clastic sediments, but also affect the differentiation degree of elements in the process of weathering, denudation, transportation and deposition, different tectonic environments correspond to different element geochemical characteristics. [PERSON] (1986) established a series of discriminant plates for distinguishing the tectonic provenance by using the content of Fe\({}_{2}\)O\({}_{3}\) + MgO, TiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), the ratio of K\({}_{2}\)O/Na\({}_{2}\)O, SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) and other parameters. The sedimentary tectonic provenance of sandstone could be divided into four types: passive continental margin (PM), active continental margin (ACM), continental island arc (CIA) and oceanic island arc (OIA) [12], and the intensity of tectonic activity gradually increased with each type. Wudun sag is a typical continental faulted lacustrine basin in  
Figure 6: UCC\(-\)normalized rare earth element spidergrams of the samples.  
the Jurassic period [11], which is different from the four kinds of sandstone sedimentary tectonic provenance established by [PERSON]. However, this study restores the structural attributes of the provenance area and tracks the material source of sediments by using the strength of the intensity of the tectonic activity of different tectonic backgrounds and the tectonic evolution background.  
With the increase in the intensity of tectonic activity in the basin, the content of the major elements Fe\({}_{2}\)O\({}_{3}\) + MgO and TiO\({}_{2}\) increases, while the ratios of SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) and K\({}_{2}\)O/Na\({}_{2}\)O decrease in the terrigenous clastic sediments [12]. Additionally, the content of trace elements such as La, Th, SC, Zr and Ti in clastic sedimentary rocks can also be used as the basis for distinguishing the tectonic environment background, due to their strong stability [15]. With the increase in tectonic activity intensity, the ratio of La/Sc decreased and the ratio of Ti/Zr increased. In the TiO\({}_{2}\)\(-\)(Fe\({}_{2}\)O\({}_{3}\) + MgO) plot (Figure 7a), K\({}_{2}\)O/Na\({}_{2}\)O\(-\)SiO\({}_{2}\) plot (Figure 7b), SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\)\(-\)K\({}_{2}\)O/Na\({}_{2}\)O plot (Figure 7c) and Ti/Zr\(-\)La/Sc plot (Figure 7d), the samples of Well D1 mainly fall in the area of the passive continental margin and the active continental margin, while the samples of Well XC1 and Well D2 mainly fall in the area of the continental island arc, indicating the tectonic activity of the provenance area of Well XC1 and Well D2 is stronger than that of Well D1.  
In the early and Middle Jurassic period, Wudun sag shown the characteristics of a faulted lacustrine basin affected by the fault activity at the southern boundary [11]. The provenance area of southern Sanwei Mountain is located in the steep slope zone of the faulted lacustrine basin, with strong tectonic activity. The provenance area of northern Beishan is located in the gentle slope zone of the faulted lacustrine basin, with the relatively stable tectonic activity. From the tectonic activity background of different well areas reflected by major and trace elements, the sediments of Well XC1 and D2 are more likely to  
Figure 7: Tectonic setting discrimination diagram of the provenance area from Wudun sag. (**a**) TiO\({}_{2}\)\(-\)(Fe\({}_{2}\)O\({}_{3}\) + MgO) plot; (**b**) K\({}_{2}\)O/Na\({}_{2}\)O\(-\)SiO\({}_{2}\) plot; (**c**) SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\)\(-\)K\({}_{2}\)O/Na\({}_{2}\)O plot; (**d**) Ti/Zr\(-\)La/Sc plot. (PM-Passive continental margin, ACM-Active continental margin, CIA-Continental island arc, OIA-Oceanic island arc).  
come from the southern Sanwei Mountain provenance area, while the sediments of Well D1 are probably derived from the northern Beishan provenance area.