## 4 The Impact of Sequence Structure on Sedimentary Filling
### The Impact of Long-Term Cycles on Sedimentary Facies Evolution  
The base-level cycle has a significant controlling effect on the sedimentary filling in the study area. There are three sedimentary systems filled within the long-term cycles (LNG) in the study area: alluvial fan, braided fluvial, and meandering fluvial facies. According to the analysis and statistics of the proportions of various sedimentary facies in the LNG, it shows that the meandering fluvial facies is most developed in the study area, followed by braided fluvial facies, and finally the alluvial fan facies. The type and characteristics of sedimentary filling have a regular variation with the fluctuation in the base-level in the LNG (Figure 11). Taking the maximum lake-flooding surface as the boundary, the LNG can be divided into a rising semi-cycle and a falling semi-cycle. The rising semi-cycle of the LNG can be further subdivided into three fourth-order cycles, namely MNG1 to MNG3, from bottom to top. The falling semi-cycle of the LNG corresponds to a fourth-order cycle named MNG4. There were significant differences in the types and development proportion of different sedimentary facies within each fourth-order cycle, which presented regular variation with changes in the base-level (Figures 11 and 12). The alluvial fan facies was most developed in the MNG1 period, accounting for 66%, and decreasing to 11% in MNG2. The proportion of braided fluvial facies in MNG1 was 34%; this proportion increased to a peak of 62.5% in MNG2, and then decreased to 38.5% in MNG3, and disappeared in the MNG4 period. The proportion of meandering fluvial facies increased from 0% in MNG1 to 26.5% in MNG2, then to 61.5% in MNG3, finally reaching 100% in MNG4. During the MNG4 period, the base-level fell and a meandering fluvial facies developed in the entire area, with the distribution area of meandering fluvial sand bodies becoming wider than that of the MNG3 period. Figure 11: Sedimentary facies changes with fluctuation in the base-level during the LNG. The MNG1 cycle is located in the early stage of the rising semi-cycle of the LNG. Skirt-shaped alluvial fan groups developed surrounding the ancient Bixian uplift near the boundary of the Binbei and Binnan faults during the MNG1 period (Figure 12d). There was no clear boundary between each alluvial fan body. The fan group diverged and connected to the braided river in the southward direction. Stream flow deposits were developed inside the alluvial fan body, extending towards the lower terrain. Part of the stream flow deposits underwent deflection near the descending plate of the syn-sedimentary faults, causing an increase in the width of the stream channel and a change in the direction to parallel to the fault. Braided fluvial facies mainly developed around the periphery of the Bixian uplift. MNG2 occurs in the middle stages of the rising semi-cycle of the LNG. As the base-level rose, the sedimentary system transitioned from alluvial fans to braided fluvial facies, and the sand body gradually decreased. Only a small number of alluvial fans developed in the central slope area near the uplift (Figure 12c). The braided fluvial channels presented in irregular strips, and their provenance changed, mainly coming from the north of the study area instead of from the Bixian uplift. The MNG3 cycle develops in the late stage of the rising semi-cycle of the LNG. During the MNG3 cycle, the Bixian uplift was basically filled to flat terrain, with the main facies being meandering fluvial and a small amount of braided fluvial (Figure 12b). The fluvial facies all came from the north. Compared to the MNG2 period, the width of the fluvial channel became narrower and the distribution range of sand bodies became smaller in MNG3, indicating a decrease in material supply. The distribution of the sand bodies became the smallest at the conversion surface between the rising semi-cycle and falling semi-cycle of the LNG. During the deposition period of MNG4, the base-level of the long-term cycle began to decline.