## 5 Discussion
### Deposition Model  
Based on the lithofacies associations, paleoenvironmental characteristics, sedimentary evolution described above, and the geological background, it can be inferred that the Qigu Formation was deposited during a period of long-term lake-level rise. However, unstable climatic conditions resulted in frequent lake-level fluctuations. The developmental patterns of shallow-water deltas in the study area can be categorized into two depositional models: the LST phase and the TST phase (Figure 14). During the deposition of the LST (SQ1-SQ3), the climate was relatively arid, and the lake basin remained shallow. Enhanced evaporation and oxidation led to the scarcity of pyrite in mudstones, while reddish-purple mudstones were widely distributed [10; 24; 26]. During this period, sediment supply was abundant, and fluvial activity was intense. The main channels transported terrigenous clastics into the lake basin, where they gradually bifurcated, forming multiple distributary channels and associated distributary channel-mouth bar complexes (Figure 14A). Frequent lake-level fluctuations and sedimentary hiatuses led to distinct microfacies distributions. Near the sediment source, the dominant microfacies was the main distributary channel (mainly lithofacies association LA1), while the distal areas were characterized by subaqueous distributary channels and mouth bars (primarily lithofacies associations LA2, LA3, and LA4). Locally, subaqueous interdistributary bays and sheet sands were also present (Figure 14A,C). Among them, the distributary channels exhibited multi-stage vertical stacking, primarily consisting of sandy conglomerates. Thesesand bodies can be characterized by considerable thickness, dendritic distribution patterns, and extensive lateral extent (Figure 14A,C). The mouth bars exhibited vertical stacking of multiple phases and lateral amalgamation, predominantly consisting of medium- to fine-grained sandstones with lobe-like distribution patterns. Terminal distributary channels were embedded within these mouth bar deposits (Figure 14A,C). Figure 14: Sedimentary model of the shallow-water delta front in the Qigu Formation. (**A**) Sedimentary model during the LST. The three solid black lines marked as AA’, BB’, and CC’ correspond to the three sections in (**C**). (**B**) Sedimentary model during the TST. (**C**) Sectional model in different parts of the delta. During the deposition of TST (SQ4-SQ6), the relatively humid climate with increased precipitation led to rapid lake-level rise and the significant expansion of lacustrine areas (Figure 14B) [31]. When the main channels transported terrigenous clastics from the source area into the lacustrine basin, deep-water conditions caused flow energy attenuation, leading to channel bifurcation processes in the lacustrine environment. Sediments accumulated rapidly in the proximal lacustrine zone, where subaqueous distributary channels and mouth bars dominate as the primary microfacies. These deposits consisted predominantly of siltstone and argillaceous siltstone, with dendritic channel patterns exhibiting limited lateral extent (Figure 14B). Vertically, the scale of channel deposits gradually diminished upward, while sandstone development was typically limited in central lacustrine areas. Compared with humid-climate shallow-water deltas, their arid-climate counterparts display marked differences in both depositional scale and internal sedimentary architecture [10; 26; 97; 98; 99]. Frequent lake-level fluctuations and climatic instability led to recurrent subaerial exposure and the oxidation of sediments, resulting in the widespread development of purplish-red mudstones. These deposits display distinct event sedimentation characteristics, particularly frequent interruptions by episodes of extreme aridity during deposition [35; 94]. Vertically, it was characterized by interbedded sandstone deposits and multiple layers of stable purplish-red mudstone.