## 5 Discussion  
### Distribution Patterns of Sandbody Genetic Types  
The genetic types of channel sandbodies within the Shaximiao Formation in the Shunan area are largely controlled by depositional processes. The formation of deeply incised abrupt-transition and gradual-transition channels is driven by strong hydrodynamic conditions coupled with abundant sediment supplies, typically developed within delta plain subfacies. By contrast, growth gradual-transition sandbodies necessitate moderately hydrodynamic conditions and are mainly developed within the transition zone between delta front and plain subfacies. Meanwhile, progradational superimposed, progradational mouth bar, and suspension accretion sandbodies develop under low hydrodynamic conditions and are mainly distributed in delta front subfacies (Figure 7).  
In the delta plain subfacies, strong hydrodynamic forces result in the formation of large channels with intense river incisions. These conditions favor the development of deeply incised abrupt-transition sandbodies within primary channels. These sandbodies are characterized by abrupt lithological contacts at both their upper and lower boundaries, substantial thickness, and a predominance of medium to coarse-grained sand deposits (Figure 6a). Within the distributary channels of the delta plain subfacies, both deeply incised gradual-transition and deeply incised abrupt-transition sandbodies develop. They form earlier due to intense river incision, which accumulates coarse-grained sediments and results in an abrupt basal contact. Over time, hydrodynamic conditions gradually weaken,  
Figure 7: Characteristics and distribution patterns of different genetic types of sandbodies.  
either due to a continued decline in sediment supply or shifts in river pathways. This change leads to a decrease in both the volume and coarseness of clastic sediments, resulting in a vertically normal-graded structure throughout the sandbody (Figure 6a). By contrast, deeply incised abrupt-transition sandbodies form under sustained strong hydrodynamic conditions, which ensure a stable supply of uniformly coarse-grained sediments. This results in a consistently even graded structure from bottom to top.  
Growth gradual-transition sandbodies form in distributary channels at the delta plain-front transition zone, where they are influenced by both fluvial and tidal processes. The hydrodynamic conditions in these settings are variable, leading to the development of a mixed normal-reverse graded pattern throughout the sandbody. By contrast, in the delta front subfacies, where hydrodynamic conditions are relatively weaker and channels are smaller, progradational superimposed, progradational mouth bar, and suspension accretion channels predominantly develop. Progradational superimposed channels initially develop along the edges of the delta front subfacies under weak hydrodynamic conditions, characterized by the deposition of fine-grained sediments. However, as hydrodynamic forces gradually intensify over time, the influx of coarse-grained sediments increases, resulting in a vertical shift toward a reverse-graded structure within the sandbody (Figure 6c). Programational mouth bars and suspension accretion channels typically form at the distal ends of delta front rivers, where hydrodynamic conditions are weak and unstable. These conditions lead to frequent interbedding of sandstone and mudstone. A key distinction between these two sandbody types lies in their vertical grain-size patterns. In progradational mouth bars, the sandstone layers display reverse grading, with grain size progressively increasing from fine to coarse upward (Figure 6d). By contrast, suspension accretion channels exhibit minimal changes in grain size vertically, showing no significant grading trends (Figure 6e). These channel types are predominantly developed within the Sha 2 member (J\({}_{2}\)S\({}_{2}\)).