## 4 Result and Discussion
### Characteristics of Sedimentary Facies
#### 4.2.2 Shallow-Water Fan-Delta Front  
1. Underwater main channel  
The lower portion of the underwater channel depositional unit is primarily composed of gray-green glutenite, exhibiting a gradual upward-fining transition toward small conglomerates and gravel-bearing coarse sandstones, all of which are particle-supported rocks (Figure 7). The deposition of a single channel is an upward-fining sequence. Depositions formed by multiple stages superimposed in the study layer. The composite channel has an estimated thickness of approximately 15-25 m. At the base, a scour surface or bottom conglomerate is observed, followed by the development of massive bedding, trough cross-bedding, and inclined bedding from bottom to top. The logging response is a combination of a high-amplitude toothed clock and box. 2. Underwater distributary channel  
The study area exhibits gentle slope topography at the front end of the delta, with numerous river branches forming a secondary underwater distributary channel. Compared to the main channel, the sedimentary hydrodynamic force is relatively weak in this area, resulting in finer grain sizes. The bottom of the main channel reveals scour surfaces and mud gravel, abruptly coming into contact with the underlying mudstone (Figure 7). As we move upward, there is a gradual transition from a small amount of glutenite to finer particles. The sequence continues with small conglomerate and medium sandstone, ultimately ending with the development of silty mudstone at the top. The single-stage river channel displays upward-fining rhythmic characteristics and has a smaller thickness than the main river channel. This channel exhibits small trough cross-bedding, inclined bedding, and small erosion surfaces, with mud gravel visible at the bottom. Alternatively, the branch channel is influenced by the fluctuations in the lake's water level, resulting in the development of mudstone in multiple stages. This channel is characterized by a combination of medium- and low-amplitude bell- and finger-shaped patterns in the SP and RT curves. 3. Estuary bar  
The river estuary bar is a thick sediment deposit resulting from sediment unloading at the end of an underwater distributary channel that extends into the lake basin. This formation results from the interaction between the distributary channel and lake water. The development of estuary bars in the shallow-water fan deltas in the study area exhibits distinct characteristics compared to typical fan deltas. The shallow-water body and gentle slope of the fan-delta front contribute to changes in the shoreline due to fluctuations in the lake's water level. As a result, a large area of river-like interaction zone forms in the fan-delta front. The estuary bar sediment demonstrates two notable features under the influence of repeated river and lake water dynamics. First, the development range is extensive, extending beyond the front end of the delta, because the estuary bars are connected to the distributary channel during periods of high water levels. Second, sand bodies in the shallow-water delta front exhibit better continuity on the plane compared to conventional estuary bars. Core and well logging analyses indicate that the estuary bar sediments in the study area consist of fine-grained materials such as siltstone, medium-fine sandstone, and gravel-bearing coarse sandstone. Vertically, these sediments exhibit antihythmic sedimentary characteristics, with the lithology becoming finer and thinner toward the edge of the sand body, where ripple bedding is developed. SP and RT analyses reveal a medium-amplitude, toothed funnel pattern. 4. Overbank channelThe features of near-source deposition within the study area result in the rapid flow of sediment-laden water through the underwater distributary channel during flooding periods, leading to the formation of underwater overbank deposition.