## 4 Results
### Fan-Delta Front
#### 4.1.2 Braided-River-Delta Depositional System  
Braided deltas, formed through the progradation of braided fluvial systems into standing water bodies, are characterized stratigraphically by vertically stacked coarsening-upward sequences [56; 57]. Conventionally, these systems are subdivided into three subfacies: delta plain, delta front, and prodelta [58], each representing distinct depositional regimes governed by fluvial-lactustrine interactions. In the study area, however, sedimentological differentiation is only robustly achievable for the delta plain and delta front subfacies. The prodelta subfacies exhibits ambiguous differentiation from fine-grained littoral-shallow lacustrine deposits due to overlapping textural characteristics, organic matter content, and bedding architectures. This diagnostic challenge arises from comparable hydrodynamic conditions and similar sediment supply mechanisms in low-gradient basin-margin settings. Consequently, all dark-colored fine-grained success--featuring laminated siltstones, organic-rich mudstones, and sporadic bioturbation structures--are systematically classified as littoral-shallow lacustrine deposits within the current facies framework. This operational categorization prioritizes lithological continuity and practical mapping consistency while acknowledging limitations in resolving subtle depositional energy gradients between prodeltaic and lacustrine environments under the study area's specific paleogeographic constraints. Braided-River-Delta Plain  
The braided delta plain, comprising braided distributary channels, floodplain mudstone deposits, and alluvial plain facies [59], exhibits extensive development of coarse-grained sand-conglomerates with cumulative thicknesses exceeding tens of meters, interbedded with gray to yellowish-brown mudstone intervals (Figure 6c). This subfacies domain is subdivided into two principal microfacies: braided distributary channel fills and associated floodplain deposits. The braided distributary channels represent the dominant sediment transport pathways, characterized by gray-brown pebbly conglomerates that display prominent clast imbrication and diverse sedimentary structures, including massive bedding, wedge-shaped cross-stratification, and tabular cross-bedding. These channel fills exhibit blocky to serrated log motifs on wireline logs, featuring high amplitude values with minor serration, typically showing gradational upper contacts and abrupt basal boundaries with underlying strata (Figure 7). Floodplain deposits, widely distributed throughout the study area, consist predominantly of purple to maroon mudstones intercalated with siltstone laminae (Figure 6f). These fine-grained successions are characterized by bell-shaped gamma-ray log responses with elevated values (Figure 7), reflecting progressive fining-upward sequences associated with waning flow energies during overbank flooding events. The significant textural contrast between high-energy channel conglomerates and low-energy floodplain silt-clay sediments underscores the dynamic interplay between channelized flow competence and episodic floodplain inundation within the braided delta plain depositional system. Figure 6: Photographs of typical Triassic rock types in the study area (for the well locations, see Figure 1a). The core in the photo is rich in abundant bedding and banded torn carbonaceous debris, indicating the complex and variable hydrodynamic environment in the study area.