## 4 Results
### Lithofacies Associations
#### 4.1.1 FA1: Delta Plain Deposits  
Description:  
FA1 exhibits a fining-upward succession comprising basal conglomerate/pebbly sandstone transitioning to coarse-grained sandstones, capped by fine-grained deposits (silstone to coal; Figures 3 and 4). Gamma-ray (GR) curves show box or bell-to-serrated compound morphologies. Two subtypes are distinguished by grain size, sand/mud ratio (SMR), and sedimentary scale: (1) FA1-1 (SMR \(\gg\) 1), which contains a coarse interval (10-50 m) with conglomerate (Gm; 0.5-5 cm gravel; Figure 3B) succeeded by pebbly sandstone (Smg; Figure 5B), trough or planar cross-bedded sandstones (St, Sp; Figure 3C), and parallel bedded sandstone (Sh; Figure 3D). The fine interval (5-20 m) comprises interbedded mudstone (Fm; Figure 5A), rippled siltstone (Fr; Figures 3E and 5A), carbonaceous mudstone (Mc; Figure 3F) with local coal (C; Figure 5A) and root clay (R; Figure 3G). (2) FA1-2 (SMR \(\leq\) 1) has a finer coarse interval (5-20 m) of pebbly sandstone (Smg) over erosional surfaces transitioning to small-scale cross-bedded sandstones (St, Sp, Sh; Figures 4B and 5G). Its thicker fine interval (10-30 m) shows distally thinning interbedded siltstone-mudstone wedges (Fl, Fm; Figures 4D and 5F) overlain by thick bedded mudstone (Fm/Fl; Figure 5F), rippled siltstone (Fr; Figure 4E), carbonaceous mudstone (Mc), and medium-thick coal seams (C; Figure 4F). Interpretation:  
FA1 is interpreted as delta plain deposits. The conglomerate or pebbly sandstone representing the coarsest sediments within the river channels indicate high-energy hydraulic conditions [44]. These thin deposits are lag deposits formed during major flood events [45]. Cross- and parallel-bedded pebbly sandstones or sandstones are indicative of high-energy fluvial channel deposits [46], whereas horizontally bedded mudstones and massive carbonaceous mudstones suggest low-energy floodplain deposits within shallow, stable water bodies [47]. The presence of coal seams and root clay (R) reflects a plant-rich, detritus-poor sedimentary setting, likely swamps within the floodplain [48,49]. FA1-1 and FA1-2 are interpreted as braided and meandering river delta plain deposits, respectively, with the former exhibiting higher sand/mud ratios and coarser-grained clastics. The thin layer of fine-grained sediments in FA1-1 suggests that the floodplain is often eroded by the instability and rapid migration of braided river channels [2,47]. The laterally migrating packages within FA1-2 coarse-grained intervals record point-bar lateral accretion processes diagnostic of meandering channels [50]. Distally thinning and fining wedges in FA1-2 fine-grained successions support natural levee deposition in meandering delta plains [51]. The wedge-shaped geometry reflects the diminishing energy of sheet flows during overbank flooding events, leading to a progressive decrease in grain size away from the channel edges [51]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Faces/Code** & **Lithology** & \begin{tabular}{c} **Sedimentary** \\ **Structures** \\ \end{tabular} & **Description** & **Interpretation** \\ \hline M & Mudstone & \begin{tabular}{c} Massive or \\ horizontal bedding \\ \end{tabular} & Relatively pure & \begin{tabular}{c} Deep and semi-deep \\ lacustrine, peat swamp \\ \end{tabular} \\ \hline Mc & Carbonaceous mudstone & Massive & Thin, interbedded with \\ mudstones or coal seams & floodplain deposits \\ \hline C & Coal & No sedimentary structures & \begin{tabular}{c} In outcrops, thin in thickness, \\ 0.2–1 m; in boreholes, significant \\ thickness variations at \\ different layers \\ \end{tabular} & \begin{tabular}{c} Peat swamp or \\ floodplain deposits \\ \end{tabular} \\ \hline R & \
\begin{tabular}{c} Muddy siltstone \\ to siltstone \\ \end{tabular} & No sedimentary structures &
\begin{tabular}{c} Development of plant root and \\ paleosols, brown in color, thin in \\ thickness, coexisting with \\ coal seams \\ \end{tabular} & Floodplain deposits \\ \hline \hline \end{tabular}
\end{table}