## 4 Results
### Lithofacies Associations
#### 4.1.2 FA2: Delta Front Deposits  
Description:  
FA2 exhibits a composite stratigraphic architecture characterized by superimposed coarsening-upward and fining-upward successions, resulting in \"reverse-to-normal\" grading patterns. GR curves show compound funnel-bell (or box)-to-serrated morphologies (Figure 6), though partial successions may occur. Two subtypes are distinguished by lithofacies, stratigraphic scales, and granulometry: (1) FA2-1 features a lower coarsening-upward succession (10-20 m) transitioning from parallel-bedded silt-fine sandstone to massive pebbly sandstone (Smg; Figure 5E). The middle fining-upward succession (5-30 m) grades from Smg (Figure 5C,D) through planar cross-bedded sandstone (Sp; Figure 5C,D) to parallel-bedded sandstone (Sh). The upper interval (5-20 m) comprises horizontally bedded (Fl; Figure 5D) and massive mudstone (Fm) with intercalated rippled mudstone (Fr; Figure 5D) and sporadic coal (C). (2) FA2-2 differs by its basal thin-bedded siltstone (Fl; 1-3 m) interbedded with mudstone, transitioning to parallel-bedded (Sh) and small-scale planar cross-bedded sandstone (Sp) in its lower coarsening-upward succession (3-10 m). The middle fining-upward succession (5-15 m) progresses from massive fine sandstone with black mud clasts (0.2-1 cm; Fmg) to Sh (Figure 5H). The upper interval (3-30 m) contains horizontally bedded (Fl; Figure 5I) and rippled silty mudstone (Fr; Figure 5I) with occasional coal (C).  
Interpretation:  
FA2 is interpreted as delta front deposits. The lower lithofacies correspond to mouth bars overlying fine-grained deposits. Vertical reverse grading suggests an abrupt reduction in the hydrodynamic, likely caused by the rapid inflow of river water into a stable  
Figure 6: Log-profile correlation of Lower-Middle Jurassic, showing sequence classification and depositional system in the study area (profile location in Figure 1). BB: Beach Bar; BD: Braided River Delta; DB: Distal Bar; DC: Distributary Channel; DSL: Deep and Semi-deep Lacustrine; IB: Interdistributary Bay; MB: Mouth Bar; MD: Meandering River Delta; PS: Peat Swamp; S: Swamp; SDC: Subaqueous Distributary Channel; SIB: Subaqueous Interdistributary Bay; SDL: Semi-deep Lacustrine; SSL: Shore-Shallow Lacustrine; LST: Lowstand Systems Tract; TST: Transgressive Systems Tract; HST: Highstand Systems Tract.  
water body, a feature commonly observed in estuarine environments [52; 53]. The middle lithofacies represent subaqueous distributary channels, characterized by finer grain sizes compared to the plain distributary channels [2]. Planar cross-bedding indicates strong hydrodynamic conditions [54]. The presence of mud clasts at the base likely results from the erosion and scouring of underlying muddy substrates by channelized flows [55]. The upper lithofacies represent a subaqueous interdistributary bay, characterized by horizontal and massive bedding. These features indicate weak hydraulic energy, with deposition primarily from settling suspended sediments [2; 54; 56]. FA2-1 and FA2-2 are interpreted as braided and meandering river delta front deposits, respectively. Key distinctions include the following: (1) the braided delta front (FA2-1) exhibits coarser-grained clastics and larger depositional scale, with pebbly coarse sandstone-dominated mouth bar deposits characterized by massive, indicative of rapid depositional regimes under high-energy conditions [57]; (2) the meandering delta front (FA2-2) displays finer-grained distal bar facies at its base, showing enhanced development of low-energy depositional elements compared to FA2-1.