## 4 Results and Discussion
### Interpretation
#### 4.2.2 Mid-Channel Downstream Accreting Bar  
Description  
The mid-channel downstream accreting bar is usually multistory and amalgamated, showing an overall fining-upward trend. It is mainly composed of fine- to coarse-grained trough cross-bedded (St), planar cross-bedded (Sp), and sigmoidal bedded (Ss) sandstone and contains a small amount of horizontal medium- to coarse-grained sandstones (m-cSh), laminated silty mudstone (zMh), gray silstone-silty mudstone (s-mg), massive intrafractional conglomerates (Gm), and planar cross-bedded conglomerate (Gp). Different periods are generally bounded by flat or concave micro-erosive surfaces, which are usually covered by thin (<15 cm) intraformational conglomerates, scattered pebbles, or silty mudstone. The grain size of each phase shows a less obvious trend of thinning upward and toward its flanks. Figure 3: The lower part of the outcrop profile photograph (**A**) and the interpretation map (**B**) show the sedimentary features of the abandoned channel and floodplain; the part above level 5 architecture interfaces shows the sedimentary features of the laterally accreting bar and channel (site IV, section 10). At site IV, a three-dimensional annular profile extends more than 50 m laterally and 30 and 40 m longitudinally, exposing a relatively complete downstream accreting bar deposit (Figure 5). The four periods of amalgamated bar deposits identified here are bounded by three eroded surfaces, with thicknesses of 1.5 m to 2.8 m for each period. The uppermost erosional boundary surface is concave, and the thickness of the axial erosion is up to 0.5 m and is covered by a layer of fine mudstone with a thickness of centimeters and a lateral extent of tens of meters (Figures 3 and 6). The core of the first period deposit is mainly composed of low-angle cross-bedded coarse sand-conglomerate (pebbles), which turns downstream into subhorizontally bedded coarse sandstone. On the transverse section of the second stage deposit, the bedding structure gradually changes from relatively flat to a small angle (3-6\({}^{\circ}\)) from the core to its flanks. In the longitudinal section, the foresets are estimated to extend forward from the core to more than 70 m (40 m for outcrop identification). This unit is mainly composed of planar-sigmoidal cross-bedded sets (1.8-2.6 m) with uniform dip angles (15-30\({}^{\circ}\)) and dip directions. In the upper parts of foresets, a near-horizontal erosion surface can be observed, becoming consistent toward the bottom along the cross-strata, and the base of the sets is not eroded and slightly concave. In the downstream direction, gray siltstone to silty mudstone deposits (S-Mg) with the same dip angles are found (Figure 3), and the height is 1/2-2/3 of the maximum height of the foresets, corresponding to the ratio between the thickness of interbar mud thicknesses and the local water flow depth observed by [PERSON] and [PERSON] (2006), [PERSON] and [PERSON] (2006) which was 1/2 to 2/3. The upper two-period phases are mainly composed of trough cross-bedding (St) and planar cross-bedding (Sp) that is 50 cm thick, bounded by an inclined surface of 10-17\({}^{\circ}\) and covered upward by floodplain mud. At profile 5 (Figure 6), upstream accreting strata are identified in the upper strata, with a thickness of approximately 1.4 m, which is not common in this study area. The slope of the formation is low, and the tendency is almost opposite to the direction of the local paleoflow. At the top of the bar, there is a 2-3 m thick horizontally bedded medium-grained sandstone  
Figure 4: Outcrop profile photograph (**A**) and interpretation map (**B**) showing the transverse profile characteristics of the erosionally based fluvial channel and bank-attached laterally accreting bar (site III, profile 6). The enlarged image (**C**) from profile A shows the lateral migration of the erosionally based channel. (**D**) is a typical vertical log of the lithofacies association in a laterally accreting bar and erosionally based channel.