## 5 Depositional Systems
This coarse-grained system exhibits distinct lithofacies cyclicity, characterized by rhythmic alternations of medium to fine-grained conglomerates (45-68% gravel content) and deep gray mudstone beds (2-5 m thick), demonstrating a strong provenance affinity. Provenance-ward grain-size coarsening is evidenced by an increase in mean gravel diameter from 8.2 mm to 23.5 mm (\(n=157\) measurements), accompanied by a 12.3 \(\pm\) 1.8% quartz content reduction. Sedimentary architecture reveals: (1) gravity-flow signatures featuring well-developed lubrication structures (Imbrication Index = 0.72-0.85), clast-supported fabric, and basal scours; (2) the absence of tractive current sedimentary structures. Wireline log responses display blocky-bell motifs with gamma ray (GR) values ranging from 80 to 120 API, indicative of normal grading sequences. Seismically, these fans exhibit wedge-shaped seismic facies with chaotic reflections proximal to fault zones (continuity index < 0.3), transitioning basinward to parallel-continuous reflections (continuity index > 0.7). This seismic and geological configuration records a depositional continuum from proximal high-energy to distal low-energy environments, controlled by decreasing sediment supply rates and increasing hydrodynamic sorting toward the sag center. The fan delta (FD), formed by alluvial fans prograding into lacustrine environments, constitutes a prominent depositional system within the southern steep slope zone of the Gaoyou Sag. The delta plain subfacies is dominated by sandy-gravel braided channel deposits, comprising matrix-supported conglomerates (sorting coefficient \(\sigma=2.8\)-3.5) interbedded with brownish-gray mudstones, exhibiting low compositional and textural maturity. Characteristic sedimentary structures include scour surfaces, tabular and trough cross-bedding, and massive bedding, forming vertically stacked upward-fining sequences. The delta front subfacies features conglomerate sandstones and sandstones intercalated with grayish-green to gray silty mudstones. These coarse-grained deposits display scour bases, large scale cross-bedding, and parallel laminations. The interbedded fine-grained facies (light grayish-green, brown, purplish-red, and pale gray mudstones/siltstones) exhibit horizontal bedding, wavy bedding, and lenticular bedding, often containing abundant mica flakes on bedding planes. Diagnostic biogenic features include plant-derived charcoal fragments and bioturbation structures. The lacustrine facies, extensively developed throughout the Dainan Formation, consist predominantly of fine-grained lithologies. Shore-shallow lacustrine deposits, influenced by frequent lake level fluctuations, are composed of brown-gray siltstones, argillaceous siltstones, silty mudstones, and dark brown mudstones. These deposits display horizontal bedding, wavy bedding, subvertical burrows, bio-perturbation structures, and plant rhizome fossils (Figure 4). Corresponding well log responses feature low-amplitude linear patterns on spontaneous potential (SP) curves and serrated medium-low resistivity values. In contrast, semi-deep to deep lacustrine facies developed under anoxic, weakly reducing to reducing conditions are characterized by dark gray to black mudstones and calcareous mudstones with horizontal lamination. Diagnostic components include pyrite nodules and well-preserved ostracod fossils (Figure 4). These deposits exhibit stable SP curve baselines and slightly undulating apparent resistivity profiles, reflecting persistent deep-water depositional environments