Sedimentary Evolution Mechanisms and Architecture Models of Sandy Braided Rivers——A Study Based on Quantitative Flume Experiments

Key words: 
• Sandy braided river, Quantitative flume experiment, Depositional evolution, Sedimentary architecture, Quantitative scale and relationship model

Abstract: [objective] During the sedimentation process of sandy braided rivers, the braided channels undergo frequent, rapid, and continuous shifts leading to erosion and reworking within the eventually preserved braided river deposits, such that deposited braid bars and braid channels are characterized by fragmented morphology, relatively small scale, and undefined quantitative relationships. Traditional sedimentary models of sandy braided river sedimentation are inadequate to effectively guide the characterization of subsurface reservoirs. [methods] To clarify the sedimentary evolution mechanisms of sandy braided rivers and establish a reliable sedimentary architecture model with quantitative size relationships, this paper conducted a flume experiment under constant boundary conditions to reproduce the formation and evolution of a sandy braided river. Using a laser scanner to obtain topographic data at regular time intervals and accurately reconstruct a three-dimensional sedimentary architecture model of the simulated braided river. Further, the study proceeds with analyses of sedimentary evolution mechanisms, dissection of sedimentary architecture, and the construction of quantitative size and relationship models for architecture elements. [conclusions and discussions] The study reveals that: (1) In the initial stage of sandy braided river sedimentation, sediments undergo bedload transportation downstream, forming initial bars. The deflected flow converges into channels, further developing these into lobe-shaped initial bars, which are then reshaped and organized into a stable network of braid channels and bar patterns through the action of the braid channels; (2) Following the formation of the braided river, the braid channels and bars co-evolve continually, primarily through three mechanisms: lateral accretion of bars driven by braid channels, abandonment and infilling of braid channels overlaying existing bars, and scouring of the confluence by braid channels, which results in downstream bar reorganization; (3) During simulation, bars typically form within 1 to 6 run steps and grow to their maximum size before experiencing continuous erosion over 3 to 8 simulation periods, ultimately leading to preservation in only 36.28% of the area; (4) Upon completion of the simulation, the internal structure of the braided river deposit is dominated by braided channel deposits, accounting for approximately 57.9%, which can be classified into complex stacked channels, incised channels, and isolated channels. Bars often undergo erosion and reworking from channels, presenting as smaller, fragmented forms; (5) Within the preserved deposit, the average width-to-thickness ratio of braided channels is 14.1, with an internal accretion ratio of 13.7, while for bars, the ratio is 19.8, with an internal accretion ratio of 25.4. This research constructed the complex sedimentary architecture formed within sandy braided river deposits after persistent and intensive erosional modification by the braid channel network. It establishes a quantitative model of the size and relationships between internal architectural elements, thereby providing a more geologically realistic and quantitative sedimentary architecture model for characterizing subsurface reservoirs.