Evolutionary processes and architectures of autogenic cycle in the Lower Cretaceous alluvial fans, Luanping Basin, Northeastern China

Autogenic cycles are well-documented in both experimental subcritical and supercritical alluvial fan settings. However， the evolution of processes and architectures within autogenic cycles in natural alluvial fans remains poorly understood. This study investigates an ancient alluvial fan succession exhibiting retrogradation-progradation cycles within the Lower Cretaceous Luanping Basin， Northeastern China. Detailed sedimentary logs and photomosaic analyses provide insights into the depositional processes and architectures of the alluvial fan. Autogenic cycles were identified through vertical stacking pattern analysis and sequence-based interpretation. Seven lithofacies associations， representing deposits formed by distinct processes， were characterized. The fan was constructed proximally by debris flows and distally by stream flows. Three distinct autogenic phases were recognized， including the progradation phase， backfilling phase， and avulsion phase. The backfilling phase is predominantly characterized by the cyclic deposition of channels and mid-channel bars under subcritical flow conditions， whereas the progradation phase is distinguished by hydraulic jump structures indicative of supercritical flows. These architectural variations are fundamentally controlled by topographic feedback inherent to each phase. During backfilling， debris flows exhibit shorter transport distances as channels progressively infill， leading to reduced fan slopes and the subsequent development of lower-flow-regime structures. Following channel avulsion， sediments preferentially accumulate in topographic lows characterized by steeper gradients， initiating progradation and forming a suite of erosional and depositional features dominated by supercritical flow conditions. This study advances our understanding of autogenic cycles in ancient alluvial fans.