Sedimentary Dynamics as a Hidden Control: Hyperpycnal Flows and Emergence of Cryogenian Iron Formations

After a ~1-billion-year hiatus in the geological record, large-scale iron formations (IFs) reappeared in the Cryogenian Period. While most studies link their genesis to marine redox conditions, their close association with glacial deposits—evidenced by diverse iron-rich clastic subfacies—implies sedimentary processes critically controlled their development. To evaluate the impact of sedimentary dynamics on Cryogenian IFs (CIFs), we conducted high-resolution sedimentological analyses of the Fulu Formation in the Nanhua Basin, South China. Results show the Fulu IFs formed in a gravity flow-dominated slope environment, with hyperpycnal flows as the key sedimentary driver. Six lithofacies were identified and grouped into three facies associations: debris-flow, hyperpycnal-flow, and chemical facies associations. The debris-flow association, occurring at the Changan Formation top and Fulu Formation Member 2, indicates frequent glaciogenic debris-flow activity. Hyperpycnal-flow deposits account for over 90% of Fulu Formation Member 1, showing frequent facies variations and hosting two IF intervals of low-energy lobe fringe deposits interbedded with chemical sediments, separated by high-energy channel deposits. Chemical facies associations formed only during “clastic hiatus” episodes. Our results demonstrate hyperpycnal flows controlled IF iron content and distribution via distinct sedimentary processes: high-energy phases diluted or eroded pre-existing iron deposits, while low-energy phases featured dominant lofting plume deposition that enhanced mixing of Fe2+-rich deep waters and oxygenated upper waters, facilitating iron oxidation and accelerating its deposition in seawater. This study confirms, for the first time, that the formation of CIFs was not solely governed by seawater redox evolution, but was also significantly influenced by high-frequency and multiphase sedimentary processes. This insight provides key constraints on the understanding the interactions among glaciation, sedimentation, and biogeochemical cycling in Neoproterozoic Earth surface system.