Saturated, Smashed, and Suppressed: The Fate of Suspended Gravel in Large Palaeofloods

Abstract Natural deposits of clastic sediments, often attributed to powerful palaeofloods, are characterized by thick parallel laminations of sand to fine‐gravel‐sized particles. These deposits traditionally have been explained by suspended sediment deposition onto rapidly aggrading beds with minimal bedload transport. However, the physical mechanisms controlling the transition from turbulent suspension to laminated deposition and the role of particle interactions in this process remain poorly constrained. In this study, we test this hypothesis within an analytical model that combines an understanding of turbulent, high‐concentration flows with sediment deposition. We propose that the turbulence of clear water is modulated by sediment particles, altering the hydrodynamics of the sediment‐laden flow and promoting laminated deposition. The results show that extremely high near‐bed shear stresses during palaeofloods can suspend both coarse gravel (>100 mm) and sand‐sized particles at high concentrations. Larger particles undergo significant comminution due to particle‐on‐particle interactions, reducing their size, while finer particles are enveloped in smaller turbulent eddies, leading to a saturated suspended sediment profile near the bed. This saturation, dominated by finer particles (around 1 mm and smaller), suppresses turbulence and facilitates the deposition of fine granules and sand as laminated bed deposits. The findings align with natural clastic deposit characteristics and demonstrate that suspended sediment dynamics, rather than bedload transport, exert the primary control on flow behavior and deposition during palaeofloods. A conceptual model is proposed to explain the development of fine‐gravel and sand laminations in high‐velocity flows, providing new insights into the formation of ancient flood deposits.