Saturated, smashed and suppressed: the fate of suspended gravel in large palaeofloods

Abstract

Natural deposits of clastic sediment, attributed to powerful palaeofloods, frequently consist of thick sequences of extensive parallel laminations consisting of sand to fine-gravel-sized particles. Within the existing literature, these stratal units have been attributed to deposition from a suspended load onto a rapidly aggrading bed with little opportunity for reworking by bedload transport. Herein, this supposition is examined and tested within an analytical model that combines an elementary understanding of turbulent, high-concentration, flows and the style of the resultant deposition.
It is proposed that the turbulence of clear water is modulated by the presence of sediment particles leading to altered hydrodynamic characteristics of the sediment-laden flow, promoting deposition of laminated sedimentary sequences. Using this premise, we show that the extremely high values of near-bed shear stress, in powerful palaeofloods, can result in coarse-gravel particles (> 100 mm) as well as sand being found in suspension at high concentrations. Due to the momentum of large particles, larger-sized suspended particles are subject to considerable comminution (smashing), due to particle-on-particle interactions, that reduces the overall particle size. Particle-on-particle interactions reduce for finer particle sizes, of less momentum, as they are enveloped within finer-scale turbulent eddies, leading to the development of a suspended sediment profile that is saturated close to the bed. The high degree of saturation predominately is related to the development of a suspension dominated by the finer size fractions (ca., 1 mm and finer), which leads to suppression of turbulence, thus allowing deposition of fine granules and sand-sized fractions as laminated bed deposits, as seen in the natural clastic deposits. The arguments supporting the development of laminae of fine-gravel and sand from saturated concentrations in natural high-velocity flows are summarized within a conceptual model.

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