A laboratory study of dynamic sorting of fine, non-cohesive particles by steady, unidirectional currents

Abstract

Abstract The conceptual model of non-cohesive ‘sortable silts’ in deep-sea muds that result from local, current-driven processes of size fractionation during entrainment and deposition has proven to be a useful tool for inferring qualitative changes in the intensities of past, deposit-forming currents. The degree to which sorting processes can be characterized quantitatively is as yet unclear. This study examines the relationship between bed shear stress and the partitioning of grain size in bed sediments and the source suspension in well-constrained laboratory channel flows. Results from flume experiments using glass microspheres with diameters in the range 13–44 μm and deposited from channel flows with bed shear stresses of 0.1–0.5 Pa indicate that little or no sorting is resolved by mean or median measures of the size of silt carried in source − suspension transport. A small effect appears in modal measures of suspended sediment size. Fine-particle beds generated under experimental flows consisted of local textural environments associated with near-bed transport phenomena; these include i) linear ‘streaks’ and ii) isolated, dune-like bedforms. Selective sampling of such features revealed, respectively: i) a minor but systematic increase in grain size of particle streaks with increasing shear stress imposed by the formative flow and ii) inconsistent textural response to shear stress among spatially-separated dunes with notable variability, ca. 10%, in central measures of grain size, such that no systematic sorting in the bedforms is apparent. These findings suggest that local sorting by grain size of fine particles in near-bed transport may be a more significant, if still subtle, contributor to silt-bed texture variability than an entrainment-derived process affecting material in the bed and parent suspension overall, as proposed in the original sortable-silt hypothesis. On the other hand, our results are not inconsistent with a mechanism of simple advective sorting, proposed by others, by which the characteristic size of a silty deep-sea deposit reflects its source-to-sink distance and the average transport velocity of the deposit-forming current.