Sorting of fine-grained sediment by currents: Testing the sortable silt hypothesis with laboratory experiments

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Jeff Culp, Kyle Strom , Andrew Parent, Brian Romans 


Accumulations of fine sediments along continental shelf and deep-sea bathymetric contours, known as contourite drifts, form a sedimentary record that is dependent on oceanographic processes such as ocean-basin-scale circulation. A tool used to aid in interpretation of such deposits is the sortable silt (SS) hypothesis. The hypothesis suggests that the mean size of the SS (silt in the 10-63 µm size range) within a deposit is linearly related to current velocity at the time of deposition. While the hypothesis has been applied to numerous drift deposits, it has not been extensively tested. Slow deposition rates of contourite drift systems make it difficult to robustly test the hypothesis in the deep ocean, and the few laboratory studies that have been conducted have yielded inconclusive results. In this study we use laboratory flume experiments to test whether or not the mean SS in a deposit is linearly related to average current velocity; we also examine how this relationship changes as a function of distance from the inlet. Tests were conducted with 4 different sediment mixtures (pure clay, pure silt, 2:1 clay:silt and 1:1 clay:silt) and current velocities typical of deep-sea settings (5-25 cm/s). Each experiment was run with a constant supply of sediment at the flume inlet for a set amount of time. Bed samples were collected at fixed locations from the flume entrance and sized. The deposit morphology was dependent on the sediment mixture and flow conditions, but deposit grain size consistently fined downstream and coarsened with velocity. Regardless of bed morphology or source sediment mixture, the mean SS was linearly related to velocity at a particular flume location across all sediment mixtures (R^2=0.7-0.94). The slope of the relationship increased with distance from the flume inlet. Our findings support the validity of mean sortable silt as a proxy for paleocurrent velocity.



Earth Sciences, Geomorphology, Oceanography, Oceanography and Atmospheric Sciences and Meteorology, Physical Sciences and Mathematics, Sedimentology


advective sorting, clay, contourite drift, silt, sortable silt


Published: 2020-05-27 19:25


GNU Lesser General Public License (LGPL) 2.1

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