Do Muds Sort? Experimental Test of a Hypothesis Key to Understanding Marine Bottom Currents

Accumulations of fine sediments in deep-ocean contourites form a sedimentary record that has been hypothesized to be directly related to bottom-current behavior. This is known as the 'sortable silt' hypothesis and states that the non-cohesive, coarse silt in the 10 to 63 µm size range within a deposit can be used as a proxy for paleocurrent velocity. Slow deposition rates on contourites (2−10 cm/kyr) make it difficult to test this hypothesis in the field and few laboratory studies have been conducted. To test the 'sortable silt' hypothesis in the laboratory, a non-recirculating flume was constructed in which silt and clay could be deposited under a variety of velocities, sediment concentrations, and silt to clay ratios. Samples of the deposited material from each experiment were analyzed to determine the grain-size distribution using a Micromeritics Sedigraph 5120 particle size analyzer. The results of these experiments were used to evaluate the following two hypotheses: 1. The proportion of sortable silt (SS%) compared to the proportion of clay is a better indicator of current velocity than the mean size of the sortable silt (SS). 2. The presence of clay will impact the movement and sorting of silt in the bed. Results show that increased velocity correlates with increased (SS), and that (SS) generally decreases downstream of the sediment source. (SS) was found to be more representative of velocity than (SS%) and, counter to the original hypothesis, clay did not have a significant effect on silt deposition.