Abstract:
Results from a small set of laboratory experiments are presented here that help further constrain the processes governing the production of turbidity currents from impulsive failures of continental shelf and slope deposits. Three mechanisms by which sediment can be transferred from a parent debris flow to a less-dense turbidity current were observed and quantified. These mechanisms are grain-by-grain erosion of sediment from the leading edge of the parent flow, detachment of thin layers of shearing material from the head of the parent flow, and turbulent mixing at the head of the parent flow. Which transfer process dominates an experimental run depends on whether the large dynamic stresses focused on the head of the debris flow are sufficient to overcome a effective yield strength for the parent sediment+water mixture and on whether the dynamic stresses are sufficient to induce the turbulent flow of the parent mixture. Analysis of data from Marr et al. [Geol. Soc. Am. Bull. 113 (2001) 1377] and Mohrig et al. [Geol. Soc. Am. Bull. 110 (1998) 387] support the use of a shear strength to dynamic stress ratio in constraining necessary critical values for occurrence of the different production mechanisms. Direct sampling of turbidity currents using racks of vertically stacked siphons was used to measure both the quantity of sediment eroded from the heads of non-mixing parent flows and the distribution of particle sizes transported by the developing turbidity currents. Acoustic backscatter imaging was used to better resolve the internal boundary separating any turbulent mixing zone near the front of a flow from unmodified parent material.
