Experiments on subaqueous sandy gravity flows: The role of clay and water content in flow dynamics and depositional structures

Deep-water deposits consisting mainly of massive sand are commonly identified as deposits of turbidity currents (i.e., turbidites). Speculation has risen in recent years as to whether some of these massive sandy deposits could have instead been deposited by debris flows. This possibility is explored here by examining the flow mechanics of sand-rich subaqueous gravity flows by means of laboratory experiments. In these experiments, sandy gravity flows were generated when well-mixed slurries of sand, clay, and water were released into a tank filled with tap water and allowed to flow under gravity over a slope that declined from 4.6° to 0.0°. The observed flow mechanics and resulting depositional features were strongly tied to the "coherence" of the debris flows (i.e., the ability of the slurry to resist being eroded and broken apart by the shear and pressure undergone by the flow). Low water content and high clay content resulted in strongly coherent debris flows, whereas high water content and low clay content resulted in weakly coherent flows. As little as 0.7 to 5 wt% of bentonite clay or 7 to 25 wt% of kaolinite clay at water contents ranging from 25 to 40 wt% was required to generate coherent gravity flows. Weakly coherent and moderately coherent flows produced significant, low-concentration subsidiary turbidity currents, and their deposits developed coarse-tail grading, water-escape structures, and minor increases in thickness at the base of the slope. Strongly coherent debris flows commonly hydroplaned and generated only minor subsidiary turbidity currents. Their deposits were structureless and ungraded, commonly showing tension cracks, compression ridges, water-escape structures, detached slide blocks, and a significant increase in thickness at the base of the slope. Application of distorted geometric scaling suggests that many aspects of these experiments appropriately scale up to the field scale of natural submarine debris flows.