Emplacement of massive turbidites linked to extinction of turbulence in turbidity currents

Submarine turbidity currents are controlled by gravity acting on suspended sediments that pull water downslope along with them 1 . In addition to suspended sediments, turbidity currents also transport sediments at the base of the flow 2 , which causes the reorganization of basal sediments prior to the settling of suspended grains 3‐6 . However, as turbidity currents reach areas with minimal slope, they cross a fall-velocity threshold beyond which the suspended sediments begin to stratify the flow. This process extinguishes the turbulence near the bed 7,8 . Here we use direct numerical simulation of turbidity currents to show that this extinction of turbulence eliminates the ability of the flow to re-entrain sediment and rework the sediment at the base of the flow. Our simulations indicate that deposits from flows without basal reworking should lack internal structures such as laminations. Under appropriate conditions, then, sustained delivery of fine sediments will therefore result in the emplacement of massive turbidites. We suggest that this mechanism can explain field observations of massive deposits 9 that were emplaced gradually by dilute but powerful turbidity currents. We also conclude that turbulence in submarine turbidity currents is more fragile than river systems, and more sensitive to damping by the stratification of suspended sediment in the flow. Turbiditesareoftencharacterizedintermsofcompleteorpartial manifestations of the Bouma sequence 10 . Units Ta to Te in Fig. 1a correspond to a single flow event, with flow waning from bottom to top. Intervals Ta and Tb, tend to be sand. Interval Tb has parallel laminations, indicating bedload reworking as sediment settles 3 . Interval Ta is ‘massive’, that is, lacking sedimentary structures. Massive deep-sea turbidites are prominent features of channelized and unchannelized submarine fans. They are enigmatic in that they show little or none of the internal structure used to interpret emplacement mechanisms. Parallel laminations are seen in fluvial deposits as well as turbidites, and have been reproduced in the laboratory 36 . Their formation is due to the organization of sand grains into streaks according to size and orientation by bedload. Mechanisms for the emplacement of massive turbidites are more speculative. These units are common in the ancient rock record of deep-water deposits, as well as in deeper parts of modern continental margins. Individual event beds can be up to metres in thickness (Fig. 1b), and extend for tens to hundreds