Deposition of ungraded muds from high-density non-turbulent turbidity currents

Turbidity current deposits of mud have recently been found up to 10m in thickness resulting from single depositional events1,2. Turbidites 1–5 m thick from the Madeira Abyssal Plain3,4 display long segments with little vertical change of grain size5,6. This suggests that the currents 'froze' in some way, making them unable to sort sediment. Here we suggest that a decline in flow speed of these currents led to a concentration gradient in silt close to the bed that was sufficiently steep to stratify the flow. A concentration gradient of 18 kg m–3 over 1.5 m height gives a layer Richardson number above the critical value of 0.25 for damping of turbulence. We propose that, with turbulence severely damped, a dense suspension of fine material consolidates like a static suspension while continuing to flow downslope. The current changes from a fully turbulent flow to a decelerating flow with suppressed turbulence and increasing density. In the latter state interparticle forces prevent differential settling of the coarser grains.