The flows that left no trace : Very large-volume turbidity currents that bypassed sediment through submarine channels without eroding the sea floor

Turbidity currents are an important process for transporting sediment from the continental shelf to the deep ocean. Submarine channels are often conduits for these flows, exerting a first order control on turbidity current flow processes and resulting deposit geometries. Here we present a detailed examination of the Madeira Channel System, offshore northwest Africa, using shallow seismic profiles, swath bathymetric data and a suite of sediment cores. This shallow (<20 m deep) channel system is unusual because it was fed infrequently, on average once every 10, 000 years, by very large volume (>100 km3) turbidity currents. It therefore differs markedly from most submarine channels which have well developed levees, formed by much more frequent flows. A northern and a southern channel comprise the Madeira Channel System, and channel initiation is associated with subtle but distinct increases in sea-floor gradient from 0.02° to 0.06°. Most of the turbidity currents passing through the northern channel deposited laterally extensive (>5 km), thin (5–10 cm) ripple cross-laminated sands along the channel margins, but deposited no sand or mud in the channel axis. Moreover, these flows failed to erode sediment in the channel axis, despite being powerful enough to efficiently bypass sediment in very large volumes. The flows were able to reach an equilibrium state (autosuspension) whereby they efficiently bypassed their sediment loads down slope, leaving no trace of their passing.