Giant flute-like scour and other erosional features formed by the 1929 Grand Banks turbidity current

Abstract

Erosional features on the floor of Eastern Valley of the Laurentian Fan, in 2800 m water depth, have been mapped with SeaMARC I side-scan sonar images and Seabeam multi-beam echo-soundings, and were directly observed during a dive with the deep submersible Alvin. The most spectacular feature is a 100-m-deep flute-shaped scour, more than 1 km long. The surrounding valley is floored by an unconsolidated coarse conglomerate, which was moulded into transverse bedforms by the turbidity current that was triggered by the 1929 Grand Banks earthquake. Direct observations and seismic-reflection profiles show that the flute-shaped scour cuts through this conglomerate and into Plio-Pleistocene valley-floor sediments, thereby exposing a section through the 1929 deposit. Application of the Allen defect theory suggests that the flute is unusually deep because general channel-floor erosion was inhibited by the conglomerate veneer. Valley-floor channels typically 1 km wide and 10m deep contain series of closed depressions that occasionally deepen to 30 m. These are also interpreted as erosional scours, analogous to pools cut on the beds of bedrock rivers. The large flute was probably formed by detached flow enlarging an initial scour depression. Such scours probably play an important role in channel-floor erosion, increasing the volume of sediment transported by large turbidity currents