1929 Grand Banks, Newfoundland earthquake

Following an earthquake in 1929 in the Grand Banks area, south of Newfoundland, 12 Atlantic submarine cables were broken in at least 23 places over a period of 12 hours. The breaks occurred progressively in a southward direction. The times at which the telegraph lines went dead suggest a flow of material moving at about 55 km per hr. What sedimentological phenomena could be responsible for these observations and what types of deposit would you expect to be formed by it?

1929 Grand Banks earthquake & Tsunami. Source: Natural Resources Canada.

1929 Grand Banks earthquake & Tsunami. Source: Natural Resources Canada.

On November 18th, 1929, at 5:02pm local time, a magnitude 7.2 earthquake ruptured approximately 250 kilometers offshore of Newfoundland. The earthquake produced noticeable ground motion in New York and Montreal, although damage was limited to Cape Breton. The earthquake produced a submarine slump on the Laurentian slope, breaking transatlantic cables and producing a tsunami that spread as far south as South Carolina and as far east as Portugal. For more information on the earthquake and tsunami damage, including some pretty incredible photography, see the Natural Resources Canada writeup on the 1929 Grand Banks earthquake.

When a landslide occurs underwater, turbulence helps keep the material suspended, allowing the gravity-driven flow to continue for extremely long distances. Because of the liquefied sediment mass has a higher density than the surrounding clear water, the two have minimal mixing, maintaining a distinctly separate flow. Eventually, the gravity-driven process slows and stops when drag forces dominate over bouyancy, with the poorly-sorted suspended material settling out of the liquid onto the seafloor (or lakebed). The Geological Society of London has a lab demonstration of a turbidity flow.

The balance of density, size, and shape of grains determines when it will settle out of the suspension, with materials of the same hydraulic equivalency settling at the same time. The heaviest, high-density largest grains will settle out first, while the lightest, finest-grained sediments remaining suspended the longest. The result is a turbidite deposit with normally-graded bedding: a coarse base with progressively finer grains on the top of the deposit. The 1929 earthquake-triggered slump produced a flow that covered 280,000 kilometers with 100 square kilometers of sediment.

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