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On February 5, 1663, perhaps the largest earthquake ever witnessed in eastern North America struck the Saguenay Fjord basin with such force that landslides and submarine slides were triggered over a wide region. Over 3 km 3 of clayey Holocene sediments collapsed from the margins of the fjord, resulting in amalgamated debris flow and slide deposits that covered much of the basin floor. These deposits reached thicknesses of up to 100 m, and extended over an area of 100 km 2 . The ambient seafloor was compressed under the impact and load of these debris masses, and/or liquefied under the cyclic stress of the seismic wave. A 0.2 km 3 landslide also occurred at this time, blocking the Saguenay River and possibly facilitating an extra large spring freshet (discharge). Biotracers (planktonic and benthic foraminifera, arcellaceans, pollen and wood), in conjuction with mass physical properties of sediment cores and high-resolution seismic and sidescan profiler data, have been used to identify local and distal sediment sources, including their original water depth of deposition, and their subsequent mode of mass gravitational transport. The landslide material was eroded by the river, and super-elevated sediment concentrations are considered responsible for a long-lasting (28 days) turbidity current that entered the Saguenay Fjord depositing a single 0.3 km 3 turbidite. The 2 to 10 m thick turbidite is considered to have been self-igniting having eroded into the surface of the collapsed basin debris. Paleo-hydraulic calculations suggest that this turbid flow averaged 0.45 m/s and reached a flow thickness of 30 m. |
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