EVIDENCE FOR AN EARTHQUAKE-TRIGGERED BASIN COLLAPSE IN SAGUENAY FJORD, CANADA

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 thicknes ses 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 mas ses, 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 R iver and possibly facilitating an extra large spring freshet (discharg e). Biotracers (planktonic and benthic foraminifera, arcellaceans, pol len and wood), in conjunction with mass physical properties of sedimen t cores and high-resolution seismic and sidescan profiler data, have b een used to identify local and distal sediment sources, including thei r original water depth of deposition, and their subsequent mode of mas s gravitational transport. The landslide material was eroded by the ri ver, and super-elevated sediment concentrations are considered respons ible for a long-lasting (28 days) turbidity current that entered the S aguenay Fjord depositing a single 0.3 km(3) turbidite. The 2 to 10 m t hick turbidite is considered to have been self-igniting having eroded into the surface of the collapsed basin debris. Paleo-hydraulic calcul ations suggest that this turbid flow averaged 0.45 m/s and reached a f low thickness of 30 m.