MICROFOSSILS SUGGEST SEPARATE TRIGGERS FOR MASS TRANSPORT DEPOSITS IN TWO LAKES WITHIN THE WESTERN QUEBEC SEISMIC ZONE, SOUTHWEST QUEBEC, CANADA

Reconstructing regional earthquake history is important for assessing earthquake hazards, especially in intraplate regions with short historic records and infrequent, but potentially high-magnitude earthquakes. A useful method of identifying paleoseismic events is through the identification of multiple mass transport deposits (MTDs) at a common stratigraphic level in a lake basin. Where such signatures are present in separate lake basins, and it can be established that they are the same age, a paleoearthquake may be the best explanation for a coeval trigger. Sub-bottom acoustic profiling revealed multiple stratigraphic levels of MTDs within early postglacial Champlain Sea deposits preserved within the basins of Lac de l’Argille and McArthur Lake, located about 25 km apart, in southwest Quebec. The sediment cores recovered at the sites overlying the two stratigraphically youngest MTDs in each lake were CT scanned, underwent ITRAX XRF analysis, and subsampled for microfossils and pollen. Microfossils (palynomorphs and testate amoebae) from the MTDs show re-sedimentation of nearshore planktonic taxa and into the deep basins of both lakes, and confirm the mass transport origin of the deposits. The pollen record leading into the MTD in McArthur Lake is rich in Picea and Pinus resinosa, indicating deposition between 11-13 ky BP, while the equivalent record overlying the Lac de l’Argille MTD is rich in Pinus strobus pollen, placing deposition between 8-10 ky BP. These two mass transport events thus are of distinctly different ages and cannot be attributed to a coeval trigger. This study demonstrates the importance of dating MTDs from relatively close lake basins, and not inferring a coeval trigger based on similar stratigraphic locations in sub-bottom profile records.