Paleomagnetic Dating of Holocene Rapidly Deposited Layers From the Saguenay Fjord, Quebec, Canada

The Saguenay Fjord (Quebec, Canada) area has been struck by several natural disasters in the last 350 years. Among these disasters are the 1663 (M~7) and 1988 (M=6) earthquakes as well as the flood of 1996. These events triggered the deposition of thick layers, ranging from a few centimetres to about sixteen metres in the Saguenay Fjord inner basin. A 38 m-long piston core (core MD99-2222) was sampled from the deepest part of the Saguenay Fjord inner basin as part of the 1999 IMAGES-V (International Marine Past Global Change Study) program in order to determine the frequency of natural hazards such as earthquakes during the Holocene. Based on physical, magnetic and sedimentological data, we identified at least 14 rapidly deposited layers (RDL) in core MD99-2222. These layers contrast sharply from the dark grey bioturbated "background" sediments and are visually recognizable by their light color due to higher CaCO3 content resulting from the incorporation of light grey, slightly calcareous, Laflamme Sea Clays. In addition, most RDL have a sandy base with corresponding peaks in density and magnetic susceptibility, along with generally lower paleomagnetic inclinations that indicate an energetic depositional process where the magnetic particles are plastered horizontally because of high flow velocity and rapid sediment accumulation. Homogeneous, light grey, clayey silt beds generally overlie the sandy bases. Unfortunately, because CaCO3 dissolution occurs in the Saguenay Fjord sediments, little material is available for radiocarbon dating, making the setting of a robust chronology difficult. Here, we will show how changes in the Earth's magnetic field intensity and direction were used, in conjunction with 210Pb dating, one AMS 14C date and the recognition of a turbidite associated with the 1663 earthquake, to construct the age model for core MD99-2222 and thus to date all the RDL.