THE MORPHOSEDIMENTARY RECORD OF GLACIAL TO POSTGLACIAL ENVIRONMENTAL CHANGES IN FJORD-LAKE MÉKINAC AND ADJACENT AREAS (SOUTHERN CANADIAN SHIELD)

Abstract Fjords can transition from marine embayments to lacustrine waterbodies under glacio-isostatic change following their deglaciation. However, little is known on how a transition from fjord to lake is influenced by glacial dynamics and how it influences sedimentation in and around the basin. Here, we analyse bathymetry imagery and subbottom imagery in Lake Mekinac in southern Quebec, and deglacial landforms from LiDAR imagery around the lake to document the evolution of a fjord basin into a lacustrine body during the transition from the Late Pleistocene to the Holocene. These analyses helped refine the glacial history of the area and show that the study area was probably under the up-ice reach of the St. Lawrence Ice Stream during full glacial conditions. The mapping of moraines in and around Lake Mekinac shows that deglaciation of the study area was marked by stabilization of the ice-margin during the Mars-Batiscan Event, which is traditionally correlated to the climate deterioration at the end of the Younger Dryas. The architecture of acoustic facies in the lake indicate a transition from glacial conditions (U1), to proglacial-paraglacial (U2) and to postglacial conditions (U3). The subbottom of Lake Mekinac also contains multiple mass movement deposits related to progradation of the Du-Milieu River delta and probably to late-Holocene earthquakes. The acoustic architecture of sediments within the lake indicates that the sedimentation regime was most dependant on the sediment input regime and that the transition from fjord to lake did not significantly influence sedimentation during deglaciation of the watershed. Further work on dating the mass movement deposits that characterise the sediment architecture should provide a better assessment of natural hazard associated with seismological events.