MIS3 FLUVIAL AND MARINE SEDIMENTS IN THE CENTRAL ST-LAWRENCE LOWLANDS- IMPLICATIONS FOR GLACIAL AND DEGLACIAL EVENTS IN SOUTHEASTERN CANADA

The recently postulated concept of a much-reduced Laurentide Ice Sheet (LIS) between 57 and 29 ka has major implications for the evolution of drainage conditions in the St. Lawrence valley (SLV). The discovery of fluvial and marine sediments dated to MIS3 implies mostly ice-free conditions in the SLV and thus a reduced ice volume, perhaps comparable to that of the Younger Dryas.

During a drilling campaign in the central SLV, one of four sonic boreholes intersected laminated fossiliferous glaciomarine silts and plant-bearing alluvial sands below the regional Gentilly Till and Champlain Sea clay. These newly observed units overlie an older till unit and laminated lacustrine sediments. The stratigraphic record of the newly discovered marine and alluvial units thus differs from the older, late MIS5 St-Pierre/La Pérade succession where the alluvial St-Pierre beds overlie the marine La Pérade clay. Plant fragments from the alluvial unit were AMS-radiocarbon-dated at 36400 – 35150 cal BP and 38680 - 37130 cal BP. The overlying marine silts record a new glaciomarine event in the central SLV prior the Last Glacial Maximum (LGM). The nature, fossil content and age of the new units indicate that the MIS3 interstadial was characterized by normal drainage conditions and a relatively short-lived marine incursion, shortly prior to the LGM advance.

This stratigraphic record resolves the long-standing question of the extent of MIS3 ice retreat in the SLV. The hitherto dominant paradigm was that the central lowlands had remained ice-covered throughout the MIS2-4 interval. This thesis was based mainly on the presumed absence of free drainage conditions in the southern Quebec Appalachians throughout the Lennoxville-Gayhurst-Chaudiere succession, then correlated with the entire Gentilly Till episode in the SLV. Infinite radiocarbon ages from sediments underlying Gentilly Till, as well as finite ages ranging from 64 000 to 74 700 years BP, provided further support for this thesis. The new alluvial-marine succession, which is consistent with earlier observations of MIS3 marine units at several localities in the Estuary (Iles-aux-Coudres, Pointe-aux-Alouettes) and Gulf of St. Lawrence (Anticosti Island, Magdalen Islands), indicates that interstadial conditions prevailed in the St. Lawrence valley during at least part of MIS3.