MIS-3 FLUVIAL AND MARINE SEDIMENTS IN THE CENTRAL ST-LAWRENCE LOWLANDS- IMPLICATIONS FOR GLACIAL AND DEGLACIAL EVENTS IN THE APPALACHIAN UPLANDS (Invited Presentation)

The recently postulated concept of a reduced Laurentide Ice Sheet (LIS) during MIS3 has major implications for the evolution of drainage conditions in the St. Lawrence River valley (SLRV). Here, as in many other lowland areas, the presence of sediments indicating normal drainage, whether fluvial, lacustrine or marine, implies mostly ice-free conditions and thus a reduced ice volume. In contrast, the presence of glaciolacustrine sediments indicates the damming and rerouting of regional drainage and therefore a substantial ice volume.

During a drilling campaign, one of four boreholes intersected, below the regional Gentilly Till and Champlain Sea clay, laminated fossiliferous glaciomarine silts and plant-bearing alluvial sands. These newly observed units overlie an older till unit and laminated lacustrine sediments lying directly on bedrock. The stratigraphic record of the newly discovered marine and alluvial units thus differs from the older, Late Sangamonian St-Pierre/La Pérade succession where the alluvial St-Pierre beds overlie the marine La Pérade clay. Plant fragments from the new alluvial unit were AMS-radiocarbon-dated at 31 270 ± 200 years BP and 33 250 ± 240 years BP. The overlying marine silts record a hitherto unrecognized glaciomarine event in the SLRV prior the Last Glacial Maximum (LGM). The nature, fossil content and age of the new units indicate that the MIS3 interstadial event was characterized by normal drainage conditions and a relatively short-lived marine incursion, just prior to the LGM advance.

This stratigraphic record resolves the long-standing question of the extent of MIS3 ice retreat in the SLRV. The hitherto dominant paradigm was that the central SLRV had remained ice-covered throughout the MIS2-4 interval. This thesis was based on the presumed absence of free drainage conditions in the southern Quebec Appalachians throughout the Lennoxville-Gayhurst-Chaudiere succession, correlated with the whole Gentilly Till episode in the SLRV. Infinite ¹⁴C ages from sediments underlying Gentilly Till, as well as a few finite ages ranging from 64 000 ± 2000 yrs BP to 74 700 +2700/-2000 yrs BP for the St. Pierre sediments, 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 the latest part of MIS3.