MID-WISCONSINAN FLUVIAL AND MARINE SEDIMENTS IN THE CENTRAL ST-LAWRENCE LOWLANDS- IMPLICATIONS FOR GLACIAL AND DEGLACIAL EVENTS IN THE APPALACHIAN UPLANDS

During a regional hydrogeological study of the central St. Lawrence Lowlands (SLL), four sonic boreholes were drilled at sites where archival data had suggested the presence of thick Quaternary sediments below a continuous cover of Champlain Sea clay. One borehole intersected fossiliferous glaciomarine silts and plant-bearing alluvial sands underlying the Champlain Sea clay-regional till succession. These units in turn overlie an older till unit and laminated lacustrine silts resting on bedrock. The stratigraphic record of the newly discovered marine and alluvial units differs from the older, Late Sangamonian St-Pierre/La Pérade succession where the alluvial St-Pierre beds overlie the marine La Pérade clay. Hand-picked plant fragments from the alluvial unit were AMS-radiocarbon-dated at 31 270 ± 200 BP and 33 250 ± 240 BP. The overlying marine silts contain a low-diversity foraminiferal fauna and record an hitherto unrecognized marine incursion in the central SLL prior the last glacial maximum (LGM). The fossil content and age of these sediments indicate that a relatively long Mid-Wisconsinan interstadial event, characterized by normal drainage conditions and followed by a short-lived marine incursion, preceded the Late Wisconsinan glacial advance.

This record resolves the long-standing question of the extent of Mid-Wisconsinan ice retreat in the SSL. The dominant paradigm was that the central SLL had remained ice-covered throughout the Early to Late Wisconsinan interval (Gentilly Till). This thesis was largely based on finite ages ranging from 64 000 ± 2000 BP to 74 700 +2700/-2000 BP for the St. Pierre sediments, and on the absence of free (northward) drainage conditions in the southern Quebec Appalachians during the Lennoxville-Gayhurst-Chaudiere succession (temporal equivalent of the Gentilly Till episode). This new alluvial-marine succession is consistent with earlier observations of Mid-Wisconsinan marine sediments units at several localities along the Estuary (Isle-aux-Coudres, Pte-aux-Alouettes) and Gulf of St. Lawrence (Anticosti I., Magdalen I.) and indicates that interstadial conditions prevailed in the SLL during the latter part of the Mid-Wisconsinan stage and that the Laurentide Ice Sheet had retreated to a position near the southern edge of the Canadian Shield prior to the LGM.