Watch this recorded virtual field trip:
Leaders: Michel Parent; and Michel Lamothe
Endorsed by: Quaternary Geology and Geomorphology Division (submitted for approval)
The inferred extent and volume of the Laurentide Ice Sheet during MIS3 has been recently proposed to be much reduced during MIS3 as a consequence of new (Dalton et al., 2016; Pico et al., 2018) but disputed (Miller and Andrews, 2019) chronostratigraphic interpretations over the Hudson Bay Lowland. The concept of a much reduced LIS between 50-30 ka has major implications for the evolution of drainage conditions in the St. Lawrence River valley (SLRV), a river system particularly sensitive to LIS dimensions. The recognition of glacial lake sediments in the SLRV implies the damming and rerouting of drainage and therefore a substantial LIS volume. In contrast, the presence of sediments indicating normal drainage, whether fluvial, lacustrine or marine, implies mostly ice-free conditions in the valley and thus a reduced ice volume. A 2020 Kirk Bryan fieldtrip is thus proposed to visit three sites providing conflicting field and geochronologic data, located at less than 50 km of each other where both types of sediments have been recognized and dated at ca 30 to 50 ka.
The first site will be located at Ulverton where late-glacial subaqueous outwash sediments allow the excursionists to discuss the impact of the presence of continental ice in the SLRV on the depositional systems in the Appalachians. The continuity of glacial cover in the Central St. Lawrence Lowlands through MIS 2-3-4 has been one of the cornerstones of the Late Quaternary stratigraphic record of southern Québec (Gadd et al., 1972; Lamothe, 1989; Lamothe et al., 1992; Occhietti et al., 1996). The second site is located near Pierreville, a classical occurrence for the St. Pierre organic (peat) sediments. At a nearby section, sub-LGM till fluvial sediments exhibiting upriver sedimentary structures are interpreted as distal glacial outwash and correlated to the mid-Wisconsinan Sables des Vieilles Forges (SVF). There, as in several other locations in the central part of the SLRV, IRSL dating of the SVF suggest fluvioglacial sediment aggradation was initiated at ca 40-50 ka (Lamothe, in preparation). The third site will allow discussing the recent discovery, below the LGM till unit and Champlain Sea clay, of a succession of marine sediments overlying plant-bearing alluvial sediments (Parent et al., 2015, 2017) AMS-radiocarbon-dated at 31 270 ± 200 years BP (36400 – 35150 cal BP) and 33 250 ± 240 years BP (38680 - 37130 cal BP). These sediments are from a borehole in which the mid-Wisconsinan marine-alluvial succession overlies an older till unit and laminated lacustrine sediments lying directly on bedrock. The stratigraphic position of the marine unit between the 35 to 39 ka-old alluvial sediments and the LGM till records an hitherto unrecognized glaciomarine event in the central St. Lawrence lowlands prior the last glacial maximum.
This Kirk Bryan fieldtrip will bring participants to classical as well as new sites exhibiting parts of the St. Lawrence valley Quaternary record and should provide them with ample opportunity to discuss the ins and outs of the revised stratigraphic record.
Primary Leader Email Address: Michel.Parent@canada.ca