Provenance Of James Bay Lowlands Glacial Deposits In The Context Of Multiple Ice-flow Events Of The Eastern Sector Of The Laurentide Ice Sheet

Paleoclimate records indicate that the Laurentide Ice Sheet (LIS) was highly dynamic throughout the last glacial cycle. Measurements of glacial striations taken from multi-faceted rock outcrops and tracing of lithological indicators also indicate important reorganizations of the ice divide system of the LIS. The sequence of shifting ice flows for the eastern sector of the LIS consists in several ice movements dating presumably from the ice-dome buildup in the early phases of the last glacial cycle to late deglaciation stages. There is, however, little information on the timing of these ice flow shifts as well as the exact trajectories and extent of the documented ice flow events. Here we focus on the evolution of the Labrador-Quebec ice dome by documenting the provenance of till units of the James Bay lowlands of Quebec. This region preserves a long record of late Pleistocene events and its location near the former geographic center of the LIS makes it particularly important for the study of ice sheet dynamics. Our investigations along the Harricana and Nottaway rivers indicate that the regional stratigraphy consists of at least 4 distinct till units. A 20-m thick varve unit also separates the lower two till units from the two upper ones. This glacial sequence lies on fluvial sands containing abundant compressed wood fragments. Distinguishing ice-flow events in multiple-till sequences is difficult and requires the development of approaches that will lead to a well-constrained interpretation of till provenance. Ice flow directional data of till was obtained through sedimentological methods (till fabrics, measurements of striations on bullet-shaped boulders, etc.), as well as the petrography of clasts, and mineralogic geochemical composition of till matrix. Clast petrological data from the four-till sequence shows variations that suggest an early ice flow towards the northwest, followed by a counterclockwise shift to the west, southwest and then south. This succession of ice flows is also supported by till fabrics. Overall, the ice flow sequence documented here is concordant with the striation record obtained from the surrounding Shield rocks. Of importance here, is the occurrence of the northwestward movement, which is documented for the first time in this part of the lowlands. Geochronological constraints obtained from the organic- bearing unit also indicate that these ice flow rearrangements occurred during the last glacial cycle. The bedrock geology surrounding the James Bay region is also suitable for the application of radiogenic tracers on the glacial deposits. We thus use bulk Sm-Nd isotopes from till matrix and 39Ar/40Ar ages of individual detrital hornblende grains to further distinguish rocks sources and gain additional insights on ice flow trajectories. Geochemical analyses are currently being processed, and the combined Sm-Nd data and 39Ar/40Ar ages should help refining ice flow trajectories. These results will contribute to identify the former locations of ice-accumulation centers and their subsequent migration paths, thereby helping resolving the post-glacial rebound history of the LIS.