Optical Dating and Sedimentary Characteristics of Holocene Dunes on the South Shore of the Central St. Lawrence lowlands, Quebec, Canada

English Abstract:

Systematic mapping of surficial sediments of the central St. Lawrence lowlands allowed the delineation of the distribution of the main Quaternary morpho-sedimentary systems. The surficial geology maps reveal the occurrence of large (multi-km) eolian sediment systems characterized by many convex parabolic dunes towards the SW. Although these sedimentary landforms cover substantial areas, uncertainties remain regarding their origin and age, as well as their stratigraphical significance within the global framework of the last deglaciation. Here we report sedimentological and chronological data from dunes located near the locality of Villeroy, Qc (NTS map sheets 21L05 and 21L12), in order to gain insights on their depositional setting and age. The distribution of these eolian features was mapped through the analysis of aerial photographs and high-resolution (LiDAR) digital elevation models, combined with field observations. Four sites were chosen for detailed investigations. Geophysical soundings (GPR) were carried out on the dunes to gain information on their internal sedimentary structures. The dunes were sampled for grain-size analysis and mineralogical identification. Sediment facies displaying clear evidence for eolian transport were sampled for optically stimulated luminescence (OSL) dating of the K-feldspar fraction. Our results indicate that this large eolian system likely derives from the reworking of beach and deltaic sands that were deposited in postglacial Champlain Sea. The main orientation of the parabolic dune fields suggests deposition by strong NE (N35°) anticyclone winds, which probably played an important role in the formation of these dunes and the remobilization of sand-sized sediments in the lowland. GPR results tend to suggest a single period of deflation. OSL investigations show that the sand particles were well exposed to sunlight prior to burial and preliminary results yielded ages of ca. 8 ka. Utlimately, results from this study will provide constraints on the period of eolian activity that followed the retreat of the Laurentide ice sheet and Champlain Sea, in addition to refine the chronostratigraphic framework for the deglaciation of the Central St. Lawrence Lowlands.