NEW INSIGHTS IN THE 3D MODELLING OF THE QUATERNARY STRATIGRAPHIC SEQUENCE OF THE SOUTHEASTERN APPALACHIANS OF CANADA

Complex successions of glacial deposits, sub-glacial, ice-marginal and ice-frontal deposits, as well as interglacial and interstadial lacustrine, fluvial, and organic-rich sediments, have been mapped and documented in the southeastern Appalachians of Quebec (Canada), north of the international border. Reconstruction of Late Pleistocene paleoenvironments is particularly challenging because the geomorphologies associated with the successions have been eroded during subsequent ice advances of the Laurentide ice sheet. Their stratigraphic architecture is, however, complex, and their subsurface extent poorly documented. The Pleistocene stratigraphy is characterized by a three-till succession, each till being underlain and overlain by glaciolacustrine sediments deposited in ice-dammed lakes during glacial advance or retreat phases. This study reports on the methodology for developing a three-dimensional numeric geologic model of surficial deposits for two major river drainage basins – the Chaudiere and St. Francois, totaling more than 20,000 km². The model is based on integrating surficial sediments maps and borehole logs using GIS and 3D geologic modeling software. This geomodeling study represents a novel approach for defining the physical lateral extension of Quaternary sediments. Stratigraphic information for the model is derived from >30,000 boreholes archived in the Quebec provincial database (Hydrogeologic Information System: SIH), Canada federal database (GSC), and from private firms. The specific methodology for the three-dimensional numeric geologic modeling of surficial deposits will be outlined. The main output of this work is refinement of the lateral extension of the Quaternary lithosomes, defined as litho- and hydrostratigraphic units, as constrained by required strict coherence between surficial distributions of geologic materials as deduced from geologic maps and borehole stratigraphic data. The 3D geologic model is a first step in understanding the regional hydrostratigraphy for evaluation of groundwater resources by local and provincial water authorities.