ABSTRACT: Modelling the retreat of the champlain sea in the mauricie area of québec using a 3d visualisation software package.

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Abstract

Previous attempts at numerically modelling the regression of the Champlain Sea have been partially unsuccessful because of the lack of reliable data on the age and synchronicity of known shorelines. Glacial tilt and postglacial isostatic recovery are also factors which are hard to take into account, since the values vary substantially across the basin. For the GAC-MAC meeting in 2002, a series of temporal snapshots were generated for the entire basin using an approach that combined 4 different software programmes, but boundary conditions such as the ice margin in the sea were graphically added to the model. This new attempt seeks to minimize the complexity inherent to using multiple software packages. The software used for the modelling is gOcad®, which allows the building of geological 3D models from various types of data. It was also decided to focus on a smaller area of the basin for which reasonably good control was available. The Mauricie area was recently re-mapped, and following new fieldwork, additional 14C dates became available. The first step in building the model resides in defining the area and choosing an appropriate digital elevation model (DEM) as the base. Then, a series of water levels are defined from assumed positions of the ice margin, known geomorphic features and reliably dated sites. This is where the glacial tilt and subsequent isostatic rebound is factored in, either from the geomorphic data, or by tilting the water plane according to an assumed value. Once the model is completed, a movie is generated showing invasion and retreat of the Champlain Sea in the area. The strength of this approach is the ability to modify the model when new data become available without having to rebuild it completely. An interesting offshoot is the capability to visualize the sea floor and perhaps begin to understand sedimentation patterns in the basin. In order to represent better the sea floor, modern bathymetric data should be incorporated with the DEM. This relatively simple procedure is a powerful teaching aid and outreach tool. Once the scientific basis of the model is established, it can be used to generate rapidly "what if" scenarios that otherwise would have been much longer to produce. However, the key to success resides primarily in the reliability of the data and the coherency of the geologic interpretations.

Bibliography of Canadian Geomorphology