ABSTRACT: 3-D numerical seismic stratigraphic model of the Lower St. Lawrence Estuary.

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A 3-D numerical seismic stratigraphic model of the Lower St. Lawrence Estuary has been derived from horizon picking of the key seismic reflectors on recently acquired high-resolution seismic sections. The model permitted the generation of a series of surfaces corresponding to the upper boundary of each seismic unit. These surfaces were then used to create a volume model of the estuary that has horizontal and vertical resolutions of 0.25 and 0.005 km respectively. The model adequately reflects first order features such as dip and thickness variations of seismic units. It is also in agreement with more subtle features such as discontinuous bedrock highs that have been independently imaged on multibeam bathymetry. Depth slices and cross-sections can also be easily extracted from the 3D model and volume calculations can be done to improve geological interpretations in the area. The model images two major bedrock troughs located between Les Escoumins and Forestville. These troughs are located in a narrow segment of the St. Lawrence platform domain bounded by steeply-dipping faults inherited from the Iapetan rifting episode. These troughs may have formed trough the glacial overdeepening of the platform domain which is less resistant to erosional processes than the surrounding Grenvillian and Appalachian domains. Second-order bedrock features mainly consist into smaller bedrock depressions acting as mini-basins located on both shoulders of the Laurentian Channel. Eight seismic units have been distinguished above the bedrock. The total sedimentary infill thickness decreases downstream from ~400 m near Les Escoumins to ~30 m near Pointe-des-Monts. Piston cores provided a geochronological framework for the upper part of the sedimentary succession. Volumetric calculations derived from the model allowed us to obtain quantitative information from the stratigraphic record of the estuary. Volume calculation for the various seismic units indicates that Holocene deposits represent ~ 70 % of the basin infill. However, the two major troughs are largely filled by deposits of pre-Holocene age. The strong correlation between the thickness of the sedimentary succession and the bedrock topography indicates that the St. Lawrence Estuary basin was an efficient sediment trap, especially during the deposition of the lower part of the sedimentary succession. The influence of the inherited bedrock topography tends to decrease over the Holocene.

Bibliography of Canadian Geomorphology