ABSTRACT: 3D architecture of the Quaternary succession in the St. Lawrence Estuary, Canada.

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Abstract

The St. Lawrence Estuary (eastern Canada) is a narrow Quaternary basin with a deep central channel, the Laurentian Channel, which comprises seven stacked sedimentary units. The bedrock geology of the Estuary has controlled the architecture of the sedimentary succession. Consequently, the detailed interpretation of the Quaternary succession has to take into account feedback effects between the inherited geometry of the Estuary, glacial erosion processes, topography, sedimentation, isostatic rebound, sea level change and seismic activity. The geological framework previously available for the Estuary relied on conceptual models developed from sparse or poor quality datasets. Recently, high-resolution seismic-reflection surveys were conducted to improve our understanding of the regional geology of the St. Lawrence Estuary. Data show that the basin is filled by a wedge-shaped Quaternary succession that thickens from less than 50 m to the northeast to > 400 m at the mouth of the Saguenay Fjord, more than 200 km to the southwest. The seismic sections image the geometry of the top of the three underlying bedrock domains (Grenvillian basement, Palaeozoic St. Lawrence Platform and Appalachians). We recognize seven Quaternary seismic units, organized in 2 main packages, lower and upper, and 2 independent units that all have variable seismic reflection characteristics. Isopach maps show that the distribution of the Quaternary seismic units is closely linked to the topography of the underlying geological domains. Within the Quaternary succession, the lower package (seismic unit 1 and, in the eastern (downstream) part of the Estuary, seismic units 1 and 2) has a highly variable thickness and fills up most of the topographic depressions of the bedrock. The upper package (seismic units 3, 4 and 5) covers the entire Estuary, including the Laurentian Channel shoulders, suggesting that it was deposited in an environment similar to the present-day physiographic setting. The other two seismic units are the result of local geological processes and mostly occur along the northern flank of the Laurentian Channel: the sedimentation of submarine fans (seismic unit 6) and mass wasting deposition (seismic unit 7). Lithological and geochronological controls are now required to correlate these seismic units and to understand their geological significance. This is the focus of current research.

Bibliography of Canadian Geomorphology