Soutenance mémoire
Study area
The Pingualuit Crater, located in the Ungava Peninsula (Nunavik, Canada; FigA) is a simple crater created by a meteoritic impact ca. lA million years ago as determined by Ardating of impactites collected nearby (Bouchard and Marsan, 1989; Grieve et al. 1991). The crater is a 410 m deep (rim-to-basin) and 3A km wide (rim-to-basin) near circular depression whose rim reaches a maximum altitude of 657 m above sea level (a.s.l), which is one of the highest peak in Ungava. This summit rises to 163 m above the surface (494 m a.s.l) of an ultraoligotrophic 246 m deep and 2.8 km wide lake hosted by the crater. Since the last deglaciation this transparent water lake (33 m Secchi depth in August 2010; Guyard et aL, 2011) has been a closed hydrological and sedimentary basin with no tributaries as the lake is only fed by atmospheric precipitation (Guyard et aL, 2011). However, 818 measurements made by OueUet et al. (1989) from water from (into) the Pingualuit Crater Lake and the adjacent Lake Laflamme suggest a potential cryptorheic drainage system across a fault plan linking both lakes (Currie, 1965). The water flowing of the Pingualuit crater area are drained by the Vachon River across the Ungava Bay catchment basin. However, the northernmost part of this area is adjacent to the Puvirnituq River, one of the main ri vers of the Hudson Bay catchment basin (Fig.4; Daigneault, 2008).
Regional glacial/deglacial history
During the last glaciation, northem Ungava remained glaciated from the end of the Sangamonian (marine isotopic substage 5e; ~ 123,000 years ago; Lisiecki and Raymo, 2005) to the Holocene deglaciation (Daigneault, 2008). In the Pingualuit Crater Lake basin, two distinct glacial movements have characterized the last glaciation: the Ungava and Payne flows (Gray and Lauriol, 1985; Bouchard and Marcotte, 1986). The first one originated from the Ungava center in the central part of the Ungava Peninsula and was oriented south-east (azimuth 110°) in the study area, whereas the second originating from the Payne center was oriented north-east (azirnuth 45°) (Fig.4; Currie, 1965; Bouchard et al. , 1989a; Daigneault and Bouchard, 2004). During this second glacial movement, the Payne flow covered the main part of the Ungava peninsula. This relative chronology is supported in the crater area, by the greatest abundance of geomorphological features being associated with the Payne flow (Bouchard et al., 1989a; Daigneault and Bouchard, 2004).Deglaciation of the Ungava peninsula began on the southem coast of the Hudson Strait between 10.5 kyr BP and 7 kyr BP, and progressed toward the Ungava plateau (Gray and Lauriol, 1985; Dyke and Prest, 1987; Lauriol and Gray, 1987; Bruneau and Gray,1997; Gray, 2001; Daigneault and Bouchard, 2004; Saulnier-Talbot and Pienitz, 2010). In the Pingualuit Crater, a sediment sequence from a 14 cm-long grab sample gathered in August 1986 presents the postglacial environment of the area (Richard et al., 1989). Characteristics at the base of the sample, dated at 5780 ± 135 cal BP (5030 ± 70 yr BP), indicate iee-free lake conditions at this time (Bouchard et al., 1989b; Richard et al., 1989; Gronlund et al. , 1990; Guyard et al., 2011). Furthermore, AMS 14C dating and multiproxy paleoenvironmental reconstruction performed by Guyard et al. (20 Il) on the – 9 m sediment core collected in the deep basin in 2007 suggests the retreat of the Laurentide Iee Sheet (LIS) from the crater area around 6844 ± 100 cal BP.
Terrestrial LiDAR mapping
In September 2012, a survey was conducted to record the high resolution topography of the internaI slopes of the crater using a terrestrial LiDAR. A total of 35 surveys were completed with the ILRlS-3D laser scanning system from Opteeh Ine. These scans were recorded from 19 different sites around the crater ring on the top of the internaI slopes; the coordinates of each site were measured by GPS (Fig. 6). Spacing among sites is at most 600 m to maximize overlap between scans for the merge step. The scanning range of a survey covers up to 1500 m of the slope in the highest reflectivity conditions (Optech Inc., 2008). The three-dimensional point clouds acquired with the terrestrial LiDAR were imported and merged with Polyworks CAD (Computer Aided Design) software from Innovmetrie Ine. To facilitate the merging process, a site was selected to generate an absolute coordinate (i.e., 0; 0; 0) and aH others sites coordinates were transformed accordingly. A scan recorded from this selected site was employed as a base for merging the other scans. The point clouds were manually aligned and then merged using the Rest-Fit function from the IMAlign Polyworks software to achieve the highest convergence. The large ASCII file generated during this process was converted to LAS (LASer) file format to create a ground surface (DEM) of 0.5 m resolution using LP360 from the QCoherent software (Fig. 7). The resulting DEM was then exported in ArcGIS® software to analyse the internaI slopes of the crater and to determine the elevation of various outlets.
Bathymetry and seismic acquisition-stratigraphy
About 50 km of sub-bottom profiles (Fig. 6) were coHected on the Pingualuit Crater Lake in August 2010 using a Knudsen Chirp 3212 system with a frequency of 3.5 kHz (Ledoux et al., 2011). The sub-bottom profiler data were integrated and analysed using The Kingdom Suite® software (lliS). Identified seismic horizons were exported into ArcGIS® to generate isopach maps for each horizon. The acoustic travel times was converted into depth using p-wave velocities of 1420 m.s1 and 1500 m.s1 for the determination of lake floor and thickness horizons, respectively (Moussawi and Tessier, 1989; Guyard et al.,2011). Interpretation of the units observed was completed using the correlation between seismic transects and stratigraphy of the -9 m sediment core described by Guyard et al. (2011). The – 300 m meshing (Fig. 6) was used to produce a mid-resolution bathymetric map of the 1ake. This bathymetry was coupled with the high-resolution topography of the internaI slopes from LiDAR data to produce for the first time the entire inner part of the PinguaIuit Crater (Fig. 7)
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Table des matières
REMERCIEMENTS
RÉSUMÉ
ABSTRACT
LISTE DES FIGURES
INTRODUCTION GÉNÉRALE
Problématique
Objectifs généraux
Expéditions et méthodologie
Organisation du mémoire et contribution
Présentations lors de congrès
CHAPITRE 1 MORPHOLOGY AND LATE-QUATERNARY EVOLUTION OF THE PINGUALUIT CRATER LAKE BASIN, NORTHERN QUÉBEC (CANADA)
1. INTRODUCTION
2. STUDY AREA
3. METHODOLOGY
4. RESULTS
5. DESCRIPTION AND INTERPRETATION OF THE SEISMIC SURVEY
6. DISCUSSION
7. CONCLUSIONS
8. ACKNOWLEDGMENTS
CHAPITRE 2 CONCLUSION
RÉFÉRENCES BIBLIOGRAPHIQUES
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