Diary of forthcoming events 2000

Abstract

The existence of former glacial lakes remains poorly documented in the remote regions of the Canadian Shield where the dense forest cover and poor access complicate classical mapping methods. Here we use a high-resolution Digital Elevation Model (DEM; 1 m/pixel) based on LiDAR data to map Quaternary deposits and landforms in the vicinity of the Gouin reservoir in the Laurentian Highlands of Quebec (Canada). A total of 598 raised shorelines were identified, providing new evidence for the presence of a large proglacial lake – Lake Obedjiwan – that formed in front of the northward retreating Laurentide ice sheet margin during the last deglaciation, a few kilometers east of the larger Lake Ojibway that covered extensive areas in northern Ontario and Quebec. The elevation of shorelines outlines a tilted water plane that depicts the influence of the glacio-isostatic adjustment across the basin, with shorelines ranging from 409 m in the south to 439 m in the north. The calculated uplift gradient of 0.78 m/km indicates that the lake developed during the early stages of the deglaciation. Our reconstruction constrains the full areal extent and maximum elevation of the lake, which covered an area of 1800 km2 and reached an elevation of 450 m. It also indicates that Lake Obedjiwan developed independently from Lake Ojibway. The lake-surface elevation was controlled by an outlet located in the southeastern part of the basin where a topographic depression lying at an elevation of 390 m routed meltwater overflow southward through the Saint-Maurice River and into the St. Lawrence River. Another important outlet is located in the northernmost part of the basin and ice retreat beyond this topographic depression (427 m) caused the drawdown of the Lake Obedjiwan, which drained westward into Lake Ojibway. This discharge added around 19 km3 of meltwater, an event that likely impacted the hydrological budget and sedimentology of Lake Ojibway. The results suggest that other glacial lakes probably developed along the southern LIS ice margin and that the approach developed in this study may contribute to the refinement of their areal coverage and improve paleogeographic reconstructions.