Eastmain-1 region Quaternary surficial deposits

Using this approach, it is imperative to characterize the different components of the terrestrial landscape prior to flooding in order to better understand the impacts of this important project on the boreal environment. The surface quaternary deposit axes of this project aims at identifying, mapping and define the paleo-geographic context of the surface deposits set in place in the Eastmain-1 region. This work will allow quantification of the inorganic carbon stored in the sediments to later estimate their potential contribution to the greenhouse gas (GHG) budget following the creation of the Eastmain-1 hydro-electric reservoir.

In order to identify and map the surface sediments, photo-interpretation of the area and a validation field campaign were conduct in 2005 prior to impoundments of the Eastmain-1 reservoir. During a field campaign, multiple sites were visited to identify the nature and origin of the deposits, measure their extension and collect samples for later laboratory analysis to determine granulometric size and carbonates content (Table 1). Glacial erosion marks measures were taken on the bedrock (striea and broutures) to determine the direction of the glacial flow when the glacier covered the territory. 

Tableau 1 : Mean carbonate content obtained trough calcimetric analysis in the surface deposits of the Eastmain-1 area.

Results show  that the major part of the surficial deposits came from events related to the last glaciation and it’s retreat, so between 20 000 and 6000 years ago. During that period, a 2 km thick ice layer formed what is called an islandsis that was covering the entire Quebec province. The displacement of this important ice mass has eroded the bedrock leaving numerous deposits of different sizes and extents. Composition of these stratigraphic layers has been identified in the region as: from bottom, glacial sediments left by the islandsis to form lodgment till, melt out till and De Geer’s moraines, following the nature and the length of the events. With the retreat of the ice from the Quebec territory, post-glacial origin sediments were set up by melting water (fluvioglacial deposits, Figure 1) and those associated with flooding of the area by marine waters such as clay deposits and shore deposits later reorganized by the wind to form dunes (Aeolian deposits, Figure 2). More recently in the post-glacial history, during the last 6000 years, organic deposits have filled the depressions to form peatlands and alluvial deposits were left by the comptemporeous fluvial processes. Flow marks left by the glacier have created microforms (striea, grooves, crescentic fractures) or mesoforms (dissymmetric rocks). Glacial flow orientation measures indicate directions between 220° and 290° (North-West and South-West).