Using discontinuous wave-cut terraces to reconstruct the history of former glacial lake levels: the example of Lake Ojibway in NW Quebec (Canada)
Abstract
The decay of the southern Laurentide ice sheet maring during the last deglaciation led to the development of Lake Agassiz and Lake Ojibway that covered large areas in the western prairies and in NE Ontario-NW Quebec, respectively. The history of glacial lakes is commonly based on the study of strandlines that generally consist of sandy beaches (and near-shore facies) or boulder ridges. However, the use of this approach is limited in the main Ojibway basin where the surficial geology consists predominantly of thick accumulation of fine-grained glaciolacustrine deposits that mask most deglacial landforms and the underlying bedrock. Nonetheless, earlier mapping programs in this flat-lying clay plain revealed a complex sequence of discontinuous small-scale cliffs that are made of Ojibway rhythmites. These terrace-like features range in size from 4 to 7 m in height and can generally be followed for 10 to 100's of meters, and sometimes for several kms. These small-scale features are interpreted to represent raised shorelines that were cut into glaciolacustrine sediments by lakeshore erosional processes (i.e., wave action). These so-called wave-cut benches (WCBs) occur at elevations ranging mostly from 3 to 30 m above the present level of Lake Abitibi (267 m), one of the lowest landmarks in the area. Here we evaluate the feasibility of using this type of shorelines to constrain the evolution of Ojibway lake levels in NW Quebec. For this purpose, a series of wave-cut terraces (WCBs) were measured along two north-south transects of about 40 km in length in the Lake Abitibi region. The absolute elevation of more than 70 WCBs was determined with a Digital Video Plotter software package using 1:15K air-photos, coupled with precise measurements (x,y,z coordinates) of control points, which were measured with a high-precision Global Navigation Satellite System tied up to known geodesic survey markers. Preliminary results suggest that Lake Ojibway experienced at least three different phases in the Lake Abitibi region, at elevations of 290 m, 297 m, and 313 m. For comparison, the near-maximum phase of Lake Ojibway lies at 460 m, about 250 km to the NE of the study area. Overall, the elevation and position of these wave-cut terraces suggest they were formed during episodes of long stands associated with late-stage phases of glacial Lake Ojibway. An additional lake level is indicated by the lowest set WCBs that lie about 6 m above modern Lake Abitibi. These terraces are also restricted to area surrounding this lake, likely reflecting the occurrence of a paleolake Abitibi. These preliminary results thus underlie the strong potential of using these lakeshore features to reconstruct former lake level history. However, the data gathered so far do not allow firm conclusions on the number, exact elevation, and regional extent of the lake-level phases documented. Additional data are required at the scale of the area submerged by Lake Ojibway. The continuation of this work should also provide constraints on the origin of these lake-level phases, thereby potentially reinforcing our understanding of the role of meltwater discharges in climate fluctuations that marked the early Holocene.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMEP51A0973D
- Keywords:
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- 0473 BIOGEOSCIENCES / Paleoclimatology and paleoceanography;
- 0746 CRYOSPHERE / Lakes;
- 1824 HYDROLOGY / Geomorphology: general;
- 9350 GEOGRAPHIC LOCATION / North America