ICE-FLOW HISTORY AND DEGLACIATION PATTERN IN THE SAGUENAY-LAC-ST-JEAN AREA (QUÉBEC, CANADA)

In 2009, a Quaternary geologic mapping project was undertaken in the municipal territory of Saguenay-Lac-Saint-Jean (SLSJ) area under a provincial groundwater program. Since most of the surficial geology information for this area date from 1970s, the main objective of this project was to recompile and update existing data into surficial geologic maps that use a standardized legend common to other recent quaternary geologic mapping projects. To achieve this goal, a new air-photo analysis (1:40 000 aerial photographs) and two field seasons (2009 and 2010) involving 4 mapping teams were conducted. Nineteen maps of surficial deposits at 1 : 50 000 scale were produced in the SLSJ. This presentation focuses on the ice-flow history based on a compilation of glacial erosional features, the deglaciation pattern and the reconstruction of the Laflamme Sea episode.

We identified more than 1300 intermediate-scale erosional forms (crag-and-tail, drumlins and rock drumlins) and about 585 small-scale erosional features (striae, grooves, crescentic fractures) across the study area. Analysis of the cross-cutting striations indicates three successive main ice-flow directions: 1) SO 2) SSE, and 3) ESE, which is only found in the lowland of the SLSJ.

The deglaciation pattern based on the spatial distribution of ice-marginal features, glaciolacustrine deposits, and marine deposits of the Laflamme Sea suggests that occupation of SLSJ graben by a late glacial ice lobe delayed invasion by the Laflamme Sea waters and blocked the drainage of several rivers, which thus formed glacial lakes in the highlands.

The maximum extent of the Laflamme Sea was established by using DGPS and photogrammetric measurements on 42 glaciomarine deltas and 130 wave washing limits. The marine limit elevation reached by this postglacial sea varies from 138 m (AMSL) SE of the study area near Petit-Saguenay and raised up to 215 m (AMSL) north of the lac Saint-Jean near Mélançon. In the study area, the Laflamme Sea covered about 7480 km² between 10,5 ¹⁴C ka BP (12,65-12,75 cal ka ) and 8 ¹⁴C ka BP (8,9 cal ka).