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The Saguenay River
The geological origins of Saguenay Fjord National Park can be traced to the Grenville orogeny during the Precambrian era. This event is considered to be the beginnings to the Laurentian mountains. Around 200 million years ago, a rock basal complex between a north fault and a south fault collapsed, creating the Saguenay Graben. The graben was 250 kilometres (160 mi) long and 50 kilometres (31 mi) wide. During the last glacial period, the region was covered by ice sheets two to three kilometers deep. The ice sheets cut deep into the Saguenay graben, gouging the fjord in the process. The weight of the ice sheets also caused the region to sink. When the claciers melted around 10,000 years ago, the graben was flooded by seawater. The subsequent post-glacial rebound lifted the terrain, shaping the fjord valleys in the process.
The Saguenay Graben is a rift valley or graben in the geological Grenville Province of southern Quebec, Canada. It is an elongated flat-bottomed basin 250 km (155 mi) long and 50 km (31 mi) wide, bounded by normal faults running parallel to its length.
Formation of the Saguenay Graben
The time of formation of the faults related to the Saugenay Graben is still under debate because it is difficult to accurately measure the age of faulting. Evidence suggests it was either the opening of the Iapetus Ocean (600-400 Ma), or the opening of the Atlantic Ocean (195-170 Ma) that caused the faulting.
During the opening of one of these oceans, fragmentation of the landmass occurred creating two fault planes, one to the North and one to the South. The resulting bedrock between dropped down along the normal faults, creating the Saguenay Graben. The extent of these faults are only known at the surface and therefore their extension and shape at depth is unknown.
The faults associated with the Saguenay Graben have been the source for earthquakes, including the 1988 Saguenay earthquake.
The area was covered by ice sheets several times throughout the Pleistocene. The graben was located relatively parallel to the ice sheet movement and therefore caused it to become a preferred travel pathway for ice.
The glaciers cut into the graben and widened it in some places as well as making it considerable deeper in others. After the retreat of the final ice sheet, there was considerable isostatic rebound. The total amount of rebound varied from 140 m on the north side and 120 m on the south side.
Present day geography
The lowlands within the graben have an altitude of between 100 and 200 m. To the east there is the Kenogami threshold which is characterized by having an altitude of 200 to 260 m. This threshold splits the graben into two physiographic regions; the Lac Saint-Jean region to the west and the Saguenay region to the east. The plateau around the Graben is between 200 and 800 m in altitude. The Saguenay River as well as the Lac Saint-Jeanare both contained within the Saguenay Graben.
The Saguenay Graben is in the Grenville Province (but was created long after the Grenville Orogeny). The Saguenay Graben is characterized primarily by the rock types: gneiss, anorthosite and granite that are Proterozoic in age. There are two outliers of limestone and shale of the Paleozoic that are found only in the graben due to its faulting.
From Wikipedia, the free encyclopedia
I've always wondered about the Geology of the Saguenay. There's lots on the internet, this is from Wikipedia!
Le graben du Saguenay est une vallée québécoise dans laquelle on retrouve la population du Saguenay–Lac-Saint-Jean au Canada. Combinaison d'un effondrement tectonique et de l'érosion glaciaire, il est situé entre les monts Valin et la chaîne de montagne des Laurentides1. Son nom est emprunté à la rivière qui la sillonne ; la rivière Saguenay. Il s'étire sur une longueur de 250 km par 50 km de largeur et plus de 275 000 personnes y habitent répartis dans 49 municipalités.
Le graben du Saguenay origine d'une profonde déchirure du bouclier canadien qui se serait produite il y a environ 950 millions d'années. Des failles secondaires se sont ensuite produites, créant un réseau de rivières qui alimentent le Graben du Saguenay. Par la suite, les glaciers ont sculpté le paysage.
From the website of "Canadian Geographic" an interesting clip about the formation of the Saguenay
À partir du site Web de «Canadian Geographic» un clip intéressant sur la formation du Saguenay