The Saguenay Graben is a

rift valley A rift valley is a linear shaped lowland between several highlands or mountain ranges created by the action of a geologic rift. Rifts are formed as a result of the pulling apart of the lithosphere due to extensional tectonics. The linear dep ...

graben In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic contex ...

in the geological

Grenville Province The Grenville Province is a tectonically complex region, in Eastern Canada, that contains many different aged accreted terranes from various origins. It exists southeast of the Grenville Front and extends from Labrador southwestern to Lake Huron. ...

of southern

Quebec Quebec ( ; )According to the Canadian government, ''Québec'' (with the acute accent) is the official name in Canadian French and ''Quebec'' (without the accent) is the province's official name in Canadian English is one of the thirtee ...

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...

. It is an elongated flat-bottomed basin long and wide, bounded by

normal fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

s running parallel to its length.

Formation of the Saguenay Graben

The time of formation of the faults related to the Saguenay 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 The Iapetus Ocean (; ) was an ocean that existed in the late Neoproterozoic and early Paleozoic eras of the geologic timescale (between 600 and 400 million years ago). The Iapetus Ocean was situated in the southern hemisphere, between the paleoco ...

(600–400 Ma), or the opening of the

Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

(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

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...

s, including the

1988 Saguenay earthquake The 1988 Saguenay earthquake struck Quebec, Canada with a moment magnitude of 5.9 on November 25. It is one of the largest recorded earthquakes in eastern Canada and eastern North America during the 20th century. The earthquake was felt by million ...

Glaciations

The area was covered by ice sheets several times throughout the

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

. 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 Post-glacial rebound (also called isostatic rebound or crustal rebound) is the rise of land masses after the removal of the huge weight of ice sheets during the last glacial period, which had caused isostatic depression. Post-glacial rebound a ...

. 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 __NOTOC__ The Saguenay River () is a major river of Quebec, Canada. It drains Lac Saint-Jean in the Laurentian Highlands, leaving at Alma and running east; the city of Saguenay is located on the river. It drains into the Saint Lawrence River. T ...

as well as the

Lac Saint-Jean Lac Saint-Jean (Canadian French: ) is a large, relatively shallow lake in south-central Quebec, Canada, in the Laurentian Highlands. It is situated north of the Saint Lawrence River, into which it drains via the Saguenay River. It covers an are ...

are both contained within the Saguenay Graben.

Local geology

The Saguenay Graben is in the Grenville Province (but was created long after the

Grenville Orogeny The Grenville orogeny was a long-lived Mesoproterozoic mountain-building event associated with the assembly of the supercontinent Rodinia. Its record is a prominent orogenic belt which spans a significant portion of the North American continent, ...

). The Saguenay Graben is characterized primarily by the rock types:

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...

anorthosite Anorthosite () is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most c ...

and

granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...

that are

Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...

in age. There are two outliers of

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

of the

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

that are found only in the graben due to its faulting.

References

{{coord, 48, 30, N, 71, 20, W, display=title, region:CA-QC Aulacogens Structural basins of Canada Rifts and grabens Landforms of Saguenay–Lac-Saint-Jean Seismic faults of Canada Valleys of Quebec