The geology of

Quebec Quebec is Canada's List of Canadian provinces and territories by area, largest province by area. Located in Central Canada, the province shares borders with the provinces of Ontario to the west, Newfoundland and Labrador to the northeast, ...

involves several different geologic provinces, made up of ancient Precambrian crystalline igneous and metamorphic rock, overlain by younger sedimentary rocks and soils. Most of southern Quebec is dominated by the

Grenville Province The Grenville Province is a tectonically complex region, in Eastern Canada, that contains many different aged Terrane, accreted terranes from various origins. It exists southeast of the Grenville Front Tectonic Zone, Grenville Front and extends fro ...

, while the vast north is divided between the large Superior Province and the Churchill Province to the east, near Labrador.

Geologic history, stratigraphy & tectonics

The Grenville Province, which dominates southern Quebec, particularly the northern shore of the St. Lawrence formed beginning in the

Archean The Archean ( , also spelled Archaean or Archæan), in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history of Earth, history, preceded by the Hadean Eon and followed by the Proterozoic and t ...

, more than 2.5 billion years ago. Geologists subdivide the Grenville Province into the

Allochthon upright=1.6, Schematic overview of a thrust system. The hanging wall block is (when it has reasonable proportions) called a window. A klippe is a solitary outcrop of the nappe in the middle of autochthonous material. An allochthon, or an alloc ...

along the river itself and the more northern Parautochthon. The Parautochthon is a band running parallel to the Grenville Front, which varies in width from Labrador to northeastern

Georgian Bay The Georgian Bay () is a large bay of Lake Huron, in the Laurentia bioregion. It is located entirely within the borders of Ontario, Canada. The main body of the bay lies east of the Bruce Peninsula and Manitoulin Island. To its northwest is t ...

Lake Huron Lake Huron ( ) is one of the five Great Lakes of North America. It is shared on the north and east by the Canadian province of Ontario and on the south and west by the U.S. state of Michigan. The name of the lake is derived from early French ex ...

. Parautochthonous rocks are made up composed of ancient Archean and Proterozoic rocks that are highly deformed plutonic and metamorphosed supracrustal (metasedimentary and metavolcanic) rocks that reached

greenschist Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically and 2–10 kilobars (). Greenschists commonly have an abundance of green minerals such as Chlorite ...

granulite Granulites are a class of high-grade metamorphic rocks of the granulite facies that have experienced high-temperature and moderate-pressure metamorphism. They are medium to coarse–grained and mainly composed of feldspars sometimes associated ...

facies on the sequence of

metamorphic facies A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures.Essentials of Geology, 3rd Edition, Stephen Marshak The assemblage is typical of what is formed in conditions corresponding ...

during 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, ...

. These rocks correlate with the least deformed rocks north of the Grenville Front in the Superior Province and show signs of east-west lateral extension. The Allochthon includes all the lands of the Grenville Province south of the Parautochtone. The Allochthon juxtaposed on the Parautochthon during the Grenville orogeny cycle from 1.09 billion to 985 million years ago. The Allochthon is composed of

Paleoproterozoic The Paleoproterozoic Era (also spelled Palaeoproterozoic) is the first of the three sub-divisions ( eras) of the Proterozoic eon, and also the longest era of the Earth's geological history, spanning from (2.5–1.6 Ga). It is further sub ...

Mesoproterozoic The Mesoproterozoic Era is a geologic era that occurred from . The Mesoproterozoic was the first era of Earth's history for which a fairly definitive geological record survives. Continents existed during the preceding era (the Paleoproterozoic ...

rocks. In the western part, it is mainly

marble Marble is a metamorphic rock consisting of carbonate minerals (most commonly calcite (CaCO3) or Dolomite (mineral), dolomite (CaMg(CO3)2) that have recrystallized under the influence of heat and pressure. It has a crystalline texture, and is ty ...

quartzite Quartzite is a hard, non- foliated metamorphic rock that was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tecton ...

, and

pelite A pelite () or metapelite is a metamorphism, metamorphosed fine-grained sedimentary rock, i.e. mudstone or siltstone. The term was earlier used by geologists to describe a clay-rich, fine-grained clastic sediment or sedimentary rock, i.e. mud or ...

platform levels and Mesoproterozoic

amphibolite Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose ...

-grade rocks. There are also

charnockite Charnockite () is any orthopyroxene-bearing quartz-feldspar rock formed at high temperature and pressure, commonly found in granulite facies’ metamorphic regions, ''sensu stricto'' as an endmember of the charnockite series. Charnockite seri ...

and

anorthosite Anorthosite () is a phaneritic, intrusive rock, 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 ...

intrusions that intersect metasediments and orthogneiss. In the center,

migmatite Migmatite is a composite rock (geology), rock found in medium and high-grade metamorphic environments, commonly within Precambrian craton, cratonic blocks. It consists of two or more constituents often layered repetitively: one layer is an old ...

, quartzo-feldspathic orthogneisses and mangerites predominate. In its eastern part, it consists mainly of gneissic rocks of varied composition and origin, metasedimentary rocks, granitoid intrusions,

gabbro Gabbro ( ) is a phaneritic (coarse-grained and magnesium- and iron-rich), mafic intrusive igneous rock formed from the slow cooling magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...

, gabbronorite and

. There are also several anorthositic intrusions scattered throughout the Allochthon belt. The

Canadian Shield The Canadian Shield ( ), also called the Laurentian Shield or the Laurentian Plateau, is a geologic shield, a large area of exposed Precambrian igneous and high-grade metamorphic rocks. It forms the North American Craton (or Laurentia), th ...

spans much of northern Quebec, which is primarily underlain by the

Superior Craton The Superior Craton is a stable fault block, crustal block covering Quebec, Ontario, and southeast Manitoba in Canada, and northern Minnesota in the United States. It is the biggest craton among those formed during the Archean period. A craton is a ...

, a 160-mile thick section of stable continental crust formed beginning 4.03 billion years ago. In the northwest and at the northern tip of the Ungava Peninsula is the large Churchill Craton, which extends into Labrador and

Nunavut Nunavut is the largest and northernmost Provinces and territories of Canada#Territories, territory of Canada. It was separated officially from the Northwest Territories on April 1, 1999, via the ''Nunavut Act'' and the Nunavut Land Claims Agr ...

as the eastern extent of the Canadian Shield. Quebec's northern bedrock is also regionally composed of the Hudson Platform along the shore of

James Bay James Bay (, ; ) is a large body of water located on the southern end of Hudson Bay in Canada. It borders the provinces of Quebec and Ontario, and is politically part of Nunavut. Its largest island is Akimiski Island. Numerous waterways of the ...

and a small section of the Nain Province, near the northern tip of Labrador. The St. Lawrence Platform occupies the lowlands where much of Quebec's urban population lives, while the

Gaspé Peninsula The Gaspé Peninsula, also known as Gaspesia (, ; ), is a peninsula along the south shore of the St. Lawrence River that extends from the Matapedia Valley in Quebec, Canada, into the Gulf of St. Lawrence. It is separated from New Brunswick on it ...

, Anticosti Island and the Magdalene Islands are part of the Appalachian Province.

Paleozoic (539–251 million years ago)

During the

Taconic orogeny The Taconic orogeny was a mountain building period that ended 440 million years ago (Ma) and affected most of modern-day New England. A great mountain chain formed from eastern Canada down through what is now the Piedmont of the east coast of the ...

that began the formation of the

Appalachian Mountains The Appalachian Mountains, often called the Appalachians, are a mountain range in eastern to northeastern North America. The term "Appalachian" refers to several different regions associated with the mountain range, and its surrounding terrain ...

, mica schist and other metamorphic rocks formed a belt running down the Gaspe Peninsula to

Sherbrooke Sherbrooke ( , ) is a city in southern Quebec, Canada. It is at the confluence of the Saint-François River, Saint-François and Magog River, Magog rivers in the heart of the Estrie administrative region. Sherbrooke is also the name of a territ ...

. Muscovite bearing granites and metamorphic rocks around Asbestos formed over a broad span from 468 to 379 million years ago. While slate and plutonic

tonalite Tonalite is an igneous rock, igneous, plutonic (Intrusive rock, intrusive) rock (geology), rock, of felsic composition, with phaneritic (coarse-grained) texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with alkali fe ...

formed in the vicinity of Becancour and

Quebec City Quebec City is the capital city of the Provinces and territories of Canada, Canadian province of Quebec. As of July 2021, the city had a population of 549,459, and the Census Metropolitan Area (including surrounding communities) had a populati ...

respectively. Around 292 million years ago, volcanism deposited tuff in the Magdalene Islands.

Mesozoic (251–66 million years ago)

Sedimentary deposition and some volcanic activity took place during the

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

, particularly in the lowlands of the St. Lawrence Platform. Central

Montreal Montreal is the List of towns in Quebec, largest city in the Provinces and territories of Canada, province of Quebec, the List of the largest municipalities in Canada by population, second-largest in Canada, and the List of North American cit ...

is underlain by 119 to 117 million year old plutonic

basanite Basanite () is an igneous, volcanic ( extrusive) rock with aphanitic to porphyritic texture. It is composed mostly of feldspathoids, pyroxenes, olivine, and calcic plagioclase and forms from magma low in silica and enriched in alkali metal ox ...

lamprophyre Lamprophyres () are uncommon, small-volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks, and small intrusions. They are alkaline silica- undersaturated mafic or ultramafic rocks with high magnesium o ...

and camptonite while to the immediate southeast amphibole gabbro and peralkaline rocks formed from 141 to 123 million years ago.

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

intrusive rocks formed hills housed in the sedimentary rocks of the St. Lawrence Lowlands Platform, notably Mount St. Bruno.

Cenozoic (66 million years ago–present)

Quebec experienced intense erosion during the past 2.5 million years of the

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), as well as the current and most recent of the twelve periods of the ...

, which often mobilized older rocks, such as Proterozoic granite boulders found offshore in the moraines which formed parts of the Magdalene Islands.

References

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