The Purcell Supergroup is composed primarily of
argillite
:''"Argillite" may also refer to Argillite, Kentucky.''
Argillite () is a fine-grained sedimentary rock composed predominantly of indurated clay particles. Argillaceous rocks are basically lithified muds and oozes. They contain variable amo ...
s,
carbonate rock
Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3), and dolomite rock (also known as doloston ...
s,
quartzite
Quartzite is a hard, non- foliated metamorphic rock which 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 tec ...
s, and
mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks includ ...
igneous rock
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or l ...
s of late
Precambrian
The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of th ...
(
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 ...
) age. It is present in an area of about 15,000 km
2 (5,800 sq. mi.) in southwestern
Alberta
Alberta ( ) is one of the thirteen provinces and territories of Canada. It is part of Western Canada and is one of the three prairie provinces. Alberta is bordered by British Columbia to the west, Saskatchewan to the east, the Northwest Ter ...
and southeastern
British Columbia
British Columbia (commonly abbreviated as BC) is the westernmost Provinces and territories of Canada, province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include ...
,
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 tota ...
, and it extends into the northwestern
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., federal district, five ma ...
where it is called the
Belt Supergroup
The Belt Supergroup is an assemblage of primarily fine-grained sedimentary rocks and mafic intrusive rocks of late Precambrian (Mesoproterozoic) age. It is more than thick, covers an area of some 200,000 km2 (77,220 sq. mi), and is considered to ...
. It was named for the
Purcell Mountains
The Purcell Mountains are a mountain range in southeastern British Columbia, Canada. They are a subrange of the Columbia Mountains, which includes the Selkirk, Monashee, and Cariboo Mountains. They are located on the west side of the Rocky Mou ...
of British Columbia by R.A. Daly in 1912.
Fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
stromatolite
Stromatolites () or stromatoliths () are layered sedimentary formations ( microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria). ...
s and
algal
Algae ( , ; : alga ) are any of a large and diverse group of photosynthetic, eukaryotic organisms. The name is an informal term for a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from un ...
structures are common in some of the Purcell Supergroup rocks,
[O'Connor, M.P. 1972. Classification and environmental interpretation of the cryptalgal organosedimentary "Molar-Tooth" structure from the Late Precambrian Belt-Purcell Supergroup. Journal of Geology, vol. 80, no. 5, p. 592-610.][McMechan, M.E. 1981. The Middle Proterozoic Purcell Supergroup in the southwestern Rocky and southeastern Purcell Mountains, British Columbia, and the initiation of the Cordilleran Miogeocline, southern Canada and adjacent United States. Bulletin of Canadian Petroleum Geology, vol. 29, no. 4, p. 583-621.] and the
Sullivan ore body at
Kimberley, British Columbia
Kimberley is a city in southeast British Columbia, Canada along Highway 95A between the Purcell and Rocky Mountains. Kimberley was named in 1896 after the Kimberley mine in South Africa. From 1917 to 2001, it was the home to the world's largest ...
, a world-class deposit of
lead
Lead is a chemical element with the Symbol (chemistry), symbol Pb (from the Latin ) and atomic number 82. It is a heavy metals, heavy metal that is density, denser than most common materials. Lead is Mohs scale of mineral hardness#Intermediate ...
,
zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic t ...
, and
silver
Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
, lies within the Alderidge Formation in the lower part of the Purcell.
Spectacular
outcrop
An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth.
Features
Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficia ...
s of Purcell and Belt Supergroup rocks can be seen in
Glacier National Park in northwestern
Montana
Montana () is a U.S. state, state in the Mountain states, Mountain West List of regions of the United States#Census Bureau-designated regions and divisions, division of the Western United States. It is bordered by Idaho to the west, North ...
[Alt, D.D. and Hyndman, D.W. 1986. Roadside geology of Montana. Mountain Press Publishing Co., Missoula, Montana, 427 p. ,] and
Waterton Lakes National Park
Waterton Lakes National Park is a national park located in the southwest corner of Alberta, Canada. It borders Glacier National Park in Montana, United States. Waterton was the fourth Canadian national park, formed in 1895 and named after Watert ...
in southwestern Alberta.
[Gordy, P.L., Frey, F.R. and Norris, D.K. 1977. Geological guide for the CSPG 1977 Waterton-Glacier Park Field Conference. Canadian Society of Petroleum Geologists, Calgary, Alberta, 93 p.]
Stratigraphy and lithology
The Purcell Supergroup consists primarily of argillites, carbonate rocks (
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 wh ...
and
dolomite), and quartzites, and includes localized occurrences of igneous rocks (
mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks includ ...
lava
Lava is molten or partially molten rock ( magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or ...
flows,
tuff
Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...
s,
pillow basalts, and
gabbro
Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ...
ic and
dioritic sills and
dykes).
Sedimentary structures
Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition.
Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different partic ...
are well preserved in the Purcell rocks despite their great age.
In the southern
Canadian Rockies
The Canadian Rockies (french: Rocheuses canadiennes) or Canadian Rocky Mountains, comprising both the Alberta Rockies and the British Columbian Rockies, is the Canadian segment of the North American Rocky Mountains. It is the easternmost par ...
(Waterton Park area), the supergroup is subdivided as follows:
In the southern Purcell Mountains (
Cranbrook area), the supergroup is subdivided as follows:
Environment of deposition
The Purcell Supergroup was probably deposited in subsiding
deltaic
A river delta is a landform shaped like a triangle, created by deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rare ...
to marine environments along the margin of the
North American craton
North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating direction or geography.
Etymology
The word ''north'' is ...
,
possibly in an
intracratonic basin where North America and another landmass were joined in a
supercontinent
In geology, a supercontinent is the assembly of most or all of Earth's continental blocks or cratons to form a single large landmass. However, some geologists use a different definition, "a grouping of formerly dispersed continents", which lea ...
called
Columbia/Nuna.
[Schieber, J., 1989. The origin of the Neihart Quartzite, a basal deposit of the mid-Proterozoic Belt Supergroup, Montana, USA. Geological Magazine, v. 126, p. 271-281.] Deposition occurred during the
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 ...
era
An era is a span of time defined for the purposes of chronology or historiography, as in the regnal eras in the history of a given monarchy, a calendar era used for a given calendar, or the geological eras defined for the history of Earth.
Com ...
, much of it probably between about 1470 and 1400
Ma (million years) ago.
[Evans, K.V., Aleinikoff, J.N., Obradovich, J.D. and Fanning, C.M. 2000. SHRIMP U-Pb geochronology of volcanic rocks, Belt Supergroup, western Montana: evidence for rapid deposition of sedimentary strata. Canadian Journal of Earth Sciences, v. 37, p.1287-1300.]
Distribution and thickness
In Canada, the Purcell Supergroup is present in an area of about 15,000 km
2 (5,800 sq. mi.) that reaches from the southern
Purcell Mountains
The Purcell Mountains are a mountain range in southeastern British Columbia, Canada. They are a subrange of the Columbia Mountains, which includes the Selkirk, Monashee, and Cariboo Mountains. They are located on the west side of the Rocky Mou ...
in southeastern British Columbia to the southern
Canadian Rockies
The Canadian Rockies (french: Rocheuses canadiennes) or Canadian Rocky Mountains, comprising both the Alberta Rockies and the British Columbian Rockies, is the Canadian segment of the North American Rocky Mountains. It is the easternmost par ...
in the southwestern Alberta.
It extends southward into the United States (western
Montana
Montana () is a U.S. state, state in the Mountain states, Mountain West List of regions of the United States#Census Bureau-designated regions and divisions, division of the Western United States. It is bordered by Idaho to the west, North ...
, northern
Idaho
Idaho ( ) is a state in the Pacific Northwest region of the Western United States. To the north, it shares a small portion of the Canada–United States border with the province of British Columbia. It borders the states of Montana and W ...
, northwestern
Washington
Washington commonly refers to:
* Washington (state), United States
* Washington, D.C., the capital of the United States
** A metonym for the federal government of the United States
** Washington metropolitan area, the metropolitan area centered o ...
, and western
Wyoming
Wyoming () is a state in the Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the southwest, and Colorado to the sou ...
) where it is called the
Belt Supergroup
The Belt Supergroup is an assemblage of primarily fine-grained sedimentary rocks and mafic intrusive rocks of late Precambrian (Mesoproterozoic) age. It is more than thick, covers an area of some 200,000 km2 (77,220 sq. mi), and is considered to ...
.
It reaches a maximum thickness of more than 10,000 metres (32,000 ft) in the Purcell Mountains.
Relationship to other units
The Purcell Supergroup is equivalent to the
Belt Supergroup
The Belt Supergroup is an assemblage of primarily fine-grained sedimentary rocks and mafic intrusive rocks of late Precambrian (Mesoproterozoic) age. It is more than thick, covers an area of some 200,000 km2 (77,220 sq. mi), and is considered to ...
of the northwestern United States. The base of the Purcell is not exposed in Canada, but it is inferred to rest
unconformably
An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...
on the
Canadian Shield
The Canadian Shield (french: Bouclier canadien ), also called 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), the anc ...
. The Purcell is unconformably overlain by the
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago.
It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is ...
Windermere Supergroup
The Windermere Supergroup is a geological unit formed during the Ordovician to Silurian periods ~, and exposed in northwest England, including the Pennines and correlates along its strike, in the Isle of Man and Ireland, and down-dip in the Southe ...
in most areas, or by younger
Cambrian
The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ag ...
or
Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, w ...
formations where the Windermere is absent.
Economic resources
The now-closed
Sullivan Mine
The Sullivan Mine is a now-closed conventional–mechanized underground mine located in Kimberley, British Columbia, Canada. The ore body is a complex, sediment-hosted, sedimentary exhalative deposit consisting primarily of zinc, lead, and i ...
at Kimberley, British Columbia, worked a world-class
sedimentary exhalative (SedEx) deposit that is hosted in the lower part of the Purcell Supergroup. During the life of the mine, the Sullivan ore body is reported to have yielded 8,412,077 tonnes of lead, 7,944,446 tones of zinc, and 9,264 tonnes of silver, as well as significant quantities of
tin
Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal.
Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, ...
and other metals.
References
{{reflist
Geologic formations of Canada
Western Canadian Sedimentary Basin
Stratigraphy of Alberta
Stratigraphy of British Columbia