The Purcell Supergroup is composed primarily of argillites, carbonate rocks, quartzites, and mafic igneous rocks of late Precambrian (Mesoproterozoic) age. It is present in an area of about in southwestern Alberta and southeastern British Columbia, Canada, and it extends into the northwestern United States where it is called the Belt Supergroup. It was named for the Purcell Mountains of British Columbia by R.A. Daly in 1912. Fossil stromatolites and algal 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, a world-class deposit of lead, zinc, and silver, lies within the Alderidge Formation in the lower part of the Purcell.
Spectacular outcrops of Purcell and Belt Supergroup rocks can be seen in Glacier National Park in northwestern MontanaAlt, D.D. and Hyndman, D.W. 1986. Roadside geology of Montana. Mountain Press Publishing Co., Missoula, Montana, 427 p. , and Waterton Lakes National Park 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 and dolomite), and quartzites, and includes localized occurrences of igneous rocks (mafic lava flows, tuffs, pillow basalts, and gabbroic and dioritic sills and dykes). Sedimentary structures are well preserved in the Purcell rocks despite their great age.
In the southern Canadian Rockies (Waterton Park area), the supergroup is subdivided as follows:
Geological unit Lithology Environment of Deposition MaximumThickness Reference Roosville Formation green and grey argillite; dolomitic argillite, siltstone and sandstone; argillaceous and stromatolitic dolomite; mudcracks and ripple marks are common. shallow marine (peritidal) Phillips Formation red, thin-bedded quartz sandstone; siltstone interbedded with argillite and conglomerate; mudcracks and ripple marks are common. marginal marine to nonmarine Gateway Formation red siltstone and argillite; green argillite; dolomitic argillite; dolomitic sandstone; dolomite; and sandy dolomite; casts of salt crystals, mudcracks, ripple marks, and rip-up clasts are locally abundant. marginal marine Sheppard Formation light grey dolomite and stromatolitic dolomite; red dolomitic siltstone and sandstone; dolomitic argillite. shallow marine Purcell Lava dark green and reddish green to purple chloritized mafic lava flows, pillow basalts, gabbroic sills and dikes; amygdules of quartz, chlorite, and calcite are common. shallow marine Siyeh Formation argillaceous limestone and dolomite; black and green argillite; dolomitic quartzite; stromatolitic dolomite. marginal marine Appekunny Formation green and maroon argillite; white, grey, green, and pale red sandstone; quartz-pebble conglomerate. marginal marine Altyn Formation grey, thin-bedded argillaceous limestone and dolomite; massive sandy dolomite and stromatolitic dolomite; dark grey to black argillite. shallow marine Waterton Formation grey, green and red argillaceous dolomite; banded and streaked limestone and dolomite; grey and green, thin-bedded argillite. marine Tombstone Mountain Formation dark grey argillite; dolomitic argillite; argillaceous dolomite and limestone. marine Haig Brook Formation light colored, cliff-forming sequence of dolomite; banded and streaked limestone and dolomite; minor argillite; base of formation is not exposed. marine
In the southern Purcell Mountains (Cranbrook area), the supergroup is subdivided as follows:
Geological unit Lithology Environment of Deposition MaximumThickness Reference Roosville Formation dolomitic argillite; siltstone; sandstone; and argillaceous and stromatolitic dolomite. intertidal Phillips Formation red, thin-bedded quartz sandstone; siltstone interbedded with argillite; ripple marks and mud cracks are abundant locally. shallow water to subaerial Gateway Formation grey-green, red, and purple siltstone; dolomitic siltstone; minor interbeds of argillite; casts of salt crystals, mudcracks, ripple marks, and rip-up clasts are locally abundant. lagoonal Sheppard Formation light grey stromatolitic dolomite, interbedded with dolomitic siltstone and argillite; stromatolite mounds up to thick; mudcracks, ripple marks, and rip-up clasts are locally abundant; casts of salt crystals are rare. intertidal Intrusive rocks fine- to coarse-grained sills and dikes of gabbro and diorite intrude the Aldridge to Van Creek Formations. intrusive Nichol Creek Formation green and purple argillite and siltstone; green volcanic sandstone and tuff interlayered with green or maroon, chloritized and sericitized basaltic to andesitic lavas, some with amygdules of quartz and chlorite. subaerial Van Creek Formation green to purple argillite and siltstone; mud cracks, ripple marks, and rip-up clasts are locally abundant. intertidal Kitchner Formation calcareous and dolomitic siltstone and argillite; silty dolomite and limestone; minor quartzite. shallow subtidal(intertidal at base) Creston Formation green, grey, and purple siltstone and argillite; mud cracks, ripple marks, and rip-up clasts are locally abundant. shallow subtidal(intertidal at top) Aldridge Formation fine-grained quartzite; argillaceous quartzite; rusty-weathering grey siltstone; dark grey argillite; base of formation not exposed. marine (subtidal at top) >
Environment of deposition The Purcell Supergroup was probably deposited in subsiding deltaic to marine environments along the margin of the North American craton, possibly in an intracratonic basin where North America and another landmass were joined in a supercontinent 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 era, 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 that reaches from the southern Purcell Mountains in southeastern British Columbia to the southern Canadian Rockies in the southwestern Alberta. It extends southward into the United States (western Montana, northern Idaho, northwestern Washington, and western Wyoming) where it is called the Belt Supergroup. It reaches a maximum thickness of more than in the Purcell Mountains.
Relationship to other units The Purcell Supergroup is equivalent to the Belt Supergroup of the northwestern United States. The base of the Purcell is not exposed in Canada, but it is inferred to rest unconformably on the Canadian Shield. The Purcell is unconformably overlain by the Neoproterozoic Windermere Supergroup in most areas, or by younger Cambrian or Devonian formations where the Windermere is absent.
Economic resources The now-closed Sullivan Mine 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 tons of lead, 7,944,446 tons of zinc, and 9,264 tons of silver, as well as significant quantities of tin and other metals.
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