Belt Supergroup

The Belt Supergroup is dominated by fine-grained sedimentary rocks, primarily mudstones, siltstones, fine-grained quartzose sandstones and limestones. Most have undergone weak metamorphism to greenschist facies, and as a result the mudrocks are commonly classified as argillites and the sandstones as quartzites.Schieber, J. 1990. Significance of styles of epicontinental shale sedimentation in the Belt basin, Mid-Proterozoic of Montana, U.S.A. Sedimentary Geology, v. 69, p. 297–312. The Belt Supergroup also includes lesser amounts of coarser grained sandstones and conglomerates.McMannis, W.J., 1963. LaHood Formation—a coarse facies of the Belt Series in southwestern Montana. Geological Society of America Bulletin, v. 74, p.407-436. Mafic intrusive rocks are present locally in the lower portion.

File:Snowslip Fm.jpg

Much of the sedimentation probably occurred between about 1450 and 1400 Ma (million years) ago. Sedimentary structures are well preserved in most of the Belt rocks despite their great age. The sedimentation is unusual in that 1) there is an abundance of fine-grained sediment and very little coarser sediment, 2) there is a lack of sequence boundaries that are common in Phanerozoic sediments, and 3) cyclic and rhythmic deposition occurred over long periods of time. The Belt Supergroup is also noted for "Molar Tooth" structures in carbonates (a bacterial degassing structure) and various types of stromatolites.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.

File:Helena Fm.jpg

Paleogeographic reconstructions indicate that the Belt Supergroup accumulated in a fault-bounded rift basin that existed where the North American craton 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. The basin appears to have been a closed "lacustrine" environment, or at least not completely open marine.{{cite web|last1=Lori Tapanila and Paul Link|title=Mesoproterozoic Belt Supergroup|url=http://geology.isu.edu/Digital_Geology_Idaho/Module2/mod2.htm|website=Digital Geology of Idaho|publisher=Idaho State University, Department of Geosciences|access-date=16 September 2016|archive-date=5 December 2019|archive-url=https://web.archive.org/web/20191205122316/http://geology.isu.edu/Digital_Geology_Idaho/Module2/mod2.htm|url-status=dead}} Depositional environments are thought to have ranged from ancient floodplains and exposed mudflats to deep water.

Evidence of the basin-bounding faults exists on all sides of the Belt basin except the west, which rifted away during subsequent continental breakup. The identity of the joined landmass remains controversial. The Siberian craton, Australia and eastern Antarctica have all been suggested based rock ages and paleomagnetic information.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.{{cite journal | last1 = Halpin| first1 = Jacqueline A.| last2 = Jensen| first2 = Torsten| last3 = McGoldrick| first3 = Peter| last4 = Meffre| first4 = Sebastien| last5 = Berry| first5 = Ron F.| last6 = Everard| first6 = John L.| last7 = Calver| first7 = Clive R.| last8 = Thompson| first8 = Jay| last9 = Goemann| first9 = Karsten| last10 = Whittaker| first10 = Joanne M.| year = 2014 | title = Authigenic monazite and detrital zircon dating from the Proterozoic Rocky Cape Group, Tasmania: Links to the Belt–Purcell Supergroup, North America | journal = Precambrian Research | volume = 250 | pages = 50–67 | doi = 10.1016/j.precamres.2014.05.025 | bibcode = 2014PreR..250...50H}}

File:Grinnell Fm.jpg

File:Appekunny Fm.jpg, green and red argillite and siltite and moderate amounts of light-gray quartzite. Divided into five informal members.]]

File:Grinnell mudcracks.jpg

The Belt Supergroup was deposited unconformably on Archean and Paleoproterozoic rocks. It reaches thicknesses of more than {{convert|15|km|mi|-1}} and is present in western Montana and northern Idaho, with minor occurrences in northeastern Washington and western Wyoming. Because of this widespread extent, the rock types and formation names of the Belt Supergroup vary depending upon location. In western Montana and northern Idaho the Belt is divided into the following four groups (youngest to oldest):

• Missoula Group – Fluvial sands and muds derived from the south.
• Piegan Group (Middle Belt Carbonate) – Carbonate muds alternating with laminae of clastic muds.
• Ravalli Group – Subaerially deposited sands and muds, mostly fluvial, derived from the southwest.
• Lower Belt – Heterogeneous coarse- to fine-grained clastic and carbonate rocks, mostly deep-water deposition with sediments derived from the southwest, and mafic sills.

The Belt Supergroup extends into Canada where the equivalent rocks are called the Purcell Supergroup, and are exposed in southeastern British Columbia and southwestern Alberta.Glass, D.J. (editor) 1997. Lexicon of Canadian Stratigraphy, vol. 4, Western Canada including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba. Canadian Society of Petroleum Geologists, Calgary, 1423 p. on CD-ROM. {{ISBN|0-920230-23-7}}.

The Belt Supergroup currently overlies softer Cretaceous age rock that is about 1300 million years younger. This apparent violation of the law of superposition was caused by the Lewis Overthrust.

File:Altyn and Appekunny Fm Slingshot Mtn.jpg limestone]]

The Belt Supergroup rocks host a variety of economically significant deposits of lead, zinc, silver, copper, gold, and other metals. These include the Coeur d'Alene lead–zinc–silver mining district in Idaho, which has produced about 7,400,000 tons of lead, 2,900,000 tons of zinc, and 35,600 tons of silver. The equivalent rocks of the Purcell Supergroup in British Columbia include the Sullivan ore body, which has also been a major producer of lead, zinc, and silver.

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{{Reflist}}

• Winston, Don and Link, Paul K., 1993, Middle Proterozoic rocks of Montana, Idaho, and Washington: The Belt Supergroup: in Reed., J., Simms, P., Houston, R., Rankin, D., Link, P., Van Schmus, R., and Bickford, P., eds., Precambrian of the conterminous United States: Boulder, Colorado, Geological Society of America, The Geology of North America, v. C-3, p. 487–521.
• [http://geology.isu.edu/Digital_Geology_Idaho/Module2/mod2.htm "Digital Geology of Idaho – Mesoproterozoic Belt Supergroup"] {{Webarchive|url=https://web.archive.org/web/20191205122316/http://geology.isu.edu/Digital_Geology_Idaho/Module2/mod2.htm |date=2019-12-05 }}

Category:Geologic supergroups of the United States

Category:Geologic supergroups of Canada

Category:Historical geology

Category:Stratigraphy of Idaho

Category:Stratigraphy of Montana

Category:Mesoproterozoic