Belt Supergroup
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| Belt Supergroup | |
|---|---|
| Stratigraphic range: | |
 Belt Supergroup strata exposed on Kintla Peak in Glacier National Park. | |
| Type | Geological supergroup |
| Sub-units | Many |
| Underlies | Flathead Formation |
| Overlies | Archean and Paleoproterozoic rocks |
| Thickness | more than 15 kilometres (10 mi) |
| Lithology | |
| Primary | Mudstone, argillite |
| Other | Sandstone, quartzite, conglomerate, intrusive rocks |
| Location | |
| Region | Montana, Idaho, Washington, Wyoming |
| Country | United States |
| Type section | |
| Named for | Big Belt Mountains, Montana |
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 15 kilometres (10 mi) thick, covers an area of some 200,000 km2 (77,220 sq. mi), and is considered to be one of the world's best-exposed and most accessible sequences of Mesoproterozoic rocks.[1] It was named after the Big Belt Mountains in west-central Montana. It is present in western Montana and northern Idaho, with minor occurrences in northeastern Washington and western Wyoming.[2] It extends into Canada where the equivalent rocks, which are called the Purcell Supergroup, are exposed in southeastern British Columbia and southwestern Alberta.[3] The rocks of the Belt Supergroup contain economically significant deposits of lead, zinc, silver, copper, gold, and other metals in a number of areas,[4] and some of the Belt rocks contain fossil stromatolites.[5]
Spectacular outcrops of Belt rocks can be seen in Glacier National Park in northwestern Montana[6] and in Waterton Lakes National Park in southwestern Alberta.[7]
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.[8] The Belt Supergroup also includes lesser amounts of coarser grained sandstones[9] and conglomerates.[10] Mafic intrusive rocks are present locally in the lower portion.[11]
Much of the sedimentation probably occurred between about 1450 and 1400 Ma (million years) ago.[11] 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.[2][8] The Belt Supergroup is also noted for "Molar Tooth" structures in carbonates (a bacterial degassing structure) and various types of stromatolites.[5]
Paleogeography and environment of deposition
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.[9] The basin appears to have been a closed "lacustrine" environment, or at least not completely open marine.[2][8] Depositional environments are thought to have ranged from ancient floodplains and exposed mudflats to deep water.[8]
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.[11][12]
