Belt Supergroup

The Belt Supergroup, is an assemblage of Mesoproterozoic sedimentary rocks, primarily mudstones, which outcrop chiefly in western Montana, but also exposed in Idaho, Washington, Wyoming, and British Columbia. It is most famous as the formation that makes up Glacier National Park in northwest Montana. It has been geologically linked to the Purcell Supergroup in British Columbia, rocks of the Udzha Basin in Siberia, and the Rocky Cape Group in Tasmania. It was named after the Big Belt Mountains, Little Belt Mountains and town of Belt, all in western-central Montana.

Characteristics

The Belt Supergroup was deposited from about 1470 to 1400 million years ago and is as much as 18 km thick. It is mostly made of fine-grained quartzites, argillite, carbonates, and mafic sills. In the primary location, western Montana and northern Idaho, the Belt is divided into four groups (youngest to oldest):

• Missoula Group - More fluvial deposition of sands and muds, similar to the Ravalli Group but from the south
• Piegan Group (Middle Belt Carbonate) - Carbonate muds alternating with lamina of clastic muds
• Ravalli Group - Subaerial-deposited sands and muds, mostly fluvial, also from the southwest
• Lower Belt - Heterogeneous coarse to fine grained, clastic to carbonate rocks, mostly subaqueous, deep water deposition, with sediments derived from the southwest, with common mafic sills

Formation

It is generally believed that the Belt Supergroup was formed in a fault-bounded (rift) basin within continental crust, and that the rocks are "lacustrine", or at least, not completely open marine. The basin was a part of a 1.45 Ga supercontinent (Columbia/Nuna) that predated Rodinia. It shows evidence of these basin-bounding faults on all sides, except the west, which rifted away in the breakup of Rodinia.

Sedimentology

The Belt Supergroup is known for the strange sedimentary structures it displays. The sedimentation in the Belt basin was strange because 1) there is an abundance of fine-grained sand, but not much sediment coarser, 2) lack of sequence boundaries that are common in Phanerozoic sediments, and 3) cyclic and rhythmic deposition over long periods of time. The Belt Supergroup is also noted for "Molar Tooth" structures in carbonates (a bacterial degassing structure) and abundant stromatolites of various types.

References

• 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.
• "Digital Geology of Idaho - Mesoproterozoic Belt Supergroup"